This paper proposes a novel low-complexity precoding weight for downlink of multi-user massive multiple-input multiple-output (MU-mMIMO) system in time-varying channel environments. Inter-user interference (IUI) suppression performance in the MU-MIMO system deteriorates due to the mismatch between the channel state used for precoding weight derivation and that during signal transmission. Leveraging the massive antenna degrees of freedom (DOFs) in mMIMO, null-space expansion (NSE) was previously conceived to improve IUI suppression performance by steering multiple nulls toward undesired user equipments (UEs), even in high-mobility environment. However, NSE has severe challenges, such as high computation complexity caused by the increasing nullification and near singular matrix structure of the Gram matrix due to high time correlation. Considering these challenges, this paper proposes Jacobi matrix-based iterative low-complexity NSE weight design. Focusing on the Gram matrix’s block diagonal structure in NSE, this paper presents a Jakes’ model-based approximation and Ridge parameter optimization using deep unfolding (DU) to enhance the convergence of the inverse matrix. The effectiveness of the proposed scheme is demonstrated through computer simulations.

The radio stripe (RS) concept has emerged as a viable and practical implementation of the cell-free (CF) massive multiple-input-multiple-output (mMIMO) paradigm, where a sequence of distributed access points (APs) sharing a common stripe cooperatively serve user-equipments (UEs) over both uplink (UL) and downlink channels. In such networks, active eavesdroppers can launch pilot-spoofing attacks, fully informed of the legitimate pilot sequences and AP-selection scheme, to contaminate UL channel estimation (CE), thereby degrading overall throughput and leaking confidential data to the adversary. To counteract spoofing, we analyze a bi-objective AP-selection framework that jointly maximizes (1) the network sum spectral efficiency (SE) and (2) the secrecy spectral efficiency (SSE) of the attacked UEs. Our results traces the Pareto frontier of sum SE versus attacked UE SE, revealing the inherent trade-offs between throughput and security.

In this paper, we propose a multi-user downlink system for two users based on the orthogonal time frequency space (OTFS) modulation scheme. The design leverages the generalized singular value decomposition (GSVD) of the channels between the base station and the two users, applying precoding and detection matrices based on the right and left singular vectors, respectively. We derive the analytical expressions for three scenarios and present the corresponding simulation results. These results demonstrate that, in terms of bit error rate (BER), the proposed system outperforms the conventional multi-user OTFS system in two scenarios when using minimum mean square error (MMSE) equalizers or precoder, both for perfect channel state information and for a scenario with channel estimation errors. In the third scenario, the design is equivalent to zero-forcing (ZF) precoding at the transmitter.

In this paper, we consider the precoder design for multiuser multiple input multiple output (MU-MIMO) systems based on orthogonal frequency domain multiplexing (OFDM) with rate splitting at the transmitter. We propose precoding and decoding schemes based on generalized singular value decomposition (GSVD), both for underloaded and overloaded communication systems that support the transmission of multiple common and private streams. To address performance degradation caused by nonlinearities from GSVD-RSMA, an iterative distortion canellation receiver-aided precoding method is introduced. The approach improves stream detection without requiring successive interference cancellation, enhancing robustness in nonlinear environments1.

Ultra-reliable low-latency IoT communications (URLLC-IoT) has recently gained a growing interest. Here the challenge in reliable low-latency uplink transmission results from the transmission power limitations and lack of multiple antennas. However, in many IoT services the data volumes are small and sensor deployments may include massive number of devices. In this work we consider a coordinated uplink transmission of clustered IoT devices. The focus is on scenarios where location-based channel knowledge map (CKM) can be applied to enable cooperation. We model and analyse the impact of hardware impairments and erroneous CKM information. In the performance evaluation we focus on the recently introduced dataoriented approach that has gathered significant attention in the context of short-packet transmissions. Specifically, it introduces a transient performance metric for small data transmissions, where the amount of data and available bandwidth play crucial roles. Results show that cooperation between clustered IoT devices may provide notable benefits in terms of increased range. Yet, the performance of the coordinated transmission system is heavily depending on the strength of the static channel component in the CKM based cooperation, the level of hardware impairments and the quality of CKM information. Analytic results are verified against simulations, showing only minor differences between analytical and experimental results.

In modern communication systems, meeting the growing demand for high-capacity transmission requires developing efficient and robust modulation techniques. To address this, we propose a low-PAPR page-style Orthogonal Time Frequency Space (OTFS) modulation framework that enhances communication capacity while maintaining a low peak-to-average power ratio (PAPR). The proposed design introduces a novel pilot signal placement and analysis method, improving channel estimation accuracy and system performance in high-mobility multi-user scenarios. This paper provides an overview of recent advancements in OTFS-based multi-user communication systems, emphasizing their contributions to enhancing spectral efficiency, reliability, and robustness. Through extensive simulations, we demonstrate the effectiveness of the proposed framework in achieving superior BER performance, improved interference mitigation, and robust transmission capabilities compared to traditional methods, validating its suitability for next-generation communication networks.

This paper proposes a low-complexity precoding weight suitable for multi-user massive multiple-input multiple-output (MU-mMIMO) in user terminals' (UTs') mobility environ-ment. Null-space expansion (NSE) achieves better inter-user in-terference (IUI) suppression performance than channel prediction schemes by steering several nulls for interfering UTs in the high mobility environment. However, NSE faces challenges in effectively managing the computation complexity of precoding. Focusing on the singularity of the Gram matrix in NSE, we incorporate Ridge regression into the Newmann series (NS) to achieve a low-complexity weight design optimized for NSE. Computer simulation demonstrates the superiority of the proposed scheme in terms of IUI suppression performance and computation complexity.

The ability to provide reliable data rates across the several intended coverage areas has made massive multiple-input multiple-output (m-MIMO) cell-free (CF) a crucial technology for future sixth-generation (6G) systems. However, CF networks also introduce new vulnerabilities, specifically in terms of security and network integrity. For that, to complement the conventional cryptographic algorithms, physical layer security (PLS) can be an effective strategy for acquiring essential wireless network information in order to create important authentication mechanisms against spoofing attacks. To this end, we propose leveraging the wireless channel and the access point selection (APS) allocation schemes as authentication methods to prevent network attacks that attempt to impersonate legitimate user communications. Our results show that monitoring these parameters provides valuable insights about the network performance and the impact under active eavesdropping influence. Consequently, this approach enables a statistical detection of illegitimate spoofing attempts through alarms, which can be further reinforced by upper-layer authentication protocols to enhance the security performance of wireless networks.

Nonlinearities are usually something to be avoided in orthogonal frequency division multiplexing (OFDM) transceivers. However, it has been demonstrated that nonlinear (NL) OFDM can outperform linear OFDM, since NL distortion has useful information on the transmitted signals that can be exploited by the optimum maximum likelihood receiver. The optimum ML receiver for NL OFDM is too complex and it is not easy to obtain its performance, although some theoretical performance bounds are available for scenarios with a large number of subcarriers and signal-to-noise ratio (SNR). However, the optimum performance of NL OFDM in small-to-moderate SNR values is still not well studied. In this paper, we consider the optimum detection of NL OFDM schemes, where we take advantage of the NL distortion to improve performance. We present a detailed study of the contributions to the signal associated with a given subcarrier from the distortion received at the other subcarriers. This allowed us to obtain a bound on the maximum likelihood (ML) performance of NL OFDM that is valid over a wide range of SNR values subcarriers and demonstrates that the performance gains observed in the asymptotic SNR region also take place for lower SNRs. Additionally, we propose a practical, iterative decision-directed receiver that exhibits large performance gains over linear OFDM in both uncoded and coded scenarios.

This paper proposes a novel and robust framework for velocity and position estimation in vehicle-to-everything (V2X) communication networks, targeting challenges that arise when traditional GPS systems and line-of-sight (LoS) channels fail due to signal blockages between vehicles, base stations (BSs), and satellites. The proposed solution integrates multiple complementary methods to enhance robustness and accuracy. Specifically, the framework first applies the MUSIC algorithm for angle-of-arrival (AoA) estimation to obtain reliable initial positioning. In parallel, a newly designed low peak-to-average power ratio (PAPR) frame structure is introduced under reconfigurable intelligent surface (RIS)-assisted Orthogonal Time Frequency Space (OTFS) modulation, enabling resilient velocity embedding against noise and interference. These methods jointly support an unscented Kalman Filter (UKF), which refines the velocity and position estimation with high reliability. Extensive numerical evaluations confirm the effectiveness of the proposed multi-method integrated framework. Various modulation techniques, including zero padding (ZP), cyclic prefix (CP), and their recursive variants (e.g., RZP, RCP), demonstrate improved bit error rate (BER) performance over traditional OFDM. The proposed low-PAPR signal design achieves a 21.5% PAPR reduction compared to conventional embedded pilot schemes, significantly improving BER in 4-QAM scenarios. Velocity estimation results also show that the UKF outperforms the extended Kalman Filter (EKF) in both straight and curved road conditions. By combining MUSIC-based AoA estimation with RIS assistance under NLoS conditions, the proposed framework remains effective even in high-Doppler and low-SNR environments.

In the last years, it has been shown that nonlinear (NL) orthogonal frequency division multiplexing (OFDM) schemes can outperform linear OFDM. This can be explained by the fact that nonlinear distortion has useful information on the transmitted signals that can be employed for detection purposes. Although the performance gains of NL OFDM can be theoretically evaluated in the asymptotic signal-to-noise ratio (SNR) region and when the number of sub carriers is large, the performance in small-to-moderate SNR values is still not clear. Moreover, receivers that try to exploit those potential gains involve very high complexity. In this paper, we present a bound on the optimum performance of NL OFDM valid for a wide range of SNR values. Moreover, a novel receiver structure that approximates the optimum performance is proposed1,

This paper considers a multiuser multiple-input multiple-output (MU-MIMO) non orthogonal multiple access (NOMA) communication scenario with one base station and two users. As a potential precoding scheme, the generalized singular value decomposition (GSVD) can be adopted in MU-MIMO-NOMA systems and has been proved to have high spectral efficiency. The power consumption at the access point (AP) should be as low as possible, making single-carrier (SC) schemes, such as single carrier with frequency-domain equalization (SC-FDE), particularly interesting. In this work, we first introduce a general uplink framework channel for GSVD based MU-MIMO-NOMA system. Then, by taking advantage of GSVD properties, we propose a novel receiver based on minimum mean square error (MMSE) equalizer offering a balanced trade-off between performance and computational complexity.

Extremely large antenna arrays (ELAAs) are being envisioned for sixth-generation (6G) to increase spatial selectivity and achieve higher capacities. The use of larger arrays combined with higher carrier frequencies increases the near-field region, where the channel model must account for the curvature of the electromagnetic waves, and the plane wave regime is inaccurate. When conventional matched-filtering (MF) beamforming is de-signed for near-field, a beam-focusing effect is observed around the focal point. This can be used to mitigate the impact of a malicious user and increase the physical layer security (PLS). In this paper, we propose an optimized beamforming design for uplink near-field communications that accounts not only for the positioning of the legitimate user but also for the position of the jammer. It is shown that the jamming rejection capabilities of the proposed beamforming are superior to that of conventional MF.1

Multiple Input Multiple Output (MIMO) architectures are now widely adopted in wireless systems, providing substantial capacity benefits by harnessing spatial diversity and multiplexing. Nonetheless, the large peak-to-average power ratio (PAPR) associated with common pre-processing techniques, like singular value decomposition (SVD), increases the system's susceptibility to nonlinear distortion. Conventional receiver designs that mitigate this distortion often neglect the fact that it has useful information on the transmitted data. Maximum likelihood (ML) detection offers the capability to take advantage of the nonlinear distortion, but its inherent complexity is prohibitively high. This paper introduces a new MIMO receiver design aimed at exploiting the diversity introduced by the transmitter nonlinearities. It also provides an approximate bound on the achievable ML bit error rate (BER) performance. Our results indicate that the proposed receiver can have a performance close to the ML receiver with just a few iterations.1

In satellite communications power consumption and reliability are key concerns. The unreliable nature of the wireless channel can cause erroneous transmissions, which must be compensated using retransmissions, at the expense of more power. This issue is aggravated by the low amplification efficiency of high data rate schemes such as Orthogonal Frequency Division Multiplexing (OFDM). Ideally, retransmissions could be avoided entirely by increasing transmission power, however, this would drastically lower the overall energy efficiency of the system. Sophisticated Hybrid Automatic Repeat Request (HARQ) techniques can simultaneously reduce the need for retransmissions and increase the success likelihood of each retransmission attempt (a critical aspect for delay-constraint services over satellite links), while minimizing average consumed power. In this work, we propose a novel HARQ technique where retransmitted signals are submitted to a soft clipping operation that lowers the Peak-to-Average Power Ratio (PAPR) of the OFDM signal, thereby increasing the amplification efficiency. A Generalized Approximate Message Passing (GAMP) based receiver makes use of the additional distortion in the retransmission to increase the likelihood of a successful decoding. It is shown that this approach can outperform Chase Combining (CC) schemes even for Super Quadrature Amplitude Modulation (Super-QAM), while decreasing the power expended during retransmissions.

This paper investigates the performance of dual-hop UAV-assisted communication system, employing a decode and forward relay architecture. The system utilizes Terahertz (THz) communication in the primary hop and Visible Light Communication (VLC) in the secondary hop. We conduct a thorough analysis by deriving closed-form expressions for end-to-end bit error rate (BER). Furthermore, a Monte Carlo approach will be employed to generate best fitting curves and validate the numerical expressions with simulation results. The outcomes of this work offer valuable insights into the feasibility and limitations of UAV-assisted THz-VLC systems.

The radio stripe (RS) concept has been emerging as a practical implementation of the cell-free (CF) massive multiple-input-multiple-output (MMIMO) network where all access points (APs) that perform the signal processing required to serve the users’ equipment (UEs) share a common stripe. In such a system, an uplink (UL) power optimization must be performed to increase the system’s spectral and power efficiencies (SE and PE). In this work, we develop a set of multi-objective UL power optimization models and algorithms based on the differential evolution (DE) meta-heuristic (MH) considering the optimal sequential linear processing (OSLP) and maximum ratio combining (MRC) detection schemes. The bi-objective approaches aim at deriving nondominated solutions in models to: 1) maximize the max-min fairness (MMF) and max-sum rate (MSR) metrics; 2) maximize the MMF and minimize transmission power (MTP); 3) maximize the MSR and minimize the MTP. A tri-objective approach is also developed to maximize the MMF and MSR and minimize the MTP. Solutions displaying interesting trade-offs are obtained with the proposed models and algorithmic approaches, compared with the case where the UEs employ full power in the UL. Results show that improved SE and PE can be attained with the proposed bi-objective and tri-objective optimization approaches.

This paper investigates rate-splitting multiple access (RSMA) networks with multiusers assisted by aerial intelligent reflecting surfaces (AIRS). To improve the sum-rate of the system, the UAV’s trajectory and phase-shift vectors are optimized, in which the mobility scenarios with static and dynamic users are explored. In particular, long short-term memory (LSTM)-based frameworks for predicting the UAV’s trajectory and the phase-shift of the reflecting elements of AIRS are proposed. For more insight, a third model is created by combining information from the static and dynamic scenarios. Furthermore, to improve the transmit beamforming at the BS, an algorithm based on alternating optimization (AO) under the assumptions of imperfect successive interference cancelation (SIC) is presented. Training progress and testing results are provided to demonstrate the efficiency of the proposed models. In addition, numerical simulations are presented to verify the performance gains in terms of sum-rate. The simulation results show that the UAV performs better in trajectory prediction and phase-shift when different investigated scenarios are not combined.

Large multiple antenna systems have been widely used in 5G for improved spectral efficiencies. For 6G, even bigger arrays are being proposed to obtain larger gains and/or to reduce interference levels. When the physical size of the array is large compared to the propagation distance, communication might occur in the near-field regime, which requires channel models that capture the curvature of the spherical waves. In this paper, we consider the near-field communication regime and study the jamming rejection features of an extremely large array (ELAA). Due to the focusing effect available in the near-field, it is shown that the jamming rejection features in this region are much stronger than the ones associated with far-field. Therefore, near-field beam-focusing constitutes an interesting physical layer security (PLS) technique for 6G communications.1

Future wireless communication systems require increasing the data-rate, tremendously, given a limited bandwidth. One solution is to use super high order constellations in high signal to noise ratio (SNR) regimes. However, in practical systems, these constellations face many issues including hardware impairments and system non-linearities. In this paper we focus on the carrier frequency offset (CFO) and investigate its effects on the performance of an orthogonal frequency-division multiplexing (OFDM) system that applies a super quadrature amplitude modulation (super-QAM). Our analysis show that although super-QAM constellations are more sensitive to CFO than low-order QAM constellations, on the other hand, we can take advantage of the high SNR regime that super-QAM constellations are used in and obtain an accurate CFO estimate and consequently reduce error vector magnitude (EVM).

Non-linear distortion imposed to orthogonal frequency division multiplexing (OFDM) signals from non-linear power amplifiers (PAs), has always been a challenging problem in wireless communications. This is due to the high peak-to-average power ratio (PAPR) of OFDM signals. To overcome this issue, receiver side compensation techniques based on distortion cancellation and iterative detection have been proposed. However, these techniques require knowledge of the non-linearity (NL) parameters. Therefore, in this paper we focus on NL parameter estimation and propose a least squares (LS) based estimation technique by transmitting a training signal. We study the performance of the proposed technique for the solid state power amplifier (SSPA) model and the Bussgang model. The studies are done considering both perfect channel knowledge and imperfect channel knowledge at the receiver. The results show that the proposed technique has good accuracy and performance for distortion cancellation and iterative detection based receivers such as the Bussgang receiver. The results also indicate that the estimation technique is more sensitive to channel estimation error than the additive Gaussian noise.

Abstract Low cost video sensors used for streaming video signals to help firefighters, require high bit rate due to uncompressed images. To increase spectral efficiency given a limited bandwidth, very high order constellations in high signal to noise ratio regimes can be used. However, noise is not the only factor effecting the high order constellations. These constellations are also sensitive to hardware impairments and system non‐linearities. Therefore, in this paper, the effect of carrier frequency offset (CFO) on the performance of an orthogonal frequency division multiplexing (OFDM) system with high order quadrature amplitude modulation (QAM) is studied. A closed form expression is derived for the maximum normalized residual CFO that an OFDM system with M ‐QAM constellation can resist to have an error free symbol detection. Finally, the suitability of common previous CFO estimation techniques such as the cyclic prefix based technique and the Moose technique in these systems are investigate. The results show that the maximum residual CFO that an OFDM system with M ‐QAM constellation can resist is proportional to the inverse of . The results also show that very large order QAM constellations such as 4096‐QAM are very sensitive to even small residual CFO values and their performance degrades, significantly. However, the bit error rate analysis indicate that the Moose CFO estimation technique can be used in these systems to compensate the CFO effect, accurately.

Multicarrier-based frequency splitting simultaneous wireless information and power transmission (FS-SWIPT) signals are very prone to nonlinear effects due to the combination of high-power energy harvesting (EH) subcarriers with low-power data subcarriers. In this article, we study the impact of nonlinear effects in FS-SWIPT signals, as well as ways to minimize these effects on the data transmission performance. We start by analytically characterizing the impact of nonlinear devices on the signals, showing that the nonlinear effects on data subcarriers are exacerbated by a factor close to the ratio between the power spectral densities of EH and data terms. This suggests that conventional receivers, that treat nonlinear distortion as an additional noise term, can have very poor performance. Then, we present an iterative receiver that iteratively estimates and cancels nonlinear distortion, and is able to have a performance close to the linear case. Finally, we study the optimum performance of FS-SWIPT signals with strong nonlinear distortion levels, showing that, by using optimum receivers, the nonlinear distortion term can be used to improve the performance of FS-SWIPT, even outperforming the linear case.

User-centric cell-free massive multiple-input multiple-output (UC CF mMIMO) is a key technology for 6G. In UC CF mMIMO each user equipment (UE) is jointly served by a small subset of a large set of cooperating access points (APs). The APs are connected through the fronthaul to a central unit (CU) for joint processing. The key to efficiency is the UE-APs association procedure together with the APs/CU cooperative signal decoding. The first is essential for reducing fronthaul signaling and the latter for removing interference. In this work, we design an algorithm to jointly perform UE-APs association and decoding. The aim is to maximize the mean squared error (MSE) between transmitted and CU decoded data, considering the fronthaul data rate limitations. In our approach, the two aspects are jointly considered, allowing improved performance while keeping the amount of fronthaul signaling low. Furthermore, the proposed method has low-complexity, requiring only the knowledge of a signal quality indicator of each UE at the CU to perform joint decoding and UE-APs association. The numerical results have shown that there is a tradeoff between performance and fronthaul requirements, where the proposed method with fewer APs may achieve the same bit error rate (BER) performance in comparison to conventional CF systems where each AP serves all UEs.

A dual-hop free space optics (FSO) and underwater visible light communication (UVLC) are a promising alternative signaling approach of next-generation wireless networking system. Nevertheless, the channel turbulence and the pointing errors are the bottlenecks in optical communication. In this regard, the performance of dual-hop mixed optical communication enabled a decode-and-forward (DF) relay protocol under consideration of strong turbulence and transceivers misalignment is proposed. In contrast, the both of the optical links are modeled by intensity modulation and direct detection (IM/DD) under consideration of pulsed amplitude modulation (PAM) scheme, and followed by Gamma-Gamma (GG) distribution model within the moderatestrong turbulence channel conditions. The Closed-form expressions in terms of probability density function (PDF) and cumulative density function (CDF) are analytically derived to obtain the outage probability and bit-error-rate (BER) performance matrices. Aggregating of the whole work, the extended generalized bivariate Meijer-G function (EGBMGF) is implemented and the results are simulated through the Monte-Carlo approach for validation of the simulated outcomes.

One of the most promising practical implementations of the cell-free massive MIMO network is based on the radio stripe system, which considers that the access points are sequentially connected, sharing the same fronthaul link while serving all user equipment using the same time-frequency resources. This paper considers an uplink power allocation technique that aims to enhance the network spectral efficiency (SE), modelled as an optimization constrained problem explicitly considering the max-min fairness case. Instead of relying on exact globally optimal approaches, with high computational complexity, three different alternative meta-heuristic (MH) based on simulated annealing, differential evolution and particle swarm optimization approaches are explored. The SE performances along with the total run times of the MH approaches are analyzed and compared with the bisection method.

In this paper, we propose a distributed hybrid analog-digital multi-user equalizer combined with an analog precoder for a wideband millimeter-wave (mmWave) cell-free massive multiple-input multiple-output (CF mMIMO) systems. A low complexity angle of departure (AoD) analog-based precoder is applied at the transmitter, while a distributed architecture is considered at the receiver side, i.e., the hybrid analog/digital beamforming processing is executed at the access points (APs), while the final data fusion is performed at the central processing unit (CPU) combiner. Herein, the average mean square error (MSE) between the transmitted and received signals is used as a metric to optimize the APs hybrid equalizer. Then, the noise variance is included for optimal data combination. The results show that the performance of the proposed hybrid scheme with the AoD analog precoder achieves a close result to the fully digital counterpart and fully centralized approach.

In wireless communication systems, the achievable data rates are always dependent on the characteristics of the wireless channel. Although a system is designed for an expected channel link quality, the channel may deteriorate for certain periods of time due to the changing nature of the propagation environment. A weak channel may lead to transmission errors, which can be corrected by employing Hybrid Automatic Repeat Requests (HARQ). In particular, type II HARQ protocols have become an appealing option since they only involve the transmission of additional parity bits, instead of repeating the original transmission. These additional parity bits can be obtained by puncturing a low rate channel code. However, finding an optimum pattern of puncturing for a specific code is non-trivial. In this work, we propose a retransmission scheme that considers various Low-Density Parity-Check (LDPC) codes of different rates that can be concatenated to perform the encoding and decoding operations, resulting in a lower encoding overhead for successful transmissions, while maintaining efficient decoding. We show that this technique can maintain high throughput and goodput, and a low outage probability, even when the received signal power is suddenly reduced due to the loss of the Line-of-sight (LOS) component.

Large intelligent surfaces (LISs) is an evolution of massive MIMO systems allowing for huge capacity gains. To reduce implementation complexity, it is convenient to implement the LIS using panels that are either activated or deactivated, and associated to terminals according to their propagation characteristics to the panels. The associated spatial resource allocation to maximise the terminals’, as well as the overall, bit rates can lead to complex optimisation problems.In this paper we consider resource allocation for panel-based LIS surfaces. We present an iterative sequential algorithm for determining the set of active panels and the respective panel-terminal association for the maximisation of the minimum terminal rate. Our algorithm is decentralised and has low complexity. Moreover, it approaches the performance of much more complex, quasi-optimum algorithms.

Unmanned aerial vehicle (UAV) systems are vulnerable to jamming from self-interested users who utilize radio devices to disrupt UAV transmissions. The vulnerability occurs due to the open nature of air-to-ground (A2G) wireless communication networks, which may enable network-wide attacks. This paper presents two strategies to identify Jammers in UAV networks. The first strategy is based on a time series approach for anomaly detection where the available signal in the resource block is decomposed statistically to find trends, seasonality, and residues. The second is based on newly designed deep networks. The combined techniques are suitable for UAVs because the statistical model does not require heavy computation processing, but is limited to generalizing possible attacks that might occur. On the other hand, the designed deep network can classify attacks accurately, but requires more resources. The simulation considers the location and power of the jamming attacks and the UAV position related to the base station. The statistical method technique made it feasible to identify 84.38% of attacks when the attacker was at a distance of 30 m from the UAV. Furthermore, the Deep network’s accuracy was approximately 99.99 % for jamming powers greater than two and jammer distances less than 2II meters.

When users exchange data with Unmanned Aerial Vehicles - (UAVs) over Air-to-Ground - (A2G) wireless communication networks, they expose the link to attacks that could increase packet loss and might disrupt connectivity. For example, in emergency deliveries, losing control information (i.e., data related to the UAV control communication) might result in accidents that cause UAV destruction and damage to buildings or other elements. To prevent these problems, these issues must be addressed in 5G and 6G scenarios. This research offers a Deep Learning (DL) approach for detecting attacks on UAVs equipped with Orthogonal Frequency Division Multiplexing - (OFDM) receivers on Clustered Delay Line (CDL) channels in highly complex scenarios involving authenticated terrestrial users, as well as attackers in unknown locations. We use the two observable parameters available in 5G UAV connections: the Received Signal Strength Indicator (RSSI) and the Signal to Interference plus Noise Ratio (SINR). The developed algorithm is generalizable regarding attack identification, which does not occur during training. Further, it can identify all the attackers in the environment with 20 terrestrial users. A deeper investigation into the timing requirements for recognizing attacks shows that after training, the minimum time necessary after the attack begins is 100 ms, and the minimum attack power is 2 dBm, which is the same power that the authenticated UAV uses. The developed algorithm also detects moving attackers from a distance of 500 m.

One of the most promising technologies to be employed in beyond 5G networks is based on the concept of cell-free (CF) massive MIMO, in which a predetermined set of access points (APs) jointly cooperate in the data transmission and reception to/from the user equipment (UE). In order to efficiently manage radio resources, the CF central processing unit implement uplink power control policies. These policies aim to optimize a given network utility function. In this paper, we investigate the max-min fairness optimization problem, in which the spectral efficiency performance of the UE with the worst channel conditions is prioritized, taking into account a per-UE power constraint and assuming linear maximum ratio combining at APs. Existing solutions have typically relied on second-order cone programming with convex approximations, which exhibit high computational complexity and scalability issues. Therefore, meta-heuristics (MHs) are explored as alternative optimization schemes, capable of providing (near)-optimal solutions with reasonable computational effort. Three MH approaches with different operation principles are compared. Numerical results show that the differential evolution algorithm exhibits the best trade-off between solution quality and run time, being able to reach (near)-optimal solutions faster than the bisection approach while coping with the scalability issues of the geometric programming-based algorithm.

The current cellular systems are conventional cell-centric based architectures, and thus the inter-cell interference greatly affects the quality of service (QoS) of users located at the border of those service areas. For that reason, future wireless systems will rely on a cell-less deployment by using an alternative promising network infrastructure known as cell-free (CF) technology. Here, in contrast to the current networks, there are no concepts of cells or cell boundaries since it is a disruptive paradigm where a large number of geographically distributed access points (APs) simultaneously and jointly serve a much smaller number of user terminals (UTs) to fulfill the exponentially increasing number of users. Therefore, this paper aims to contribute on this field and considers the design of hybrid analog/digital multi-user equalizer for broadband millimeter wave (mmWave) CF massive multiple-input multiple-output (mMIMO) systems, using a distributed beamforming approach (processing is distributed by APs and central unit (CU)) to reduce the fronthaul requirements. First, only a low complexity analog precoder is applied to the transmitter, while a joint design of APs hybrid equalizer and CU combiner are considered by minimizing the average mean square error (MSE) between the transmitted and CU estimated signals. To achieve this joint design, a cyclic minimization method is used to iterate between APs hybrid equalizer and CU combiner, where the information is exchanged between APs and CU based on a set of error variances to improve detection performance. The simulation results show that the proposed precoder and the hybrid linear equalizer can achieve performance results very close to those of the fully digital system. Moreover, just two iterations of the proposed algorithm are already enough to match the full centralized approach performance.

The use of efficient high-power amplifiers (HPAs) is critical for future millimeter-wave (mmWave) systems. HPAs can become highly nonlinear when operated near saturation, distorting the transmission signal. Therefore, the design of efficient solutions to mitigate nonlinear distortion (NLD) is of paramount importance. This article proposes a hybrid multiuser equalizer for the uplink of wideband mmWave massive MIMO systems. The equalizer is designed to mitigate both the multiuser interference (MUI) and inter-symbol interference (ISI), introduced by the multiple access channel, and the NLD, introduced by the user terminals HPA. The digital part of the hybrid equalizer is iterative and computed on a per subcarrier basis, to minimize the mean squared error between the transmitted and detected data. In the iterative processing, a feedback loop estimates and removes the distortion. The analog part is frequency-flat and its design assumes that the digital part efficiently removes the MUI, ISI and NLD. Performance results show a huge gain of the proposed hybrid nonlinear equalizer, when compared with linear ones. The nonlinear effects are almost completely removed, without significant error propagation effects, allowing a more efficient operation of the HPAs.

This paper studies the performance of Non-Orthogonal Multiple Access (NOMA), for both conventional and cooperative NOMA, associated to Single Carrier with Frequency Domain Equalization (SC-FDE) block transmission technique, and to massive Multiple-Input Multiple-Output (MIMO). 5G makes use of millimeter wave communications (mm-Wave) and massive MIMO (m-MIMO). Moreover, NOMA is a potential solution, to be incorporated in future releases of 5G, to increase the capacity required to support extremely high number of Internet of Things (IoT). It is shown that combining NOMA with SC-FDE and mMIMO only suffers a moderate degradation, while a capacity gain is achieved due to the sharing of the spectrum.

Multiple-input, multiple-output (MIMO) techniques usually involve some kind of pre-processing at the transmitter, namely when singular value decomposition (SVD) techniques are employed. However, the precoding operations in general and the ones associated to SVD techniques in particular can substantially increase the envelope fluctuations of the transmitted signal, which leads to an increased sensitivity to transmitter nonlinearities and inherent performance degradation. However, recent research demonstrates that nonlinear distortion can be seen as useful information, provided that an optimum detection is considered. In this work, we present an analytical approach to obtain the asymptotic optimum performance of MIMO-SVD systems with strong nonlinear distortion effects1.

Assuming a cooperative non-orthogonal multiple access (NOMA) system with rate fairness in a scenario with multiple users and arbitrary relays, this paper investigates adaptive power allocation (PA), relay selection (RS), and user pairing (UP) policies. Specifically, two adaptive PA optimization problems are formulated, one at the base station (BS) and another at the selected relays. Closed-form expressions for the power allocation factors are derived as well as an algorithm that provides the optimal solution at the BS. In order to show the superiority of the proposed study, our results are compared with other benchmark schemes in terms of outage probability, Jain's fairness index, and average sum rate.

The design of a wireless catheterization laboratory (CathLab) presents considerable challenges, especially for intravascular ultrasound (IVUS) systems. In fact, the huge bit rates, together with the highly demanding quality of service (QoS) requirements such as high reliability, very low latency and secure communications, make the design of a wireless CathLab particularly difficult, especially due to the limited signal processing capabilities at the transmitter side. In this paper, we consider the design of a secure wireless CathLab system using a physical layer security scheme that takes advantage of the Multi-Input Multi-Output (MIMO) channel characteristics. It is shown that the physical layer security scheme can have excellent secrecy characteristics, even for an eavesdropper close to the transmitter or receiver. Moreover, the signal processing requirements are very small, especially for the data transmission phase, making this technique particularly interesting for a wireless IVUS scenario.

Cell-free (CF) massive multiple input multiple output (MaMIMO) has been suggested to reach future wireless networks demands. Analyzing the effect of hardware impairments on performance of CF MaMIMO systems is of vital importance. In this paper, we investigate the effect of residual carrier frequency offset (CFO) on the uplink sum-rate of CF MaMIMO systems. Therefore, we derive closed form expression for the uplink achievable sum-rate of orthogonal frequency domain multiplexing (OFDM) based CF MaMIMO systems. Our results show that in a fully synchronized CF MaMIMO system, the sum-rate increases unlimitedly as the number of access points (AP) increases. However, when the system is hit by CFO the sum-rate is limited even when the number of APs tends to infinity. Our results also indicate that for finite number of APs, when the number of antennas at each AP tends to infinity, the sum-rate of the system in the presence of CFO is bounded. While, for the fully synchronized system, as the number of antennas at each AP grows large, the sum-rate increases, unlimitedly when the antennas of an AP are independent and is bounded when there is a correlation between the antennas of an AP.

In this paper we consider a multipath channel subject to variations in time due to the presence of a Doppler drift which imposes a phase rotation on the original channel impulse response. Accordingly, a decision-directed state-space approach based on the extended Kalman filter (EKF) is proposed to track the channel variation. This approach is particularly efficient since we are able to reduce the number of training symbols used for channel tracking to be as few as two times the number of transmitters. These training symbols are required to initialize the EKF. Furthermore, using a decision-directed approach we are able to provide the EKF with state-observations without additional training symbols. Our approach is specially designed to work with hybrid analog/digital multiple-input multiple-output (MIMO) architectures.

The need for higher data rates in wireless communications has resulted in a significant effort in what regards to the design of transmission techniques and waveforms. This design can be very challenging thanks to the strong trade-off between the spectral efficiency and the energy efficiency, which comes from the fact that spectral-efficient waveforms usually presents a very large peak-to-average power ratio (PAPR), requiring a linear amplification for preserving the linearity of the transmitted signals. Nevertheless, obtaining spectral- and energy-efficient wireless communications will undoubtedly be one of the main challenges of both 5G and beyond 5G (B5G) systems. In this work, we propose a new amplification scheme named quantized digital amplification (QDA). The QDA is based on the decomposition of a high-PAPR signal into constant-envelope components and allows to obtain very high energy efficiencies, being suitable for spectrally-efficient transmission techniques such as orthogonal frequency-division multiplexing (OFDM). By presenting a set of results of the QDA’s prototype, we demonstrate that it is possible to obtain amplification efficiencies way above the ones of the state-of-the art amplification schemes, while preserving the linearity of the transmitted signals.1

A Large Intelligent Surface (LIS) is a recently proposed concept, especially suitable for high speed indoor communications and industrial internet of things (IoT) applications. Basing the LIS on smaller panels has clear advantages in terms of flexibility and mass production of its elements. In this paper we consider a panel-based LIS and we study the interplay of the panel size, the number of baseband outputs per square meter of deployed surface, the total activated surface area, the number of baseband outputs per panel, the terminal density and the ensuing minimum terminal rate. Our performance results show that it is desirable to employ smaller panels when the terminal density increases, but this means more outputs per m2, and higher overall LIS implementation complexity. It was observed that we can surpass such increase by working with higher fractions of the LIS area. Furthermore, we present an empirical equation stating the number of outputs per panel needed to ensure that all terminals are reasonably served. These results are useful for the LIS design in practical scenarios.

Besides traditional orthogonal multiple access (OMA), 5G and other future wireless communication systems will also involve non-orthogonal multiple access (NOMA). In that context, the combination of NOMA systems with the popular orthogonal frequency-division multiplexing (OFDM) will arise naturally in highly spectral efficient communications. Since OFDM signals are very prone to nonlinear distortion effects, the receiver design of NOMA-OFDM systems should be modified accordingly. In this work, we consider an efficient, nonlinear power amplification for NOMA-OFDM systems and we propose different reception strategies to cope with the nonlinear distortion. It is shown that the performance of conventional power-domain NOMA is very poor under nonlinear transmission scenarios. However, it is also demonstrated that Bussgang receivers combined with optimum detection schemes might be an excellent alternative1.

The Time-Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing (TIBWB-OFDM) waveform enables to achieve greater confinement in the signal spectrum. This improves with the increase of the window roll-off since the out of band (OOB) radiation drops. However, the TIBWB-OFDM block duration also increases, which leads to a decrease in the transmission rate for a given band, limiting, therefore, the spectral efficiency gains of the system. In this paper, we propose an alternative method for TIBWB-OFDM symbol construction by allowing a partial overlap between adjacent windowed OFDM symbols that compose it. This improves the spectral efficiency at the expense of introduced interference between the signal’s blocks, that deteriorates the Bit Error Rate (BER) performance. To avoid these BER performance losses, we propose a modified receiver design that includes the cancellation of block overlapping effects in the time domain. The proposed receiver is shown to be able to achieve excellent performance, even with the strong overlapping effects of a TIBWB-OFDM system with high spectral efficiency.

Future ultrahigh-speed wireless communication systems face difficult challenges due to the fundamental limitations of current technologies working at microwave frequencies. Millimeter-wave (mmWave) technologies can offer large underutilized bandwidths and ease the implementation of large antenna arrays which are required to help overcome the severe signal attenuation that occurs at these frequencies. The high cost and power consumption of a fully digital mmWave precoder and combiner can be reduced by the adoption of hybrid analog/digital designs based on analog phase shifters. The implementation can be simplified even further if each transceiver is connected to an independent antenna subarray and if the phase shifters are replaced by switches, with the possible aid of inverters. In this paper we present an hybrid design algorithm for orthogonal frequency division multiplexing (OFDM) based mmWave MIMO systems, which can cope with different analog architectures that can be adopted at either the RF precoder or combiner, in order to achieve reduced complexity and power consumption. Numerical results show that using the proposed algorithm, all the architectures can achieve good trade-offs between spectral efficiency and simplified implementation.

In this paper we consider a multi-input multi-output (MIMO) channel subject to time variations. These channel variations are due to the presence of a Doppler term which imposes a phase rotation on the original channel impulse response (CIR). On one hand, modern equalization schemes require the knowledge of the CIR or, equivalently, the channel frequency response (CFR). On the other hand, channel estimation procedures consume significant resources, particularly when transmitting training sequences. Therefore, we propose the use of a statespace approach, namely the extended Kalman filter (EKF), to interpolate the channel estimates obtained during the training stage onto the data transmission stage. Our approach is validated considering single-carrier frequency-domain equalization (SCFDE) transmissions.

In this paper we consider a multipath channel subject to variations in time due to the presence of a Doppler term which imposes a phase rotation on the original channel impulse response. Accordingly, a decision-directed state-space approach based on the extended Kalman filter (EKF) is proposed to track the channel variation. This approach is particularly efficient since we are able to reduce the number of pilot symbols used for channel tracking by using a decision-directed approach. In fact, only two training symbols are required to initialize the EKF. Our approach is validated considering orthogonal frequency-division multiplexing (OFDM) schemes.

Simultaneous wireless information and power transmission (SWIPT) is one of the popular technique used in radio frequency energy harvesting (EH). Frequency splitting based SWIPT (FS-SWIPT) is a novel SWIPT technique that can be adopted with frequency domain multiple access signal transmission protocol for the near field EH application in biomedical sensors. In this paper, we implement FS-SWIPT in orthogonal frequency division multiplexed (OFDM) signal, the OFDM signal has separate sub carriers for the data symbols and the energy carrying symbols. At the receiver, the energy and information symbols are based on their sub carriers by using FS-SWIPT technique. The amplification of high energy OFDM signal at transmitter by using a high power amplifier are susceptible to non linear distortion (NLD), which degrades the bit error rate (BER) performance. We analyze the impact of NLD on the BER performance of FS-SWIPT and validate the analytical results by using simulations.

The high capacity demands of 5G communications require multiple input multiple output (MIMO) technologies and millimeter wave communications to enables the required multi-Gbps. The small wavelengths of millimeter wave communications allow packing a huge number of antennas in a small space, enabling massive MIMO schemes, with the associated high capacity gains. The hardware impairments inherent in low complexity hardware implementations are characteristic of these systems. This means strong nonlinear effects throughout the transmission chain that will have negative impact on the system's performance. In this paper we study the impacts of these nonlinear effects on the overall performance of an OFDM massive MIMO communication employing a hybrid analog/digital architecture. For this purpose, we develop analytical and semi-analytical tools for performance evaluation of nonlinear effects in those systems. These tools are shown to be very accurate. They make it possible to predict the nonlinearities' penalties in hybrid systems and/or to assess the optimal operation point for a specific nonlinear amplifier.

This paper considers a hybrid two-step space-frequency receiver structure for multiuser uplink millimeter-wave (mmWave) massive MIMO based systems. We adopt constant envelope OFDM (CE-OFDM) since it is a promising modulation technique for future systems, which allows very efficient power amplification. We consider low-complexity user terminals employing a single RF chain and using an analog precoding based only on the average angles of departure of the channel. At the base station a non-linear multi-user equalizer, based on the iterative block decision feedback equalization (IB-DFE) principle, and using a hybrid analog-digital structure, is designed to efficiently remove the inter-user and inter-carrier interferences. This equalizer is designed to minimize the sum of the mean square errors of all subcarriers. Numerical results have shown that the proposed scheme outperforms the linear equalizers. Moreover, performance is already close to the full digital counterpart after only a few iterations.

The pre-processing required at the transmitter side by many MIMO (Multi-Input, Multi-Output) schemes can lead to signals with substantial envelope fluctuations, making them very prone to nonlinear distortion effects. These nonlinear distortion effects can lead to significant performance degradation and signals with high irreducible error floor when conventional receivers are employed. However, the nonlinear distortion has information on the transmitted signals than can be employed to improve the performance, provided that an optimum receiver is employed. Nevertheless, the huge complexity of optimum receivers precludes its use in most practical scenarios. In this paper we consider MIMO schemes with strong nonlinear effects at the transmitter side and we propose an iterative receiver that is able to cope with nonlinear effects, allowing excellent performance even with strong nonlinear distortion effects. In fact, our results show that MIMO schemes with strong nonlinear effects can even perform better than linear MIMO system.

In radio access network planning and optimisation, the estimation and evaluation of cell coverage is crucial. For it, operators use several methodologies, ranging from theoretical propagation models for planning cell coverage, to drive test measurements for evaluating the performance within a cell. A novel propagation model is proposed which combines drive test measurements, cell reach statistics, antenna radiation patterns, terrain morphologies, and classical theoretical propagation models. Considering a realistic scenario of an LTE cell, the use of a classical propagation model such as Walfish- Ikegami results in an average error of -27 dB of estimations with respect to measurements, evidencing a pessimistic estimation of signal. By opposition, the proposed model fits with DT measurements.

Non-orthogonal multiple access (NOMA) is being considered a key technology for 5G systems. In this work, we consider power-domain NOMA schemes for downlink orthogonal frequency-division multiplexing (OFDM) transmissions. Moreover, we consider the use of an efficient, nonlinear power amplification. We analyze the nonlinear distortion effects associated with the power amplifier and show that the nonlinear distortion can be severe, preventing the use of conventional NOMA receivers. In that context, we propose a receiver that estimates and cancels the nonlinear distortion and enables the implementation of power-domain NOMA schemes in strongly nonlinear OFDM scenarios.

In this paper we consider the use of a single- carrier with frequency-domain equalization (SC-FDE) scheme for the uplink in a cellular radio system. To cope with the doubly-selective nature of the channel we propose a receiver structure for the joint channel equalization and tracking. We use a low-complexity iterative frequency-domain equalizer based on the iterative block decision-feedback equalization (IB-DFE) concept for the channel equalization and an extended Kalman filter (EKF) for the channel tracking procedure. The tracking procedure works by, first, observing the channel through channel estimation based on training blocks and, then, by predicting the channel during data transmission. Alternatively, decision-directed channel estimation can be used also, resulting in an improved performance.

In order to fulfil the high capacity demands of 5G communications, massive MIMO and millimetre wave are two enabling key technologies considered for future wireless communication. Although these technologies allow huge capacity gains, the radio frequency transceivers employed in this systems induce several hardware impairments as noise quantization (due to resolution quantizers), nonlinear power amplifiers, frequency and phase offsets and in-phase and quadrature imbalance, which lead to a strong nonlinear distortion in the transmitted signal. The impact of these nonlinear effects on the overall performance of hybrid analog-digital massive MIMO millimetre wave OFDM systems is studied in this work, highlighting the importance of its knowledge to improve communication quality. Theoretical analysis of the distortion spectral signal in transmission is assessed and validated for these communication systems, using the quasi-Gaussian nature of the input nonlinear signals. The nonlinearity impact in these systems was observed for different scenarios, where the value of saturation level shows to have a strong influence on the overall system performance.

It is generally assumed that nonlinear distortion effects in multicarrier systems can lead to substantial performance degradation. However, it was recently shown that the nonlinear distortion component contains information on the transmitted signal and can be used to improve the performance. Nonetheless, all known receivers that are able to take advantage of this fact are extremely complex. In this paper, we present receivers based on the generalized approximate message passing concept for orthogonal transform multiplexing (OFDM) signals with strong nonlinear effects. Our simulation results show that the proposed receivers are very powerful, allowing excellent performance in the presence of strong nonlinear distortions. In fact, nonlinear OFDM schemes using those receivers can outperform linear OFDM schemes by several dBs in ideal additive white Gaussian noise (AWGN) channels, and by tens of dBs in frequency-selective fading channels.

In this work, the authors consider phase shift keying (PSK)‐type signals with very compacted spectrum and low envelope fluctuations obtained through the use of magnitude modulation (MM) techniques and stringent roll‐off pulse shaping filtering. The analytical bit error rate (BER) of PSK‐type signals combined with MM is assessed, for both flat fading and time‐dispersive channel scenarios. The authors use a Gaussian model to approximate the statistical distribution of the distortion error introduced by MM to enable a closed‐form evaluation of the BER. The proposed expression depends only on the constellation order and the Kullback–Leibler divergence of the MM factors' probability density function from the Gaussian one, and it was found to be very accurate.

This study considers massive multiple‐input multiple‐output (m‐MIMO) schemes employing orthogonal frequency division multiplexing (OFDM) or single‐carrier with frequency‐domain equalisation (SC‐FDE) modulation, concerning an uplink transmission. The authors study the performance and complexity for both conventional zero forcing (ZF) and minimum mean squared error (MMSE) (which require the inversion of channel matrix) and iterative frequency domain equalisation receivers based on equal gain combining (EGC) and maximum ratio combining (MRC) concepts, that do not require matrix inversions. It is shown that, although matrix inversions are generally the more complex operations for relatively balanced systems (i.e. with similar number of transmit and receive antennas), this is not generally true for systems where the channel matrix is ‘very tall’ (i.e. much more receive than transmit antennas), at least in terms of the number of multiplications and sums. This means that the advantage on a reduced complexity of MRC/EGC‐based equalisers with respect to ZF/MMSE is, in fact, limited. However, the performance advantages combined with the possibility of parallel receiver implementations, make those techniques particularly interesting for m‐MIMO schemes, either employing OFDM or SC‐FDE modulations.

Massive multiple-input multiple-output (mMIMO) systems are very important for 5G since they are one of the main enablers of the targeted huge capacity gains. On the other hand, the power consumption at the mobile terminals should be as low as possible, making single-carrier waveforms, such as single carrier with frequency-domain equalization (SC-FDE), particularly interesting, since the transmit signals can have a low peak-to-average power ratio (PAPR), allowing a highly efficient power amplification. However, the combination of mMIMO systems with SC-FDE modulations gives rise to implementation difficulties at the receiver side. On one hand, the equalization procedure should avoid matrix inversions, but this might lead to performance degradation. On the other hand, low-resolution analog-to-digital converters (ADCs) should be employed in each receive branch to reduce the implementation complexity, which might lead to severe nonlinear distortion effects. In this work, we study analytically the performance of mMIMO systems based on SC-FDE schemes employing low-complexity, iterative FDE receivers, and low-resolution ADCs. It is shown that the performance degradation associated to low-resolution ADCs can be tolerable if the number of receive antennas is larger than the number of transmit antennas, even when low-complexity FDE receivers are employed.

In this paper, we apply a generalized form of the alternating direction method of multipliers (ADMM) and derive a multiple-input multiple-output (MIMO) detection algorithm for single carrier transmissions in time dispersive channels. The proposed algorithm supports different penalty parameters for each individual subcarrier and antenna and also includes a relaxation coefficient in the iterations. Besides evaluating the impact of these parameters, a method is presented for the automatic selection of the penalty. It is shown through simulations that very competitive performances can be obtained with the proposed approach for systems with high-order modulation combined with large antenna settings.

Decentralized PHY/MAC techniques for the uplink of Multi-Packet Reception (MPR) schemes are of great interest in future wireless communication systems (e.g., IEEE 802.11ax). In MPR schemes the average number of successful received packets depends on the radio propagation effects, the number of nodes simultaneously transmitting and the capture threshold that characterizes the PHY-layer MPR technology. In this paper we adopt a MPR capture model capable of describing the average number of successful packets simultaneously received. The capture model is then used in an optimal reservation-based medium access control (MAC), designed to operate in a decentralized way through a random access policy. This work describes the PHY/MAC cross-layer design, as well as the rationale required to understand its optimality. While the main objective of the proposed MAC is to achieve the optimal number of simultaneous transmissions that maximize the PHY-layer performance, a cost due to the MAC reservation period must also be considered in the optimization process. A model of the PHY/MAC performance is derived, which allows the optimization of the parameters that regulate the nodes' random access. Different simulation results are described and compared with numerical results obtained with the proposed PHY/MAC model. The paper ends up discussing possible solutions to implement the proposed methodology in practical wireless networks formed by a high number of competing nodes.

The constant need of operators to ensure a high quality of service requires an adequate estimation of signal level in the service area. Drive Tests (DTs) are used to monitor signal level. Still this is an expensive task, not being able to cover all areas. A methodology to build complete path loss grids for a given site is proposed. Starting from available DTs measurements for certain pixels, path loss is estimated for the remaining ones by tuning a propagation model and extrapolating the path loss for neighboring pixels. This allows to provide a complete path loss estimation for the desired area around the cell.

Non-orthogonal multiple access (NOMA) has been acknowledged as an emerging technique for multiple access techniques in fifth generation (5G) wireless network. It offers progressive and attractive features such as low latency, ultra- dense coverage, high fairness and new waveform design as compared to the previous schemes. In this paper, recent advances and future research challenges of NOMA is mainly considered. We start by summarizing the existing works with emphasis on recent literature in resource allocation, performance analysis, principal features related to NOMA, and various types of NOMA techniques are discussed. We offer comparative features on demonstrating how NOMA impacts device to device (D2D) communications, cognitive radio networks, massive multiple input multiple output (MIMO), radio frequency (RF) energy harvesting, and other emerging technologies in 5G. Finally, we present the future research challenges and recommendations in order to mitigate the existing issues and advance this unique feature in 5G.

This paper deals with the cross-layer design of a decentralized medium access control (MAC) scheme to coordinate the access of multiple transmitters adopting a Multi-Packet Reception (MPR) physical (PHY) layer. Knowing that in MPR systems the average number of packets successfully decoded depends on the number of simultaneous transmissions, we propose an integrated PHY/MAC cross-layer design that explores the advantages of MPR communications in a decentralized network. The joint performance of the PHY/MAC cross-layer scheme is characterized to allow the maximization of the average number of packets successfully decoded. While the main objective of the proposed MAC is to randomly grant access to the channel in order to achieve the optimal number of simultaneous transmissions, the decentralized MAC introduces a cost due to the time required to coordinate the multiple transmitters. In a first step we characterize the throughput achieved by the cross-layer scheme, by modeling the performance of the PHY-layer and the random MAC scheme. The formal characterization of the throughput is then used to optimize the duration of the first stage of the MAC scheme. In this way, the number of transmitters is regulated to optimize the cross-layer operation, taking into consideration the features of the MPR PHY-layer and the maximum performance achieved with the proposed MAC design. Several results obtained through simulation are presented, which validate the numerical results obtained with the proposed characterization. Finally, the results confirm the advantages of the proposed PHY/MAC design.

Single-carrier with frequency-domain equalization (SC-FDE) modulations are a natural choice for the uplink of massive multiple intput, multiple output (MIMO) systems, due to the low peak-to-average power ratio (PAPR) of their signals. However, equalization techniques should avoid matrix inversions. On the other hand, energy-efficient analog-to-digital converters (ADCs) should be used in each receive branch to reduce the power consumption and the implementation complexity, which might lead to strong nonlinear distortion effects. In this work, we present the analytical performance evaluation of nonlinear massive MIMO systems based on SC-FDE systems employing low-complexity, iterative FDE receivers1.

In this paper, we propose a new methodology for soft forwarding which is based on a novel technique known as soft angular modulation (SAM). This is effective even under poor source-relay link conditions. The new treatment presented here allows a more flexible system design. This makes soft forwarding, which is not practical with current infrastructure, a feasible approach. In this work, we convert soft information from the soft domain into the angular domain. We then forward unity amplitude symbols using wireless fading channels. Due to the analogue nature of the probabilistic content (or soft information), the soft information relaying (SIR) scheme is not treated as a practically feasible relay protocol. We offer a remedy to this by forwarding unity amplitude symbols from the relay. Using the expression of soft noise variance, we deduct an approximate log likelihood ratio (LLR) expression for a parallel relay network employing SAM scheme. It is shown that the derived BER of SAM and numerical BER are closely aligned. The proposed scheme yields improved BER performance as compared to other conventional schemes.

This paper considers massive multiple-input, multiple output (MIMO) schemes for single-carrier with frequency-domain equalization (SC-FDE) modulations. We present low-complexity turbo multi-user receivers based on the maximum ratio combining (MRC) and equal gain combining (EGC) techniques. Our receivers are implemented in the frequency-domain and allow excellent performances, even for moderate numbers of antennas.

Vehicles need to locate other vehicles and network infrastructure elements on unmanned autonomous vehicle (UAV) systems. Human passengers also need to locate and be located by the vehicles, preferentially using a portable device, such as a smartphone. This paper analyses the accuracy of several localization algorithms in the remote location of entities running WiFi access points, using measurements collected in moving vehicles using a new application developed by us. The algorithms analysed include closed form estimators and one based on second order cone programming (SOCP) relaxation, which exhibits the best accuracy and is capable of estimating the path loss exponent and the transmission power. Although, due its lower complexity, the Levenberg-Marquardt algorithm was better suited for the stand-alone Android prototype application. The results show that real-time accurate positioning of static/slow moving remote entities is possible, even though the accuracy degrades when the measuring vehicle's speed increases.

Time-Interleaved Block Windowed Burst-OFDM (TIBWB-OFDM) was recently proposed as a new waveform candidate for future 5G systems and beyond. In this paper, it is shown that this technique is, in fact, easily combined with MIMO systems, offering promising gains in terms of power and spectral efficiency. In addition, since TIBWB-OFDM is built on the mature OFDM technique, it enables fast forward implementation of a frequency domain equalization with inter-stream MIMO suppression, maintaining the simplicity of the equalizer at an affordable cost. Bearing the complexity requirement in mind, three equalization techniques are studied, namely low-complexity iterative maximum rate combining (MRC) and equal gain combining (EGC) receivers, that do not require costly matrix operations, and an iterative block decision feedback equalizer (IB-DFE) receiver that, although requiring matrix inversions, can provide excellent results by taking advantage of single-carrier type nature of the TIBWB-OFDM block. We show that, on the one hand, IB-DFE receiver exhibits superior gains and can be successfully employed in scenarios with moderate number of antenna-elements, where its complexity can be actually manageable. On the other hand, it is shown that iterative MRC and EGC offers excellent performances for high values of receiving antenna-elements.

This paper proposes a way of turning the equalisation of multiple- input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) signals simpler through the employment of low complexity iterative frequency domain equalisation (FDE) receivers based on equal gain combining (EGC) and maximum ratio combining (MRC) concepts, that do not require computing the inverses of high dimensional channel matrices. Performance results show that MIMO- OFDM schemes employing this type of techniques can achieve excellent bit error rate (BER) performances over severe time- dispersive channels and, therefore, approach or even outperform receivers that require inverting channel matrices, such as zero forcing (ZF) and minimum mean squared error (MMSE), after a few iterations.

In this paper, we consider massive multiple input multiple output systems that employ linear precoding techniques and are impaired by transmitter-side nonlinearities and channel estimation errors. We present an analytical method for the statistical characterization of the transmitted signals and develop a general framework to obtain analytically, the performance of different precoding techniques under the assumption of imperfect channel state information and for different types of nonlinearities. Regardless of the particular nonlinear transmission scenario, it is shown that the performance penalty associated to the nonlinear distortion can be tolerable if the number of transmit antennas is greater than the number of independent data streams.

In this paper, we investigate the suitability of single carrier frequency domain processing (SC-FDP) as the downlink transmission scheme in a massive multiple-input multiple-output (MIMO) system. By deriving the sum-rate of the SC-FDP massive MIMO system theoretically, we show that this method obtains a sum-rate similar to that of orthogonal frequency division multiplexing (OFDM) massive MIMO. We also derive the theoretical sum-rate of both SC-FDP and OFDM in a non-synchronized massive MIMO scenario and show that the rate of the former is significantly larger than that of the latter. Moreover, we theoretically analyze the sum-rate of both systems in the presence of power amplifier non-linearity. All the sum-rates are derived for both zero forcing and matched filter precoding schemes. The results show that the effect of power amplifier non-linearity on the sum-rate of both systems is similar when the number of users is large. We also compare SC-FDP with OFDM from the peak to average power ratio (PAPR) and complexity viewpoints. Although the PAPR of SC-FDP signals is lower than that of OFDM signals, for MIMO systems, the difference between their PAPR decreases as we increase the number of users. Thus, both techniques can have similar PAPR in massive MIMO systems. The overall complexity of SC-FDP and OFDM is similar. Due to the mentioned facts, SC-FDP can be considered as a promising transmission scheme for the downlink of the massive MIMO systems in the presence of carrier frequency offset and power amplifier non-linearities.

In this paper, we characterize the wireless interference of a mobile ad hoc network, where the nodes move according to the random waypoint model. The interferers are assumed to be located within an interference region that is defined as a circular region centered in a fixed node located at a given point of the mobility scenario. The main contribution of this paper is the characterization of the aggregate interference caused to the fixed node by mobile interferers located within the interference region. The distribution of the interference is analyzed taking into account the stochastic nature of the path loss due to the mobility of the nodes, as well as fast fading and shadowing effects. The derivation of the characteristic function of the aggregate interference is used in two different estimators, which successfully characterize the interference using only a small set of samples. The theoretical approach is validated through simulations, which confirm its effectiveness. Finally, we assess the accuracy of the proposed estimators, demonstrating the practical value of this paper.

In this paper we characterize the radio frequency (RF) spectrum opportunities available in a common GSM (Global System for Mobile Communications) channel to support the operation of a cognitive radio network (CRN). In a first step, we describe the technical details involved to sample the channel using a Software Defined Radio (SDR) device. Adopting a simple Energy-based detector, we identify the two energy regions where the GSM system is active or inactive. Based on the output of the detector, we show that the distribution of the durations of busy and idle periods are approximated by geometric distributions. Finally, we validate a theoretical model for the distribution of the service time. The validation results indicate that the service time can be successfully represented by a discrete Generalized Pareto (dGP) distribution, which is confirmed by the Kolmogorov-Smirnov test. Because the throughput of the CRN is represented by the inverse of the service time, the proposed analysis provides an upper bound for the networks' throughput, indicating the maximum throughput that can be attained when a single SU transmits over a GSM cellular channel. The results presented in the paper are validated with real data, confirming the accuracy of the proposed service time model.

It is well-known that Massive MIMO systems (Multiple-input multiple-output) have high potential for future wireless broadband systems. Massive MIMO (m-MIMO) relies on spatial multiplexing, and for that reason the base station needs a precise channel knowledge at uplink and downlink. Pilots can be used for channel state information (CSI) estimation, but common estimation processes imply a matrix inversion which can be a heavy computational process for m-MIMO system where the number of antennas used in the communication is very high. To alleviate computational requirements, reduce latency and to improve battery life capacity of mobile devices matrix inversion operations should be avoided. Having in mind these constrains, a new channel estimation method based on Zadoff-Chu (ZC) sequences is presented here, that achieves similar or better performance than least squares (LS) or minimum mean-Square Error (MMSE) channel estimators. It is also presented a set of performance results that sustain our assumption.

In this paper we propose two decentralized medium access control (MAC) schemes to coordinate the access of multiple transmitters adopting a Multi-Packet Reception (MPR) physical (PHY) layer. The proposed MAC schemes operate over a MPR-based PHY-layer and are divided in two stages. In the first stage the nodes indicate their willingness to transmit, while in the second one they jointly transmit. The MAC schemes differ by considering a Single-Packet Reception (SPR) or a MPR PHY-layer in the first stage. We analyze the throughput achieved by each MAC in a theoretical way. Several results obtained through simulation are also presented, which validate the numerical results obtained with the theoretical model. The results demonstrate the effectiveness of the proposed MAC schemes, and allow us to draw valuable conclusions about the design of distributed MPR systems.

In recent schemes proposed to the physical layer of wireless systems the receiver node has the capability of simultaneously receiving multiple transmissions, which is commonly referred to as a Multi-Packet Reception (MPR) scheme. Assuming that the transmitters are randomly located in the far-field region and a generic signal-to-interference-plus-noise ratio (SINR) threshold-based criterion is used as the packet capture model, we derive the probability of successful reception of a packet considering path loss, shadowing and fading effects. Our approach relies on the characterization of the aggregate interference caused by the transmitters, which is used in the capture model in a simplified way. The probability of successful reception of a packet is then used to approximate the average number of packets simultaneously received. The proposed analysis is evaluated for different scenarios and compared with several results obtained through simulation. The simplicity of the model, as well as its accuracy, makes it a useful tool to assist the design of future medium access control mechanisms for MPR wireless systems.

This work addresses the target tracking problem that makes use of combined measurements, namely received signal strength (RSS) and angle of arrival (AoA). By linearizing the measurement models and incorporating the prior knowledge obtained from target state transition model, we show that the application of the Kalman filter (KF) to the considered tracking problem is straightforward. Then, an extension of the linearization approach to the case where the target transmit power is not known is introduced and applied to the measurement model to obtain an estimate of the transmit power. By taking advantage of this estimated value, we show that the proposed KF algorithm can easily be generalized to the case of unknown transmit power. Our simulation results confirm the efficacy of the proposed algorithms in comparison with the existing one, as well as the robustness of the proposed approach to not knowing the transmit power. Finally, the supremacy of using the Bayesian approach in comparison with the classical one which disregards the prior knowledge information is also validated through computer simulations.

Massive MIMO (m-MIMO) schemes operating at mmWave bands are being proposed to be used in next generation wireless systems, bringing new challenges both in terms of detection techniques and the definition of channel models. In this paper, frequency-domain iterative detection schemes which do not require matrix inversions and are suitable for m-MIMO systems employing Single- Carrier with Frequency-Domain Equalization (SC- FDE) modulations are considered. These techniques are evaluated using a general clustered channel model for multilayer m-MIMO systems. Performance results show that these techniques can approach the Matched Filter Bound (MFB) with just a few iterations, even with significant correlation between different antenna elements.

Massive multiple input, multiple output (MIMO) schemes have been considered to support the physical layer of 5G systems and its combination with single-carrier with frequency-domain equalization (SC-FDE) schemes is particularly interesting for the uplink. However, the receiver complexity increases with the number of antennas, and it is important to have low complexity massive MIMO schemes. In this paper we consider the receiver design for the uplink of massive MIMO schemes where SC-FDE techniques are employed by the user terminals. To achieve this, we empoly low resolution analog-to- digital converters (ADCs) at each receive branch of the BS, combined with low complexity FDE techniques. It is shown that, although the nonlinear distortion levels inherent to the use of low resolution ADCs can be very high, we can have excellent performance, even with low complexity FDE receivers, provided that the number of receiver antennas is higher than the number of user terminals.

Next generation of mobile communication systems must support astounding data traffic increases, higher data rates and lower latency, among other requirements. All these requirements should be met while assuring energy efficiency for mobile devices and base stations. Most likely, the future 5G systems will include massive MIMO (Multiple Input Multiple Output) schemes with dozen or hundreds of antennas enabling beamforming while operating in the millimeter wave spectrum. As soon as the millimetric wave propagation difficulties are overcome, the full potential of massive MIMO structures can be tapped. This paper presents a transmission system with bi-dimensional antenna arrays, based on a double layer structure combining beamforming with a multi-branch power amplification that achieves simultaneously power efficiency in amplification of multilevel constellations without penalties on system performance when compared with common schemes using only beamforming.

Orthogonal frequency division multiplexing (OFDM) schemes have high envelope fluctuations and peak- to-average power ratio (PAPR), making them very prone to nonlinear distortion effects, which can affect significantly the performance when conventional receivers are employed. However, it was recently shown that strong nonlinear distortion effects on OFDM signals do not necessarily lead to performance degradation. In fact, nonlinear OFDM schemes can outperform linear ones when optimum maximum likelihood (ML) receivers are employed. In this paper, we considered OFDM schemes with strong nonlinear distortion effects and we proposed a low- complexity detection scheme able to approach the optimum ML performance. Our technique is based on the fireworks algorithm (FWA) and allows excellent trade-offs between performance and complexity.

The use of massive multiple input multiple output (MIMO) techniques combined with orthogonal frequency division multiplexing (OFDM) schemes in the downlink of broadband wireless systems is considered in this paper. It is assumed that the transmitter supports up to R simultaneous links and employs T≫R antenna elements, and we study the feasibility of very low complexity transmitters with low resolution digital-to-analog converters (DACs). It is shown that the nonlinear distortion levels for detection purposes decrease with the ratio T/R, which means that we can employ very low resolution quantizers.

In this paper we consider massive multiple input multiple output (MIMO) schemes where each transmitter branch has very low resolution digital to analog converters (DACs). We present an analytical method for obtaining the statistical characterization of the transmitted signals that allows a simple and accurate performance evaluation even when there is imperfect channel state information (CSI). It is shown that, although the performance penalty associated to the use of low resolution DACs can be considerable, it can be mitigated by employing more antennas at the transmitter than at the receiver.

The aim of this manuscript is to propose a new hybrid nonlinear block space-time receiver structure for millimeter wave (mmW) systems, to efficiently separate the spatial streams. We consider a fully connected analog-digital architecture. We assume that both the transmitter and the receiver are equipped with a large antenna array and the number of radio frequency (RF) chains is lower than the number of antennas. The analog and digital parts of the equalizer are jointly optimized using as a metric the mean square error (MSE) between the transmitted data vector and its estimate after the digital equalizer. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently deal with the constraints the analog part is selected from a dic-tionary based on the array response vectors. The performance results have shown that the performance of our hybrid nonlinear space-time equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently proposed for hybrid mmW massive MIMO architectures.

In this paper, we investigate the effect of carrier frequency offset (CFO) on the sum-rate of block-wise single carrier(SC) massive MIMO systems. Based on the user knowledge regarding the channel conditions, we consider three different scenarios. In the first scenario, the users know the expectation of the effective channel including the CFO effect in the time domain. In the second scenario, the users know the expectation of the effective channel including the effect of the CFO in the frequency domain. In the third scenario, the users know the expectation of the effective channel without the effect of the CFO. We show that in the first scenario, the effect of the CFO can be completely compensated. We also show that for finite number of base station (BS) antennas, the sum-rate in the second scenario is higher than that of the third scenario. However, as the number of BS antennas tends to infinity, the sum-rate in the former case is bounded and in the later case it tends to infinity.

This paper investigates the bit error probability of a wavelet-time diversity transmission system, which exploits the orthogonality properties between wavelet matrix rows to spread the information of QPSK symbols before transmission. Numerical results of performance comparisons show that the proposed theoretical analysis very accurately matches with the Monte Carlo simulation results. Besides, analytical and simulation results show much higher performance under flat-fading conditions, while remains unaffected the spectral efficiency of the original modulation scheme, which offers significant advantages for next-generation broadband wireless communication systems.

The combination of millimeter wave (mmW) with massive MIMO is a promising approach to achieve the multi Gb/s required by future wireless systems. Fully digital architectures are not feasible due to hardware limitations, and thus, the design of signal processing techniques for hybrid analog-digital architectures is of paramount importance. In this paper, we propose a new hybrid iterative block space-time receiver structure for multiuser mmW massive MIMO systems. We consider low-complexity user terminals employing analog-only random precoding and a single RF chain. At the base station, a hybrid analog-digital equalizer/detector is designed to efficiently remove the multiuser interference. The analog and digital parts of the equalizer are jointly optimized using as a metric the average bit-error-rate. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently handle these constraints, the analog part is selected from a dictionary based on the array response vectors. We also propose a simple, yet an accurate semi-analytical approach for obtaining the performance of the proposed hybrid receiver structure. The results show that the performance of the hybrid iterative equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently considered for hybrid mmW massive MIMO architectures.

This paper considers the use of the linear amplification with nonlinear components (LINC) technique for the power amplification of spectrally compact offset quadrature phase shift keying (OQPSK) signals allowing the use of highly efficient, low cost, and strongly nonlinear high power amplifiers (HPAs). However, the performance of the LINC signal separation and power combining procedures decreases with the rise of the signal's peak-to-average power ratio (PAPR). A new ring-type magnitude modulation (RMM) method is proposed for OQPSK signals that limits both its maximum and minimum complex envelope excursions avoiding zero crossings, without spreading the transmitted signal's spectrum. The performance results show that band-limited OQPSK signals whose envelope has low fluctuations produce LINC components with a narrower spectrum, with a considerable impact on the LINC transmitter regardless of the type of combiner chosen: when using a passive/matched combiner, the transmitter's power efficiency is significantly increased without spreading the combined signal's spectrum; for the highly efficient non-linear Chireix combiner, there is a reduction of the amount of spectral leakage produced by nonlinearly combining the LINC signal components. Finally, an iterative decoding scheme is also proposed, which employs estimates of the received symbols' RMM coefficients to compensate the RMM distortion.

Millimeter waves and massive MIMO are a promising combination to achieve the multi-Gb/s required by future 5G wireless systems. However, fully digital architectures are not feasible due to hardware limitations, which means that there is a need to design signal processing techniques for hybrid analog-digital architectures. In this manuscript, we propose a hybrid iterative block multiuser equalizer for subconnected millimeter wave massive MIMO systems. The low complexity user-terminals employ pure-analog random precoders, each with a single RF chain. For the base station, a subconnected hybrid analog-digital equalizer is designed to remove multiuser interference. The hybrid equalizer is optimized using the average bit-error-rate as a metric. Due to the coupling between the RF chains in the optimization problem, the computation of the optimal solutions is too complex. To address this problem, we compute the analog part of the equalizer sequentially over the RF chains using a dictionary built from the array response vectors. The proposed subconnected hybrid iterative multiuser equalizer is compared with a recently proposed fully connected approach. The results show that the performance of the proposed scheme is close to the fully connected hybrid approach counterpart after just a few iterations.

In this paper, we consider a T \times R massive MIMO system with up to R simultaneous links (T>> R>>1) and nonlinear amplifiers in each transmitter branch. The impact of a nonlinear amplification is analytically studied and accurate expressions for the spectral characterization of the transmitted signals, as well as the bit-error-rate (BER) are derived. The accuracy of our analysis is validated by simulations. It is also shown that the nonlinear distortion levels for detection purposes decrease with the ratio T/R, which means that the impact of the nonlinear distortion effects on the system's performance can be alleviated provided that T>>R. Therefore, the combination of massive MIMO systems with nonlinear, highly efficient power amplifiers can be an interesting option for low-cost, broadband wireless systems.

The expectations for fifth generation (5G) systems involve handling huge throughputs together with power consumption. In order to address this issue, massive multiple-input and multiple-output (MIMO) schemes were developed exploring the fact that any M-ary constellation can be decomposed as several offset quadrature phase shift keying (OQPSK) signals, to be amplified by efficient saturated amplifiers and transmitted separately. Since nonlinear amplifiers should be employed with constant envelope signals to avoid nonlinear distortion, a ring-type magnitude modulation (RMM) technique was proposed for bandlimited OQPSK signals to mitigate its envelope fluctuations while maintaining its spectral characteristics. This technique was shown to improve the power amplification efficiency of linear amplification with nonlinear components (LINC) transmitters, although with some degradation of the of the detection performance. This paper proposes an iterative decoding scheme relying on the accuracy of the first iteration's log likelihood ratio (LLR) computation to estimate the RMM coefficients of the received symbols. Several LLR computation methods are compared with the ideal case where the receiver knows each symbol's RMM coefficients, and it is shown that a few iterations are enough to obtain a bit error rate (BER) performance close to the best case scenario.

In this paper we design a new iterative space-frequency equalizer, based on iterative block decision feedback equalization (IB-DFE) principle, for multiplexing MIMO CE-OFDM millimeter wave (mmW) based systems. Linear frequency domain equalizers are not the most efficient to separate the spatial streams due to inter-carrier interference. Therefore, we design a non-linear equalizer that takes into account both inter-block and residual inter-carrier interferences by efficiently exploit the high space-frequency diversity order inherent to these systems. The feedforward and feedback matrices for the proposed iterative space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our iterative scheme significantly outperforms the linear frequency domain equalizers, recently considered for CE-OFDM, with only a few iterations.

In this paper we propose a decentralized medium access control (MAC) scheme to coordinate the access of multiple transmitters adopting a multi-packet reception (MPR) physical (PHY) layer. MPR receivers are able to decode multiple transmissions, and depending on the technique adopted at the PHY-layer, there is an optimal number of simultaneous transmissions that maximize the number of packets successfully decoded in a simultaneous way. The proposed MAC scheme admits an MPR PHY-layer and is divided in two stages. In the first one the nodes indicate their willingness to transmit, while in the second one they jointly transmit. Adopting a generic model for the PHY-layer, we characterize the throughput achieved by the proposed MAC when both MAC and PHY layers are considered. The formal characterization of the throughput is then used to optimize the duration of the first stage of the MAC scheme in order to optimize the throughput. In this way, the number of transmitters is regulated to optimize the cross-layer operation, taking into consideration the features of the MPR PHY-layer and the maximum performance achieved with the proposed MAC design. Several results obtained through simulation are presented, which validate the numerical results obtained with the proposed characterization. The proposed scheme is a first step to develop MAC protocols specifically designed to distributed MPR systems that operate in a cross-layer and improve the exploitation of current and future MPR techniques proposed for the PHY-layer.

In the next generation wireless systems, where m-MIMO and mmWave should be combined with beamforming techniques, array optimization at receiver ensuring a good SINR level can become a very large problem. Meta-heuristics can be an adequate approach to solve this problem with a mild computational effort. In this work, different meta-heuristics are implemented and applied to this problem, with Differential Evolution presenting promising connection setup time results that expectedly would enable its use in m-MIMO systems.

In this work, we evaluate the performance of Wavelet-Coding into offering robustness for OFDM signals against the combined effects of varying fading and noise bursts. Wavelet-Code enables high diversity gains with a low complex receiver, and, most notably, without compromising the system's spectral efficiency. The results show that the Wavelet-Coded OFDM system achieves a BER of 1E-3 with nearly 6 dB less SNR than the convolutional coded OFDM system in frequency selective channels with a normalized channel response variation rate of ζ=1E-4.The proposed system fits as a key enabler for the use of mm-wave frequencies in future generation mobile communication due to its robustness against multipath fading.

The use of massive multiple input multiple output (MIMO) techniques combined with orthogonal frequency division multiplexing (OFDM) modulations is being proposed for future broadband wireless systems such as 5G cellular networks. However, although these combination brings large capacity gains, it also suffers from two important drawbacks: the high implementation complexity inherent to the large number of antenna elements and the high sensitivity to nonlinear distortion effects due to the large peak-to-average power ratio (PAPR) of OFDM signals. In this paper, we study the capacity of massive MIMO-OFDM systems with strong nonlinear distortion effects. We derive theoretical expressions for the channel capacity considering different downlink scenarios where a base station with T nonlinear transmitting branches communicate with R receive antennas. It is shown that, although nonlinear distortion effects can reduce substantially the system capacity, this capacity loss can be reduced by increasing the number of transmit antennas (i.e., by using T≫ R).1.

The growing progress in wireless communication services lead to a demand in high data rates, spectral efficiency and flexibility requirements. The recently proposed Block-Windowed Burst Orthogonal Frequency Division Multiplexing (BWB-OFDM) transceiver proved to be a reliable alternative scheme to face these current demands. BWB-OFDM employs smoother, non- rectangular windows, allowing a power spectral density similar to the filtered OFDM approach; also, it packs together several OFDM symbols, with the addition of a sole zero- padding to accommodate the multipath channel's propagation delay. This means better overall power and spectral efficiencies. Nevertheless, the system has the same drawback of OFDM when transmitting over hostile channel conditions, such as deep fading in time-dispersive channels. To overcome this problem, a new Time-Interleaved BWB- OFDM (TIBWB- OFDM) transceiver is proposed. This scheme employs interleaving on the time-samples of each BWB- OFDM block, thus creating a sort of diversity at the frequency domain, aiming to preserve the data symbols severely corrupted by the channel's deep fades. The new TIBWB-OFDM transceiver presents considerable power gains relatively to the BWB-OFDM,while maintaining their spectral efficiency.

Multiple input multiple output (MIMO) schemes combined with orthogonal frequency division multiplexing (OFDM) modulations are widely employed in broadband wireless systems. However, the transmitted signals are prone to nonlinear distortion effects due to their high envelope fluctuations. In this paper, we present a simple analytical method for studying nonlinear effects in MIMO-OFDM systems. Our method allows accurate results for the power spectral density (PSD) of the transmitted signals, as well as the signal- to- interference (SIR) levels at the detection level. It is shown that the robustness to nonlinear distortion effects increases with the number of transmit antennas (for a fixed number of independent data streams).

The combination of massive multiple input multiple output (MIMO) schemes with orthogonal frequency division multiplexing (OFDM) modulations are being considered for 5th generation (5G) wireless broadband systems. However, both the high complexity associated to massive MIMO transceivers as well as the amplification problems of OFDM constitute important drawbacks of such combination. In this paper we propose a reduced complexity, energy efficient massive MIMO OFDM scheme. For that purpose, we consider a maximal ratio transmit (MRT) technique combined with a simple time-domain clipping operation. We present an analytical method for obtaining both the power spectral density (PSD) of the transmitted signals, as well as the signal-to-interference (SIR) at the detection level. It is shown that having a large number of transmit antennas not only provides the known power and capacity gains, but also increases the robustness to nonlinear distortion effects.

BS (Base Station) cooperation architectures, combined with block transmission techniques, such as SC-FDE (Single-Carrier with Frequency-Domain Equalization) for the uplink and OFDM (Orthogonal Frequency Division Multiplexing) for the downlink, are proven to be suitable for broadband wireless transmission systems. In BS cooperation schemes, MTs (Mobile Terminals) in adjacent share the same physical channel, in which the different signals are sent to a CPU (Central Processing Unit) where iterative frequency-domain receivers based on the IB-DFE (Iterative Block Decision Feedback Equalization) concept perform the combination and separation of the different transmission sources. This paper presents an extension of the BS cooperation system considering a C-RAN (Centralized-Radio Access Network) approach, allowing a large scale clustering centralized deployment.

BS (Base Station) cooperation schemes allow for an improved overall system's capacity and spectral efficiency. This is due to the possibility of MTs (Mobile Terminals) in adjacent cells sharing the same physical channel. The BSs send a quantized version of the received signals to a central unit that performs the separation of the signals associated to different MTs. In this paper we consider the uplink of BS cooperation schemes with SCFDE (Single-Carrier with Frequency domain Equalization) modulations and receivers based on the IB-DFE (Iterative Block Decision Feedback Equalization) concept for the multi-user detection process. We design robuts receivers that take advantage of the statistical characteristics of the quantization noise.

Strongly nonlinear amplifiers have lower implementation complexity, together with much higher amplification efficiency than linear amplifiers. However, they are only recommendable when the signals at their input have very low envelope fluctuations, something that in general is incompatible with signals with high spectral efficiency (i.e. employing high-order constellations). It is proved, although, that spectral efficient M-ary constellations can be decomposed as a sum of several OQPSK signals that can be amplified separately, thus bringing a new life for this modulation scheme. Constant- envelope OQPSK-type signals can be obtained using its equivalent minimum shift keying (MSK) representation, but don't possess the required compact spectrum close to the minimum Nyquist frequency. This can only be achieved by employing narrowband Nyquist filters, which on the other hand causes the signal's envelope to fluctuate. Considering that, this paper proposes a ring-type magnitude modula- tion (RMM) method for narrowband offset quadrature phase shift keying (OQPSK) signals that considerably reduces its envelope fluctuations with- out spreading the transmitted signal's spectrum. Simulation results show that the reduction of the signal's dynamic range and peak-to-average power ratio (PAPR) allows the use of highly efficient, nonlinear saturated high power amplifiers (HPAs) without spreading the signal's spectrum and with minimum bit error rate (BER) performance degradation.

In this study, the authors consider a peak‐to‐average power ratio (PAPR) reduction for orthogonal frequency‐division multiplexing systems based on the decomposition of the set of subcarriers in subsets of subcarriers, denoted resource blocks, each one weighted by a different complex factor. They present a new iterative sphere‐geodesic descent method for obtaining these weighting factors so as to minimise the PAPR of the transmitted signals. This method, which they term geodesic descent method, efficiently makes use of the Riemannian structure of the power constraint. The authors’ performance results show that the proposed technique provides good trade‐off between the PAPR reduction and the bit error rate performance, for both uncoded and coded scenarios.

In this letter, a multiple-input multiple-output detection algorithm based on the alternating direction method of the multipliers (ADMM) is proposed for single-carrier transmissions in time dispersive channels. The ADMM is applied as a heuristic to solve the maximum likelihood detection problem for arbitrary signal constellations formulated in the frequency domain. This approach allows most of the decoding steps to be implemented in the frequency domain with an overall reduced complexity cost. Simulation results show that the performances close to the matched filter bound can be attained in large problem sizes, even in overloaded scenarios.

Base station (BS) cooperation architectures provide meaningful capacity gains, as they allow mobile terminals (MTs) in neighbor cells to share the same physical channel. Combining uplink signals received by the different BSs by employing iterative frequency-domain algorithms allows for the correct separation of the signals transmitted by each MT. To transmit those signals, MTs rely on single-carrier (SC) modulation with frequency-domain equalization (FDE). Those SC-FDE transmissions combined with iterative block decision feedback equalization (IB-DFE) techniques have been shown to be a effective mean to transmit cyclic-prefix (CP) assisted blocks within broadband wireless systems. In this paper, we aim to combine the aforementioned concepts of uplink transmission based on IB-DFE with BS cooperation in a clever manner. Our proposed scheme improves both the frequency reuse factor and the MTs' performance at cell edges, with further improved performance when low-density parity-check (LDPC) codes and turbo IB-DFE equalization schemes are considered.

In wireless communication systems privacy among users contents is a crucial security requirement such as power and spectral efficiency. Spectral and power efficiencies are attainable with power efficient transmission schemes where constellations are decomposed into several BPSK (Bi Phase Shift Keying), being each one amplified and transmitted independently by an antenna. Besides that, due to constellation shaping on the desired transmission direction performed by these transmitters, some kind of physical layer security is assured as well. Under this approach, security is achieved by constellation shaping of the transmitted constellation since each user must know the transmitter configuration parameters associated to the constellation shaping, i.e., the direction in which the constellation is optimized, otherwise the received data would be meaningless. Commonly, security is assured by encrypted algorithms implemented by higher layers, such as private and public encrypted keys. However, system security can be improved with physical layer security schemes since they can be complemented with other security schemes from higher layers. Simulation results show the effectiveness of the proposed approach.

We address the received signal strength (RSS) based target localization problem in large-scale cooperative wireless sensor networks (WSNs). Using the noisy RSS measurements, we formulate the localization problem based on the maximum likelihood (ML) criterion. Although ML-based solutions have asymptotically optimal performance, the derived localization problem is non-convex. To overcome this difficulty, we propose a convex relaxation leading to second-order cone programming (SOCP) estimator, which can be solved efficiently by interior-point algorithms. Moreover, we investigate the case where target nodes limit the number of cooperating nodes by selecting only those neighbors with the highest RSS. This simple procedure can reduce the energy consumption of an algorithm in both communication and computation phase. Our simulation results show that the proposed approach outperforms significantly the existing ones in terms of the estimation accuracy and convergence. Furthermore, the new approach does not suffer significant performance degradation when the number of cooperating nodes is reduced.

We consider 3-D target positioning in noncooperative wireless sensor network (WSN) by employing received signal strength (RSS) and angle-of-arrival (AoA) measurements. Based on the least squares (LS) criterion, we derive a novel objective function for solving the hybrid localization problem. Despite the fact that the resulting optimization problem is non-convex, we show that it can be approximated into a convex problem by applying second-order cone programming (SOCP) relaxation. Moreover, we show that the objective function can be written in the form that belongs to the generalized trust region subproblems (GTRS), which can be solved exactly. Our numerical results exhibit remarkable performance of the new approaches, reducing the estimation error for more than 5 m and 3 m, in comparison to the state-of-the-art approach.

FTN (Faster Than Nyquist) signaling allows capacity gains but requires substantially complex equalization schemes, even for an ideal AWGN (Additive White Gaussian Noise) channel. Extending conventional FTN receivers for severely time- dispersive channels is not an option, since the receiver complexity becomes prohibitively high. In this paper we address the design of a receiver for FTN signaling over severely time-dispersive channels. To cope with the severe ISI (Inter-Symbol Interference) associated to the combined effects of FTN signaling and the time-dispersive channel we consider an iterative FDE scheme (Frequency-Domain Equalization) combined with a HARQ (Hybrid Automatic Repeat reQuest) especially designed taking into account the characteristics of FTN signals. We also present a simple, yet accurate model for the performance evaluation, considering multiple retransmissions per block. Our performance results show that we can have significant throughput gains, while maintaining essentially the receiver complexity of conventional, Nyquist-rate signaling schemes, even for severely time-dispersive channels.

Iterative block decision feedback equalization (IB-DFE) algorithms are proven to be powerful tools when detecting several uplink signals sharing the same physical channel. However, those algorithms may face some constraints -- the IB-DFE has some difficulty picking certain signals up if they are considerably weaker than the rest and the computational effort grows exponentially with the number of signals to be detected. In this paper we present a clustered multiuser detection scheme, based on an IB-DFE algorithm. It is demonstrated that if the mobile terminals (MTs) have enough power and are distributed in clusters, with considerable power difference between them, it is possible to successfully detect them all while containing the computational complexity. Our scheme also requires fewer receiving antennas than the original IB-DFE implementations and, with the aid of low-density parity- check (LDPC) and turbo- equalization, it is able to reach high performance levels with low power requirements.

The development of new mobile communications systems faces several important challenges, namely demanding for high bit rates, high spectral efficiency, low energy consumption and the usage of hostile channels. In order to fulfill these requirements for block-based single carrier transmissions, we propose to use high order QAM constellations combined with envelope control Magnitude Modulation (MM) techniques and bit-interleaved coded modulation with LDPC codes. Also, the receiver uses powerful MM specialized IB-DFE solutions to combat the channel distortion and noise. The obtained results have shown significant improvements in terms of energy efficiency and bit error rate.

The study of nonlinear effects on orthogonal frequency division multiplexing (OFDM) signals can be made taking advantage of their Gaussian characteristics. This is relatively simple and straightforward for smooth, polynomial nonlinear characteristics, since the required number of intermodulation products (IMPs) can easily be obtained. However, the situation is more complex for clipping characteristics, since they are not differentiable and their polynomial approximation is a function of the clipping level. In this paper we consider the effects of clipping on sampled OFDM signals and we define an equivalent nonlinear characteristic that is polynomial, with small degree, whose output has the same spectral characteristics of a clipping operation. Our results show that we can use this equivalent nonlinear characteristic for studying the impact of the effects of clipping on sampled OFDM signals, allowing very accurate estimates of the power spectral density (PSD) of the output signals, as well as the corresponding signal-to-interference ratio (SIR). It can also be employed to obtain the performance of optimum receivers for clipped OFDM.

Orthogonal frequency division multiplexing (OFDM) modulations are very popular in wireless communications due to their flexibility and good performance over severely time-dispersive channels. However, OFDM signals have large peak-to-average power ratio (PAPR), which leads to amplification problems. Constant envelope OFDM (CE-OFDM) techniques use an OFDM signal to modulate the phase of a given carrier, allowing a 0 dB PAPR, with all inherent advantages. However, the phase modulator is a nonlinear device that can lead to performance degradation. In this paper, we take advantage of the Gaussian characteristics of OFDM signals to characterize analytically CE-OFDM signals, both in terms of power spectral density (PSD) and optimum asymptotic performance.

Orthogonal frequency division multiplexing (OFDM) modulations are very popular in wireless communications due to their flexibility and good performance over severely time-dispersive channels. However, OFDM signals have large peak-to-average power ratio (PAPR), which leads to amplification problems. Constant envelope OFDM (CE-OFDM) techniques use an OFDM signal to modulate the phase of a given carrier, allowing a 0 dB PAPR, with all inherent advantages. However, the phase modulator is a nonlinear device that can lead to performance degradation. In this paper, we take advantage of the Gaussian characteristics of OFDM signals to characterize analytically CE-OFDM signals, both in terms of power spectral density (PSD) and optimum asymptotic performance.

Mobile communication systems must support multiple users achieving at same time privacy. Thus security is a crucial requirement to assure privacy of the data transmitted from and for each user. Several solutions can be adopted to implement security. Commonly they are based on encrypted algorithms implemented by higher layers, such as private and public encrypted keys. Also security can be implemented at physical layer and complemented with other security schemes from higher layers. Directivity introduced at the transmitted constellation can be also used to assure some kind of physical layer security. This is the case of power efficient transmission schemes where constellations are decomposed into several BPSK (Bi-Phase Shift Keying), being each one amplified and transmitted independently by an antenna. It can be shown that privacy among user contents is achieved under this approach, since each user must know the transmitter parameters associated to the constellation shaping, i.e., the direction in which the constellation is optimized, otherwise receives a meaningless data. If transmitter parameters are unknown, mutual information will be null which means a secrecy level of 100%. Simulation results show the effectiveness of the proposed approach.

This paper presents a novel solution for joint turbo equalization and carrier synchronization in SC-FDE schemes. The proposed solution resorts to the Bayesian principle for the synchronization operation. In fact, the so called sensor factor is conditioned with respect to the phase drift and the transmitted data symbol. The phase drift is the quantity of interest and the data symbol is, for synchronization purposes, considered a nuisance parameter. After marginalizing this nuisance parameter the sensor factor is expressed in terms of the observation and the phase drift only. This allows us to implement the recursive Bayesian filtering technique known as sequential importance sampling (SIS). Finally, we compare our solution with an alternative solution, namely, the a posteriori (MAP) estimator.

SC (Single-Carrier) modulations are known to be suitable for high data rate transmission over severely time-dispersive channels such as the ones inherent to wireless systems. When combined with an appropriate CP (Cyclic Prefix), they present an efficient alternative to MC (Multi-Carrier) modulations such as OFDM (Orthogonal Frequency Division Multiplexing), with similar overall complexity. In this paper we study the theoretical performance of SC-FDE schemes (SC with Frequency-Domain Equalization) employing iterative receivers based on the IB-DFE (Iterative Block Decision Feedback Equalization). We present a simple, yet accurate analytical model for obtaining the corresponding BER (Bit Error Rate) performance.

Orthogonal frequency division multiplexing (OFDM) schemes are being considered for both optical fibre and wireless optical communications mainly because their ability to cope with the high inter‐symbol interference levels associated to dispersive channels. DC biased optical OFDM (DCO‐OFDM) is a promising technique that meet the particularities of intensity modulated OFDM schemes. To reduce the required DC component OFDM signals need to be submitted to an asymmetric clipping. However, the resulting non‐linear distortion effects can lead to significant performance degradation and/or undesirable out‐of‐band radiation. Therefore it is important to evaluate the impact of an asymmetric clipping on DC‐biased optical OFDM signals. In this work, the authors take advantage of the Gaussian‐like nature of OFDM signals and they study analytically the impact of an asymmetric clipping operation in the performance of DCO‐OFDM schemes. The authors present accurate theoretical expressions for the power spectral density and the non‐linear distortion at the subcarrier level, which can be used for obtaining the bit error rate of conventional receivers. The authors also study the optimum performance of these non‐linear OFDM schemes and the authors show that for an optimum maximum likelihood detection the non‐linear distortion instead of leading to performance degradation can actually improve it, even outperforming conventional, linear OFDM schemes.

In this study the authors propose a novel joint detection and phase noise estimation scheme suited for severely time‐dispersive channels. The authors consider single‐carrier modulations combined with frequency domain equalisation schemes where the wireless transmission is impaired with phase noise. An iterative frequency‐domain equaliser is assumed on the receiver side and the phase noise is estimated and compensated for after the equalisation step and within each iteration of the equaliser. In fact, by exploiting the Gaussianity of the equaliser output the authors are able to track the phase noise using stochastic recursive filtering techniques. These techniques share the same dynamic state‐space (DSS) model. The observation equation corresponds to the measurement of the phase noise of a digitally‐modulated signal affected by additive white Gaussian noise, and the dynamics equation corresponds to the Wiener–Lévy model for the phase noise. Supported on this DSS model the authors aim at estimating the unknown phase noise value given all observations up to the current time instant. In a Bayesian context this represents estimating recursively in time the filtering and the predictive distributions. From these distributions a minimum mean‐squared error estimate of the phase noise is determined.

In this manuscript, we consider the uplink of a single carrier frequency division multiple access (SC-FDMA) based system, where a set of small-cells coexist with a macro-cell using the same spectrum. To deal with both the inter-carrier interference (ICI) and inter-system interference, we combine interference alignment (IA) precoding at the small-cell transmitters with iterative decision feedback equalization at the macro-receiver. The transmitter and receiver design is performed jointly by considering both full-coordination and limited inter-system information exchange between macro- and small-cells. At the small cells side, we assume that the access points are connected through a limited capacity backhaul network to a central unit, where the separation of a quantized version of the small-cell signals is performed. For this case, the iterative feedback equalizer is designed by explicitly taking into account the impact of signal quantization. Moreover, a semi-analytical approach for the performance of the proposed schemes is provided. The results show that the proposed schemes are robust to both inter-system and inter-carrier interferences and thus are able to efficiently separate the macro- and small-cells spatial streams under limited information exchange.

Prefix-assisted direct-sequence code-division multiple access (DS-CDMA) is a viable transmission technique for high-data-rate wireless broadband systems, capable of coping with highly dispersive channels at the uplink. Packet combining (PC) ARQ with an iterative block decision feedback equalization (IB-DFE) technique can be used to cope with errors. However, extracting the IB-DFE's packet error rate (PER) for MAC-layer simulations can be time consuming. Most works that model the DS-CDMA behavior with PC ARQ use simplified asymptotic PER models in the study of the system performance, which are not precise for low signal-to-noise ratios and for a low number of mobile terminals (MTs). In this paper, the MAC performance is modeled using a discrete-time Markov chain that relies on a PER model. As an example, this paper describes a PER analytical model for an uncoded prefix-assisted DS-CDMA system with PC ARQ that considers the following inputs: the number of MTs accessing the channel, the MTs' number of transmissions, and the channel realizations for a given bit energy over noise ratio. The results show that the model portrays with better accuracy the simulations' results, compared with an alternative asymptotic model.

It is widely recognized that orthogonal frequency division multiplexing (OFDM) signals are very prone to nonlinear distortion effects, which can lead to significant performance degradation. However, recent results have showed that nonlinear distortion effects do not necessarily mean performance degradation and can actually lead to performance improvements relative to conventional linear OFDM schemes. In this paper, we consider the effects of bandpass memoryless nonlinear devices on OFDM signals and study the optimum asymptotic performance for both nondispersive channels and severely time-dispersive channels. We present analytical methods for obtaining the Euclidean distance between two OFDM signals that are subjected to different nonlinear characteristics. These results are then employed for obtaining the average asymptotic gain relative to conventional linear OFDM schemes in nondispersive channels and the gain distribution for severely time-dispersive channels with Rayleigh-distributed fading on the different multipath components. Our analytical results, which are shown to be very accurate, indicate that the optimum detection of OFDM schemes with strong nonlinear distortion effects allows significant gains when compared with conventional linear OFDM schemes.

In this paper we present a solution for joint equalization and phase drift compensation for the underwater acoustic (UWA) channel. In particular, we aim at compensating the vehicular component of the Doppler effect. This component is modeled resorting to a random walk model with a linear drift. It will be estimated using a Bayesian filtering technique, namely, the sequential importance sampling (SIS) algorithm, also known as particle filter (PF). The equalization will be attained using an iterative frequency-domain equalizer (IFDE). Performance results, obtained using state-of-the-art UWA channel models, are presented.

With this paper we aim to show that Gaussian sums particle filtering (GSPF) combined with iterative frequency-domain equalization (IFDE) for single-carrier (SC) modulations offer an appropriate solution for data transmission over highly frequency-selective channels with strong phase noise and linear drift. We present a promising IFDE scheme dubbed iterativeblock decision feedback equalization (IB-DFE) and propose a post-equalization phase noise estimation scheme within the iteration chain. In fact, by exploiting the Gaussianity of the equalizer output we are able to track the phase noise value by updating the state posterior distribution using the GSPF algorithm. Additionally, simulation results are presented considering QPSK, 16-QAM, and 64-QAM signalling in highly frequency-selective channels and different values for the phase noise variance and linear drift.

Radio Spectrum sensing has been a topic of strong research in the last years due to its importance to Cognitive Radio (CR) systems. However, in Cognitive Radio Networks (CRNs) with multiple Primary Users (PUs), the Secondary Users (SUs) can often detect PUs that are located outside the sensing range, due to the level of the aggregated interference caused by that PUs. This effect, known as Spatial False Alarm (SFA), degrades the performance of CRNs, because it decreases the SUs' medium access probability. Adopting Energy-based spectrum sensing (EBS) in each SU, this work starts to characterize the interference caused by multiple PUs located outside the sensing area. The interference formulation is then used to derive the probabilities of detection (PD) and false alarm (PFA), and closed form expressions are presented. Several results show that the probabilities PDand PFAare successfully validated through simulation. Finally, the work shows that the SFA can be almost neglected, depending on the path loss factor and on the number of samples collected by the energy detector to decide the spectrum's occupancy state.1

This paper presents a new technique for the design of receiver architectures, based on the use of variable reference voltages in ADCs. The main goal is to improve the SNDR of a receiver chain when compared to traditional architectures like Low-IF or Subsampling, while performing down-conversion without the need of a Mixer block. The proposed technique takes advantage of the inherent capability of the ADC to perform the multiplication operation. High-level model simulations show that this technique is capable of improving the SNDR by about 5 dB (from 50 to 55 dB), while avoiding out-of-band noise aliasing, when compared to the referred techniques.

In this paper we design two multi-user receivers based on the iterative block decision feedback equalization (IB-DFE) concept for MC-CDMA systems with closed-form interference alignment (IA). The receivers are designed in two steps: first a linear equalizer is used to mitigate the inter-user aligned interference; then iterative frequency-domain receivers are designed to efficiently separate the spatial streams, in the presence of both residual inter-user and inter-carrier interferences at the output of the first equalizer. Two strategies are considered: in the first one the matrices are obtained by minimizing the mean square error (MSE) of each individual data stream at each subcarrier, while in the second approach the matrices are computed by minimizing the overall MSE of all data streams at each subcarrier. Our schemes achieve the maximum degrees of freedom provided by the IA precoding, while allowing a close-to-optimum space-diversity gain, with performance approaching the matched filter bound (MFB).

The PAPR (Peak-to-Average Power Ratio) is widely employed to measure the magnitude of the envelope fluctuations of a given signal. It is particularly used to define the required amplifier back- off for multi-carrier modulations such as OFDM (Orthogonal Frequency Division Multiplexing) schemes. However, the PAPR increases with the block size (i.e., the number of subcarriers, in the OFDM case), but the signal's distribution converges to a Gaussian distribution for a large number of subcarriers. Therefore, one might ask if the PAPR is indeed the best parameter to define the amplifier's back-off. In this paper we address this issue. It is shown that the required amplifier's back-off for a given non-linear distortion level, i.e. referred to the input amplifier saturation power, is almost independent of the number of subcarriers and, therefore, the PAPR is not the most suitable to define it. Therefore, we recommend the use of a parameter related to the instantaneous envelope power distribution to define the amplifier back-off.

Polyphase magnitude modulation (MM) has been shown to be a robust and effortless mean to improve the efficiency of a transmitter's high power amplifier (HPA), due to the real-time reduction of the peak to average power ratio (PAPR). MM's technique flexibility allows us to include the MM system on any existing single-carrier (SC) based transmission system with clear benefits on the achieved bit error rate \emph{vs} overall signal power to noise ratio. This paper analyzes the efficiency of MM when added to a multi- input multi-output (MIMO) system, using a block-based SC transmission combined with iterative block decision feedback equalization (IB-DFE). To improve the IB-DFE performance for low power signals, we consider an additional scheme where low-density parity-check (LDPC) coding and turbo equalization are added. Simulation results show a net power efficiency enhancement, particularly for systems with channel coding, confirming MM as a major asset for high performance communication systems.

Amplify-and-forward relay techniques combined with OFDM (Orthogonal Frequency Division Multiplexing) modulations are being considered for broadband wireless systems since they allow simple relay systems suitable for severely time-dispersive channels. However, since they rely in OFDM-based signals, these techniques are prone to nonlinear distortion effects. In this paper we consider OFDM- based amplify-and- forward relay systems with strong nonlinear distortion effects at both the transmitter and the relay node and we present a simple, but accurate, analytical method for characterizing the distortion levels at the subcarrier level.

In this paper, we present a theoretical analysis for obtaining the minimum Euclidean distance between two nonlinearly distorted OFDM (Orthogonal Frequency Division Multiplexing) signals. This analysis takes advantage of the Gaussian behavior of OFDM signals with a large number of subcarriers and is a key step to get the asymptotic gain of the optimum receiver. We consider both polar and Cartesian memoryless nonlinearities, showing that in general the optimum performance of nonlinear OFDM is better than linear OFDM.

It was shown recently that we can decompose multilevel constellations as the sum of constant-envelope components which can be amplified and transmitted by separate antennas, allowing power-efficient transmitters, together with directivity at the constellation level without changing on the radiation pattern associated to the set of antennas. However, errors in the direction estimates can lead to substantial performance performance degradation since the constellations seen at the receiver can be substantially distorted. In this paper we present an improved receiver that is designed taking into account constellation distortion effects inherent to errors in direction estimates. It is shown that these "smart" receivers, optimized taking into account the apparent constellation at the receiver side can substantially outperform conventional receivers that assume that assume undistorted constellations.

In this paper we consider the use of single carrier modulations combined with frequency-domain equalization schemes (SC-FDE) for underwater acoustic (UWA) communications. The propagation channel is doubly selective, with both time dispersion effects inherent to multipath propagation and phase drifts associated to Doppler effects. We propose a joint equalization and phase drift estimation and compensation scheme. Our performance results show that the proposed receiver is able to accurately estimate phase drifts and compensate them, allowing significant performance improvements in the presence of strong phase drifts.

In this paper we show that Gaussian sum particle filters (GSPFs) present an effective solution for the problem of estimating phase noise in digital communications. We start by describing the problem of estimating the phase noise in terms of its general Bayesian formulation and then present the solution offered by the GSPF. Our filter is based in two central features. On the one hand, the system model comprising both the dynamics model and the observations model and on the other hand the sensor factor, a periodic function with respect to the current observation of the state. Although phase noise estimation algorithms are widely represented in the literature to our knowledge it has never been considered to use GSPFs for this purpose. Furthermore, other solutions using conventional particle filters assumed the modulation symbols to be PSK, i.e., phase-defined, while we assume phase- and amplitude-defined symbols, e.g., M-QAM.

The linear amplification with nonlinear components (LINC) amplification concept is very important on the developing of the modern mobile and satellite communications systems due to the use of inexpensive high power amplifiers (HPA). This paper analyses the effectiveness of combining an efficient PAPR reducing technique based on magnitude modulation (MM) when applied to a LINC amplification system. The simulation results show that the inclusion of the MM on the LINC processing chain is very favourable in terms of spectral regrowth introduced by signal's clipping distortion. The system is also more robust to gain and phase imbalances between amplifiers with only a marginal bit error rate (BER) loss for an uncoded transmission scenario.

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative frequency-domain detection receivers based on the IB-DFE concept (Iterative Block Decision Feedback Equalization) can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this paper we combine iterative IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The receiver is designed in two steps: first a linear filter is used to mitigate the inter-user aligned interference, and then an iterative frequency-domain receiver is designed to efficiently separate the spatial streams in the presence of residual inter-user aligned interference at the output of the filter. The matrices for this non-linear space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our receiver structure is explicitly designed taking into account the residual inter-user interference, allowing both an efficient separation of the spatial streams and a reduction in the number of iterations of the IA procedure. We also propose a simple, yet accurate analytical approach for obtaining the performance of the proposed receiver structure. Our scheme achieves the maximum degrees of freedom provided by the IA precoding, while allowing an almost optimum space-diversity gain, with performance close to the matched filter bound (MFB).

Single carrier transmissions with frequency domain equalization (SC-FDE) have gained widespread use in emergent broadband wireless systems becoming an attractive alternative to popular Orthogonal Frequency Division Multiplexing (OFDM) schemes, particularly at the uplink. Since coherent receivers are usually employed with SC-FDE, accurate channel estimates are required so as to avoid substantial performance degradation. Several channel estimation strategies have been proposed for SC-FDE, but a thoroughly evaluation of the degradation caused by channel estimation errors and a comparison against OFDM is still lacking. In this paper we study the impact of imperfect channel knowledge on SC transmission with focus on the linear frequency domain equalizer (FDE) and on the Iterative Block Decision Feedback Equalizer (IB-DFE). We propose a modified IB-DFE which incorporates knowledge of the channel estimation error model and show that its performance becomes more robust against the presence of strong error components in the channel estimates. We also evaluate, analytically and through simulations, the degradation caused by imperfect channel estimation in SC-FDE and compare it against OFDM schemes (Orthogonal Frequency Division Multiplexing). It is shown that the channel estimation requirements for SC-FDE are higher than for OFDM unless a channel estimation error aware receiver is employed.

Nonlinear phase distortion can severely degrade the performance of optical signals, both in terms of spectral characteristics and BER (Bit Error Rate) performance. This is especially serious for multicarrier optical signals. In this paper we study analytically the impact of nonlinear phase distortion on Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) schemes with large number of subcarriers. The nonlinearly-distorted signal can be decomposed as the sum of useful and self-interference components. Conventional CO-OFDM implementations treat the self-interference as an undesirable noise term that leads to performance degradation. However, it is shown that this distortion component has information on the transmitted signals that can be employed to improve the performance. We show that the nonlinear phase distortion can lead to performance improvements relatively to the ideal linear case, contrary to what one could expect. We also present a sub-optimum receiver that is able to take advantage of this potential performance improvement.

Random access approaches in Low Earth Orbit (LEO) satellite networks are usually incompatible with the Quality of Service (QoS) requirements for multimedia traffic, especially when hand-held terminals must operate with a very low signal-to-noise ratio. This paper proposes the Satellite Random Network Diversity Multiple Access (SR-NDMA) protocol that handles multimedia traffic under this context through the combination of a random and scheduled access scheme. The protocol uses a multi-packet receiver, for Single Carrier with Frequency Domain Equalization (SC-FDE) in the uplink, that gradually reduces the packet error rate with additional transmissions. The paper proposes analytical performance models for the throughput, delay and energy efficiency - as long as terminals have finite queues. System parameters are defined to enhance the energy efficiency while satisfying the QoS requirements for limited queue and bit-rate constraints. Results show that the proposed system is energy efficient and provides enough QoS to support multimedia services such as video telephony.

Multilevel modulations can ensure high spectral efficiency but at cost of energy efficiency and envelope fluctuations that can compromise the amplification efficiency at the transmitter. Due to strict power and bandwidth constrains it is also desirable to adopt energy efficient amplification combined with powerful equalizer to reduce interference impact. In this paper we propose a method for amplification of multilevel constellations compatible with grossly nonlinear amplifiers based on the constellation's decomposition as a sum of BPSK (Bi-Phase Shift Keying) sub-constellations. Our approach relies on an analytical characterization of the mapping rule were the constellation symbols are written as a linear function of the transmitted bits. This analytical method is then employed to design iterative receivers such as IB-DFE (Iterative Block Decision Feedback Equalization), that can cope with higher sensitivity to ISI effects (InterSymbol Interference) of the resulting high order constellations. It is also defined a pragmatic method for designing IB-DFE receivers that can be employed with any constellation. It is also shown that the resulting method for designing an SC-DFE receiver (Single Carrier-Decision Feedback Equalization) does not require a significant increase in system complexity and can be used for the computation of the receiver parameters for any constellation.

Traditionally, Low Earth Orbit (LEO) satellite networks use hard handovers when a Mobile Terminal (MT) moves between the footprints of two satellites. Such approach introduces a transitory disconnection interval, which may limit the capacity to provide demanding real-time requirements. A soft- handover approach that applies distributed packet combining on a satellite diversity scenario is proposed in this paper. Considering inexpensive MT hardware and low transmission powers, the paper analyses the performance gains and the difficulties that are associated to different Doppler and time-of-arrival shifts when the MT signal is broadcast to two satellites during the short handover period where both satellite's footprint overlap. A Single-Carrier with Frequency Domain Equalization (SC-FDE) transmission technique is considered in this paper. It is shown that it is possible to have an inter-satellite handover almost without quality of service (QoS) degradation for a Satellite Network Diversity Multiple Access (S-NDMA) scheme.

In this paper, we present a theoretical analysis for obtaining the minimum Euclidean distance between two nonlinearly distorted OFDM signals (Orthogonal Frequency Division Multiplexing). This analysis takes advantage of the Gaussian-like behavior of OFDM signals with a large number of subcarriers and can be employed to provide the asymptotic gain of the optimum receiver. This approach is then employed with several different nonlinear characteristics, showing that in general the optimum performance of nonlinear OFDM is better than linear OFDM.

Recent results indicate that the optimum performance of uncoded OFDM (Orthogonal Frequency Division Multiplexing) in the presence of strong nonlinear distortion effects can be better than with linear transmitters. However, the uncoded OFDM performance is not very useful since typically OFDM schemes are combined with channel coding (also denoted coded OFDM), with substantially different uncoded and coded OFDM performances. In this paper we consider the optimum performance of coded OFDM schemes in the presence of strong nonlinear distortion effects. It is shown that nonlinear distortion effects not only do not lead to performance degradation, but they also might lead to substantial performance gains relatively to coded OFDM with linear transmitters.

The Iterative Block Decision Feedback Equalizer (IB-DFE) is a promising technique for single carrier (SC) block transmissions whose performance can approach the matched filter bound in severely time dispersive channels. In this paper we evaluate analytically and through simulations the impact of imperfect channel estimation on the performance of this equalizer and propose a modified IB-DFE which is shown to be more robust against the presence of strong error components in the channel estimates.

Imperfect channel estimation can have a severe impact on the performance of coherent detection systems. In this paper we study the effect of imperfect channel knowledge in the performance of SC-FDE (Single Carrier with Frequency Domain Equalization). We propose a low complexity linear FDE which incorporates knowledge of the channel estimation error variance and derive analytical BER expressions. It is shown that its performance becomes more robust in the presence of strong channel estimation error components compared with a conventional FDE receiver. We also compare the degradation caused by imperfect channel estimation in SC-FDE and in OFDM schemes (Orthogonal Frequency Division Multiplexing) and show that the channel estimation requirements for SC-FDE are higher than for OFDM.

Traditionally, an unsuccessful packet reception, either due to channel conditions or interference from multiple sources (collisions), is discarded and followed by a new reception of the same packet. Network diversity multiple access (NDMA) handles collisions by using time diversity multipacket reception: the base station forces terminals to transmit P copies of each packet when P terminals collide. This rigidity of having P copies is unsuitable for poor propagation conditions. It might be desirable to have more than P copies of a given packet, adopting a technique for single packet reception called hybrid-ARQ. Hybrid-ARQ effectively decreases the packet error rate. This paper proposes such mechanism for multi-packet reception, called hybrid-ARQ NDMA (H-NDMA), a gated slotted random access protocol, where the access mechanism forces the terminals involved in a collision with reception errors to transmit more than P times. Analytical models for the throughput, delay and energy per useful packet are proposed. The energy performance is analyzed for bounded average delay constraints. The proposed system's performance is evaluated for a single-carrier with frequency domain equalization (SC-FDE) scheme, and compared with the classical NDMA protocol. The comparison highlights that H- NDMA is scalable for an increasing number of terminals and network load.

In this paper we consider the optimum detection of OFDM (Orthogonal Frequency Division Multiplexing) signals with strong nonlinear distortion effects. It is shown that the optimum performance with strong nonlinear distortion effects is not as bad as one might expect and can even be better than the performance with conventional, linear transmitters. To achieve these excellent performances we should employ receivers able to take advantage of the information associated to transmitted data symbols that is inherent to the nonlinear distortion component, in opposition to traditional OFDM implementations where nonlinear distortion effects are regarded as an undesirable noise-like component. We study the achievable gains of the optimum receiver both analytically and by simulation. Since the complexity of optimum receivers is extremely high when we have nonlinear distortion effects, even for OFDM signals with a small number of subcarriers, we propose several sub-optimum receivers and evaluate their performance. Our sub-optimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

Offset signals can be designed to have reduced envelope fluctuations allowing high amplification efficiency. When combined with SC-FDE (Single-Carrier with Frequency-Domain Equalization) schemes, they become excellent candidates for broadband wireless systems with severe power constraints. However, the performance of these modulations with conventional FDE receivers is sub-par, even for receivers specifically designed for offset modulations. This is especially serious when large offset QAM (Quadrature Amplitude Modulation) constellations are used. In this paper we consider SC-FDE schemes combined with offset modulations and study the reason behind the poor performance of conventional FDE implementations. We also present pragmatic FDE receivers for offset modulations that have low complexity and excellent performance.

This paper presents a Medium Access Control (MAC) protocol solution designed to properly handle collisions when in the presence of a multi-packet detection receiver for Single-Carrier (SC) modulations with Frequency-Domain Equalization (FDE). It is considered an iterative frequency-domain receiver that jointly performs equalization, multi-packet separation and channel decoding operations, for up to Qmaxmobile terminals transmitting in one slot. In this work, it is proposed and evaluated a p-persistent Network Diversity Multiple Access (NDMA) random MAC protocol designed to cope with a total number of mobile terminals J, for a maximum decoding capability of Qmaxsimultaneous packets. An accurate analytical model is presented to optimize two different scenarios: in the first one, a saturated network is considered and it is determined the packet transmission probability that maximizes the uplink throughput; the second represents a non-saturated network and the goal is to compute the optimal transmission probability associated to each mobile terminal that minimizes the packet transmission delay. In the end, analytical results obtained through physical and MAC layer simulations are discussed.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Hybrid Automatic Retransmission reQuest (H-ARQ) and time diversity multipacket reception approaches, such as Network Diversity Multiple Access(NDMA), improve the system performance by requesting additional retransmissions and combining all the signals received together. However, the high round trip delay time associated to satellite networks introduces limitations in the number of retransmission requests that may be issued by the terminals to fulfill the Quality of Service (QoS) requirements. This paper considers the design of hybrid protocols combining H-ARQ and NDMA for satellite networks with demand-assigned traffic. The satellite NDMA (S-NDMA) protocol is presented and analytical models are proposed for its performance. Energy efficient QoS provisioning is also analyzed. The proposed system's performance is evaluated for a Low Earth Orbit (LEO) network with a Single-Carrier with Frequency Domain Equalization (SCFDE) scheme, and compared to H-NDMA. Results show that the proposed system is energy efficient and can provide enough QoS to support high demand services such as video telephony.

In this paper we consider the use of single carrier modulations combined with frequency-domain equalization schemes (SC-FDE) in doubly selective channels where the channel variations are due to both phase noise and carrier frequency offset (CFO). Since carrier phase variations within the block duration can compromise significantly the performance of block transmission techniques. We propose an iterative receiver with joint symbol detection and phase-noise decision-directed estimation where we approximate the a posteriori phase noise probability density function by a weighted sum of Gaussian functions.

In this paper we consider the uplink transmission in BS (Base Station) cooperation schemes where users in adjacent cells share the same physical channel and the signals received by each BS are sent to a CPU (Central Processing Unit) that combines the different received signals associated to a given user and/or performs the user separation. We consider SC-FDE signals (Single-Carrier with Frequency-Domain Equalization) combined with iterative frequency-domain receivers based on the IB-DFE concept (Iterative Block Decision Feedback Equalization). Our performance results show that the proposed receivers allow significant macro-diversity gains as well as an efficient user separation, with performance close to the MFB (Matched Filter Bound), making these techniques an excellent choice for the uplink transmission in future broadband wireless systems employing BS cooperation schemes.

A novel algorithm for transmit beamforming to single cochannel multicast group is presented in this paper. We consider the max-min fairness (MMF) based beamforming problem where the maximization of the smallest receiver signal-to-noise ratio (SNR) over the secondary users subject to constraints on the transmit power and interference caused to the primary users. It is shown that this problem, which is nonconvex NP-hard, can be approximated by a convex second-order cone programming (SOCP) problem. Then, an iterative algorithm which successively improves the SOCP approximation is presented. Simulation results show the superior performance of the proposed approach, together with a reduced computational complexity, as compared to the state-of-the-art approach.

Multipacket Reception (MPR) techniques employing either Direct Sequence Code Division Multiple Access (DS- CDMA) or Network Diversity Multiple Access (NDMA) have great potential to deal with packet collisions from multiple Mobile Terminals (MTs). However, their performance evaluation in prac- tical systems is difficult because lengthy simulations are needed to obtain the packet error rates for specific channel conditions. This paper considers MPR receivers based on the Iterative Block Decision Feedback Equalization (IB-DFE) concept. An analytical tool that computes the Bit Error Rate (BER) and Packet Error Rate (PER) for DS-CDMA is proposed, where its theoretical results have a good accuracy. This analytical approach, that provides a simple and relatively fast way to compute the BER and PER performance for DS-CDMA, is then employed for a comparison with NDMA.

In this paper we consider broadband wireless systems with high power efficiency. For this purpose we employ MM (Magnitude Modulation) techniques to reduce the PAPR (Peak-to-Average Power Ratio) of the transmitted signals, allowing efficient power amplification. Designed transceivers combine MM with SC-FDE (Single-Carrier with Frequency-Domain Equalization) methods upon iterative-block decision feedback equalization (IB-DFE). Since IB-DFE schemes designed for conventional SC signals are not suitable when we employ PAPR-reduction techniques, the effect of MM is taken into account on the estimation reliability factor that drives IB-DFE convergence. Results for the new proposed MM- IBDFE transceiver present gains above 1.5dB in the overall power efficiency, even when considering severely time-dispersive channels.

In this paper we consider the optimum ML (Maximum-Likelihood) detection for OFDM signals (Orthogonal Frequency Division Multiplexing) with iterative clipping and filtering. It is shown that the nonlinear distortion does not necessarily mean significant performance degradation and, in fact, the optimum performance could even better than the performance with ideal, linear transmitters. We also present sub-optimum receivers that allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

The high envelope fluctuations of OFDM signals (Orthogonal Frequency Division Multiplexing) make them very prone to nonlinear distortion effects. In typical OFDM implementations the nonlinear distortion component is regarded as a noise-like term that leads to performance degradation. To achieve optimum performance we should employ a ML (Maximum Likelihood) receiver where we take into account all the information associated to the transmitted signals that is in the nonlinearly-distorted signal. Although the performance of an ML is substantially better than traditional receivers, its complexity is prohibitively high. In this paper we consider nonlinearly distorted OFDM schemes and we present sub-optimum receivers that try to approach the ML performance. It is shown that, contrarily to what was expectable, the ML performance with nonlinear transmitters can be better than with ideal, linear transmitters. Our suboptimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the ML performance and the performance of typical OFDM receivers.

In this paper we propose distributed power allocation schemes for the downlink of distributed precoded multicell MISO-OFDM systems. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation schemes minimize the average virtual bit error rate over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed schemes are compared against other power allocation schemes that have been recently proposed for multicell systems.

Offset modulations such as OQPSK (Off- set Quaternary Phase Shift Keying) and OQAM (Offset Quadrature Amplitude Modulation), combined with SC- FDE (Single-Carrier with Frequency-Domain Equalization) are an interesting choice for broadband wireless systems, that require low-complexity transmitters and efficient power amplification. However, the FDE perfor- mance can be rather poor due to interference between the in-phase and quadrature components at the sampling instants, especially when large constellations are employed. In this paper we propose pragmatic iterative and non- iterative FDE designs for offset modulations. Our performance results show that both iterative and non-iterative pragmatic FDE receivers have excellent performance, even for large OQAM constellations.

This paper considers the uplink transmission in BS (Base Station) cooperation schemes employing SC-FDE schemes (Single-Carrier with Frequency-Domain Equalization), where MTs (Mobile Terminals) in adjacent cells share the same physical channel. We study the quantization requirements when sending the received signals at different BSs to a central unit that performs the separation of different MTs using iterative frequency-domain receivers. Our performance results show that a relatively coarse quantization, with only 4 or 5 bits in the in-phase and quadrature components of the complex envelope can be sufficient to allow close-to-optimum macro- diversity gains as well as an efficient separation of the transmitted signals associate to each MT.

Nonlinear OQPSK-type (Offset Quadrature Phase Shift Keying) modulations can be designed to have almost constant envelope, making it suitable for systems requiring highly-efficient and low-cost grossly nonlinear power amplifiers. In this paper we consider the use of pseudo-random spreading sequences for channel estimation purposes in systems employing nonlinear OQPSK-type modulations. We consider Maximum-Length sequences, Kasami sequences, Gold sequences and Tomlinson, Cercas, Hughes sequences, all known to have good autocorrelation properties, making them appropriate for channel estimation purposes. The cross-correlation between different OQPSK components that constitute nonlinear OQPSK-type signals is obtained and its impact on the channel estimation performance is evaluated. It is shown that Tomlinson, Cercas, Hughes sequences outperform the other sequences in the channel estimation context.

In this paper we consider single-carrier with frequency-domain equalization (SC-FDE) schemes where the transmission bandwidth is above the symbol rate. To allow high spectral efficiencies, several channels share the same bandwidth. Since the co-channel interference (CCI) levels can be very high, we propose iterative FDE receivers where we jointly detect all users sharing the same channel. Our performance results show that we can have excellent performances, even with several users sharing the same channel. In fact, we can have the maximum theoretical spectral efficiency even with signals that have bandwidth substantially above the symbol rate.

Software radio signals with several channels have high dynamic range, making them very prone to nonlinear distortion effects, namely those inherent to an efficient power amplification. In this paper we present analytical approach evaluation for evaluating the impact of nonlinear devices in software radio signals. As an application, we will consider the nonlinear amplification of software radio signals where we have a high number of channels with substantially different powers.

Orthogonal frequency division multiplexing (OFDM) schemes are the choice modulation for broadband wireless broadcast and multicast systems. However, OFDM schemes have important limitations such as high-envelope fluctuation of the transmitted signals and its sensibility to carrier frequency errors. When these limitations prove critical the authors consider single-carrier frequency-domain equalisation (SC-FDE) schemes, that allow much higher power efficiency because of lower envelope fluctuations. The overall performance can be further improved if the conventional linear FDE is replaced by an iterative FDE such as an iterative block decision feedback equaliser (IB-DFE). Conventional IB-DFE are usually designed for a quadrature phase shift keying constellation. However, it is strongly recommended that broadcast and multicast systems employ hierarchical constellations with several classes of bits with different error protection. In this study the authors consider the use of SC-FDE schemes combined with IB-DFE receivers in broadband wireless broadcast and multicast systems. The authors emphasise the advantages of these schemes and the authors present IB-DFE designs suitable for hierarchical constellations with several classes of bits with different error protection.

The authors derive a lower bound on the performance of generalised multilevel quadrature amplitude modulations (M-QAM) in time-discrete multipath Rayleigh fading environments with imperfect channel estimation. The bound can be regarded as an extension of the well-known matched filter bound (MFB), taking into account the impact of imperfect channel knowledge. The analytical expressions derived allow the computation of the bit error rate for the different bit streams that can be mapped simultaneously onto M-QAM symbols using a hierarchical approach when unequal bit error protection is desired.

The almost Gaussian nature of OFDM (Orthogonal Frequency Division Multiplexing) signals with high number of subcarriers N is widely employed to characterize nonlinearly distorted OFDM signals and to evaluate the corresponding performance. In this paper we study the accuracy of the Gaussian approach when evaluating nonlinear effects in OFDM signals with finite number of subcarriers, showing the strengths and limitations of this approach. It is shown that the decomposition in useful and self-interference components is valid even for a reduced number of subcarriers. The Gaussian approximation of the nonlinear self-interference at the subcarrier level is very accurate provided that N2is high. However, the nonlinear distortion levels slightly lower than the ones obtained with the Gaussian approximation, with relative errors dropping with 1/N, leading to somewhat pessimistic SIR levels (Signal to Interference Ratio).

Diversity Combining (DC) and Multipacket Detection (MPD) are efficient techniques that cope with packet errors due to severe propagation conditions and/or collisions. To evaluate their network performance on a network simulator, an accurate characterization of the Bit Error Rate (BER) and Packet Error Rate (PER) can successfully replace the behavior of the PHY layer. This choice is better in terms of computational time than implementing a full PHY layer to be jointly simulated with the upper layers. In this paper, an analytical model is proposed to compute the BER and PER of DC, MPD and hybrid techniques, considering a Single-Carrier with Frequency-Domain Equalization (SC-FDE) scenario. The paper also considers two frequency-domain detection methods: a simple linear receiver and a powerful iterative receiver. The proposed model is simple and versatile, since it can support nonhomogeneous transmitting powers, and different channel conditions concerning retransmissions and channel randomization techniques. Several simulations show that the proposed model provides accurate BER and PER results for a wide range of scenarios.

It is widely accepted that SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an excellent candidate for broadband wireless systems, especially when an efficient power amplification is intended. If grossly nonlinear power amplifiers are employed, conventional QPSK (Quaternary Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) modulations should be replaced by offset modulations such as OQPSK (Offset QPSK) and OQAM (Offset QAM). In fact, offset signals have much lower dynamic range than non-offset signals, with OQPSK signals being able to have an almost constant envelope. This paper considers frequency-domain receiver design for OQPSK and OQAM schemes. It is shown that FDE (Frequency Domain Equalization) designed for non-offset modulations are not suitable for offset modulations due to the residual IQI (In-phase/Quadrature Interference), i.e. the interference between in-phase (I) and quadrature (Q) components at the sampling instants. Therefore, we propose several FDE designs where there is no IQI in the sampling instants as well as iterative FDE receivers with IQI cancellation. Our receivers have excellent performance, with the linear designs significantly outperforming linear FDE for non-offset modulations and the iterative designs with performance close to the MFB (Matched Filter Bound).

A solution for the problem of estimating the PN (Phase Noise) from the observation of the channel output in burst communications is to establish a state-model for the PN and determine the a posteriori probability density function (pdf) of the state conditioned on all measurement data, thus providing the means to compute an optimal estimate with respect to any criterion, e.g., Minimum Mean-Squared Error (MMSE). However, except in the Gaussian case, it is extremely difficult to determine and propagate this density function. As a result,and since the observation model of the PN is non-linear, the a posteriori pdf becomes non-Gaussian, and a sub-optimal solution for the problem must be found. In this paper we approximate the non-Gaussian pdf by a weighted sum of Gaussians. Additionally, we compare the performance results obtained when modeling the density function by a weighted sum of Gaussians with those obtained with a single Gaussian using a SC-FDE (Single Carrier-Frequency Domain Equalizer) scheme.

In this paper we consider OFDM (Orthogonal Frequency Division Multiplexing) and SC-FDE (Single-Carrier with Frequency-Domain Equalization) schemes where the channel is estimated with the help of training blocks multiplexed with data blocks. It is shown that SC-FDE schemes need more accurate channel estimates than OFDM schemes with the same constellation. To improve the performance, we consider iterative receivers with joint detection and channel estimation. Our receivers allow significant performance improvements, for both OFDM and SC-FDE, with OFDM slightly outperforming SC-FDE.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Network Diversity Multiple Access (NDMA) handles collisions by combining a multipacket detection scheme with time diversity. In NDMA, the base station (BS) forces mobile terminals (MTs) to transmit J copies of each packet when J MTs collide. Diversity combining is limited to J copies of the packets, not allowing it to adapt to severe errors due to channel noise. This paper considers a multipacket detection scheme recently proposed, which reduces the packet error rate (PER) when more than J copies of the packets are available. In this paper, an Hybrid-ARQ NDMA access mechanism is proposed. The access mechanisms forces MTs with reception errors during a collision resolution epoch to transmit more than J times. Analytical models are proposed for the goodput and delay, considering Poisson traffic. The proposed system's performance is evaluated for a Single-Carrier with Frequency Domain Equalization (SC-FDE) scheme, and compared to classical NDMA.

The correlation properties of some spreading sequences when applied to a system using nonlinear OQPSK-type signals is the main focus of this paper. These signals include, as special cases, CPM schemes (Continuous-Phase Modulation), which can be designed to have very low envelope fluctuations or even a constant envelope, making them compatible with a very low cost and power efficient grossly nonlinear amplification. We use an analytical method to derive both the autocorrelation and the cross-correlation of those signals. The resulting expressions are then applied to Gold Codes, Maximum-Length Sequences (MLS) and TCH (Tomlinson, Cercas, Hughes) sequences. It is shown that TCH sequences have better autocorrelation and cross-correlation properties than the other spreading sequences.

In SFN (Single Frequency Network) broadcasting systems the equivalent CIR (Channel Impulse Response) can be very long but typically it has a sparse nature. This means that the equivalent CIR has several clusters of paths, each one associated to the CIR between a given transmitter and the receiver. These clusters have several multipath components and are typically well separated in time. In this paper we consider OFDM-based (Orthogonal Frequency Division Multiplexing) broadcasting systems with SFN operation and we propose an efficient channel estimation method that takes advantage of the sparse nature of the equivalent CIR. For this purpose, we employ low-power training sequences and we employ an iterative receiver with joint detection and channel estimation. The receiver can know the location of the different clusters that constitute the overall CIR or not. Our performance results show that we can have excellent performance, close to the performance with perfect channel estimation, even with employ low-power training blocks and also for the case where the receiver does not know the location of the different clusters that constitute the overall CIR.

SC-FDE (Single Carrier Frequency-Domain Equalization) modulations are known to be suitable for broadband wireless communications due to their robustness against severe time-dispersion effects and the relatively low envelope fluctuations of the transmitted signals. In this paper we consider the use of SC-FDE schemes in broadcast and multicast systems with SFN (Single Frequency Network) operation where we do not have perfect carrier synchronization between different transmitters. We study the impact of different CFO (Carrier Frequency Offset) between the local oscillator at the receiver and the local oscillator at each transmitter. We also propose receiver structures able to reduce the performance degradation caused by different CFO at different transmitters. Our receivers can be regarded as modified turbo equalizer implemented in the frequency-domain, where a frequency offset compensation is performed before the iterative receiver.

Packet Combining (PC) and Multipacket Reception (MPR) techniques have great potential to deal with lost packets since both take full advantage of the signals associated to all failed transmission attempts. However, their performance evaluation in practical systems is difficult because lengthy simulations are needed to obtain the packet error rates for specific channel conditions. The paper presents an analytical tool to obtain the Bit Error Rate (BER) and Packet Error Rate (PER) performances for Packet Combining (PC), Multipacket Reception (MPR) and the hybrid of both schemes, based on Iterative Block Decision Feedback Equalization (IB-DFE) receivers for Single-Carrier with Frequency-Domain Equalization (SC-FDE). A set of simulation results show that the proposed analytical approach has accurate Bit Error Rate (BER) and Packet Error Rate (PER) results with a simple and relatively fast method.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Two different approaches are considered to cope with lost packets, a low-complexity packet combining ARQ scheme (Automatic Repeat reQuest) and a multipacket detection scheme. These approaches are employed on a SC-FDE scheme (Single-Carrier with Frequency Domain Equalization). Suitable receiver structures are described, employing packet combining, multipacket detection and hybrid schemes. An accurate analytical tool is presented for predicting the performance of the different schemes.

Current applications and services for ad hoc net works heavily rely in advertisements, which are transmitted using broadcast. In the current Medium Access Control (MAC) schemes, namely in IEEE 802.11, broadcast co-exists with unicast frames. While the transmission scheme used for unicast frames is considered reliable, the IEEE 802.11 does not offer any possibility of retransmission for broadcast frames. This work presents a novel reliable MAC scheme, which is designed to maximize the network throughput. In our scheme all nodes adopt a common optimal contention window, which is based on a common view of the channel. Contrary to IEEE 802.11 we use the same contention window for both unicast and broadcast transmissions. Thus the conditional collision probability is independent of the traffic type. Departing from the optimal throughput for a saturated network, we estimate the number of nodes, which is a prime parameter to regulate the medium access control. We show that each node is suitable to estimate the number of competing nodes by using both its own medium access probability and the idle slot probability observed from the channel. Several simulation results evaluate the reliability and the throughput of our proposal, for different amounts of unicast/broadcast traffic. The results are compared to the well known IEEE 802.11 MAC, indicating that even without retransmitting the broadcast frames, the probability of successful transmission can be doubled. Moreover, the throughput of our approach outperforms IEEE 802.11 MAC and the results show it can be boosted up to 100%.

This paper considers the use of several grossly nonlinear amplifiers for an efficient amplification of Mary-OQAM signals (Offset Quadrature Amplitude Modulation) with FDE (Frequency Domain Equalization) and the impact of phase and gain unbalance between different amplifiers. It is shown that for some 16-QOQAM signals it is possible to employ two grossly nonlinear amplifiers to perform an amplification that is formally equivalent to a linear amplification. However, the phase and gain of these amplifiers need to be carefully selected, since we can only accept gain errors below 10% and phase errors below 0.1 rad (roughly 6 degrees).

In this paper we present a performance analysis of XOR1-based flat routing protocols in high mobility conditions, considering a vehicular ad hoc network (VANET) formed in a highway scenario. First, we describe an XOR-based protocol that incorporates several adaptations of the existing XOR-based routing algorithms for wired networks, in order to cope with the network mobility. Then we propose an improved version of it, XORi, which modifies the protocol's information gathering process to accommodate the specific dynamic nature of VANETs topology. Finally, we evaluate the performance of XOR-based protocols with other topology-based routing protocols. Simulation results allow us to characterize the performance of this class of protocols through the comparison of the packet delivery ratio, end-to-end path delay and average number of path hops2. When a moderate density of nodes is considered, simulations show that XOR-based algorithms achieve almost the same packet delivery rate as link state algorithms, such as OLSR, while for high density of nodes XOR-based algorithms scale better in terms of delay when compared to source routing algorithms, such as DSR3.

In this paper we propose the use of QAM constellations, specifically 4, 16, and 64-QAM, in severely time-dispersive channels and with moderate CFOs (Carrier Frequency Offsets). We consider an SC-FDE (Single Carrier Frequency-Domain Equalization) and assume that previously to the equalization procedure a coarse CFO estimation and compensation is performed. Since the coarse CFO estimate can have a significant error leading to a residual CFO we present and evaluate a receiver with joint detection and carrier synchronization in order to guarantee a good BER. We employ frequency-domain iterative DFE (Decision Feedback Equalization) receivers denoted by IB-DFE (Iterative Block-Decision Feedback Equalization), which are combined with a DD (Decision-Directed) CFO estimator. Our aim is to determine the synchronization requirements for 4, 16, and 64-QAM and compare several methods for the DD CFO estimators when using these QAM constellations.

A DS-CDMA (Direct Sequence-Coded Division Multiple Access) system has maximum spectral efficiency if the system is fully loaded (i.e., the number of users is equal to the spreading factor) and we employ signals with bandwidth equal to the chip rate. However, due to implementation constraints we need to employ signals with higher bandwidth, decreasing the system's spectral efficiency. In this paper we consider prefix-assisted DS-CDMA systems with bandwidth that can be significantly above the chip rate. To allow high spectral efficiency we consider highly overloaded systems where the number of users can be twice the spreading factor or even more. To cope with the strong interference levels we present an iterative frequency-domain receiver that takes full advantage of the total bandwidth of the transmitted signals. Our performance results show that the proposed receiver can have excellent performance, even for highly overloaded systems. Moreover, the overall system performance can be close to the maximum theoretical spectral efficiency, even with transmitted signals that have bandwidth significantly above the chip rate.

In the last years, there has been an increasing interest in developing and testing Medium Access Control (MAC) protocols for ad hoc networks based on optimal methods. Several solutions do not take into account the throughput-fairness tradeoff, presenting high throughput performance but under-performing in terms of medium access fairness. This work presents a novel MAC scheme, which is designed to maximize the network throughput-fairness performance. In our scheme, all nodes adopt a common optimal contention window, which is based on a common view of the channel. Our proposal improves the access fairness because the contention window is similar to all nodes, and its behavior does not depend on previous MAC states. Departing from the optimal throughput for a saturated network, we devise a scheme to estimate the number of nodes, which is a prime parameter to regulate the medium access control. We show that each node is suitable to estimate the number of competing nodes by using both its own medium access probability and the idle slot probability observed from the channel. Several simulation results evaluate the throughput-fairness performance of our proposal with several state-of-the-art MAC protocols, such as AOB, Idle Sense, GDCF and the well known IEEE 802.11. The results indicate that our approach presents the highest value of medium access fairness among the simulated protocols, meaning that its throughput is closer to the optimal theoretical throughput obtained for a fair network.

This paper considers the use of low-complexity diversity combining hybrid ARQ schemes (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. Its low complexity makes it particularly interesting for battery-powered nodes of a wireless sensor network (WSN). TDMA is commonly used for WSNs applications that require high throughput or constrained packet delay. This paper analyzes the energy per useful packet, packet delay and goodput performances of the TDMA system. Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations.

Lost packets, either due to errors or collisions, are usually discarded and need to be retransmitted, leading to performance losses. In this paper we compare the performance of two different approaches to cope with lost packets. One alternative is a multi-packet detection approach able to separate up to Qmaxmobile terminals transmitting in one slot. The other alternative is a low-complexity soft combining ARQ scheme (Automatic Repeat reQuest) employed in a TDMA (Time Division Multiple Access) architecture. This paper evaluates the uplink non-saturated packet delay and goodput for a Poisson traffic generator. Our analytical results are validated by simulation using joint PHY (physical layer) and MAC (Medium Access Control) behavior.

The almost Gaussian nature of OFDM (Orthogonal Frequency Division Multiplexing) signals with high number of subcarriers is widely employed to characterize nonlinearly distorted OFDM signals and to evaluate the corresponding performance. In this paper we study the accuracy of the Gaussian approach when evaluating nonlinear effects in OFDM signals with finite number of subcarriers, showing the strengths and limitations of this approach.

In this paper we consider SC-FDE schemes (Single-Carrier with Frequency-Domain Equalization) employing multi-resolution hierarchical constellations and we consider the design of turbo receivers, implemented in the frequency-domain, for systems where we have different error protections. We present a versatile method to design the receiver that is suitable to any constellation.

This paper considers the use of a code combining hybrid ARQ scheme (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. A multi-rate turbo-code is proposed to implement the H-ARQ scheme, allowing improved performances. This paper analyzes the energy per packet successfully received, the packet delay and goodput performances for the TDMA system on a HAP (High Altitude Platform). Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations for a typical HAP Network uplink channel.

This paper considers the design of linear frequency-domain receivers with oversampling for OQPSK (Offset Quaternary Phase Shift Keying) signals. Based on a receiver structure able to eliminate the residual interference between in-phase (I) and quadrature (Q) components at the sampling instants, a method that minimizes the overall residual ISI (Inter-Symbol Interference) plus IQI (I-Q interference) is proposed. Therefore, this receiver significantly outperforms previous frequency-domain OQPSK receivers.

This paper proposes the Mobile MH-MAC (Multimode Hybrid - Medium Access Control) protocol, an extension of MH-MAC with enhanced support for mobility on wireless sensor networks. It provides an asynchronous mode for energy efficiency during idle (unconnected) periods, and a synchronous mode for high throughput periods. Mode change is controlled by the application in a cross-layer approach. MMH-MAC was specially designed to tolerate asynchronous mobile nodes in the neighborhood of synchronous nodes, providing fast transition and interference mitigation mechanisms. Current MAC protocols rely on a passive method to associate a mobile node to a static cluster, triggered by the reception of a beacon. We propose an active approach that allows a mobile node to start sending data packets shortly after entering into data range of a static node, with negligible data loss in existing synchronous data connections. MMH-MAC was implemented in TinyOS and tested on TelosB motes and TOSSIM simulator.

Nonlinear operations are usually regarded as something undesirable that should be avoided in digital communication systems since they lead to performance degradation, namely spectral widening and decrease in power efficiency, for a given modulations. In this paper we show that the adoption of appropriate nonlinear devices can allow an improvement on the performance of some modulation schemes, either for OQPSK-type schemes (Offset Quadrature Phase Shift Keying) or TC-OQAM (Trellis Coded Offset Quadrature Amplitude Modulation). We present an analytical characterization of these signals after a bandpass memoryless nonlinear device. It is shown that an encoder followed by a nonlinear amplifier can be viewed as a serial concatenated coding scheme and the coding gain of such scheme can be improved relatively to the linear transmission scheme.

Several routing protocols for Mobile Ad Hoc Networks (MANETS), including the well known Ad hoc On-Demand Distance Vector Routing (AODV) and Optimized Link State Routing (OLSR), propose the use of periodic messages (Hello messages) to detect neighbor nodes. After receiving the first Hello message from one of its neighbors, a node starts the link sensing task by setting up a sensing timer. Each time a new Hello message is received from the same neighbor, the sensing timer is restarted and the link duration is prolonged. If the sensing timer expires, it indicates a long time interval without receiving an Hello message and, consequently, the link is considered broken. The transmission frequency of the Hello messages and the expiration value of the sensing timer truly depends on node's mobility: if the nodes are moving quickly and the Hello messages are rarely transmitted, the neighbor nodes can be in communication range but they are not detected; in the same scenario, if the expiration value of the sensing timer is too high, a link is sensed broken too late. In this paper, we consider a MANET under the Random Waypoint mobility model. We investigate the relationship between the transmission frequency of the Hello messages and the sensing timer expiration value with the network node's mobility. We formally deduce the probability of link existence after β periods of transmission of the Hello message. The probability is later used to define the sensing timer expiration value, considering a given probability that the Hello message transmission fails. Finally, we evaluate our study through both numerically analysis and simulations, which confirms the effectiveness and accuracy of our approach.

This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency- Domain Equalization). We consider iterative frequency-domain receivers that jointly perform equalization, multi-packet separation and channel decoding operations, for up to Qmaxmobile terminals transmitting in one slot. In this paper we propose an accurate analytical model for a p-persistent random access MAC protocol, designed to handle a total number of mobile terminals J above Qmax. It evaluates the uplink non-saturated packet delay and calculates the optimal configuration for the packet transmission probability associated to each mobile terminal that minimizes the delay. Our analytical results are validated using physical and MAC layer simulations.

This paper considers the use of SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization) with low-complexity soft combining ARQ schemes (Automatic Repeat reQuest). With our technique, packets associated to different transmission attempts are combined in a soft way, allowing improved performances. Its low complexity makes it particularly interesting for the uplink of wireless systems. This paper proposes an accurate analytical model for a TDMA (Time Division Multiple Access) scheme where packet combining ARQ is applied. It evaluates the uplink non-saturated packet delay and goodput for a generic message arrival process. Our analytical results are validated using physical and MAC layer simulations.

SC modulation (Single-Carrier) with FDE (Frequency-Domain Equalization) combined with iterative (turbo) FDE schemes has excellent performance in severely time-dispersive channels, making it a promising candidate for future broadband wireless systems. In fact, it was observed that the performance can be close to the MFB (Matched Filter Bound). In this paper we consider a class of iterative FDE schemes and we study the impact of the number of multipath components and the diversity order on its performance. It is shown that for a high number of separable multipath components the asymptotic performance approaches the MFB, even without diversity. When we have diversity the performance approaches the MFB faster, even when we have just a small number of separable multipath components.

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.

In this paper, we derive matched filter bound (MFB) expressions for the performance of hierarchical quadrature amplitude modulations (M-HQAM) over time-discrete multipath Rayleigh fading environments with diversity. The expressions are valid for multipath environments where different paths and antennas can be correlated. Since hierarchical constellations are usually employed to achieve unequal bit error protection, the MFB expressions are given in terms of the individual bit error rate (BER) for the different bit streams. Although typically the MFB is obtained assuming slow time varying channels, fast varying channels are also considered in this paper.

This paper presents a multipacket detection technique for dealing with packet collisions in OFDM schemes (orthogonal frequency division multiplexing). Instead of discarding collided packets as it is done in traditional MAC approaches (medium access control) additional packets are transmitted and separated using multiple input multiple output (MIMO) based techniques. Reliable detection and high throughputs can be achieved as long as different interleavers are used for different retransmissions. We also include a method for estimating the users involved in the collision.

As with other multicarrier modulations, OFDMA signals (Orthogonal Frequency Division Multiple Access) have large envelope fluctuations making them very prone to nonlinear distortion effects. In this paper we study the impact of nonlinear devices on OFDMA signals. We present an analytical characterization of the signals at the output of a nonlinear device that can be used for obtaining the spectral characterization of the transmitted signals, as well as the computation of the nonlinear interference levels on the received signals. It is shown that the power allocated to each user has a key impact on the nonlinear distortion effects. For this reason, we should avoid having low-power and high-power users in adjacent subcarriers. It is also shown that nonlinear distortion levels are lower when just a small fraction of the subcarriers is used (i.e., when the system load is small).

This paper considers frequency-domain receiver design for OQPSK-type schemes. Since the I-Q interference can lead to significant performance degradation we propose an FDE design (frequency-domain equalization) where there is no I-Q interference in the sampling instants for the predominant OQPSK constellation. We also propose iterative FDEs with I-Q interference cancelation, as well as the cancelation of unintended OQPSK components.

Whenever a DS-CDMA signal is the sum of several components associated to different spreading codes it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR) setting strong linearity requirements for the power amplifiers. The use of clipping and filtering techniques (which can be repeated several times) was shown to be an effective way of reducing the PMEPR of DS-CDMA signals. We consider an improved receiver where there is an iterative estimation and cancellation of nonlinear distortion effects. The proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects.

In this paper we derive a lower bound on the bit error rate (BER) for the individual bit streams that can be mapped simultaneously in an hierarchical way onto generalized multilevel quadrature amplitude modulations (M-QAM). The expressions are valid for time-discrete multipath Rayleigh fading environments with imperfect channel estimation and can be regarded as an extension of the well-known matched filter bound (MFB).

This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization). We consider iterative frequency- domain receivers that jointly perform the equalization, multi-packet separation and channel decoding operations. Our receivers allow a high throughput provided that the number of mobile terminals that is trying to access a given slot is equal to or below the multi-packet detection capacity Qmax. We propose a new MAC protocol that allows a total number of mobile terminals J above Qmaxby controlling the number of mobile terminals contending to transmit. The paper evaluates the uplink saturated system throughput and proposes an optimal configuration for the packet transmission probability associated to each mobile terminal. Our analytical results are validated using physical and MAC layer simulations.

In this paper, we consider iterative frequencydomain receivers for block transmission techniques with rate-1 Space Time Block Coding (STBC) for two or four transmit antennas. We consider both Orthogonal Frequency Division Multiplexing (OFDM) and SingleCarrier (SC) schemes. Since our STBC with 4 transmit antennas is not orthogonal, our receiver includes the cancellation of the residual interference, allowing performances close to the ones of an orthogonal code. Our performance results show that combining STBC with block transmission schemes allows excellent performances, even for channels with small multipath diversity.

Whenever there is a collision involving several transmitted packets, the traditional approach for MAC (Medium Access Control) protocols is to discard all packets involved and request their retransmission, with a consequent loss in the overall throughput. Based on conventional Multiple Input Multiple Output (MIMO) techniques, in this paper we propose a multipacket detector for OFDM schemes (Orthogonal Frequency Division Multiplexing) that allows an efficient packet separation in the presence of collisions and can achieve high throughputs. We show that reliable detection is possible as long as different interleavers are used for different retransmissions. We also include a method to estimate the users involved in the collision, as well as well as the corresponding channel characteristics.

In this paper we derive matched filter bound (MFB) bit error rate (BER) expressions for the individual bit classes of multilevel hierarchical quadrature amplitude modulations (M-HQAM). The expressions are valid for time-discrete multipath Rayleigh fading environments with diversity where the different paths can be correlated. Though typically MFB are obtained assuming slow-varying channels in this paper we also consider fast varying channels.

In this paper, we consider MC-CDM schemes (MultiCarrier Code Division Multiplexing) where clipping techniques are employed to reduce the envelope fluctuations of the transmitted signals. Both CP-assisted (Cyclic Prefix) and ZP (Zero- Padded) MC-CDM schemes are studied. We develop frequencydomain turbo equalizers combined with an iterative estimation and cancelation of nonlinear distortion effects, with relatively low complexity since they allow FFT-based (Fast Fourier Transform), frequency-domain implementations. Our performance results show that the proposed receivers allow significant performance improvements at low and moderate SNR (Signal-to-Noise Ratio), even when strongly nonlinear transmitters are employed. The receiver for ZP MC-CDM is of special interest for systems where the duration of the channel impulse response is not a small fraction of the duration of the MC-CDM blocks, being suitable to MC-CDM systems with very large blocks (hundreds or even thousands of subcarriers), since they do not require the inversion nor the multiplication of large matrixes.

The traditional approach to cope with collisions is to discard the packets involved in it and to ask for their retransmission. However, since the signal associated to a collision has important information concerning the packets involved, we can efficiently resolve collisions with proper retransmissions. In this paper we consider severely time-dispersive channels and we propose a technique that allows efficient packet separation. We employ SC-FDE schemes (single-carrier with frequency-domain equalization) and we propose an iterative frequency-domain multi-packet detection. Since our technique requires uncorrelated channels for different retransmissions, we also propose an SP technique (Shifted Packets) for retransmissions in fixed channels. Since the total number of transmissions is equal to the number of packets involved in the collision (even when the channel remains fixed for the retransmissions), our technique allows high throughputs. The analysis of our detection technique combined with the NMDA (network-assisted diversity multiple access) MAC (medium access control) protocol shows significant throughput and delay improvements for low Eb/N0values compared to a TDMA (time division multiple access) approach, where collisions are avoided. This technique is particularly appealing for the uplink of broadband wireless systems, since we consider SC-FDE schemes and the complexity is concentrated in the receiver. By employing the SP scheme we can use the same channel for the retransmissions, with only a small performance degradation.

This paper considers the use of SC-FDE techniques (single-carrier with frequency-domain equalization) with low-complexity soft combining ARQ schemes (automatic repeat request). With our technique, packets associated to different transmission attempts are combined in a soft way, allowing improved performances. Moreover, the transmitter and receiver complexities are low and we can have excellent performances, especially when we have deep fading effects in slow-varying channels. By combining the advantages of SC-FDE schemes with soft combining ARQ, the proposed technique is particularly interesting for the uplink of wireless systems.

In this paper we consider the evaluation of nonlinear distortion effects on the uplink transmission of systems employing OFDMA signals (orthogonal frequency division multiple access). Our results allow an analytical spectral characterization of the transmitted signals, as well as the computation of the nonlinear interference levels on the received signals. It is shown that the power allocated to each user has a key impact on the nonlinear distortion effects. It is also shown that nonlinear distortion levels are lower when just a small fraction of the subcarriers is used (i.e., when the system load is small).

SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization) are promising candidates for future broadband MIMO (Multiple Input, Multiple Output) wireless systems. These modulations allow excellent performances in severely time-dispersive channels, provided that accurate channel estimates and performing decoding algorithms are available at the receiver. For this purpose, pilot symbols and/or training sequences are usually multiplexed with data symbols. However, since this leads to overhead and some spectral degradation, the use of implicit pilots (i.e., pilots superimposed to data) should be considered. In this paper we consider MIMO SC-FDE systems where the channel estimation is based on either explicit or implicit pilots. Since the interference levels between data and pilots might be very high (for the implicit bits), we propose an iterative receiver with joint equalization, turbo decoding and channel estimation. Our performance results show that the use of implicit pilots, combined with the proposed receiver, allows performances close to the case with perfect channel estimation, even for severely time-dispersive channels.

SC modulations (single-carrier) with FDE (frequency-domain equalization) are promising candidates for future broadband wireless systems. These modulations allow excellent performances in severely time-dispersive channels, provided that accurate channel estimates are available at the receiver. For this purpose, pilot symbols and/or training sequences are usually multiplexed with data symbols. Since this leads to overhead and some spectral degradation, the use of superimposed pilots (i.e., pilots added to data) was recently proposed. In this paper we consider SC-FDE systems where the channel estimation is based on superimposed pilots. Since the interference levels between data and pilots might be very high, we propose an iterative receiver with joint equalization and channel estimation. Our performance results show that the use of superimposed pilots, combined with the proposed receiver, allows performances close to the case with perfect channel estimation, even for severely time-dispersive channels.

The traditional approach for MAC protocols (Medium Access Control) is to discard all packets involved in a collision and request their retransmission, which precludes high throughputs. However, since the signals associated to collisions have important information concerning the packets involved, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a multipacket detection technique for OFDM schemes (Orthogonal Frequency Division Multiplexing) that allows an efficient packet separation in the presence of collisions. This technique allows high throughputs, since the total number of transmissions can be equal to the number of packets involved in the collision. By using different interleavers for different retransmissions we can successfully separate packets involved in a collision even when the channel remains fixed for the retransmissions.

Usually, packets involved in a collision are lost, requiring their retransmission. However, the signal associated to collisions has important information concerning the packets involved. In fact, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a frequency-domain multi- packet detection technique for SC-FDE schemes (Single- Carrier with Frequency-Domain Equalization) that allows an efficient packet separation in the presence of successive collisions. This technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision, even when the channel remains fixed for the retransmissions. Since we consider SC-FDE schemes and the complexity is concentrated in the receiver, this technique particularly appealing for the uplink of broadband wireless systems.

Usually, packets involved in a collision are lost, requiring the retransmission of all packets. However, if we do not discard collided packets and we use proper retransmissions we can efficiently resolve collisions. In this paper we propose a technique that allows an efficient packet separation in the presence of successive collisions. We consider an SC (Single-Carrier) modulation with FDE (Frequency-Domain Equalization) and we propose a frequency-domain multi-packet detection scheme. Since our technique requires uncorrelated channels for different retransmissions, we also propose a PS technique (Packet-Shift) for retransmissions using the same channel. Our technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision. Moreover, the complexity is concentrated in the receiver, making this technique particularly appealing to the uplink of broadband wireless systems. By employing the PS scheme we can use the same channel for the retransmissions, with only a small performance degradation.

This paper focuses on the use of specific interleaving techniques for use in W-CDMA (wideband code division multiple access). The linear equalization algorithm usually performs a matrix inversion on a matrix with a high level of sparseness, and such sparseness shouldn't be compromised with the use of a random interleaver; specific interleaver designs should be employed in order to keep the complexity of the equalization algorithm down to an acceptable level. The used equalizer is based on the linear MMSE (minimum mean square error) algorithm and is tested using the UMTS (universal mobile telecommunications system) HSDPA (high speed downlink packet access) standard as a basis, including the reference UMTS environments.

In this paper we derive general expressions for the performance of hierarchical multilevel quadrature amplitude modulation constellations (M-QAM), over flat Rayleigh fading environments with imperfect channel estimation. Cases of diversity reception with identical channels and with dissimilar channels, employing maximal ratio combining (MRC) are considered. Since hierarchical constellations are usually employed to achieve unequal bit error protection, the individual bit error rate (BER) of each bit stream is analyzed and it is shown that poor channel estimation has a more severe impact in the performance of the least protected bit streams.

In this paper, we consider SC-FDE schemes (Single Carrier with Frequency-Domain Equalization) employing OQPSK (Offset Quaternary Phase Shift Keying) signals with quasi-constant envelope. To allow high spectral efficiency, different frequency channels are separated by the symbol rate, regardless of their bandwidth (which is much higher than the symbol rate for quasi-constant envelope OQPSK signals), and we consider a multi-channel frequency-domain receiver with ACI (Adjacent Channel Interference) suppression. Our performance results show that it is possible to have high spectral efficiency with quasi-constant envelope OQPSK signals. This technique is especially interesting for the uplink of future broadband wireless systems since we can employ low-cost, grossly nonlinear power amplifiers at the mobile terminals and almost all signal processing is shifted to the base station.

We consider MC-CDMA schemes, with reduced envelope fluctuations. Both CP-assisted (cyclic prefix) and ZP (zero-padded) MC-CDMA schemes are addressed. We develop turbo FDE (frequency-domain equalization) schemes, combined with cancelation of nonlinear distortion effects. The proposed turbo receivers allow significant performance improvements at low and moderate SNR, even when the transmitted signals have reduced envelope fluctuations.

A coherent detection of OFDM signals (Orthogonal Frequency Division Multiplexing) allows good performances in severely time-dispersive channels. However, coherent receivers require good channel estimates, a difficult task when the channel impulse response is long and/or when space diversity techniques are employed. Usually, the channel estimates are obtained with the help of pilot symbols multiplexed with data symbols, which reduces the useful bit rate, decreasing the spectral efficiency of the systems. To avoid this, we consider the use of implicit pilots instead of multiplexed pilots. Since the interference levels between data and pilots might be very high, we propose an iterative receiver with joint detection and channel estimation. Our performance results show that our technique allows the use of low-power pilots, with performances close to the ones with perfect channel estimation, even when space diversity techniques are employed.

The LMMSE (linear minimum mean square error) algorithm is one of the best linear receivers for DS-CDMA (direct sequence-code division multiple access). However, for the case of MIMO/BLAST (multiple input, multiple output/Bell Laboratories layered space time) with high loading, the perceived complexity of the LMMSE receiver is taken as too big, and thus other types of receivers are employed, yielding worse results. In this paper, we investigate the complexity of the solution to the MIMO LMMSE receiver's equations using block-Fourier algorithms, for both steady and unsteady channel situations.

For conventional cyclic-prefix (CP)-assisted block transmission systems, the CP length is selected on the basis of the expected maximum delay spread. With regard to single-carrier (SC)-based block transmission implementations, a full-length CP is recommendable, since it allows good performances through the use of simple frequency-domain equalization (FDE) techniques. In this letter, a soft-decision-directed correction (SDDC)-aided turbo FDE technique is presented for reduced-CP SC-based block transmission systems using conventional frame structures. The relations with some already known iterative FDE techniques are established, and a set of performance results is reported and discussed. The advantages of the proposed approach are emphasized, namely, the possibility of approximately achieving (besides the obvious bandwidth efficiency gain) the maximum power efficiency gain that a strong CP reduction allows.

For conventional cyclic prefix (CP)-assisted single-carrier/frequency-domain equalization (SC/FDE) implementations, as well as for orthogonal frequency-division multiplexing (OFDM) implementations, the CP length is known to be selected on the basis of the expected maximum delay spread. Next, the data block size can be chosen to be large enough to minimize the CP overhead, yet small enough to make the channel variation over the block negligible. This paper considers the possibility of reducing the overall CP assistance, when transmitting sequences of SC blocks, while avoiding an excessively long fast Fourier transform window for FDE purposes and keeping good FDE performances through low-complexity, noniterative receiver techniques. These techniques, which take advantage of specially designed frame structures, rely on a basic algorithm for decision-directed correction (DDC) of the FDE inputs when the CP is not long enough to cope with the time-dispersive channel effects. More specifically, we present and evaluate a novel class of reduced-CP SC/FDE schemes, which takes advantage of a special frame structure for replacing “useless” CP redundancy by fully useful channel coding redundancy, with the help of the DDC algorithm. When using the DDC-FDE technique with these especially designed frame structures, the impact of previous decisions, which are not error-free, is shown to be rather small, thereby allowing a power-efficiency advantage (in addition to the obvious bandwidth-efficiency advantage) over conventional block transmission implementations under full-length CP. Additionally, the DDC algorithm is also shown to be useful to improve the power efficiency of these conventional implementations.

The analytical bit error rate of hierarchical quadrature amplitude modulation formats, which include uniform and nonuniform constellations, over flat Rayleigh fading environments is studied in this paper. The analysis takes into account the effect of imperfect channel estimation and considers diversity reception with both independent identically and nonidentically distributed channels, employing maximal ratio combining.

For conventional CP-assisted (Cyclic Prefix) SC/FDE implementations (Single- Carrier/Frequency-Domain Equalization), as well as for OFDM implementations (Orthogonal Frequency Division Multiplexing), the CP length is known to be selected on the basis of the expected maximum delay spread. Next, the data block size can be chosen to be large enough to minimize the CP overhead, yet small enough to make the channel variation over the block negligible. This paper considers the possibility of reducing the overall CP assistance, when transmitting sequences of SC blocks, while avoiding an excessively long FFT block for FDE purposes and keeping good performances through a moderate increase of the FDE receiver complexity. Firstly, we present an algorithm for a Decision-Directed Correction (DDC) of the FDE inputs when the CP is not long enough to cope with the time-dispersive channel effects. The resulting FDE performance is then evaluated in worst-case (CP-free) conditions, and the impact of previous decisions which are not error-free is shown to be rather small. In the following, we present and evaluate a novel class of reduced-CP SC/FDE schemes, which takes advantage of the DDC algorithm for replacing "useless" CP redundancy by fully useful channel coding redundancy: highly power/bandwidth efficient block transmission schemes, especially recommendable for both strongly time-dispersive and time-varying channel conditions, are then achieved.

In this paper we consider the uplink transmission in MC-CDMA systems (multicarrier coded division multiple access). To reduce the envelope fluctuations of the transmitted signals, the MC-CDMA signal associated to each MT (mobile terminal) is submitted to a clipping device, followed by a frequency-domain filtering operation. We define an iterative receiver that jointly performs the MUD (multiuser detection) and the estimation and cancellation of the nonlinear distortion effects that are inherent to the transmitted signals. Our performance results show that the proposed receiver structures allow good performances, even for severely time-dispersive channels and/or when a low-PMEPR is intended for each MT

Whenever a DS-CDMA signal (direct sequence code division multiple access) is the sum of several components associated to different spreading codes (e.g., the DS-CDMA signal to be transmitted by the BS (Base Station) in the downlink, or any multicode DS- CDMA signal), it has high envelope fluctuations, and a high PMEPR (Peak-to-Mean Envelope Power Ratio) setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. A promising class of nonlinear signal processing schemes, combining a nonlinear operation in the time- domain with a linear, frequency-domain filtering operation, was recently proposed for reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. However, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be relatively high, especially when a low PMEPR is intended. This can be especially serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider improved receiver where there is an iterative estimation and cancelation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects.

We investigate channel estimation for single-carrier-frequency domain equalization (SC-FDE) system using the techniques typically used for an orthogonal frequency domain multiplexing (OFDM) system. Two techniques of frequency domain multiplexed (FDM) pilot insertion using interleaved frequency domain multiple access (IFDMA) signal with a Chu sequence are considered. One called frequency domain superimposed pilot technique (FDSPT) scales data-carrying tones and then superimposes them with pilot tones. This technique preserves spectral efficiency at the expense of performance loss. The other, called frequency expanding technique (FET), shifts groups of data frequencies for multiplexing of pilot tones at the expense of spectral efficiency. Our results show that both techniques increase peak to average power ratio (PAPR) although it is still lower than that of an OFDM system. The application of FDSPT is limited by the pilot overhead ratio, resulting from the removal of data frequencies for pilot frequencies. It is shown that channel estimation using conventional time domain multiplexed pilots and FET pilot tones produce the same BER, while the FDSPT requires about 1.5 dB more power for the same performance. Using FDM pilots in SC system facilitates flexible and efficient assignment of signals to available spectrum.

In this paper we consider the uplink transmission within a DS-CDMA system employing CP-assisted (Cyclic Prefix) block transmission techniques combined with spatial multiplexing techniques that require multiple antennas at both the transmitter and the receiver. We present an efficient frequency-domain receiver structure with iterative MUD (Multiuser Detection). The performance of the proposed receiver can be close to the single-user matched filter bound, even for fully loaded systems and/or severely time-dispersive channels.

In this paper we consider the use of CP-assisted (cyclic prefix) DS-CDMA schemes (direct sequence code division multiple access) in broadband wireless systems. We present an iterative, frequency-domain MUD (multiuser detection) receiver for the uplink transmission that combines IB-DFE (iterative block decision feedback equalization) principles with serial interference cancellation. It is shown that the performance proposed receiver can be close to the single-user MFB (matched filter bound), even for fully loaded systems, in severely time-dispersive channel and/or in the presence of strong interfering signals

This paper focuses on the usage of an enhanced equalization-based receiver for WCDMA (wideband code-division multiple access) MIMO (multiple input, multiple output) BLAST (Bell Labs layered space time)-type systems. The receiver is based on the MMSE (minimum mean square error) algorithm coupled with an IPC (iterative partial cancellation) scheme. The scheme is tested in both an uncoded and coded setting, using the UMTS (Universal Mobile Telecommunications System) HSDPA (high speed downlink packet access) standard as a basis, and the reference UMTS environments.

The MC-CDMA signals (multicarrier coded division multiple access) have strong envelope fluctuations which make them very prone to nonlinear effects. These effects can be both intentional (such as the ones inherent to a nonlinear signal processing for reducing the envelope fluctuations) or not (such as the ones inherent to a nonlinear power amplification). In this paper we present an analytical tool for the performance evaluation of nonlinear effects in MC-CDMA signals which takes advantage of the Gaussian-like behavior of MC-CDMA signals with a large number of subcarriers and employs results on memoryless nonlinear devices with Gaussian inputs so as to characterize statistically the signals at the output of the nonlinear device. This characterization is then used for an analytical computation of the SIR levels (signal-to-interference ratio) and the BER performance (bit error rate). A set of numerical results is presented and discussed, showing the accuracy of the analytical BER performance analysis in the presence of nonlinear effects

In this paper, we consider the uplink of SC-based (single-carrier) block transmission systems where the transmission bandwidth associated to each channel, exceeds the symbol rate and we can have significant overlapping between adjacent channels. We present an iterative frequency-domain receiver for joint equalization and ACI (adjacent channel interference) suppression taking advantage of spectral correlations for the separation of cyclostationary signals. Our performance results show that the BER (bit error rate) performances with strong ACI levels can be similar to the ones without ACI. Moreover, after a few iterations these performances can be close to MFB (matched filter bound)

The proposed decorrelating multi-user detector (MUD) scheme allows the cancellation of both multiple access interference (MAI) and interpath interference (IPI). The current approach considers the downlink environment of universal mobile telecommunication systems (UMTS), with a RAKE receiver in the mobile station (MS). A comparison is made with a subtractive interference cancellation MUD. Moreover, to improve the performance, we consider a combination of the decorrelating MUD with the subtractive interference cancellation MUD

Due to the multipath propagation, the Wideband-Code Division Multiple Access (W-CDMA) channels are typically frequency-selective. For this reason, conventional Vertical - Bell Laboratories Layered Space-Time (V-BLAST) detection techniques are not suitable for W-CDMA-based MIMO systems (Multiple Input, Multiple Output). In this paper, we consider the use of MIMO schemes within W-CDMA systems and we propose a V-BLAST detector suitable for multipath, frequeny-selective channels where the individual detection of each path is performed using a simple decorrelator, and the resulting signals are combined through an MSE-based (Mean Square Error) multipath combining algorithm. Due to the high sensitiveness of this detector to interpath interference, a Multipath - Parallel Interference Cancellation (MPIC) is also considered, followed by either Successive Interference Cancellation (SIC) and Parallel Interference Cancellation (PIC).

In this paper, we present a class of nonlinear signal-processing schemes to reduce the envelope fluctuations in the downlink transmission within code-division multiple-access (CDMA) systems. These schemes are compatible with conventional CDMA receivers since they just require some modifications at the transmitter side. The proposed transmitter structure includes a nonlinear operation in the time-domain followed by a linear frequency-domain filtering operation. We also include a semi-analytical statistical characterization of the transmitted signals, which takes advantage of the Gaussian-like nature of CDMA signals with a high number of users. This characterization is used for an analytical performance evaluation of the proposed techniques. A set of performance results is presented showing that we can reduce significantly the envelope fluctuations of the transmitted signals, while maintaining the spectral occupation of the corresponding conventional CDMA signals. Moreover, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be kept relatively low.

The software radio concept is a topic of widespread interest in wireless cellular systems. Due to the high sampling ratio and quantization requirements, the ADC (analog-to-digital converter) is as key component of any software radio architecture. In this paper we present an analytical tool for evaluating the quantization requirements within the ADC used in software radio architectures. For this purpose, we take advantage of the Gaussian behavior of the multi-band/multi-user signal at the input of the wideband ADC and employ results on Gaussian signals and memoryless nonlinearities. This characterization is then used for the performance evaluation and optimization of different quantization characteristics.

In this paper we consider a broadband wireless system employing SC (single-carrier) modulations combined with FDE (frequency-domain equalization). We propose a receiver structure for joint carrier synchronization and equalization which combines IB-DFE (iterative block decision feedback equalization) and post-equalization carrier synchronization schemes. The proposed receiver can be regarded as a modified IB-DFE where we perform a decision-directed frequency offset estimation within each iteration of the equalizer. Our results show that the proposed receiver structure has a good performance, allowing BER (bit error rate) close to the MFB (matched filter bound), even with moderate frequency offsets and severe time-dispersive channels.

We present a multiple access scheme for the uplink of broadband wireless systems. We consider SC (single carrier) based block transmission, with all users transmitting continuously, regardless of their data-rate. The transmitted signals can have very low envelope fluctuations. Moreover, the different users remain orthogonal, even for severe time-dispersive channels. By employing IB-DFE (iterative block decision feedback equalization) techniques we can have detection performances close to the matched filter bound, and even for fully loaded systems and severe time-dispersive channels.

The use of multiple antennas at the BS (base station) allows significant improvements on the spectral efficiency of wireless communication systems by increasing the number of simultaneous users in a given cell. We introduce a new iterative multiuser detection scheme for systems requiring high-rate transmission in severe time-dispersive channels. We consider the use of single-carrier modulation combined with frequency-domain equalization techniques, which are known to be excellent candidates for severe time-dispersive channels. The BS employs multiple antennas and consists of an iterative LST (layered space-time) receiver combined with frequency-domain equalization techniques. Our performance results show that the proposed receiver structure has excellent performance, which can be very close to the matched filter bound, even for severe time-dispersive channels and in the presence of strongly interfering channels.

This work presents a wide class of digital signal-processing schemes for orthogonal frequency-division multiplexing (OFDM) transmission which combine a nonlinear operation in the time domain and a linear filtering operation in the frequency domain. The ultimate goal of these schemes is to reduce the envelope fluctuation of ordinary OFDM, while keeping its high spectral efficiency and allowing a low-cost, power-efficient implementation. An appropriate statistical model concerning the transmitted frequency-domain blocks is developed, which is derived from well-established results on Gaussian stochastic processes distorted by memoryless nonlinearities. This model can be employed for performance evaluation by analytical means, with highly accurate results whenever the corresponding conventional OFDM signals exhibit quasi-Gaussian characteristics. Cases where the signal-processing scheme is repeatedly used, in an iterative way, are treated through an extension of the proposed statistical modeling. A set of numerical results is presented and discussed so as to show the practical interest of both the proposed schemes and the analytical methods for evaluation of their performance. For the sake of comparisons, this paper includes numerical results concerning the partial transmit sequence technique, which is an alternative peak-to-mean envelope power ratio-reducing technique of higher complexity, often recommended due to its distortionless nature. The superior performance/complexity tradeoffs through the proposed class of nonlinear signal-processing schemes is emphasized.

Some promising techniques to reduce the envelope fluctuations of OFDM signals with a high number of subcarriers combine a nonlinear operation (e.g., an envelope clipping) with a frequency-domain filtering operation, so as to ensure the low-PMEPR (peak-to-mean envelope power ratio) goal while maintaining the spectral efficiency of the corresponding conventional OFDM scheme. However, the filtering operation leads to some envelope fluctuation regrowth, increasing the PMEPR of the transmitted signals. In this paper we consider the repeated use, in an iterative way, of an envelope clipping and a frequency-domain filtering for low-PMEPR OFDM transmission with high spectral efficiency. Based on semi-analytical, statistical characterization of the transmitted frequency-domain blocks, a set of performance results is presented and discussed, showing that the proposed iterative technique can provide improved tradeoffs between power efficiency, spectral efficiency and implementation simplicity.

In this paper we develop signal processing techniques to reduce the PMEPR for downlink transmission (i.e., from the BS to the mobile terminals) within synchronous CDMA systems using orthogonal spreading codes. These techniques are compatible with conventional CDMA receivers since just require some modifications at the transmitter side. The proposed transmitter structure includes a nonlinear operation in the time-domain followed by a linear, frequency-domain filtering operation. We also include a semi-analytical statistical characterization of the transmitted signals, which takes advantage of the Gaussian-like nature of CDMA signals with a high number of users. This characterization is used for an analytical performance evaluation of the proposed techniques. Our performance results show that we can reduce significantly the PMEPR of the transmitted signals, while maintaining the spectral occupation of the corresponding conventional CDMA signals.

This paper is concerned with the use of frequency-domain equalization (FDE) and space diversity within block transmission schemes for broadband wireless communications. The expected performance with both multicarrier (MC) and single-carrier (SC) modulations is emphasized, when a cyclic prefix, long enough to cope with the maximum relative channel delay, is appended to each transmitted block. A set of numerical results is presented and discussed, with the help of appropriate, analytical performance bounds which are conditional on a given channel realization. These bounds are used to explain the performance advantage of the SC/FDE option, the benefits of space diversity, and the impact of the criterion for computing the FDE parameters.

We compare, in terms of both performance and complexity, two types of spectrally efficient techniques for low-PMEPR OFDM transmission: PTS techniques (partial transmit sequence) and a class of techniques which combines a nonlinear operation in the time domain and a linear, filtering operation in the frequency domain. From our performance results, when considering M-QAM constellations on N /spl ges/ 64 subcarriers, the advantage of the techniques which involve a nonlinear operation in the time domain is made clear, especially for small M and high N.

This paper considers a wide class of digital signal processing schemes, previously proposed by the authors, which combines a nonlinear operation in the time domain and a linear operation in the frequency domain, so as to reduce the envelope fluctuations and allow good power/bandwidth tradeoffs with OFDM transmission; it also considers both conventional and two-branch, LINC (linear amplification with nonlinear components) transmitter structures. The paper uses the ideas behind that class so as to show that it includes "peak cancellation" schemes, such as that proposed by other authors, and also in order to derive signal processing schemes for LINC decomposition purposes, while keeping low out-of-band radiation levels. The paper addresses the statistical characterization of the frequency domain blocks effectively transmitted, so as to evaluate by analytical means, in a unified manner, either those peak cancellation schemes (to be used within a conventional transmitter structure) or the signal processing schemes to be employed within a LINC transmitter structure. A set of numerical results, which have been obtained with the help of this analytical method, is presented and discussed.

OFDM techniques, known to be well-suited for high rate transmission in time-dispersive channels, are currently being considered for broadband wireless LAN systems. However, the strong envelope fluctuation of conventional OFDM signals is hardly compatible with an efficient, somewhat nonlinear, power amplification. A new multicarrier modulation technique, called CEPB-OFDM (constant envelope paired burst OFDM), was proposed to allow high transmit power efficiencies. This paper is concerned with the performance evaluation of CEPB-OFDM and includes a performance comparison, when a grossly nonlinear power amplifier is employed, with other previously proposed OFDM techniques which are able to reduce the envelope fluctuation. The performance results clearly indicate that the CEPB-OFDM technique is recommendable for the above-mentioned wireless systems.