Acquiring accurate channel state information (CSI) is critical for reliable and efficient wireless communication, but challenges such as high pilot overhead and channel aging hinder timely and accurate CSI acquisition. CSI prediction, which forecasts future CSI from historical observations, offers a promising solution. Recent deep learning approaches, including recurrent neural networks and Transformers, have achieved notable success but typically learn deterministic mappings, limiting their ability to capture the stochastic and multimodal nature of wireless channels. In this paper, we propose a novel CSI prediction scheme based on diffusion models. We decompose the CSI prediction task into two components: a temporal encoder, which extracts channel dynamics, and a diffusion-based generator, which produces future CSI samples autoregressively. Extensive simulations demonstrate that our diffusion-based models significantly outperform state-of-the-art baselines.

Non-orthogonal multiple access (NOMA) is a promising technology for next-generation wireless communication systems due to its enhanced spectral efficiency. However, wireless communication is facing increasing requirements for security. To that end, jamming mitigation using multi-antennas has emerged as an important research topic. In this paper, we consider an uplink NOMA system with a reconfigurable intelligent surface (RIS) that assists the uplink users and, at the same time, mitigates the jammer. Our goal is to minimize the total users’ transmitted power under signal-to-interference-plus-noise ratio constraints at the base station. To be effective, typically a high-dimensional RIS is needed, leading to a large optimization problem, which in general faces convergence problems. We propose an iterative algorithm for this high-dimensional non-convex optimization problem that converges with a jammer comprising as many as 64 antennas, and an RIS with 128 elements. More specifically, we introduce a design and optimize the performance of an absorptive RIS (A-RIS). Compared to a standard RIS, we show that an A-RIS can dramatically reduce the users’ required transmit power and successfully mitigate the jammer. The A-RIS is in particular useful in cases when the number of jammer antennas is of the same order as the number of A-RIS elements.

Acquiring accurate channel state information (CSI) is critical for reliable and efficient wireless communication, but challenges such as high pilot overhead and channel aging hinder timely and accurate CSI acquisition. CSI prediction, which forecasts future CSI from historical observations, offers a promising solution. Recent deep learning approaches, including recurrent neural networks and Transformers, have achieved notable success but typically learn deterministic mappings, limiting their ability to capture the stochastic and multimodal nature of wireless channels. In this paper, we introduce a novel probabilistic framework for CSI prediction based on diffusion models, offering a flexible design that supports integration of diverse prediction schemes. We decompose the CSI prediction task into two components: a temporal encoder, which extracts channel dynamics, and a diffusion-based generator, which produces future CSI samples. We investigate two inference schemes—autoregressive and sequence-to-sequence—and explore multiple diffusion backbones, including U-Net and Transformer-based architectures. Furthermore, we examine a diffusion-based approach without an explicit temporal encoder and utilize the DDIM scheduling to reduce model complexity. Extensive simulations demonstrate that our diffusion-based models significantly outperform state-of-the-art baselines.

Integrated access and backhaul (IAB) is one of the promising techniques for 5G networks and beyond (6G), in which the same node/hardware is used to provide both backhaul and cellular services in a multi-hop architecture. Due to the sensitivity of the backhaul links with high rate/reliability demands, proper network planning is needed to ensure the IAB network performs with the desired performance levels. In this paper, we study the effect of infrastructure planning and optimization on the coverage of IAB networks. We concentrate on the cases where the fiber connectivity to the nodes is constrained due to cost. Thereby, we study the performance gains and energy efficiency in the presence of free-space optical (FSO) communication links. Our results indicate hybrid fiber/FSO deployments offer substantial cost savings compared to fully fibered networks, suggesting a beneficial trade-off for strategic link deployment while improving the service coverage probability. As we show, with proper network planning, the service coverage, energy efficiency, and cost efficiency can be improved.

Contemporary wireless communication systems rely on Multi-User Multiple-Input Multiple-Output (MU-MIMO) techniques. In such systems, each Access Point (AP) is equipped with multiple antenna elements and serves multiple devices simultaneously. Notably, traditional systems utilize fixed antennas, i.e., antennas without any movement capabilities, while the idea of movable antennas has recently gained traction among the research community. By moving in a confined region, movable antennas are able to exploit the wireless channel variation in the continuous domain. This additional degree of freedom may enhance the quality of the wireless links, and consequently the communication performance. However, movable antennas for MU-MIMO proposed in the literature are complex, bulky, expensive and present a high power consumption. In this paper, we propose an alternative to such systems that has lower complexity and lower cost. More specifically, we propose the incorporation of rotation capabilities to APs equipped with Uniform Linear Arrays (ULAs) of antennas. We consider the uplink of an indoor scenario where the AP serves multiple devices simultaneously. The optimal rotation of the ULA is computed based on estimates of the positions of the active devices and aiming at maximizing the per-user mean achievable Spectral Efficiency (SE). Adopting a spatially correlated Rician channel model, our numerical results show that the rotation capabilities of the AP can bring substantial improvements in the SE in scenarios where the line-of-sight component of the channel vectors is strong. Moreover, our proposed system is robust against imperfect positioning estimates.

Parametric millimeter-wave (mmWave) channel estimation involves modeling the channel matrix by combining direction-dependent signal paths, exploiting the sparse nature of mmWave channels. In our study, we propose a deep learning-based approach to estimate angle-of-arrival (AoA) and angle-of-departure (AoD) parameters from input observations in the frequency domain. To address this challenge, we have adapted a residual convolutional neural network (ResNet) to this specific problem and incorporated a technique from topological data analysis, enabling us to accurately retrieve the angular frequencies. Furthermore, we have extended this basic architecture by incorporating a posterior model fitting to enhance the system performance in terms of probability of detection. In our research, we compare the ResNet and extended ResNet approaches with state-of-the-art signal processing techniques and the Crámer-Rao lower bound through simulation. Our results indicate significant improvements in system robustness by increasing the probability of detection while maintaining a reduced estimation error.

In this paper, we study integration strategies for reconfigurable intelligent surfaces (RISs) in distributed MIMO (D-MIMO) systems as one of the promising techniques for evolving 5G networks and beyond (6G). The scalability of D-MIMO systems to large networks in a practically feasible way is challenging. The exploitation of cost-effective and energy-efficient dynamic access point (AP) clustering techniques and densification enablers is crucial for such systems to sustain the required level of performance and reliability while mitigating the environmental and economic challenges related to future network operations. RIS is well regarded as a low-cost, rapidly deployable, energy-efficient candidate offering extra diversity in the spatial domain. This work explores RIS-aided multi-AP systems attaining certain prescribed key performance indicators (KPIs) while considering the energy consumption in the system.We concentrate on indoor use cases with large user equipment (UE) populations where the service coverage is low due to the high obstacle density. To that end, we propose dynamic user-centric AP clustering and RIS placement techniques adapted for serving multi-user indoor systems. Here, we use an alternating optimization method as a sub-optimal solution for RIS phase-shift configuration, keeping the computation complexity low. We show that the integration of RISs in D-MIMO systems is an attractive approach to enhance energy efficiency and provide the scalability required in 5G and beyond (6G).

Holographic Multiple-Input Multiple-Output (HMIMO), which densely integrates numerous antennas into a limited space, is anticipated to provide higher rates for future 6G wireless communications. The increase in antenna aperture size makes the near-field region enlarge, causing some users to be located in the near-field region. Thus, we are facing a hybrid near-field and far-field communication problem, where conventional far-field modeling methods may not work well. In this paper, we propose a near-far field channel model that does not presuppose whether each path is near-field or far-field, different from the existing work requiring the ratio of the number of near-field paths to that of far-field paths as prior knowledge. However, this gives rise to a new challenge for accurately modeling the channel, as conventional methods of obtaining channel model parameters are not applicable to this model. Therefore, we propose a new method, Expectation-Maximization (EM)-based Near-Far Field Channel Modeling, to obtain channel model parameters, which considers whether each path is near-field or far-field as a hidden variable, and optimizes the hidden variables and channel model parameters through an alternating iteration method. Simulation results show that our method is superior to conventional near-field and far- field algorithms in fitting the near-far field channel in terms of outage probability.

Integrated access and backhaul (IAB) is one of the promising techniques for 5G networks and beyond (6G), in which the same node/hardware is used to provide both backhaul and cellular services in a multi-hop fashion. Due to the sensitivity of the backhaul links with high rate/reliability demands, proper network planning is needed to make the IAB network performing appropriately and as good as possible. In this paper, we study the effect of deployment optimization on the coverage of IAB networks. We concentrate on the cases where, due to either geographical or interference management limitations, unconstrained IAB node placement is not feasible in some areas. To that end, we propose various millimeter wave (mmWave) blocking-aware constrained deployment optimization approaches. Our results indicate that, even with limitations on deployment optimization, network planning boosts the coverage of IAB networks considerably.

Non-orthogonal multiple access (NOMA) has become a promising technology for next-generation wireless communications systems due to its capability to provide access for multiple users on the same resource. In this paper, we consider an uplink power-domain NOMA system aided by a reconfigurable intelligent surface (RIS) in the presence of a jammer that aims to maximize its interference on the base station (BS) uplink receiver. We consider two kinds of RISs, a regular RIS whose elements can only change the phase of the incoming wave, and an RIS whose elements can also attenuate the incoming wave. Our aim is to minimize the total power transmitted by the user terminals under quality-of-service constraints by controlling both the propagation from the users and the jammer to the BS with help of the RIS. The resulting objective function and constraints are both non-linear and non-convex, so we address this problem using numerical optimization. Our numerical results show that the RIS can help to dramatically reduce the per user required transmit power in an interference-limited scenario.

6G will likely be the first generation of mobile communication that will feature tight integration of localization and sensing with communication functionalities. Among several worldwide initiatives, the Hexa-X flagship project stands out as it brings together 25 key players from adjacent industries and academia, and has among its explicit goals to research fundamentally new radio access technologies and high-resolution localization and sensing. Such features will not only enable novel use cases requiring extreme localization performance, but also provide a means to support and improve communication functionalities. This paper provides an overview of the Hexa-X vision alongside the envisioned use cases. To close the required performance gap of these use cases with respect to 5G, several technical enablers will be discussed, together with the associated research challenges for the coming years.

In this paper, a coordinated multi-point (CoMP) based mmWave communications operating over access links impaired by blockage is considered. By assuming realistic assumptions such as line-of-sight and non-line-of-sight channel components, fading, and random blockage, we propose a tractable analytical framework for mmWave communications. Moreover, the performance of the CoMP-based mmWave communication is evaluated by deriving the distribution of the received signal and presenting closed form expression for outage probability. Simulation results show the correctness of the analytical results and impact of blockage on the reliability of the mmWave communications.

In this paper, we study the performance of cooperative millimeter wave (mmWave) networks with hybrid precoding architectures. Considering joint transmissions and BS silence strategy, we propose hybrid precoding algorithms which minimize the sum power consumption of the base stations (BSs), for both fully- and partially-connected hybrid precoding (FHP and PHP, respectively) schemes, for single-carrier and orthogonal frequency-division multiplexing systems. We reformulate the analog precoding part as an equal-gain transmission problem, which only depends on the channel information, and the digital precoding part as a relaxed convex semidefinite program subject to per-user quality-of-service constraints that gives the optimal sum power consumption in terms of the BS silence strategy. In order to reduce the complexity of the hybrid precoding algorithm with optimal BS silence strategy, we propose a sub-optimal hybrid precoding algorithm that iteratively put BSs with small power into the silent mode. The simulation results show that, depending on the parameter settings, the power consumption of the PHP may be dominated by the RF transmit power and it may result in a larger power consumption than the FHP. For the cases with 2 BSs and 4 users, implementation of the FHP and the PHP in cooperative networks reduces the required RF transmit power, compared to the case in a non-cooperative network, by 71% and 56%, respectively.

Collaborative localization over vehicular networks is challenging if quality varies among the collected multiple sources of information. These information sources are either from vehicle on-board sensors or remote sensing using vehicular communications. The variation in the quality of remote sensor information may cause estimation performance deterioration, even threatening the system security. In this paper, we propose a distributed localization framework with truth discovery for heterogeneous and dynamic vehicular networks. Firstly, it allows vehicles to learn which neighboring vehicles they should cooperate with. Secondly, it is resilient against the quality variation of the shared information between the connected vehicles.

In a wirelessly-powered communication network (WPCN), an energy access point (E-AP) supplies the energy needs of the network nodes through radio frequency wave transmission, and the nodes store their received energy in their batteries for possible data transmission. In this paper, we propose an online control policy for energy transfer from the E-AP to the wireless nodes and for data transfer among the nodes. With our proposed control policy, all data queues of the nodes are stable, while the average energy consumption of the network is shown to be within a bounded gap of the minimum energy required for stabilizing the network. Our proposed policy is designed using a quadratic Lyapunov function to capture the limitations on the energy consumption of the nodes imposed by their battery levels. We show that under the proposed control policy, the backlog level in the data queues and the stored energy level in the batteries fluctuate in small intervals around some constant levels. Consequently, by imposing a negligible average data drop rate, the data buffer size and the battery capacity of the nodes can be significantly reduced.

Massive multiple-input multiple-output (M-MIMO) is recognized as a promising technology for the next generation of wireless networks because of its potential to increase the spectral efficiency. In initial studies of M-MIMO, the system has been considered to be perfectly synchronized throughout the entire cells. However, perfect synchronization may be hard to attain in practice. Therefore, we study a M-MIMO system whose cells are not synchronous to each other, while transmissions in a cell are still synchronous. We analyze an asynchronous downlink M-MIMO system in terms of the coverage probability and the ergodic rate by means of the stochastic geometry tool. For comparison, we also obtain results for the corresponding synchronous system. In addition, we investigate the effect of the uplink power control and the number of pilot symbols on the downlink ergodic rate, and we observe that there is an optimal value for the number of pilot symbols maximizing the downlink ergodic rate of a cell. Our results also indicate that, compared to the synchronous system, the downlink ergodic rate is more sensitive to the uplink power control in the asynchronous mode.

In a wireless powered communication network, an energy access point (EAP) supplies the energy needs of the network nodes through radio frequency wave transmission, and the nodes store the received energy in their batteries for their future data transmission. In this paper, we propose an online stochastic policy that jointly controls energy transmission from the EAP to the nodes and data transfer among the nodes. Our proposed policy is designed using a quadratic Lyapunov function to capture the limitations on the energy consumption of the nodes imposed by their battery level. The proposed policy is adaptive to different channel statistics of the network. We provide theoretical analysis for the performance of the proposed policy and show that it stabilizes the network. Moreover, the average energy consumption of the network under this policy is within a bounded gap of the minimum energy level required for stabilizing the network.

The sectored multi-probe anechoic chamber (MPAC) is regarded as a promising millimeter-wave (mmWave) over-the-air (OTA) testing solution. However, there are still debates in the literature about which channel emulation method, i.e., pre-faded synthesis (PFS) or plane wave synthesis (PWS), should be used for the sectored MPAC testing system. In this paper, a thorough comparison between the two channel emulation methods is conducted. It is found that for sectored MPAC (where the active probes are unlikely to be uniformly distributed over the probe panel) the PFS is more accurate than the PWS. Explanations are given.

This paper studies the sum throughput of the multi-user multiple-input-single-output networks in the cases with a large-but-finite number of transmit antennas and users. Considering continuous and bursty communication scenarios with different users' data request probabilities, we derive quasi-closed-form expressions for the maximum achievable throughput of the networks using optimal schedulers. The results are obtained in various cases with different levels of interference cancellation. Also, we develop an efficient scheduling scheme using genetic algorithms (GAs) and evaluate the effect of different parameters, such as channel/precoding models, number of antennas/users, scheduling costs, and power amplifiers' efficiency, on the system performance. Finally, we use the recent results on the achievable rates of finite block-length codes to analyze the system performance in the cases with short packets. As demonstrated, the proposed GA-based scheduler reaches (almost) the same throughput as in the exhaustive search-based optimal scheduler, with substantially less implementation complexity. Moreover, the power amplifiers' inefficiency and the scheduling delay affect the performance of the scheduling-based systems significantly.

This paper studies the performance of delay-constrained hybrid automatic repeat request (HARQ) protocols. Particularly, we propose a fast HARQ protocol where, to increase the end-to-end throughput, some HARQ feedback signals and successive message decodings are omitted. Considering quasi-static channels and a bursty communication model, we derive closed-form expressions for the message decoding probabilities as well as the throughput, the expected delay, and the error probability of the HARQ setups. The analysis is based on the recent results on the achievable rates of finite-length codes and shows the effect of the codeword length on the system performance. Moreover, we evaluate the effect of various parameters, such as imperfect channel estimation and hardware on the system performance. As demonstrated, the proposed fast HARQ protocol reduces the packet transmission delay considerably compared with the state-of-the-art HARQ schemes. For example, with typical message decoding delay profiles and a maximum of 2,..., 5 transmission rounds, the proposed fast HARQ protocol can improve the expected delay compared with standard HARQ by 27%, 42%, 52% and 60%, respectively, independently of the code rate/fading model.

This paper presents a Rate-based Cell Range Expansion (CRE) for mmWave massive Multiple Input Multiple Output (MIMO) enabled two-tier heterogeneous networks (Het-Nets) where macro base stations (BSs) operate at sub-6 GHz and small BSs operate at mmWave band. We design a new user association scheme with the consideration of disparities between macro BSs and small BSs, chiefly bandwidths combined with Signal to Interference plus Noise Ratio (SINR), although conventional CRE provides a fixed Cell Selection Offset (CSO) for all user equipments (UEs). A downlink model is put forth to analyze the performance of the proposed Rate-based CRE. Our scheme consists of two parts: CRE between small cells and CRE between macro cells and small cells. We also investigate the proposed method by evaluating the average user throughput and cell edge user throughput. System-level computer simulation results such as average user throughput and 5-percentile user throughput are provided. The results confirm that the proposed method can improve the average user throughput compared with the conventional CRE scheme and maintain the cell edge user throughput for such multi-frequency cooperative networks. Thus, this scheme is an effective solution for user association in 5G mmWave HetNets.

We study the sum throughput of multiple-input-multiple-output (MIMO) networks in the cases with large but finite number of transmit and receive data terminals. We develop an efficient scheduling scheme using genetic algorithms (GAs), and evaluate the effect of various parameters, such as channel/precoding models, number of antennas/users, scheduling costs and power amplifiers efficiency, on the system performance. Also, considering continuous and bursty communication scenarios with different users' data request probabilities, we derive closed-form expressions for the maximum achievable throughput of the MIMO networks using optimal schedulers. As we show, our proposed GA-based scheduler reaches (almost) the same throughput as in the exhaustive search-based optimal scheduler, with substantially less implementation complexity. Also, the power amplifiers inefficiency affect the network throughput significantly. For instance, consider a MIMO setup with a 40-antenna base station, 60 users, total consumed power of 26 dB, continuous communications and the typical parameter settings of the power amplifiers. Then, the network throughput decreases by 50% when the power amplifiers efficiency reduces from 75% to 25%.

This paper develops a stochastic geometry-based approach for the modeling and analysis of single- and multi-cluster wireless networks. We first define finite homogeneous Poisson point processes to model the number and locations of the transmitters in a confined region as a single-cluster wireless network. We study the coverage probability for a reference receiver for two strategies; closest-selection, where the receiver is served by the closest transmitter among all transmitters, and uniform-selection, where the serving transmitter is selected randomly with uniform distribution. Second, using Matern cluster processes, we extend our model and analysis to multi-cluster wireless networks. Here, two types of receivers are modeled, namely, closed- and open-access receivers. Closed-access receivers are distributed around the cluster centers of the transmitters according to a symmetric normal distribution and can be served only by the transmitters of their corresponding clusters. Open-access receivers, on the other hand, are placed independently of the transmitters and can be served by all transmitters. In all cases, the link distance distribution and the Laplace transform (LT) of the interference are derived. We also derive closed-form lower bounds on the LT of the interference for single-cluster wireless networks. The impact of different parameters on the performance is also investigated.

The effects of phase noises (PNs), power imbalances, and correlations on multiuser orthogonal frequency division multiplexing (OFDM) multiple‐input multiple‐output (MIMO) systems are studied. It is assumed that each user is equipped with a single antenna, whereas the base station (BS) has multiple antennas and use zero‐forcing (ZF) decoder for multiuser detection. Since each user has an independent oscillator, the received uplink (UL) signal at each BS antenna is corrupted by all of these independent PNs. Furthermore, there may be power imbalances and correlations (due to common scatterers) between different users. These impairments are jointly analyzed in this work. A closed‐form expression of the mean square error (MSE) performance of the multiuser MIMO‐OFDM system is derived. The analytical results are verified by simulations.

Initial access (IA) is identified as a key challenge for the upcoming 5G mobile communication system operating at high carrier frequencies, and several techniques are currently being proposed. In this paper, we extend our previously proposed efficient genetic algorithm‐ (GA‐) based beam refinement scheme to include beamforming at both the transmitter and the receiver and compare the performance with alternative approaches in the millimeter wave multiuser multiple‐input‐multiple‐output (MU‐MIMO) networks. Taking the millimeter wave communications characteristics and various metrics into account, we investigate the effect of different parameters such as the number of transmit antennas/users/per‐user receive antennas, beamforming resolutions, and hardware impairments on the system performance employing different beam refinement algorithms. As shown, our proposed GA‐based approach performs well in delay‐constrained networks with multiantenna users. Compared to the considered state‐of‐the‐art schemes, our method reaches the highest service outage‐constrained end‐to‐end throughput with considerably less implementation complexity. Moreover, taking the users’ mobility into account, our GA‐based approach can remarkably reduce the beam refinement delay at low/moderate speeds when the spatial correlation is taken into account. Finally, we compare the cases of collaborative users and noncollaborative users and evaluate their difference in system performance.

In this paper, we propose a sequential hybrid beamforming design for multi-link transmission over mmwave frequency bands. As a starting point, a baseline data communication link is established via traditional analog beamforming at both the BS and UE. If an extra RF chain is available at the UE, it can continue to probe the propagation environment at the same frequencies. In case the environment is favorable and system resources allow, a secondary data communication link is established to enable multi-stream transmission. In principle, the secondary link could be served by the same BS and/or one or several other BS(s). To initialize the secondary data communication link, a parallel beam search scheme is proposed, which helps the UE/BS to find a suit-able beam pair with given optimization criteria without interrupting the baseline data communication. By applying the proposed two-step approach, hybrid beamforming becomes an add-on feature that can be easily switched on over an analog beamforming enabled system without interrupting its operation whenever system requires. Meanwhile, the information obtained by deploying the proposed parallel beam search scheme can also be used for deciding a back-up beam pair if signal blockage occurs to the baseline data communication link.

In this paper, we perform a coverage analysis of a cellular massive multiple-input multiple-output (MaMIMO) network which adopts 3D beamforming. In contrast to the previous works on 3D beamforming which assume that all users are placed on the ground (i.e., in a 2D environment), we consider a more practical scenario where the users of the network are dropped in a 3D environment with different heights. In this scenario, by adopting a stochastic geometry framework, first we calculate the coverage probability of the network as a function of the users height. Then considering this coverage probability as the objective function, the optimum tilt angle of the base stations antenna pattern is found and the effect of users' heights on this tilt angle is investigated. Our numerical results show that by taking the users' heights into account and using the proposed method, a considerable improvement is achieved in the performance of the network compared to other similar 3D beamforming methods that ignore the users' height distribution.

Initial access (IA) is identified as a key challenge for the upcoming 5G mobile communication system operating at high carrier frequencies, and several techniques are currently being proposed. In this paper, we extend our previously proposed genetic algorithm (GA)-based beam refinement scheme to include beamforming at both the transmitter and the receiver, and compare the performance with alternative approaches in the millimeter wave multi-user multiple-input-multiple-output (MU-MIMO) networks. Taking the millimeter wave communications characteristics and various metrics into account, we investigate the effect of different parameters such as the number of transmit antennas/users/per-user receive antennas, beamforming resolution as well as hardware impairments on the system performance employing different beam refinement algorithms. As shown, our proposed GA-based approach performs well in delay-constrained networks with multi-antenna users. Compared to the considered state-of-the-art schemes, our method reaches the highest service outage-constrained end-to-end throughput with considerably less implementation complexity. Moreover, taking the users' mobility into account, GA-based approach can remarkably reduce the beam refinement delay at low/moderate speeds when the spatial correlation is taken into account.

We study the performance of multi-hop networks composed of millimeter wave (MMW)-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in the cases with and without hybrid automatic repeat request (HARQ). Taking the MMW characteristics of the RF links into account, we derive closed-form expressions for the network outage probability. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we present mappings between the performance of RF- FSO multi-hop networks and the ones using only the RF- or the FSO-based communication, in the sense that with appropriate parameter settings the same outage probability is achieved in these setups. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, the HARQ can effectively improve the outage probability#x002F;energy efficiency, and compensate the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual- hop networks, outage probability 10^{-4} and code rate 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to the cases with no HARQ, by 13 and 17 dB, respectively.

Cellular networks operating at millimeter wave (mmWave) frequencies are able to achieve multi-gigabit-per-second data rates due to the large bandwidth available. However, the data transmission range will be shorter and significant signal power difference will be observed between line-of-sight (LOS) and non-line-of-sight (NLOS) links. This paper considers interference management and useful signal enhancement in the uplink transmission of small-cell mmWave networks. Taking blockages into account, we analyze the coverage performance of the partial-zero-forcing (PZF) receiver which utilizes a number of antennas to cancel out the strongest uplink interferers and uses the rest of the antennas for boosting the useful signal. Using stochastic geometry, we derive analytical expressions for the coverage probability of the PZF receiver under a LOS probability function based path loss model. For a broad range of parameter settings, the maximum coverage probability is achieved by using most antennas for array gain and only canceling a few strongest interferers. Particularly, compared to zero-forcing, the PZF scheme can improve the coverage probability significantly.

In this paper, we study the performance of initial access beamforming schemes in the cases with large but finite number of transmit antennas and users. Particularly, we develop an efficient beamforming scheme using genetic algorithms. Moreover, taking the millimeter wave communication characteristics and different metrics into account, we investigate the effect of various parameters such as number of antennas/receivers, beamforming resolution as well as hardware impairments on the system performance. As shown, our proposed algorithm is generic in the sense that it can be effectively applied with different channel models, metrics and beamforming methods. Also, our results indicate that the proposed scheme can reach (almost) the same end-to-end throughput as the exhaustive search-based optimal approach with considerably less implementation complexity.

This paper introduces a new metric, referred to as delay-sensitive area spectral efficiency (DASE), to analyze the performance of delay-constrained wireless networks. We study DASE in different point-to-point, spectrum sharing, interference, and Poisson-point process-based dense network configurations and with different levels of channel state information (CSI) at the transmitters. Also, we determine the optimal rates/powers optimizing DASE. Finally, we use some recent results on finite block-length codes to analyze the effect of the codewords length on the network DASE. As demonstrated, DASE is a useful metric for analyzing delay-sensitive green networks. Moreover, adaptive power allocation and partial CSI feedback are powerful tools for performance improvement in green networks.

This paper studies the performance of multi-hop and mesh networks composed of millimeter wave-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in cases with and without hybrid automatic repeat request (HARQ). Using the central limit theorem as well as other state-of-the-art approximation schemes, we derive closed-form expressions for the networks' outage probability and ergodic achievable rates. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we determine the minimum number of the transmit antennas in the RF link such that the same rate is supported in the RF- and the FSO-based hops. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, HARQ can effectively improve the outage probability/energy efficiency, and compensate for the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability of 10-4and code rate of 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to cases with open-loop communication, by 13 and 17 dB, respectively.

Denser deployments of heterogeneous networks (HetNets) lead to more frequent handovers, which results in a decline of user experience as well as heavy signaling overheads to the network. Therefore, the study on handover is of great importance especially in dense HetNets. In this paper, we focus on the analysis of cross-tier handover from the macro cell tier to the small cell tier, which shows the highest handover failure rate in current standards and industry studies. Using the stochastic geometry, we propose an analytical model for the cross-tier handover processes in the HetNet. Based on the derived analytical model, the closed-form expressions for the key handover performance metrics, including the handover rate, handover failure rate, and ping-pong rate, are deduced as functions of the base station density, time to trigger and user mobility. Simulation results verify the accuracy of the proposed analytical model and closed-form theoretical analyses, which provide guidance for the deployments of HetNets and corresponding handover strategies.

The effects of oscillator phase noises (PNs) on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems are studied. It is shown that PNs of common oscillators at the transmitter and at the receiver have the same influence on the performance of (single-stream) beamforming MIMO-OFDM systems, yet different influences on spatial multiplexing MIMO-OFDM systems with singular value decomposition (SVD) based precoding/decoding. When each antenna is equipped with an independent oscillator, the PNs at the transmitter and at the receiver have different influences on beamforming MIMO-OFDM systems as well as spatial multiplexing MIMO-OFDM systems. Specifically, the PN effect on the transmitter (receiver) can be alleviated by having more transmit (receive) antennas for the case of independent oscillators. It is found that the independent oscillator case outperforms the common oscillator case in terms of error vector magnitude (EVM).

Wireless caching using device-to-device (D2D) communications is a promising approach for reducing delivery delay and improving spectrum efficiency and energy efficiency. In addition to the physical link condition, social behaviors are also of great importance for effectiveness enhancement of the D2D-based wireless caching schemes. In this work, we focus on resource allocation including power and spectrum to improve the system efficiency in terms of jointly considered spectrum efficiency and energy efficiency, by leveraging both physical and social characteristics. Furthermore, a heuristic-based mixed integer nonlinear programming algorithm is proposed to solve the joint optimization problem, including the establishment of D2D links, spectrum allocation and power optimization. Numerical results demonstrate the effectiveness of the proposed scheme in terms of different pairing algorithms.

This paper investigates the outage probability and the throughput of relay networks with joint information and energy transfer where the relay harvests energy from transmitted radio-frequency (RF) signal of the source. Considering different power consumption models, we derive the outage probability of the systems for both adaptive and non-adaptive power allocations at the relay. With a total energy consumption constraint at the source, we provide closed-form expressions for the optimal time sharing and power allocation between the source energy and information transfer signals. The analytical and simulation results demonstrate the efficiency of joint energy and information transfer systems in different conditions.

This paper studies the performance of hybrid radio-frequency (RF) and free-space optical (FSO) links in the cases with and without hybrid automatic repeat request (HARQ). Considering millimeter wave (mmwave) characteristics in the RF link and pointing errors in the FSO link, we derive closed-form expressions for the message decoding probabilities as well as the throughput and the outage probability of the RF-FSO setups. We also evaluate the effect of various parameters such as power amplifiers efficiency, different transmission techniques in the FSO link, pointing errors in the FSO link as well as different coherence times/symbol rates of the RF and the FSO links on the throughput and outage probability. The results show the efficiency of the RF-FSO links in different conditions. Moreover, the HARQ can effectively improve the outage probability/energy efficiency, and compensate the effect of hardware impairments in RF-FSO links.

This paper provides an overview and preliminary comparison of several multi-carrier and single-carrier waveforms that are potential candidates for future 5G mobile radio communications above 6 GHz. The waveforms are assessed primarily based on the established and known results as well as recent findings keeping in view the design requirements that are relevant to using frequencies above 6 GHz, especially the millimeter wave frequencies. The Key Performance Indicators and degrees of freedom in the design of different waveforms and their potential applications for mm-wave communications are discussed. Certain features that are particularly interesting for mm-wave communication and require further investigations are also highlighted. Furthermore, a common framework for synthesizing different waveform candidates has been developed. Finally, a preliminary qualitative comparison of different multi-carrier and single carrier waveforms has been derived.

In this paper, we develop a green joint power and information transfer system. To guarantee reliable communication, we implement hybrid automatic repeat request (HARQ) feedback both in the power and information transmission phases. Moreover, with a limit on the packet transmission delay, we derive the optimal power allocation minimizing the energy-constrained outage probability. The simulation and analytical results indicate that the diversity gain of the joint power and information transfer system is the same as the diversity gain of the conventional non-green information transfer systems. Also, the implementation of HARQ improves the power efficiency significantly. For instance, consider a Rayleigh fading channel, outage probability 10-3, code rate 1 nats-per-channel-use and the case where the receiver requires to receive 0.1 Joule-per-channel-use energy per codeword length from the transmitter to process the information. Then, with a maximum of two retransmissions in the power and information transfer phases, the HARQ reduces the average required energy by 4 dB, compared to the open-loop communication setup.

mmMAGIC (Millimetre-Wave Based Mobile Radio Access Network for Fifth Generation Integrated Communications) is an EU funded 5G-PPP project, whose overall objective is to design and pre-develop a mobile radio access technology (RAT) operating in the 6–100 GHz range, capable of impacting standards and other relevant fora. The focus of the project is on extreme Mobile Broadband, which is expected to drive the 5G requirements for massive increase in capacity and data-rates. This paper elaborates on some 5G key research areas such as: identification of the most compelling use-cases and Key Performance Indicators (KPIs) for future 5G systems, advantages and challenges of millimeter-wave (mmWave) technologies, channel measurements and channel modeling, network architecture; and the design of a new mobile radio interface including multi-node and multi-antenna transceiver architecture.

Recently, substantial attention has been paid to analyse the performance of wireless networks in the cases where the positions of the base stations are modelled by stochastic geometry. In this study, the authors analyse the performance of Rayleigh‐fading Poisson‐point‐process (PPP)‐based networks. They derive the optimal transmission rate that maximises the per‐user throughput. Also, they determine sufficient conditions for positive diversity gain in PPP‐based networks, develop tight approximations for the effective density of the network and give a simplified expression to calculate the outage probability for the interference‐limited case. The analytical and the numerical results demonstrate considerable potential for efficient data transmission in PPP‐based networks.

This paper investigates the performance of the point-to-point multiple-input-multiple-output (MIMO) systems in the presence of a large but finite numbers of antennas at the transmitters and/or receivers. Considering the cases with and without hybrid automatic repeat request (HARQ) feedback, we determine the minimum numbers of the transmit/receive antennas, which are required to satisfy different outage probability constraints. Our results are obtained for different fading conditions and the effect of the power amplifiers efficiency/feedback error probability on the performance of the MIMO-HARQ systems is analyzed. Then, we use some recent results on the achievable rates of finite block-length codes, to analyze the effect of the codewords lengths on the system performance. Moreover, we derive closed-form expressions for the asymptotic performance of the MIMO-HARQ systems when the number of antennas increases. Our analytical and numerical results show that different outage requirements can be satisfied with relatively few transmit/receive antennas.

In this paper, we propose and analyze a wireless energy and information transfer system. To reduce the outage probability, compared with open-loop communication, we implement retransmission protocols both in the energy and in the information transmission phases. We use some recent results on finite block-length codes to analyze the effect of the energy and information signals lengths on the system outage probability/throughput. Finally, under a packet transmission delay constraint, we derive the optimal power allocation and time sharing between the energy and information signals, such that the energy-constrained outage probability is minimized. The simulation and analytical results demonstrate that the retransmission-based protocols are efficient techniques to reduce the energy-limited outage probability of wireless energy and information transmission systems.

A centralized coordinated multipoint downlink joint transmission in a frequency division duplex system requires channel state information (CSI) to be fed back from the cell-edge users to their serving BS, and aggregated at the central coordination node for precoding, so that interference can be mitigated. The control signals comprising of CSI and the precoding weights can easily overwhelm the backhaul resources. Relative thresholding has been proposed to alleviate the burden; however, this is at the cost of reduction in throughput. In this paper, we propose utilizing the long-term channel statistics comprising of pathloss and shadow fading in the precoder design to model the statistical interference for the unknown CSI. In this regard, a successive second-order cone programming (SSOCP)-based precoder for maximizing the weighted sum rate is proposed. The accuracy of the solution obtained is bounded with the branch and bound technique. An alternative optimization framework via weighted mean square error minimization is also derived. Both these approaches provide an efficient solution close to the optimal, and also achieve efficient backhauling, in a sense that the precoding weights are generated only for the active links. For comparison, a stochastic approach based on particle swarm optimization is also considered.

This paper studies the performance of hybrid radio-frequency (RF) and free-space optical (FSO) links assuming perfect channel state information (CSI) at the receiver. Considering the cases with and without hybrid automatic repeat request (HARQ), we derive closed-form expressions for the message decoding probabilities as well as the throughput and the outage probability of the RF-FSO setups. We also evaluate the effect of adaptive power allocation and different channel conditions on the throughput and the outage probability. The results show the efficiency of the RF-FSO links in different conditions.

Spatial wireless channel prediction is important for future wireless networks, and in particular, for proactive resource allocation at different layers of the protocol stack. Various sources of uncertainty must be accounted for during modeling and to provide robust predictions. We investigate two channel prediction frameworks, classical Gaussian processes (cGP), and uncertain Gaussian processes (uGP), and analyze the impact of location uncertainty during learning/training and prediction/testing, for scenarios where measurements uncertainty are dominated by large-scale fading. We observe that cGP generally fails both in terms of learning the channel parameters and in predicting the channel in the presence of location uncertainties. In contrast, uGP explicitly considers the location uncertainty. Using simulated data, we show that uGP is able to learn and predict the wireless channel.

This paper studies the performance of the hybrid radio-frequency (RF) and free-space optical (FSO) links assuming perfect channel state information (CSI) at the receiver. Considering the cases with and without hybrid automatic repeat request (HARQ), we derive closed-form expressions for the message decoding probabilities as well as the throughput and the outage probability of the RF- FSO setups. We also evaluate the effect of different channel conditions on the throughput and the outage probability. The results show the efficiency of the RF-FSO links in different conditions.

In this paper, we make theoretical analysis on cross-tier handover in Small Cell Networks (SCNs). The cross-tier handover is defined as the handover occurred from a macro cell tier to a small cell tier, or vice versa. Based on stochastic geometry, we first propose a linear approximation approach to study the coverage of small cells. Using the proposed method, we then derive analytical expressions for cross-tier handover rate and sojourn time inside a small cell, which are the key parameters in mobility performance evaluation of an SCN. According to the analysis, we show that the small cell coverage area can be well represented by a biased circle, and the expected value of small cell size is inversely proportional to the square root of the BS density in the macro cell tier. Another important observation is that the handover rate and sojourn time change linearly with the reciprocal of small cell radius. These analytical results could provide fundamental supports for improving mobility management in SCNs.

In this paper, we consider the uplink of a single-cell massive multiple-input multiple-output (MIMO) system with inphase and quadrature-phase imbalance (IQI). This scenario is of particular importance in massive MIMO systems, where the deployment of lower-cost, lower-quality components is desirable to make massive MIMO a viable technology. Particularly, we investigate the effect of IQI on the performance of massive MIMO employing maximum-ratio combining (MRC) receivers. In order to study how IQI affects channel estimation, we derive a new channel estimator for the IQI-impaired model and show that IQI can substantially downgrade the performance of MRC receivers. Moreover, a low-complexity IQI compensation scheme, suitable for massive MIMO, is proposed which is based on the IQI coefficients' estimation and it is independent of the channel gain. The performance of the proposed compensation scheme is analytically evaluated by deriving a tractable approximation of the ergodic achievable rate and providing the asymptotic power scaling laws assuming transmission over Rayleigh fading channels with log-normal large-scale fading. Finally, we show that massive MIMO effectively suppresses the residual IQI effects, as long as, the compensation scheme is applied.

This paper considers the downlink of a heterogeneous network, where multiple base stations (BSs) can serve the users by non-coherent multiflow beamforming. We assume imperfect channel state information at both BSs and users. The objective is to jointly optimize the precoding, load balancing, and BS operation mode (active or sleep) for improving the energy efficiency of the network. The considered problem is to minimize the weighted total power consumption (both circuit power and dynamic transmit power), while satisfying per-user quality of service constraints and per-BS transmit power constraints. This problem is non-convex, but we prove that for each combination of BS modes, the considered problem has a hidden convexity structure. Thus, the global optimal solution is obtained by an exhaustive search over all possible BS mode combinations. Furthermore, by iterative convex approximations of the non-convex power consumption functions, a heuristic algorithm is proposed to obtain a local optimal solution with low complexity. Simulation results illustrate that our proposed algorithms significantly reduce the total power consumption, compared to the scheme where all BSs are continuously active. This implies that putting a BS into sleep mode by proper load balancing is an important solution for energy savings in heterogeneous networks.

This paper studies the sum throughput maximization of the return-link in multi-beam satellite systems. Considering bursty communication scenarios with different users' data request probabilities, we develop an efficient scheduling scheme using genetic algorithms (GAs). Moreover, we consider co-channel interference (CCI) and adjacent channel interference (ACI). We consider a receiver with and without interference cancelation. Using a simplified channel model, we evaluate the proposed scheduler in a multi-beam system. The proposed GA-based scheduler approaches the throughput of an optimal scheduler based on exhaustive search with substantially less implementation complexity.

This paper studies the throughput of spectrum sharing networks utilizing rate adaptation. We use some recent results on the achievable rates of finite block-length codes to analyze the secondary user (SU) throughput with a constraint on the primary user (PU) codeword drop probability. With codewords of finite length, we derive closed-form expressions for the SU activation probability and throughput. The results are obtained in different scenarios with no channel state information at the SU transmitter. As demonstrated numerically and analytically, using finite-length codewords there is considerable potential for the data transmission of unlicensed secondary users under different quality-of-service requirements of the licensed primary users.

Hybrid automatic repeat request (HARQ) plays an important role in improving the transmission efficiency and the robustness of wireless networks. Considering K-tier heterogeneous networks (HetNets) and modelling the locations of the base stations (BSs) as a homogeneous Poisson point process (PPP), this paper investigates the performance of HetNets implementing HARQ. We give closed- form expressions for the quality of service (QoS) coverage probability which is defined in terms of whether the received signal quality is above a predetermined threshold, and the per-user throughput with HARQ. We show that using HARQ can indeed improve the QoS coverage probability. However, depending on the channel conditions, the per-user throughput of the HetNets may decrease by the implementation of HARQ. Furthermore, we show that the small cell density has negligible effect on the QoS coverage probability and the per-user throughput, and the per-user throughput may increase with the small cell path loss.

This paper studies the problem of feedback resource allocation in multiple-input-single-output (MISO) channels utilizing partial channel state information (CSI) feedback. Considering low/moderate signal-to-noise ratios (SNRs), the optimal quantizers and the feedback bit allocation maximizing the throughput are obtained in the asymptotic case where the number of feedback bits increases. Moreover, the results are utilized to derive the optimal retransmission rates in the automatic repeat request (ARQ) protocols and joint CSI-ARQ schemes are proposed for the MISO setups. We show that uniform channel amplitude quantization is asymptotically optimal in terms of throughput. Also, the optimal retransmission rates of the incremental redundancy (INR) ARQ protocols follow an arithmetic progression in the exponential domain. Under certain conditions, a MISO system using quantized CSI can be mapped to a MISO or a SISO (S: single) setup using ARQ or joint CSI-ARQ feedback in the sense that they lead to the same throughput. Finally, to maximize the throughput, the optimal number of channel direction quantization bits should be (M-1) times the number of amplitude quantization bits, where M is the number of transmit antennas.

This paper studies the throughput of spectrum sharing networks utilizing rate adaptation. We use some recent results on the achievable rates of finite block-length codes to analyze the secondary user (SU) throughput with a constraint on the primary user (PU) codeword drop probability. With codewords of finite length, we derive closed-form expressions for the SU activation probability and throughput. The results are obtained in different scenarios with perfect or no channel state information at the SU transmitter. As demonstrated numerically and analytically, using finite-length codewords and rate adaptation, there is considerable potential for the data transmission of unlicensed secondary users under different quality-of-service requirements of the licensed primary users.

This paper considers a downlink heterogeneous network, where different types of multiantenna base stations (BSs) communicate with a number of single-antenna users. Multiple BSs can serve the users by spatial multiflow transmission techniques. Assuming imperfect channel state information at both BSs and users, the precoding, load balancing, and BS operation mode are jointly optimized for improving the network energy efficiency. We minimize the weighted total power consumption while satisfying quality-of-service constraints at the users. This problem is nonconvex, but we prove that for each BS mode combination, the considered problem has a hidden convexity structure. Thus, the optimal solution is obtained by an exhaustive search over all possible BS mode combinations. Furthermore, by iterative convex approximations of the nonconvex objective function, a heuristic algorithm is proposed to obtain a suboptimal solution of low complexity. We show that although multicell joint transmission is allowed, in most cases, it is optimal for each user to be served by a single BS. The optimal BS association condition is parameterized, which reveals how it is impacted by different system parameters. Simulation results indicate that putting a BS into sleep mode by proper load balancing is an important solution for energy savings.

This paper studies the energy-limited performance of multi-relay networks. Taking the properties of the power amplifiers (PAs) into account, we derive closed-form expressions for the optimal power allocation, the outage probability and the throughput in the cases with a sum consumed energy constraint. Moreover, we analyze the diversity and the multiplexing gains of the PA-aware systems. Finally, we investigate the performance of large-scale multi-relay networks and develop efficient multi-relay systems with low cooperation overhead. The numerical and the analytical results show that the inefficiency of the PAs affects the outage probability and the throughput of the multi-relay systems substantially.

This paper studies the impact of in-phase and quadrature-phase imbalance (IQI) in two-way amplify-and-forward (AF) relaying systems. In particular, the effective signal-to-interference-plus-noise ratio (SINR) is derived for each source node, considering four different linear detection schemes, namely, uncompensated (Uncomp) scheme, maximal-ratio-combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) based schemes. For each proposed scheme, the outage probability (OP) is investigated over independent, non-identically distributed Nakagami-m fading channels, and exact closed-form expressions are derived for the first three schemes. Based on the closed-form OP expressions, an adaptive detection mode switching scheme is designed for minimizing the OP of both sources. An important observation is that, regardless of the channel conditions and transmit powers, the ZF-based scheme should always be selected if the target SINR is larger than 3 (4.77dB), while the MRC-based scheme should be avoided if the target SINR is larger than 0.38 (-4.20dB).

This paper studies the effect of optimal power allocation on the performance of communication systems utilizing automatic repeat request (ARQ). Considering Type-I ARQ, the problem is cast as the minimization of the outage probability subject to an average power constraint. The analysis is based on some recent results on the achievable rates of finite-length codes and we investigate the effect of codewords length on the performance of ARQ-based systems. We show that the performance of ARQ protocols is (almost) insensitive to the length of the codewords, for codewords of length ≥ 50 channel uses. Also, optimal power allocation improves the power efficiency of the ARQ-based systems substantially. For instance, consider a Rayleigh fading channel, codewords of rate 1 nats-per-channel-use and outage probability 10-3. Then, with a maximum of 2 and 3 transmissions, the implementation of power-adaptive ARQ reduces the average power, compared to the open-loop communication setup, by 17 and 23 dB, respectively, a result which is (almost) independent of the codewords length. Also, optimal power allocation increases the diversity gain of the ARQ protocols considerably.

We investigate the performance of dual-hop two-way amplify-and-forward (AF) relaying in the presence of inphase and quadrature-phase imbalance (IQI) at the relay node. In particular, the effective signal-to-interference-plus-noise ratio (SINR) at both sources is derived. These SINRs are used to design an instantaneous power allocation scheme, which maximizes the minimum SINR of the two sources under a total transmit power constraint. The solution to this optimization problem is analytically determined and used to evaluate the outage probability (OP) of the considered two-way AF relaying system. Both analytical and numerical results show that IQI can create fundamental performance limits on two-way relaying, which cannot be avoided by simply improving the channel conditions.

Recently, substantial attention has been paid to improve the spectral efficiency of communication setups using different spectrum sharing techniques. This paper studies the throughput of spectrum sharing channels utilizing hybrid automatic repeat request (HARQ) protocols. Considering different HARQ schemes, the unlicensed user throughput is obtained under an outage probability constraint for the licensed user. The outage-limited throughput is obtained for both independent and spatially-correlated fading conditions, where there is spatial dependency between the fading coefficients. The results show that, using HARQ and adaptive power allocation, the maximum throughput is achieved by combination of simultaneous transmission and interference-avoiding spectrum sharing paradigms. The performance of the spectrum sharing networks is not sensitive to spatial correlation, within the practical range, and the throughput changes are negligible at low/moderate correlations. Finally, there is considerable potential for data transmission of the unlicensed user with limited performance degradation of the licensed user.

We analyze the performance of amplify-and-forward dual-hop relaying systems in the presence of in-phase and quadrature-phase imbalance (IQI) at the relay node. In particular, an exact analytical expression for and tight lower bounds on the outage probability are derived over independent, non-identically distributed Nakagami-m fading channels. Moreover, tractable upper and lower bounds on the ergodic capacity are presented at arbitrary signal-to-noise ratios (SNRs). Some special cases of practical interest (e.g., Rayleigh and Nakagami-0.5 fading) are also studied. An asymptotic analysis is performed in the high SNR regime, where we observe that IQI results in a ceiling effect on the signal-to-interference-plus-noise ratio (SINR), which depends only on the level of I/Q impairments, i.e., the joint image rejection ratio. Finally, the optimal I/Q amplitude and phase mismatch parameters are provided for maximizing the SINR ceiling, thus improving the system performance. An interesting observation is that, under a fixed total phase mismatch constraint, it is optimal to have the same level of transmitter (TX) and receiver (RX) phase mismatch at the relay node, while the optimal values for the TX and RX amplitude mismatch should be inversely proportional to each other.

We analyze the performance of dual-hop two-way amplify-and-forward relaying in the presence of in-phase and quadrature-phase imbalance (IQI) at the relay node. In particular, two power allocation schemes, namely, fixed power allocation and instantaneous power allocation, are proposed to improve the system reliability and robustness against IQI under a total transmit power constraint. For each proposed scheme, the outage probability is investigated over independent, non-identically distributed Nakagami- m fading channels, and exact closed-form expressions and bounds are derived. Our theoretical analysis indicates that, without IQI compensation, IQI can create fundamental performance limits on two-way relaying. However, these limits can be avoided by performing IQI compensation at source nodes. Compared with the equal power allocation scheme, our numerical results show that the two proposed power allocation schemes can significantly improve the outage performance, thus reducing the IQI effects, particularly when the total power budget is large.

We consider downlink transmission in a cellular network consisting of multi-antenna base stations (BSs), single-antenna mobile users, directional antennas each with a vertically adjustable beam, and control signaling delays in feedback and backhaul links. We propose a novel transmission technique in which intercell interference management is performed via coordinating the beamforming jointly in the horizontal and vertical planes of the wireless channel, denoted as coordinated 3D beamforming. In the horizontal plane, we focus on intercell interference cancelation (ICIC) and investigate its performance when the provided channel state information (CSI) is impaired due to delay/mobility. It is demonstrated that the superiority of ICIC over conventional intracell maximum ratio transmission is highly dependent on the CSI accuracy. In the vertical plane, we consider intercell interference control via coordinatively adapting the elevation angle of the BS antenna pattern, denoted as tilt, to the locations of the scheduled users. It is shown that with perfect CSI interference management should be performed in the horizontal plane using ICIC, while at high delay/mobility it should be done in the vertical plane via coordinated tilt adaptation. For intermediate values of delay/mobility, joint interference management in both planes is required.

We study the effect of unreliable backhaul links on the performance of Coordinated Multi-Point (CoMP) techniques. CoMP has emerged as a powerful scheme to mitigate co-channel interference. Economically viable deployment of Heterogeneous Networks (HetNets) will require the use of lower-performance backhaul options, e.g. non-line-of-sight (NLOS) microwave links. Motivated by HetNets, a backhauling model is introduced, by assigning Link Failure Probability (LFP) to backhaul links, for the cooperative clusters. In this paper we analyze the centralized and semi-distributed CoMP architectures. We investigate the probability of deficient backhaul links reducing quality of service, by impeding transmission. By evaluating the average sum rate of users within a CoMP cluster, we show how backhaul link reliability affects the performance of the cooperative cluster. We conclude, that the performance gains offered by CoMP quickly diminish, as the unreliability of the backhaul links grows.

Coordinated multipoint (CoMP) transmission and reception techniques have been proposed to combat inter-cell interference in cellular systems and, hence, to increase the data rates. Due to the overhead introduced, not all users may be served with CoMP. In this paper, we focus on how to select users for joint processing CoMP under limited backhaul capacity. The evaluated user selection schemes take user experience into account, quantified by the utility of Internet application types. We propose a heuristic algorithm utility based user selection that has low computational complexity. Utility based user selection and maximize utility, which maximizes the sum utilities, are compared to maximize rate, which maximizes the sum data rates. A range of traffic mixes and user locations are evaluated. The simulation results indicate that resources required for CoMP are more efficiently used if the user selection is based on utility rather than on maximizing the total data rate. If utility is taken into account a higher total utility can in many cases be achieved for the same limited backhaul capacity.

In future wireless networks a significant number of wireless broadband users will be vehicular, i.e., they will be in public transportation vehicles like buses, trams or trains. In this paper, we show that the efficient use of relay nodes to serve vehicular users can greatly improve the energy efficiency of the network while maintaining the required quality-of-service (QoS). We consider vehicular users moving along a road within the coverage of a base station (BS). Communication can take place in a single-hop fashion (baseline case) or can be assisted by a single relay node (dual-hop), which can either be a fixed relay node (FRN) deployed at a specific position on the road or a moving relay node (MRN) mounted on top of the vehicle. We compare the required overall transmit energy for direct transmission, FRN and MRN assisted transmission in a noise limited system under Rayleigh fading while assuming an outage probability (OP) target. A lower bound is derived for the required energy of the FRN assisted transmission. It is shown that as the vehicular penetration loss (VPL) increases, both FRN and MRN assisted transmission can significantly lower the overall transmit energy compared to the conventional one-hop case. Moreover, transmission relying on an MRN outperforms the FRN assisted case when VPL is moderate to high.

Coordinated multi-point (CoMP) transmission has received a lot of attention, as a way to improve the system throughput in an interference limited cellular system. For joint processing in CoMP, the user equipments (UEs) need to feed back the channel state information (CSI), typically to their serving base stations (BSs). The BS forwards the CSI to a central coordination node (CCN) for precoding. These precoding weights need to be forwarded from the CCN to the corresponding BSs to serve the UEs. In this work, a feedback load reduction technique is employed via partial joint processing to alleviate the CSI feedback overhead. Similarly, to achieve backhaul load reduction due to the precoding weights, scheduling approaches are proposed. The state of the art block diagonalization solution is compared with our proposed constrained and unconstrained scheduling. Our main contribution is the method of choosing the best subset of the BSs and UEs at the CCN that yields the best sum rate under the constraint of efficient backhaul use. In particular, with constrained scheduling, the choice of a smaller subset proportionally reduces the backhaul load. Simulation results based on a frequency selective WINNER II channel model, show that our proposed constrained scheduling outperforms the block diagonalization approach in terms of the average sum rate per backhaul use.

We study the power allocation problem for the downlink of a cooperative system with different node activeness, i.e., each receiving node requests for data transmission according to a certain probability. Data symbols of the active nodes are jointly transmitted from cooperative transmission points using zero-forcing precoding. The problem is cast in form of maximizing the ergodic achievable rate subject to per-transmit-point average total power constraints. The derived power allocation indicates that depending on the channel conditions and receiving nodes' activation probability, the optimal solution takes the form of either greedy power allocation or power sharing allocation.

In this paper, we address the classical problem of maximum-likelihood (ML) detection of data in the presence of random phase noise. We consider a system, where the random phase noise affecting the received signal is first compensated by a tracker/estimator. Then the phase error and its statistics are used for deriving the ML detector. Specifically, we derive an ML detector based on a Gaussian assumption for the phase error probability density function (PDF). Further without making any assumptions on the phase error PDF, we show that the actual ML detector can be reformulated as a weighted sum of central moments of the phase error PDF. We present a simple approximation of this new ML rule assuming that the phase error distribution is unknown. The ML detectors derived are also the aposteriori probabilities of the transmitted symbols, and are referred to as soft metrics. Then, using the detector developed based on Gaussian phase error assumption, we derive the symbol error probability (SEP) performance and error floor analytically for arbitrary constellations. Finally we compare SEP performance of the various detectors/metrics in this work and those from literature for different signal constellations, phase noise scenarios and SNR values.

Coordinated Multi-Point transmission is a promising technique to improve the performance of the users at the cell-edge. To achieve this, in case of a centralized approach, users need to unicast the quantized channel state information (CSI), typically to the anchor base station (BS), and then each BS forwards this information to a central coordination node for precoding and scheduling. In the case of a decentralized approach, users broadcast the quantized CSI such that the coordinating BSs could simultaneously receive the CSI. The advantage of a decentralized approach is that it does not require a central coordination node, thereby not imposing stringent latency constraints on the backhaul. The CSI transmission over the erroneous feedback channel in the uplink gives rise to precoding loss and scheduling loss. In the decentralized framework, the feedback errors could result in BSs receiving a different version of the CSI. In this work, we propose a decentralized opportunistic scheduling approach, which only requires a minimal sharing of scheduling information between BSs. The results show that the sum rate achieved with the proposed method is comparable to that of the centralized approach even when there is a high bit error probability introduced by the feedback channel. We also show that when the bit error probabilities in the feedback channel are less than 10-4, the decentralized approach achieves the sum rate of the centralized approach.

Coordinated multi-point (CoMP) transmission is considered as an efficient technique to improve cell-edge performance as well as system spectrum efficiency. In CoMP-enabled systems, a cluster of coordinated base stations (BSs) are typically assumed to be connected to a control unit (CU) via backhaul links, and the provided performance gain relies heavily on the quality of the channel state information (CSI) available at the CU side. In this paper, we consider the downlink of a CoMP cluster and compare three different CoMP transmission schemes: zero-forcing coherent joint transmission, non-coherent joint transmission and coordinated scheduling. Moreover, for each of the analyzed schemes, the performance in terms of average sum rate of the CoMP cluster is studied with predicted CSI, considering the effects of the feedback and backhaul latency, as well as the user mobility. Compared to zero-forcing coherent joint transmission, we show that non-coherent joint transmission and coordinated scheduling are more robust to channel uncertainty. In addition, depending on the latency, user mobility and user locations, different schemes would achieve the highest average sum rate performance. Hence, a system could switch between the transmission schemes to improve the sum rate.

Time varying phase noise is a limiting factor in highspeed wireless communication systems, e.g., microwave backhaul links in cellular networks. This paper seeks to investigate the performance of circular M-ary quadrature amplitude modulations (M-QAMs) in the presence of time varying phase noise. A new approximate union bound expression for the symbol-error probability (SEP) of a specific circular M-QAM constellation is derived. Numerical results show that this expression is accurate at medium-to-high signal-to-noise ratios (SNRs) for different constellation orders, M. Next, exact closed-form expressions for the SEP of binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) modulations in the presence of phase noise are derived. Extensive simulations are carried out to compare the performance of rectangular and circular QAM modulations in the presence of time varying phase noise, where it is demonstrated that circular QAM modulations can outperform their rectangular counterparts when considering the effect of time varying phase noise.

In this paper, the issues upon user scheduling in the downlink packet transmission for multiple services are addressed for coordinated multi-point (CoMP) OFDMA networks. We consider mixed traffic with voice over IP (VOIP) and best effort (BE) services. In order to improve cell-edge performance and guarantee diverse quality of service (QoS), a utility-based joint scheduling algorithm is proposed, which consists of two steps: ant colony optimization (ACO) based joint user selection and greedy subchannel assignment. We compare the proposed algorithm with the optimal algorithm and the greedy user selection (GUS) based scheme. Via simulation results, we show that the performance of our proposed algorithm well approaches to that of the optimal solution. It is also observed that 95% of BE users are satisfied with average cell-edge data rate greater than 200kbps by using either of the two algorithms. Whereas, our proposed algorithm ensures that more than 95% of VoIP users are satisfied with packet drop ratio less than 2%, compared to 78% by the GUS based algorithm.

Joint processing between base stations is a promising technique to improve the quality of service to users at the cell edge, but this technique poses tremendous requirements on the backhaul signaling capabilities, such as the distribution of channel state information and the precoding weights to the base stations involved in joint processing. Partial joint processing is a technique aimed to reduce feedback load, in one approach the users feed back the channel state information of the best links based on a channel gain threshold mechanism. However, it has been shown in the literature that the reduction in the feedback load is not reflected in an equivalent backhaul reduction, unless additional scheduling or precoding techniques are applied. The reason is that reduced feedback from users yields sparse channel state information at the Central Coordination Node. Under these conditions, existing linear precoding techniques fail to remove the interference and reduce backhaul, simultaneously, unless constraints are imposed on scheduling. In this paper, a partial joint processing scheme with efficient backhauling is proposed, based on a stochastic optimization algorithm called particle swarm optimization. The use of particle swarm optimization in the design of the precoder promises efficient backhauling with improved sum rate.

This paper addresses the problem of limited CSI feedback in coordinated multi-point (CoMP) networks. Specifically, the system throughput is obtained for block-fading CoMP channels with quantized CSI feedback, and the effect of feedback bit allocation on the system throughput is investigated for different user locations and fading distributions. The analytical and simulation results show that substantial throughput increment is achieved via CoMP joint transmission with very limited number of feedback bits per base station. The effect of optimal bit allocation becomes more important for the user that is located in the CoMP cluster edge areas. Also, the standard Zonal-sampling scheme provides the best bit allocation strategy in many cases, maximizing the system throughput.

The potential of relay selection (RS) has been well appreciated. However if the control channels, conveying information on the selected relay node (RN), are unreliable, the gains of RS diminish. This is because the transmitting RN might not be the one maximizing the employed utility function. Furthermore, more than one of the RNs might end up transmitting, an event that could be performance degrading. It is also likely that no RN transmits, which is very undesirable. In this paper we assume decode-and-forward and present closed form expressions for the outage probability (OP) under Rayleigh fading when two potential RNs are available and the control information related to RS is corrupted by errors. We show that when control channels are unreliable, the OP performance degrades significantly and reaches a floor as the signal-to- noise ratio of the RN-to-destination data channel grows. Furthermore when control channels are very unreliable, suboptimal schemes not requiring control information, like repetition coding, outperform RS.

Future wireless systems are likely to comprise a dense grid of fixed relay nodes (FRNs) and contain a large number of user terminals (UTs) that can act as user relay nodes (URNs) under certain circumstances. In this paper, we argue that significant gains can be attained if FRNs and URNs act together under the umbrella of a hybrid architecture. To this end, we perform an exact and asymptotic outage probability (OP) analysis of a dual hop system with single relay selection under Rayleigh fading. Furthermore, we derive an ergodic capacity upper bound as a function of the number of FRNs and URNs. We show that in low signal-to-noise ratio (SNR), FRNs are much more effective in reducing OP than URNs. Although it is always more beneficial to employ FRNs instead of URNs, in high SNR URNs are almost equally as effective as FRNs in reducing OP. In terms of ergodic capacity, FRNs achieve higher performance than URNs in all SNR regimes. We conclude that from a system design viewpoint, if the aim is to meet certain quality-of-service constraints, FRNs can be reserved to serve low SNR users, while URNs could be employed when channel conditions to destination UTs are more favorable.

In this paper we show the potential of moving relay nodes (MRNs), i.e., relays mounted on top of public transportation vehicles, to improve capacity, reliability and coverage for vehicular users in future wireless networks. To this end, we study and compare the performance of dual-hop MRN assisted transmission, dual-hop transmission assisted by a fixed relay node (FRN) deployed on the street, as well as of the baseline single-hop direct transmission. For an accurate comparison, we optimize the FRN position in terms of outage probability (OP). This position is a function of the pathloss, transmit power and vehicle penetration loss (VPL). The problem is investigated for Rayleigh fading under a fixed cell coverage. When VPL is moderate to high, MRN assisted transmission greatly outperforms transmission assisted by an FRN as well as direct transmission, hence it is very promising for future wireless systems.

In this paper, we study the subchannel (SC) power allocation for orthogonal frequency division multiplexing (OFDM) multiple access points (APs) systems with non-coherent cooperative transmission. The objective is to maximize the total capacity under per-AP power constraints. It can be proved that the optimal solution can be obtained by the combination of an optimal SC partition search and the power allocation across SCs for each feasible partition. Existing work exhaustively searched the optimal SC partition and used Lagrange dual method to compute the power allocation across SCs. Since the entire complexity increases exponentially with the number of SCs, the existing method is unsuitable for practical implementation. In this paper, we propose a novel optimal power allocation algorithm for non-coherent cooperative transmission with a much lower complexity. Firstly, a concept of “cut-off SC” is proposed for searching the optimal SC partition. Then, an efficient optimal power allocation algorithm across SCs is proposed for any given cut-off SC. Simulation results demonstrate that the proposed algorithm is optimal with a polynomial complexity, and ends within an acceptable number of iterations.

In this work, the problem of receiver design for phase noise estimation and data detection in the presence of oscillator phase noise in a point-to-point multiple-input multiple-output (MIMO) system is addressed. First, we discuss some interesting and challenging aspects in receiver design for MIMO systems in the presence of Wiener phase noise. Then, using the variational Bayesian (VB) framework, a joint iterative phase noise estimator and symbol detector are developed based on inverse Gibbs or variational free energy maximization. Further, the symbol error probability (SEP) of the newly proposed iterative scheme is compared with the optimal maximum likelihood (ML) detector with perfect phase information for 16-phase shift keying (PSK) and 16-quadrature amplitude modulation (QAM) schemes.

Previous results in the literature have shown that derivation of the optimum maximum-likelihood (ML) receiver for symbol-by-symbol (SBS) detection of an uncoded data sequence based on all received signals in the presence of random phase noise is an intractable problem. This is because it involves the computation of the conditional probability distribution function (PDF) of the phase noise process. In this paper, we seek to minimize symbol error probability (SEP), which is achieved by SBS detection of the sequence based on all received signals. We show that the ML detector for this problem can be formulated as a weighted sum of central moments of the conditional PDF of phase noise. Given that the central moments of the conditional PDF of phase noise can be estimated, this new optimal structure is tractable with respect to the previously known optimal ML receiver. Furthermore, based on the new receiver structure, we propose a simple approximate method for SBS detection and investigate its scope and applicability. Simulation results demonstrate that SEP performance close to optimality can be obtained through the proposed method for a wide range of phase noise variances and low signal-to-noise ratio (SNR).

Heterogeneous networks have the potential to improve coverage, throughput, and energy efficiency of wireless networks through the use of specialized cellular structures, in particular femtocells and macrocells. However, to reduce interference between different cells, ensure smooth hand-offs from cell to cell, and achieve seamless operation the overall network needs to be synchronized. In this paper a new distributed clock synchronization scheme for heterogeneous networks is proposed that employs the clock drift ratio (CDR) information available at user-equipments (UEs) to achieve synchronization between non-interacting femtocells and macrocells. Simulation results show that the proposed scheme can significantly reduce the clock drift between macrocells and femtocells and result in timing synchronization throughout the network without introducing significant overhead.

In this paper, we address the problem of designing a joint scheduling and power control algorithm in a downlink coordinated multi-point (CoMP) cluster supporting CoMP joint transmission. The objective is to maximize the cell-edge throughput under per-point power constraints. By an analytical derivation, binary power control is proved to be the optimal solution for any given selected user group. Utilizing this analytical result, a centralized and a semi-distributed version of joint user selection and power control algorithms are proposed. Compared to algorithms without considering joint transmission and algorithms without considering power control, simulation results show that the proposed algorithms achieve a good trade-off between joint transmission and interference coordination, which helps to improve the cell-edge performance.

This paper presents a novel multiple-access modulation scheme which combines key characteristics of single carrier frequency division multiple access (SC-FDMA) with continuous phase modulation (CPM) in order to generate a power efficient waveform. CPM-SC-FDMA is developed based upon the observation that the samples from a CPM waveform may be treated as "data symbols" taken from a constant-envelope encoder. As with any encoder output, these samples may be precoded using the Discrete Fourier Transform and transmitted using SC-FDMA. Having originated from a constant envelope CPM waveform, CPM-SC-FDMA can potentially retain much of the power efficiency of CPM-thus resulting in a lower peak-to-average power ratio (PAPR) than conventional SC-FDMA. In this paper, we account for the information rate, memory, power efficiency, bit error rate (BER) performance and spectral occupancy of CPM-SC-FDMA. In addition, we investigate the impact of amplifier nonlinearity on BER performance as the number of users increases. Finally, we provide a detailed numerical comparison with a commensurate convolutionally coded QPSK-SC-FDMA scheme (CC-QPSK-SC-FDMA). We show a CPM-SC-FDMA scheme that provides an overall gain of up to 4 dB relative to the CC-QPSK-SC-FDMA scheme over a frequency-selective channel.

This article presents a summary of 3GPP Long Term Evolution (LTE) radio system performance during the execution of a multi-site field trial. This exhaustive testing campaign, performed over the lower 700 MHz and 2 GHz bands, includes a battery of stationary and mobility throughput cases, multi-user capacity scenarios, handover performance and latency tests, that have been designed to reveal LTE radio performance under a variety of realistic user scenarios. Tests have been performed using a Category 2 LTE UE, which can handle a maximum transport of 50 Mbps in the downlink and 25 Mbps in the uplink. As the results indicate, the system boasts a peak downlink single-user throughput in excess of 49 Mbps and a peak uplink single-user throughput observed of 19 Mbps, which is quite close to the theoretical uplink limit of 21 Mbps. In multi-user scenarios with four active users, in the best case, we have measured a cell throughput of 47 Mbps in the downlink and 20 Mbps in the uplink, which is close to the theoretical best. Finally, as we show, the system achieves a handover success rate of 100% and a best case average user plane latency of 22ms.

In this paper, three joint processing schemes for the downlink are characterized and compared within a cluster of base stations. The motivation of this study is to analyze the performance of these schemes over the cluster area, as a first step towards designing an adaptive joint processing scheme supporting dynamic usage scenarios. Each one of the analyzed schemes, the centralized, partial and distributed joint processing approaches, requires a different amount of available channel knowledge at the transmitter side, inter-base information exchange and feedback from the users. In addition, these schemes show varying capabilities to serve the users depending on their location in the cluster area. Therefore, in a real scenario, an adaptive joint processing scheme encompassing the three schemes could be used by the cluster of base stations. Simulation results show that, assuming coherent transmission, the centralized joint processing scheme outperforms with 25% the partial joint processing scheme and with 50% the distributed joint processing approach in the cell edge when a backhaul-load weighted average sum-rate per cell metric is taken into account.

In this paper, we consider a static cluster of base stations where joint processing is allowed in the downlink. The partial joint processing scheme is a user-centric approach where subclusters or active sets of base stations are dynamically defined for each user in the cluster. In frequency selective channels, the definition of the subclusters or active set thresholding of base stations can be frequency adaptive (per resource block) or non-adaptive (averaged over all the resource blocks). Frequency adaptive thresholding improves the average sum-rate of the cluster, but at the cost of an increased user data interbase information exchange with respect to the non-adaptive frequency thresholding case. On the other hand, the channel state information available at the transmitter side to design the beamforming matrix is very limited and rank deficiency problems arise for low values of active set thresholding and users located close to the base station. To solve this problem, an algorithm is proposed that defines a cooperation area over the cluster where the partial joint processing scheme can be performed, frequency adaptive or non-adaptive, for a given active set threshold value.

Coordinated Multi-Point (CoMP) transmission is considered in 3GPP LTE-Advanced as a key technique to improve the cell-edge performance. In order to support joint resource allocation among coordinate cells in CoMP systems, efficient frequency reuse schemes need to be designed. However, most of the existing frequency reuse schemes are not suitable for CoMP transmission due to not considering multi-cell joint transmission scenario in their frequency reuse rule. To solve this problem, a cooperative frequency reuse (CFR) scheme is proposed in this paper, which divides the cell-edge area of each cell into two types of zones, and defines a frequency reuse rule to support CoMP transmission for users in these zones. Compared with the conventional soft frequency reuse (SFR) scheme, simulation results demonstrate that the CFR scheme reduces the blocking probability by more than 50%, and improves the cell-edge throughput by 30~40%, with 5~9% additional cell-average throughput.

We have investigated the performance in terms of overall efficiency and maximum output power of a real high power amplifier with various modulated signals, ranging from constant envelope signals to orthogonal frequency division multiplexing based signals having large envelope variations. We show that there are large differences on the overall amplifier efficiency as well as its maximum output power depending on the modulated signal and also on the operation mode. In particular, we show that for high overall efficiency it is important to operate the amplifier close to the maximum output power level as often as possible, and we show that this has consequences for the resource allocation in especially packet based multicarrier systems. We also show that the amplifier performance with various Single-Carrier Frequency Division Multiple Access signals is very similar, and that the peak-to-average-power ratio can be used to predict the overall efficiency for the traditional amplifier drive method. Finally, we show that with advanced, today infeasible, amplifier operation methods the overall amplifier efficiency depends less on the modulation scheme, thus motivating further research in that domain.

In this paper, we investigate the power efficiency and bit error rate performance of two spectrally efficient CPM-SCFDMA (Continuous Phase Modulated Single Carrier Frequency Division Multiple Access) waveforms. CPM-SC-FDMA is derived by sampling a Continuous Phase Modulated (CPM) waveform and then DFT-precoding the resulting signal samples for transmission using SC-FDMA. Having originated from a constant envelope CPM waveform, CPM-SC-FDMA can potentially retain much of the power efficiency of CPM-thus resulting in a lower peak-to-average power ratio than conventional SC-FDMA. As we show in this paper, when taking the difference in power amplifier backoff requirements into account, CPM-SC-FDMA can provide an overall gain of up to 4 dB relative to convolutionally encoded QPSK-SC-FDMA over a frequency-selective channel.

The next generation wireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA) need to operate in widely different deployment and usage scenarios. Thus, support for flexible resource allocation is important. In this paper we investigate the performance of different memory- less and memory-based channel estimators for different OFDMA subcarrier allocation schemes and different pilot patterns. We evaluate the performance in various fading environments and for different user terminal velocities. The results show that channel estimation can perform well enough for time-frequency localized resources as small as 22 channel symbols with two pilots in many important scenarios. The results provided can be used to identify appropriate subcarrier allocations for the next generation OFDMA based wireless systems.

A performance trade-off investigation is carried out between different possible uplink multiple access schemes, that are based on Orthogonal Frequency Division Multiplexing (OFDM), for International Mobile Telecommunication (IMT) Advanced systems. Between the Discrete Fourier Transform (DFT) preceded systems with different subcarrier allocation mappings and systems lacking DFT-precoders, Block Interleaved Frequency Division Multiple Access (B-IFDMA) is shown to provide a good trade-off between the frequency diversity collected, envelope properties achieved, and channel estimation performance compared to the other mapping schemes. The schemes are analyzed in the presence of the different possible modules which include equalizers, modulators, interleavers, and channel codes. In particular, robust codes such as Turbo codes are able to collect the diversity provided by such schemes, and B-IFDMA systems is shown to be able to beat the other systems in bit error rate (BER) performance terms.

In this paper, a novel channel allocation method is proposed to reduce the time-varying effects in downlink OFDMA systems. This method makes use of a Kalman filter in order to predict the user SINR and rate in the next time slot. Based on this estimated rate, the available subcarriers are assigned according to a rate greeding algorithm. Compared with the rate-craving greedy (RCG) algorithm and the rate allocation with fixed increase (RAFI) algorithm, the numerical results show that the proposed algorithm can effectively improve the throughput for a single user, averagely 3 to 7 % more than the RCG algorithm and 1 to 3 % more than the RAFI algorithm. Moreover, the total outage probability is also reduced as the number of users increases, averagely 8 to 11% more than the RCG algorithm and 3 to 7% more than the RAFI algorithm.

The EU WINNER projects have studied OFDM-based packet data systems beyond 3G that use adaptivity on all time- scales to obtain high flexibility and performance. The adaptive transmission in both downlink and uplink is scheduled and controlled at base stations and relay nodes and requires frequent transmission of control information over the downlink. The use of scheduling, adaptive modulation and coding, with fine granularity in both time and space, could potentially result in unrealistic bandwidth demands for such downlink control signaling. The present paper describes how this problem has been handled within WINNER in two cases: frequency-adaptive transmission, which allows individual link adaptation within time-frequency resource units and non-frequency adaptive transmission, which averages over the channel variations in the frequency domain. An important tool for limiting the associated control information is to broadcast only a small essential set of control data to all user terminals, using a safe but therefore bandwidth-demanding code rate. The remaining control information is multicast to groups of users with different signal to interference and noise ratios (SINRs). The modulation and code rates of these transmissions are adjusted to the SINRs of these groups. The over-all coded data rate of the control transmission can thereby be reduced to acceptable levels.

High data rates, high spectral efficiency, flexibility, and low delays over the air interface will be important features in next-generation wireless systems. The overall challenge will be packet scheduling and adaptive radio transmission for multiple users, via multiple antennas and over frequency-selective wideband channels. This problem needs to be structured to obtain feasible solutions. The basic simplifying assumptions used here are clustering of antennas into cells, orthogonal transmission by use of cyclic-prefix orthogonal frequency-division multiplexing (OFDM) and a time-scale separation view of the total link adaptation, scheduling and intercell coordination problem. Based on these assumptions, we survey techniques that adapt the transmission to the temporal, frequency, and spatial channel properties. We provide a systematic overview of the design problems, such as the dimensioning of the allocated time-frequency resources, the influence of duplexing schemes, adaptation control issues for downlinks and uplinks, timing issues, and their relation to the required performance of channel predictors. Specific design choices are illustrated by recent research within the Swedish Wireless IP program and the EU IST-WINNER project. The presented results indicate that high-performance adaptive OFDM transmission systems are indeed feasible, also for challenging scenarios that involve vehicular velocities, high carrier frequencies, and high bandwidths.

The European IST research projects WINNER and WINNER II aim at developing a single new ubiquitous radio access system concept that can be adapted for use in a wide variety of mobile communication scenarios, from wide area coverage to hot spots. This cellular packet data system uses a multicarrier-based air interface. Its medium access control (MAC) system layer is designed for either FDD or TDD cellular transmission. It supports adaptive transmission by tight interaction with the physical layer, and provides efficient resource allocation and scheduling within relay-enhanced cells. It also supports separate allocation of antenna resources per packet flow and furthermore provides support for spectrum sharing and flexible spectrum use. The paper outlines the resource allocation within a super-frame and the use of either opportunistic scheduling or diversity-based transmission within frames.

Within the EU FP6 Integrated Project WINNER, adaptive transmission is investigated as a key technology for boosting the spectral efficiency of a new radio interface for 4G systems. Adaptive allocation and link adaptation of time-frequency chunks based on channel prediction in an OFDM-based system offers a significant potential to design a spectrally efficient system. The chunk size is typically defined based on the minimum coherence time and coherence bandwidth of the targeted channels. It is important to allow efficient channel coding and link retransmission schemes without restricting the resource scheduler, even for systems using small chunk sizes, to achieve multi-user diversity gains. In this paper we introduce some possible approaches to implement FEC coding and hybrid ARQ and analyze their interplay with resource scheduling in packet oriented adaptive TDMA/OFDMA

Using an OFDM based uplink for future cellular networks has been a controversial issue due to difficulties in carrier frequency synchronization. In this paper, a model is provided to quantify the effects of Gaussian distributed carrier frequency offsets in a multiuser OFDM uplink. Both theoretical evaluations and simulations on spectral efficiency are performed in a slotted OFDM uplink, in which time-frequency bins are allocated adaptively to different users based on their channel conditions. The results show that the loss in spectral efficiency is small when the standard deviation of each user's carrier frequency offset is no more than 2% of the subcarrier spacing