This work addresses the problem of precoding optimization for a multi-user and multi-target ISAC system from a communication-centric perspective. Specifically, the paper considers a scenario where a multiple-antenna base station (BS) serves several single-antenna communication user equipments (UE) while also steering beams for target detection. The objective is to maximize the signal-to-interference plus noise ratio (SINR) subject to power and radio-sensing constraints. The problem is challenging due to its non-convexity. To address this, we propose a sub-optimal solution by zero-forcing (ZF) multi-user interference. A closed-form solution is derived, which efficiently computes the precoder with low computational cost. Simulation results validate the effectiveness of our proposed precoder design which outperforms previous precoder design methods for ISAC.

Ultra-dense networks (UDNs) have been proposed to achieve high data rates and energy efficiency, but their performance is limited by the increase of inter-user interference. To overcome this problem, interference alignment (IA) algorithms have been widely researched. However, reported IA techniques neglect the nonlinear distortion (NLD) induced by power amplifiers. Thus, their performance is severely degraded in power-efficient transmissions operating close to the saturation point. Distortion-aware precoding techniques have been studied to reduce NLD in single transmitter scenarios either single-user or multi-user. Nevertheless, its extension to UDNs with multiple transmitters and users is not straightforward. In this work, a novel IA algorithm, named NLD-IA, is proposed to alternatively design the precoding and combining vectors to reduce the interference and NLD by maximizing the sum-rate through the signal-to-interference-plus-noise ratio (SINR). Precoding vectors are obtained via a non-convex optimization problem that models the NLD correlation. An analytical solution based on the Newton method over the complex field is developed to solve this problem with low computational complexity. Simulation results show that the proposed NLD-IA significantly reduces interference and NLD outperforming previous IA algorithms. The proposed method is an attractive solution for UDNs commonly found in the context of Internet of Things applications.

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.

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.

This paper considers a cooperative bistatic scenario for integrated sensing and communication (ISAC) paradigm, where transmitter and receiver base stations (BS) are connected through the front-haul to a central unit (CU), enabling cooperation. A multiple-input multiple-output (MIMO) configuration is considered in combination with orthogonal frequency division multiplexing (OFDM). The communication and radio-sensing functions are performed in orthogonal sets of sub-carriers, where the sub-carriers allocated to radio-sensing are distributed among antennas in an interleaved way. The interleaved frame structure is exploited in the design of a new parameter estimation method. The proposed method is a two-stage algorithm to estimate the delay/Doppler shift and angular position of objects (paths) in the channel. First, the multiple signal classification (MUSIC) algorithm estimates the angle-of-arrival (AoA) of the paths. Then, the delay, Doppler shift and angle-of-departure (AoD) are jointly estimated for each AoA combining an extended search over the AoD domain and a two-dimensional periodogram. The simulations show that the delay, Doppler shift and AoA are estimated correctly, while the estimation of the AoD may report error, although it can be compensated.

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.

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.

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.

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.

The secrecy challenge of protecting a power domain non-orthogonal multiple access (NOMA) system is associated with the intrinsic superposition structure of the transmitted information. Because of this superposition operation, the interference generated by the other user's information must be first decoded by the receiver in order to allow the intended information to be acquired. Therefore, a robust secrecy solution for this type of channel access techniques must consider not only the existence of attacks coming from undetectable passive eavesdroppers located outside of the network, but also attacks carried out by eavesdroppers registered as legitimate users inside the system. In this work, a cooperative jamming technique is combined with a secure channel training solution in order to protect a two user power domain NOMA system against eavesdropping attacks coming simultaneously from inside and outside of the network. The proposed secrecy solution is evaluated numerically and the evaluation demonstrates that weak secrecy is achieved against the inside and outside eavesdroppers.

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.

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.

Accommodating the coverage and capacity requirements of wireless cellular communication systems evolved contemporary cellular architectures to comprise two tiers: A macro-cell overlay coverage tier encompassing a small-cell tier offloading user traffic in high traffic demand areas. While two-tier cellular architectures can effectively meet coverage and capacity requirements when operating on independent frequency bands, radio spectrum scarcity entails sharing spectrum by both tiers which substantially complicates interference management for such architectures. In order to address uplink interference in two-tier cellular architectures, this paper considers the joint application of dual space-frequency block codes (SFBC) and signal alignment (SA) enabled physical network coding (PNC). When compared to designs adopting interference alignment (IA) or PNC, the proposed joint dual-SFBC with SA-PNC design substantially reduces interference and enables connecting a larger number of users. Numerical results also verify that the proposed method outperforms the SA-PNC static method without any information exchange requirement between the two tiers while achieving close to optimal performance for macro-cells.

In this paper, we consider the application of signal alignment (SA) enabled physical network coding (PNC) for the uplink of heterogeneous networks. The joint design of SA and PNC has the potential of fully exploiting the precoding space and can achieve better spatial diversity of a multi-input multi-output (MIMO) networks that provides higher system degrees of freedom (DoF), than the case where only PNC or interference alignment (IA) is employed individually. We consider a scenario where a set of small-cells coexist under the same spectrum with the macro- cell. For this scenario, we design new joint SA and PNC based schemes under different levels of inter-tier coordination. We compare the proposed joint SA-PNC schemes with the IA based techniques, recently designed for uplink heterogeneous systems. Simulation results show that the proposed SA-PNC is quite efficient to remove the inter- tier/system interference while allowing to increase the overall data rate, by serving more users, as compared with the IA based methods.

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.

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.

In this paper, we consider the K-user singleinput single-output interference channel with delayed channel state information at the transmitters (CSIT). Our main contribution is to show that even with delayed CSIT and distributed transmitters, the achievable degrees of freedom (DoFs) still scale with the number of users. More specifically, we propose a method that achieves K/(2√K -1) ≥ √K/2 DoF. The main idea behind the proposed method is to use interference alignment (IA) at the receiver side in conjunction with repetition coding at the transmitters. Repetition coding repeats the data R times while IA makes the interference generated by a given transmitter identical: 1) at all non-intended receivers and 2) all transmission repetitions. Therefore, one retransmission per transmitter is sufficient to cancel all the interference. This reduces significantly the overhead required for interference removal since the number of interference terms is reduced from RK(K - 1) to K.

In this paper, we consider the downlink communication of an arbitrary number of transmitters over broadcast interference channels. In order to suppress interference, we investigate a network-level linear precoding scheme dubbed precoding reuse. Such a technique is leveraged on the interference alignment (IA) principle, where the undesired signals overlap in a reduced-dimension subspace at the receivers. Precoding reuse relies on two cascaded precoders to suppress intra- and inter-cell interference and requires feedback only within a cell. We designed precoding reuse schemes for which interference suppression can be attained in a coordinated or uncoordinated fashion over a fixed number of quasi-static channel uses. We provide expressions for the parametrization of the precoders and investigate the advantages, limitations, suitable scenarios, and applications of the proposed IA scheme. Finally, we pointed out the open problems, where the proposed IA scheme and parametrization can be implemented with little effort, demanding minimum changes to an existing cellular system supporting multi-user MIMO.

The exploration of the physical layer characteristics of the wireless channel is currently the object of intensive research in order to develop advanced secrecy schemes that can protect information against eavesdropping attacks. Following this line of work, in this manuscript we consider a massive MIMO system and jointly design the channel precoder and security scheme. By doing that we ensure that the precoding operation does not reduce the degree of secrecy provided by the security scheme. The fundamental working principle of the proposed technique is to apply selective random rotations in the transmitted signal at the antenna level in order to achieve a compromise between legitimate and eavesdropper channel capacities. These rotations use the phase of the reciprocal wireless channel as a common random source between the transmitter and the intended receiver. To assess the security performance, the proposed joint scheme is compared with a recently proposed approach for massive MIMO systems. The results show that, with the proposed joint design, the number of antenna elements does not influence the eavesdropper channel capacity, which is proved to be equal to zero, in contrast to previous approaches.

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 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.

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.

In this paper, we consider a K-user M × N MIMO interference channel, with outdated channel state information at the transmitters (CSIT). Based on the technique introduced by Cadambe and Jafar, for the IC with perfect and up-to-date CSIT, we propose and analyze a scheme that guarantees interference alignment with outdated CSIT. The interference is aligned by the transmitters, and removed by the receivers using zero-forcing by Cadambe and Jafar. However, this is not feasible for the IC with outdated CSIT, since there is no up-to-date CSIT to compute the interference alignment precoders. To overcome this limitation, we propose that the interference should be aligned by the receivers instead. With this approach, the interference caused by a transmitter spans the same subspace at all receivers, i.e., the interference at receiver k ≠ m may be obtained through a linear map from the interference at receiver m. Therefore, one interference retransmission per transmitter is enough to cancel all the interference. This reduces the overhead required for interference removal from K(K -1) to K interference terms.

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.

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.

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).

Small‐cells are considered as an effective solution to increase capacity and offload traffic from, the current macro‐cell cellular system. Owing to the difficulty and costs involved in acquiring new spectrum licenses, small‐cells are expected to coexist with their respective macro‐cells, in the same spectrum. This leads to considerable interference between the two systems. Optimum performance, at the macro‐cell, is achieved when the small‐cell terminals transmit their information over the null‐space of the macro‐cell link. However, availability, at the small‐cell terminals, of the macro‐cell channel null‐space information requires full‐cooperation and thus a high overhead of information exchange. In this study, the cognitive precoding schemes are designed under a limited inter‐system information exchange and the constraint that the performance of the macro‐cell link is kept close to the case where no small‐cell network does exist. Two techniques are considered: a two‐bit quantisation precoded and a dual space‐frequency coding precoded approach. It is demonstrated that the first achieves a performance close to the full cooperation approach recently proposed, but with very low information exchange requirements. For the second, it is show that both the systems are able to coexist without any inter‐system cooperation and with a performance close to the non‐coexistence 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).

To increase capacity and offload traffic from the current macro-cell cellular system, operators are considering the deployment of small cells. It is expected that both the small and macro-cells will coexist in the same spectrum, resulting in unsustainable levels of interference. Interference alignment is considered as an effective method to deal with such interference. By using interference alignment, the small cells align their transmission along a common direction to allow the macro-cell receiver to completely remove it. It is clear that, if the two systems have no limitations on the information that may be exchanged between them to perform the signal design, then the performance may be improved in comparison to the case of no or partial cooperation. However, this full-cooperation strategy requires a high-rate connection between the macro and small cells, which may not be available. To overcome this problem, we consider that the alignment direction is selected from a finite set, known to both macro- and small-cell terminals. We provide sufficient conditions for this set that guarantee full diversity, at the macro-cell, and propose an efficient method to optimize the set elements. Results show that an alignment set with a description length of 1 bit is enough to achieve the same diversity as in the case where an infinite amount of information is exchanged between both systems. The proposed set optimization method achieves better performance than random vector quantization and similar performance to Grassmannian quantization.

To increase capacity and offload traffic from the current macro cell cellular system, small cells have been deployed extensively. In the current deployments small cells coexist with their respective macro cells in the same spectrum, due to the difficulty and costs involved in acquiring new spectrum licenses. This leads to considerable interference between the two systems. In this context, we propose an underlay cognitive interference alignment technique to remove the interference from the small cell to the macro cell system. In the proposed scheme the small cells sense the unused resources in the space dimension of the macro cell, i.e. its null space, and precode their signals such that they lie in the sensed null space. We demonstrate that only the sign of the null space components is necessary to obtain close to full null space information performance. This results in less overhead in the sensing process making the system more efficient and practically feasible.

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.

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.

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.

The aim of this paper is to propose and evaluate multiuser linear precoding techniques for downlink of multicell MIMO based networks. We consider a high-speed backhaul network where the base stations are transparently linked by optical fiber to a central unit. This architecture allows an efficient joint multicell multiuser processing. The proposed precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by proposing three power allocation algorithms with per-BS power constraint and different complexity tradeoffs: one optimal to minimize the BER and two suboptimal. The performance of the proposed schemes is evaluated, considering typical pedestrian scenarios based on LTE specifications. Numerical results show that the proposed suboptimal power allocation schemes achieve a performance close to the optimal.

In this paper we propose and evaluate distributed multiuser linear precoding techniques for the downlink of multicell MIMO OFDM systems. The precoder is designed in two phases: first the intercell interference is removed by applying a set of precoding vectors computed in a distributed manner at each BS; then the system is further optimized through power allocation. Three centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed: one optimal to minimize the average BER and two suboptimal. The performances of the proposed schemes are compared to the centralized multicell based approaches recently proposed, and evaluated considering typical pedestrian scenarios. Numerical results show that the proposed suboptimal schemes achieve a performance very close to the optimal. Also, it is shown that for the same degrees of freedom the performance is closed to the one of centralized precoding schemes but with much lower feedback load over the backhaul network.

In this work we propose and evaluate a pilot and data-based channel estimation scheme for SFBC OFDM-based cooperative scenario. We consider that both the base station and the relay node are equipped with double antenna arrays whereas the user terminal is a single antenna device. The relay protocol considered is equalise-and- forward. In this paper we propose a channel estimation method that iteratively uses the information carried by the pilots and data to improve the estimate of the equivalent channels for the path Base Station - Relay Node - User Terminal. The MMSE criterion is used in the design of the estimator for both the pilot-based and data-aided iterations. The proposed method is evaluated in terms of the channel estimation MSE and the results show that the proposed scheme allows, with a single data iteration, a reduction of the number of pilots by a factor of approximately two when compared with non-data-aided schemes.

This paper deals with a new multi-user linear precoding technique for downlink of distributed MIMO OFDM systems. We consider a distributed broadband wireless system where some base stations are transparently linked by optical fiber to a central unit. We further assume that the user terminals are equipped with an antenna array and the distributed base stations equipped with either single or an antenna array. This architecture provides a high speed backhaul channel allowing an efficient joint multi-cell processing. The proposed power allocation algorithm is based on minimization of the sum of inverse signal-to-noise ratio (SNR-1) on each user terminal over the available subcarriers. The motivation to minimize the sum of SNR-1 instead of bit error rate (BER) is in the fact that the first criterion achieves a closed-form solution, which is more interesting from practical point of view. The aim is to propose a practical distributed precoding technique to remove the inter-cell interference and improve the user's fairness at the cell-edges. The performance of the proposed scheme is evaluated, considering typical pedestrian scenarios based on LTE specifications.

We propose a data precoded relay-assisted scheme, for a system cooperating with L relays, each one equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where either the direct path is not available or has poor quality. However because of the half duplex constraint at the relays, transmission of a data rate that would be possible using a modulation technique with m bits per symbol in the case a continuous link was available from the BS to the UT, requires the use of a constellation with 2m bits per symbol. This implies a penalty in the power efficiency. In this communication we propose a simple precoding scheme that exploits the relation between QPSK and 4L-QAM, by alternately transmitting through the L relays, keeping the diversity of order L. Numerical results are presented for the case where 2 relay nodes are used, for realistic scenarios with LTE parameters and using turbo coding, showing significant performance improvements relatively to the case of a relay based distributed Alamouti employing 16-QAM.

In this paper we propose a scheme with a dedicated relay node for an OFDM-based system. We consider an antenna array at the base station and a single antenna at both the user terminal and the relay node. The relay-assisted protocol considered is equalize-and-forward. The proposed scheme requires the computation of parameters which depend on unknown variables at the destination and we show that under some assumptions the scheme is suitable to provide all those requirements. Also, we show that since the pilots at the relay node are properly designed the TD-MMSE estimator can still provide the required channel estimate. The system performance is evaluated by considering a typical pedestrian scenario based on WiMAX specification. The results show that the degradation in the estimator performance is negligible.

We propose a data precoded relay-assisted scheme, for a system cooperating with 2 relays. We assume that all the terminals are equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where a direct path is not available. However because of the half duplex constraint at the relays, transmission of a data rate equivalent to that of a modulation technique with m bits per symbol in the case a continuous link would be available from the BS to the UT requires the use of a constellation with 2m bits per symbol. This implies a penalty in power efficiency. The proposed simple precoding scheme exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, keeping the diversity of order 2. Numerical results show significant performance improvements relatively to the case of a relay-based distributed Alamouti employing 16-QAM, and a performance very close to the non-cooperative system employing 2x1 QPSK Alamouti coding with a continuous link available, when all the links have the same SNR.

In this paper we investigate the performance of single-relay cooperative scheme, where the source is equipped with a 2-antenna array and both the relay and destination with a single antenna. We consider Amplify-and-Forward, Decode-and-Forward and Selective Decode-and-Forward relaying protocols and a space-frequency block coding designed for DL OFDM based systems is employed. Signals expressions are derived for each relay protocol under various scenarios, considering power constraint imposed on cooperating nodes. The aim of this work is to evaluate this Virtual MIMO scheme in realistic scenarios (e.g. correlated antennas, precise channel models, Doppler effects) for implementation purposes in a system with LTE based parameters and considering channel turbo coding. The coded performance of the proposed Virtual MIMO scheme is evaluated, and compared against non-cooperative OFDM based systems, for typical pedestrian scenarios based on LTE specifications. The proposed cooperative scheme outperforms the co-located MISO and MIMO for some scenarios.

The aim of this paper is to adapt and evaluate the performance of two classical pilot-aided channel estimation schemes designed for OFDM systems: least square and minimum mean square error, in a relay-assisted scenario. We consider the amplify-and-forward, which is the simplest relay protocol for cooperative systems. Also, we consider the use of a single antenna at all the terminals: base station, user terminal and relay node. In our scenario, it is assumed that some of the user terminals deployed in a certain area could act as relaying-able terminals for the communications of other users. In this communication we show that the classical channel estimation techniques designed for conventional wireless communications can be extended for relay-assisted based systems under some modifications.

This paper aims to compare the performance of the different transmit signal design techniques that have been proposed recently for DL MC-CDMA systems and presents a joint space-frequency multi-user pre-filtering and spreading scheme. We consider the use of antenna arrays at the base station (BS) and a single antenna at the mobile terminal (MT) and derive the proposed multi-user space-frequency pre-filtering technique that formats the transmitted signals to eliminate the effects of multiple access interference (MAI) and channel distortions at the mobile terminals, whilst maintaining low MT complexity. Thus, two types of detectors are considered at the MT: simple despreading and single user equalizers. The performance of the different schemes is evaluated, considering either perfect or non-perfect CSI and channel coding based on UMTS specifications, in order to select the best strategy for DL TDD MC-CDMA systems.

In this paper we consider and compare several strategies that can be employed to constrain the transmit power at base station (BS) and compare the performance of the different space-frequency pre-filtering techniques that have been proposed recently for DL MC-CDMA systems. We consider the use of antenna arrays at the base station and a single antenna at the mobile terminal (MT). These schemes format the transmitted signals to eliminate the effects of multiple access interference (MAI) and channel distortions at the mobile terminals, while maintaining low MT complexity. The performance of the different pre-filtering schemes is evaluated, considering both uncoded data and channel turbo coding based on UMTS specifications

This paper deals with downlink space-frequency block coding with a pre-filtering technique for time division duplex MC-CDMA. We consider the use of antenna arrays at the base station (BS) and a single antenna at the mobile terminal (MT). The space-frequency block codes allow that the spatial diversity be efficiently exploited and the linear multi-user pre-filtering technique allows to format the transmitted signals so that the multiple access interference (MAI) and channel distortions at mobile terminals are completely removed. The aim of this technique is to transfer the most computational complexity from MT to the BS, keeping the first one at very low complexity. Simulations results are carried out in high data rate scenarios with the channel turbo code specified for the current UMTS system, to demonstrate the effectiveness of the proposed multi-user pre-filtering algorithm with SFBC

The cyclostationary properties of direct sequence spread spectrum signals are well known. These cyclostationary properties imply a redundancy between frequency components separated by multiples of the symbol rate. We present a multiple access interference canceller that explores this property and applies it to UMTS-TDD. This frequency domain canceller acts on the spread signal in such a way that minimizes the interference and noise at its output (minimum mean squared error criterium). The performance is evaluated in two detector configurations: one including the frequency shift canceller (FSC) and the other plus a parallel interference canceller (PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector.