This paper investigates the impact of hardware impairments in terms of low-cost radio frequency (RF) chains and dynamic-resolution analog-to-digital/digital-to-analog converters (ADCsIDACs) over a reconfigurable intelligent surface (RIS) enhanced cell-free massive multiple-input multiple-output system. We aim to model a practical scenario by considering phase noise and spatial correlation at the RIS. We utilize the well-known linear minimum mean square error estimation to obtain the channel estimates and present a closed-form expression for the downlink spectral efficiency (SE). To reduce the fronthaul load, we implement an access point (AP) selection strategy that assigns each user to a specific set of APs rather than all the APs. Our analysis reveals that i) the SE with RIS-aided links is significant in scenarios with low transmit power, especially when the direct link is weak, ii) the effect of ADCslDACs imperfection is more acute than RF impairments at higher transmit power levels, iii) the energy efficiency shows minimal impact at lower impairment levels but becomes significantly affected as the impairment level increases, and iv) dynamic-resolution ADCslDACs playa vital role on the system performance.

We consider a reconfigurable intelligent surface (RIS) assisted cell-free massive multiple-input multiple-output non-orthogonal multiple access (NOMA) system, where each access point (AP) serves all the users with the aid of the RIS. We practically model the system by considering imperfect instantaneous channel state information (CSI) and employing imperfect successive interference cancellation at the users’ end. We first obtain the channel estimates using linear minimum mean square error approach considering the spatial correlation at the RIS and then derive a closed-form downlink spectral efficiency (SE) expression using the statistical CSI. We next formulate a joint optimization problem to maximize the sum SE of the system. We first introduce a novel successive Quadratic Transform (successive-QT) algorithm to optimize the transmit power coefficients using the concept of block optimization along with quadratic transform and then use the particle swarm optimization technique to design the RIS phase shifts. Note that most of the existing works on RIS-aided cell-free systems are specific instances of the general scenario studied in this work. We numerically show that i) the RIS-assisted link is more advantageous at lower transmit power regions where the direct link between AP and user is weak, ii) NOMA outperforms orthogonal multiple access schemes in terms of SE, and iii) the proposed joint optimization framework significantly improves the sum SE of the system.

In this paper, we consider an indoor visible light communication (VLC) system with multiple "white" light emitting diodes serving to form overlapping wireless communication cells. In order to maintain seamless connectivity to mobile users, a handover procedure should be implemented. In particular, practical conditions such as blockages due to obstacles inside the room environment and the mobility of users can affect direct VLC connectivity. The use of reconfigurable intelligent surfaces (RISs) in optical wireless systems allows to exploit non-direct connectivity links, thus providing efficient communication links. In this paper, we present a proactive handover mechanism that exploits the presence of a RIS, in order to redirect the communication links in case of blockages. The proposed approach has been implemented both in hard and soft modes and assessed in terms of achievable data rate and handover latency for a user walking in a given reference room at different user speeds and blockage conditions. Our presented results and comparisons with conventional handover methods (i.e., without RIS) are helpful in showing the superiority of the presented algorithm.

In this article, we consider a simultaneous lightwave information and power transfer-enabled indoor visible light/radio frequency (RF) hybrid communication system. In this system, several luminaries are mounted on the ceiling of a room and sensors receive lightwave power and information from luminaries. Each sensor harvests energy from lightwave signals and uses them for uplink communication using RF signals. In general, blockages due to the movement of humans are common in indoor systems which results in severe performance degradation. We consider blockages due to such human movements in our system. An optimization problem is formulated to maximize the uplink weighted sum rate considering uplink orthogonal multiple access (OMA) and non-OMA cases. To find the optimal beamforming matrix and time allocation parameters, a lightweight artificial neural network architecture is proposed. Our solution is capable of predicting the human blockages and accordingly optimizing the beamforming matrices and time allocation parameters leading to a near-optimal uplink sum rate. Specifically, up to 30% rate improvement is observed compared to zero-forcing beamforming when the number of blockages is more than five. Further, the use of NOMA for uplink results in up to 25% sum rate improvement.

In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interference cancellation, addressing both model-based and emerging machine learning-based methods. Then, we turn our attention to new wireless systems with many degrees of freedom for which the traditional IBFD designs do not gracefully scale and, hence, require many innovations to enable IBFD. We provide an extensive review of basic concepts and state of the art in massive multiple-input–multiple-output IBFD. Then, we consider the mmWave band IBFD and review advanced physical-layer architectures. The above review provides the proper context to discuss IBFD innovations and new challenges for sixth-generation networks and beyond, where wireless networks are envisioned to be multifunctional, combining communications with functions such as sensing, cognitive radios, physical-layer security, and wireless power transfer. We conclude this article with a status update on the adoption of IBFD in communication standards.

Unmanned aerial vehicles (UAVs) are envisioned to provide diverse services from the air. The service quality may rely on the wireless performance which is affected by the UAV’s position. In this paper, we focus on the UAV placement problem in the Internet of Vehicles, where the UAV is deployed to monitor the road traffic and sends the monitored videos to vehicles. The studied problem is formulated as video resolution maximization by optimizing over the UAV’s position. Moreover, we take into account the maximal transmission delay and impose a probabilistic constraint. To solve the formulated problem, we first leverage the techniques in extreme value theory (EVT) and Gaussian process regression (GPR) to characterize the influence of the UAV’s position on the delay performance. Based on this characterization, we subsequently propose a proactive resolution selection and UAV placement approach, which adaptively places the UAV according to the geographic distribution of vehicles. Numerical results justify the joint usage of EVT and GPR for maximal delay characterization. Through investigating the maximal transmission delay, the proposed approach nearly achieves the optimal performance when vehicles are evenly distributed, and reduces 10% and 19% of the 999-th 1000-quantile over two baselines when vehicles are biased distributed.

We propose a deep reinforcement learning (DRL) approach for a full-duplex (FD) transmission that predicts the phase shifts of the reconfigurable intelligent surface (RIS), base station (BS) active beamformers, and the transmit powers to maximize the weighted sum rate of uplink and downlink users. Existing methods require channel state information (CSI) and residual self-interference (SI) knowledge to calculate exact active beamformers or the DRL rewards, which typically fail without CSI or residual SI. Especially for time-varying channels, estimating and signaling CSI to the DRL agent is required at each time step and is costly. We propose a two-stage DRL framework with minimal signaling overhead to address this. The first stage uses the least squares method to initiate learning by partially canceling the residual SI. The second stage uses DRL to achieve performance comparable to existing CSI-based methods without requiring the CSI or the exact residual SI. Further, the proposed DRL framework for quantized RIS phase shifts reduces the signaling from BS to the RISs using 32 times fewer bits than the continuous version. The quantized methods reduce action space, resulting in faster convergence and 7.1% and 22.28% better UL and DL rates, respectively than the continuous method.

The capability of nodes to transmit and receive data simultaneously within the same frequency band, referred to as in-band FD, disrupts the conventional assumptions underlying wireless network design. This new feature enhances spectral efficiency and reduces latency, which are essential drivers in advancing next-generation networks. In the past few years, full-duplex (FD) has evolved from being a laboratory idea to being incorporated into telecommunications standards and proof of concepts. From 2010 to 2020, considerable research and development efforts were devoted to advancing FD wireless communications. By 2015, the cable modem industry had already implemented in-band FD technology to establish the DOCSIS 4.0 standard, enabling next-generation cable modems to operate in FD mode. By 2020, FD wireless products started to emerge in the market.

The use of in-band full-duplex (FD) enables nodes to simultaneously transmit and receive on the same frequency band, which challenges the traditional assumption in wireless network design. The full-duplex capability enhances spectral efficiency and decreases latency, which are two key drivers pushing the performance expectations of next-generation mobile networks. In less than ten years, in-band FD has advanced from being demonstrated in research labs to being implemented in standards, presenting new opportunities to utilize its foundational concepts. Some of the most significant opportunities include using FD to enable wireless networks to sense the physical environment, integrate sensing and communication applications, develop integrated access and backhaul solutions, and work with smart signal propagation environments powered by reconfigurable intelligent surfaces. However, these new opportunities also come with new challenges for large-scale commercial deployment of FD technology, such as managing self-interference, combating cross-link interference in multi-cell networks, and coexistence of dynamic time division duplex, subband FD and FD networks.

The sum rate performance of simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR- RIS) assisted full-duplex (FD) communication systems is investigated. The reflection and transmission coefficients of STAR- RIS elements are optimized for the energy splitting and mode switching protocols to maximize the weighted sum rate of the system. The underlying optimization problems are non-convex, and hence, the successive convex approximation technique has been employed to develop efficient algorithms to obtain suboptimal solutions. Thereby, the maximum average weighted sum rate and corresponding coefficients at the STAR-RIS subject to predefined threshold rates and unit-modulus constraints are quantified. The performance of the proposed system design is compared with the conventional reflecting/transmitting-only RISs and half-duplex counterparts via simulations where it is observed that STAR-RIS can boost the performance of FD systems.

Intelligent reflecting surfaces (IRSs) have the potential of increasing the coverage and energy efficiency of future wireless networks. In this paper, we study the performance of an IRS-aided network in the presence of blockages from a large-scale point-of-view, where multiple IRSs are randomly distributed. We model the blockages and IRSs with a line segment process and take into account the fading channels from both the direct and indirect links. The performance of a receiver is studied based on specific association policies, i.e. which IRS(s) should be activated to reflect the signal to the receiver. Specifically, we consider the association with a random IRS, the closest IRS or with all available IRSs in the cell. A complete analytical framework in terms of the outage probability is developed using tools from stochastic geometry. We show that each association policy has its advantages, which heavily depend on the network’s parameters such as the cell-radius, the number of IRS elements and the blockage density.

In this paper, we consider a wireless communication system, where a full-duplex hybrid access point (HAP) transmits to a set of cellular users (CUs) in the downlink channel, while receiving data from a set of energy-constrained communication devices like user equipments (UEs) in the uplink channel. The HAP has a massive antenna array, while all CUs and UEs nodes are equipped with single antenna each. Time switching protocol is adopted, where channel estimation, wireless power transfer, and information transfer between UEs, CUs and full-duplex HAP are performed in two phases. By adopting maximum ratio combining/maximum ratio transmission (MRC/MRT) and zero-forcing (ZF) processing at the HAP, the uplink and downlink achievable rate expressions in the large-antenna limit and approximate results that hold for any finite number of antennas are derived. Moreover, the optimum energy beamformer and time-split parameter at the HAP are found to maximize the downlink sum-rate under a constraint on uplink sum-rate. Our findings reveal that our proposed energy beamforming with ZF and MRC/MRT processing for information transfer achieves up to 47% and 14% average sum rate gains as compared with the suboptimum energy beamformer, respectively.

In this paper, we consider a non-orthogonal multiple access (NOMA) underwater optical wireless communication system in which a full-duplex decode-and-forward relay is used to assist the far user with weak channel conditions. Full-duplex operation introduces interference to the near user with strong channel conditions and our model covers several practical assumptions such as imperfect interference cancellation and random receiver orientation at the near/far user. In particular, we derive the exact outage probability at the near user and far user valid for log-normal weak turbulence conditions. An accurate outage approximation for the near user is also derived in closed-form. The correctness of the analysis has been verified through simulation results where they both match closely. Moreover, the derived expressions are useful to investigate the impact of various system and channel parameters such as the access point/relay transmit power, node locations, strength of the residual interference and random receiver orientation at the near and far user. Our results reveal that the outage probability of the near user is more sensitive to receiver orientation errors as compared to the far user.

The Internet of Things (IoT) for smart ocean is a promising paradigm that will support emerging applications in the areas of maritime transport, emergency search and rescue, security and border surveillance, environmental protection, etc. There has been a surging amount of data acquired from different maritime terminals, such as vessels, buoys, and offshore platforms. As a result, the demand for high-speed, ultrareliable, and low-latency maritime communications and data processing is proliferating. In this context, transmission and processing of maritime data have become a research hotspot. IoT technologies are expected to dramatically enhance the capacity, safety, and efficiency of connected vessels and other maritime terminals. Meanwhile, the unique characteristics of smart ocean applications create heterogeneous challenges in achieving viable, reliable, and secure communications and data processing. Addressing the challenges calls for novel approaches and consideration for the deployment of next-generation maritime communication networks. Therefore, it is essential to pursue research on new theories, architecture, and technologies to fully exploit the capability that is delivered by IoT for smart ocean to form efficient and intelligent maritime communication systems. This special issue aims to create a platform for researchers from both academia and industry to disseminate state-of-the-art results and to advance the applications of IoT for the smart ocean.

We analyze the effect of channel aging on the achievable rate of time division duplexed massive multiple input multiple output systems serving a number of users under aging channels, using nonorthogonal multiple access (NOMA) and orthogonal multiple access (OMA). Using the recently proposed shared uplink pilot based channel estimation for NOMA, we derive bounds on the channel estimation error variance for the two schemes. We then derive the achievable spectral efficiencies of the two schemes. Using numerical results, we show that, in slowly varying channels, using NOMA with shared pilots is preferable over OMA, while the reverse is true under fast varying channels.

We consider downlink non-orthogonal multiple access transmission where an access point communicates with a set of near and far users via a full-duplex multiple antenna relay. To deal with the inter-user interference at the near user and self-interference at the relay, we propose the optimum and suboptimal beamforming schemes. In addition, we consider two different user selection criteria, namely: 1) random near user and random far user (RNRF) selection and 2) nearest near user and nearest far user (NNNF) selection, and we derive the outage probabilities of the near and far users. Our findings reveal that as compared to half-duplex operation, full-duplex relaying can reduce the outage probability of the near users up to 63% in the case of NNNF user selection. With suboptimal beamforming schemes, the NNNF user selection shows a superior performance as compared to the RNRF user selection for all choices of transmit power, while with the optimum beamforming, the performance of the RNRF user selection converges to the NNNF user selection at high transmit power. The simulation results are provided to confirm the accuracy of the developed analytical results and facilitate a better performance comparison.

We investigate the problem of antenna selection (AS) in full-duplex (FD) cooperative non-orthogonal multiple access (NOMA) systems, where a multi-antenna FD relay assists transmission from a multi-antenna base station (BS) to a far user, while at the same, the BS transmits to a near user. Specifically, based on the end-to-end signal-to-interference-plus-noise ratio at the near and far users, two AS schemes to select a single transmit antenna at both the BS and the relay, respectively, as well as a single receive antenna at relay are proposed. In order to study the ergodic sum rate and outage probability of these AS schemes, we have derived closed-form expressions assuming Rayleigh fading channels. The sum rate and outage probability of the AS schemes are also compared with the optimum selection scheme that maximizes the performance as well as with a random AS scheme. Our results show that the proposed AS schemes can deliver a near-optimal performance for near and far users, respectively.

In this paper, we consider a non-orthogonal multiple access cognitive radio network, where a full-duplex (FD) multi-antenna relay assists transmission from an access point (AP) to a cognitive far user, while at the same time, the AP transmits to a cognitive near user. Our objective is to maximize the rate of the near user under a constraint that the rate of the far user is above a certain threshold. To this end, a non-convex joint optimization problem of relay beamforming and the transmit powers at the AP and FD relay is solved as a semi-definite relaxation problem, in conjunction with an efficiently solvable line-search approach. We also consider a low complexity fixed beamformer design, where the optimum power allocation between the AP and FD relay is solved. Several fixed beamforming designs based on the zero-forcing criterion are proposed for which exact and asymptotic outage probability expressions corresponding to the near and far users are derived. Our results demonstrate that the proposed joint optimization can significantly reduce the self-interference impact at the FD relay and inter-user interference in the near user case.

In this paper, we study the effect of channel aging on the uplink and downlink performance of an FDD massive MIMO system, as the system dimension increases. Since the training duration scales linearly with the number of transmit dimensions, channel estimates become increasingly outdated in the communication phase, leading to performance degradation. To quantify this degradation, we first derive bounds on the mean squared channel estimation error. We use the bounds to derive deterministic equivalents of the receive SINRs, which yields a lower bound on the achievable uplink and downlink spectral efficiencies. For the uplink, we consider maximal ratio combining and MMSE detectors, while for the downlink, we consider matched filter and regularized zero forcing precoders. We show that the effect of channel aging can be mitigated by optimally choosing the frame duration. It is found that using all the base station antennas can lead to negligibly small achievable rates in high user mobility scenarios. Finally, numerical results are presented to validate the accuracy of our expressions and illustrate the dependence of the performance on the system dimension and channel aging parameters.

This paper considers a cloud radio access network, where full-duplex (FD) users communicate with remote radio heads (RRHs) that are spatially distributed. We consider all participate RRH association (ARA) and single nearest RRH association (SRA) policies with optimal, maximum ratio combining/maximal ratio transmission (MRT), and zero-forcing/MRT (ZF/MRT) processing schemes and derive analytical expressions useful to compare the average uplink/downlink (UL/DL) sum rate among association schemes as a function of the number of RRHs antennas and UL/DL RRH density. We also study a dense network setting with multiple FD users and derive exact expressions for the average UL/DL rates, where a user-centric clustering technique is adopted and each user is served by its nearest UL and DL RRHs. Furthermore, by maximizing the instantaneous sum rate, we develop an optimum power allocation scheme for the single-user case. We observe that ARA results in a rate region that is strongly biased toward the UL or DL, but using SRA results in a more balanced rate region. Moreover, SRA policy with ZF/MRT processing achieves up to 32% and 42% average sum rate gains as compared with the HD SRA and FD ARA counterparts, respectively.

In this paper, we consider a power beacon (PB) assisted two-way relaying network, where two single antenna energy constrained users first harvest energy from a multi-antenna PB and then communicate with each other with the assistance of a relay. The key aim of the paper is to design the energy beamforming vector and time split parameter for optimizing the system performance. In particular, two different design objectives are investigated, namely, max-min rate design and sum rate maximization design. Due to the non-convex nature of the optimization problems, the global optimal solutions are difficult to obtain. Instead, we propose an alternating optimization design framework where near optimal performance can be achieved through iterative optimization. In addition, to further reduce the computation complexity, closed-form suboptimal designs are provided. Simulation results are presented to validate the effectiveness of the proposed alternating optimization design and suboptimal design. The outcomes of the paper suggest that adopting the proposed energy beamforming vector at the PB can substantially boost the system performance. Also, the topology of the network has a significant impact on the achievable performance.

This paper investigates a power beacon (PB) assisted two-way relaying network, in which two single antenna energy constraint users first harvest energy from a multi-antenna PB and then communicate with each other with the assistance of a relay. Considering user fairness, we propose the max-min design that maximizes the minimum rate of the two users and investigate the corresponding optimal energy beamformer and time split. Due to the non-convex nature of the optimization problems, an alternating optimization design is proposed in which the optimal beamformer and time split can be solved via a low-complexity algorithm and convex optimization, respectively. Our results show that the proposed alternating optimization design yields near optimal performance, and implementing multiple antennas at the PB can significantly improve the system performance.

We consider a cooperative non-orthogonal multiple access (NOMA) network in which a full-duplex (FD) multi-antenna relay assists transmission from a base station (BS) to a set of far users with poor channel conditions, while at the same time the BS transmits to a set of near users with strong channel conditions. We assume imperfect self- interference (SI) cancellation at the FD relay and imperfect inter-user interference cancellation at the near users. In order to cancel the SI at the relay a zero-forcing based beamforming scheme is used and the corresponding outage probability analysis of two user selection strategies, namely random near user and random far user (RNRF), and nearest near user and nearest far user (NNNF), are derived. Our finding suggests that significant performance improvement can be achieved by using the FD multi- antenna relay compared to the counterpart system with a half-duplex relay. The achieved performance gain depends on network parameters such as the user density, user zones, path loss and the strength of the inter-user interference in case of near users. We also show that the NNNF strategy exhibits a superior outage performance compared to the RNRF strategy, especially in the case of near user.

In this paper, we consider a non-orthogonal multiple access cognitive radio network, where a full-duplex multi-antenna relay assists transmission from a base station (BS) to a cognitive far user, whereas, at the same time, the BS transmits to a cognitive near user. Our objective is to enlarge the far-near user rate region by maximizing the rate of the near user under a constraint that the rate of the far user is above a certain threshold. To this end, a non- convex joint optimization problem of relay beamforming and the transmit powers at the BS and cognitive relay is solved as a semi-definite relaxation problem, in conjunction with an efficiently solvable line-search approach. For comparisons, we also consider low complexity fixed beamformer design, where the optimum power allocation between the BS and cognitive relay is solved. Our results demonstrate that the proposed joint optimization can significantly reduce the impact of the residual self-interference at the FD relay and inter-user interference in the near user case.

We consider a multiuser wireless system with a full-duplex hybrid access point (HAP) that transmits to a set of users in the downlink channel, while receiving data from a set of energy-constrained sensors in the uplink channel. We assume that the HAP is equipped with a massive antenna array, while all users and sensor nodes have a single antenna. We adopt a time-switching protocol where in the first phase, sensors are powered through wireless energy transfer from HAP and HAP estimates the downlink channel of the users. In the second phase, sensors use the harvested energy to transmit to the HAP. The downlink-uplink sum-rate region is obtained by solving downlink sum-rate maximization problem under a constraint on uplink sum-rate. Moreover, assuming perfect and imperfect channel state information, we derive expressions for the achievable uplink and downlink rates in the large-antenna limit and approximate results that hold for any finite number of antennas. Based on these analytical results, we obtain the power-scaling law and analyze the effect of the number of antennas on the cancellation of intra-user interference and the self-interference.

We propose techniques for optimizing transmit beamforming in a full-duplex multiple-input-multiple-output wireless-powered communication system, which consists of two phases. In the first phase, the wireless-powered mobile station (MS) harvests energy using signals from the base station (BS), whereas in the second phase, both MS and BS communicate to each other in a full-duplex mode. When complete instantaneous channel state information (CSI) is available, the BS beamformer and the time-splitting (TS) parameter of energy harvesting are jointly optimized in order to obtain the BS-MS rate region. The joint optimization problem is non-convex, however, a computationally efficient optimum technique, based upon semidefinite relaxation and line-search, is proposed to solve the problem. A sub-optimum zero-forcing approach is also proposed, in which a closed-form solution of TS parameter is obtained. When only the second-order statistics of transmit CSI is available, we propose to maximize the ergodic information rate at the MS while maintaining the outage probability at the BS below a certain threshold. An upper bound for the outage probability is also derived and an approximate convex optimization framework is proposed for efficiently solving the underlying non-convex problem. Simulations demonstrate the advantages of the proposed methods over the sub-optimum and half-duplex ones.

This paper investigates the impact of the channel state information (CSI) and antenna correlation at the multi-antenna relay on the performance of wireless powered dual-hop amplify-and-forward relaying systems. Depending on the available CSI at the relay, two different scenarios are considered, namely, instantaneous CSI and statistical CSI where the relay has access only to the antenna correlation matrix. Adopting the power-splitting architecture, we present a detailed performance study for both cases. Closed-form analytical expressions are derived for the outage probability and ergodic capacity. In addition, simple high signal-to-noise ratio (SNR) outage approximations are obtained. Our results show that, antenna correlation itself does not affect the achievable diversity order, the availability of CSI at the relay determines the achievable diversity order. Full diversity order can be achieved with instantaneous CSI, while only a diversity order of one can be achieved with statistical CSI. In addition, the transmit antenna correlation and receive antenna correlation exhibit different impacts on the ergodic capacity. Moreover, the impact of antenna correlation on the ergodic capacity also depends heavily on the available CSI and operating SNR.

In this paper, we consider a three-node cooperative wireless powered communication system consisting of a multi-antenna hybrid access point (H-AP) and a single-antenna relay and a single-antenna user. The energy constrained relay and user first harvest energy in the downlink and then the relay assists the user using the harvested power for information transmission in the uplink. The optimal energy beamforming vector and the time split between harvest and cooperation are investigated. To reduce the computational complexity, suboptimal designs are also studied, where closed-form expressions are derived for the energy beamforming vector and the time split. For comparison purposes, we also present a detailed performance analysis in terms of the achievable outage probability and the average throughput of an intuitive energy beamforming scheme, where the H-AP directs all the energy towards the user. The findings of the paper suggest that implementing multiple antennas at the H-AP can significantly improve the system performance, and the closed-form suboptimal energy beamforming vector and time split yields near optimal performance. Also, for the intuitive beamforming scheme, a diversity order of N+1/2 can be achieved, where N is the number of antennas at the H-AP.

In this paper, we consider two fundamental full-duplex (FD) architectures, two-node and three-node, in the context of cellular networks where the terminals employ directional antennas. The simultaneous transmission and reception of data in non-orthogonal channels makes FD radio a potential solution for the currently limited spectrum. However, its implementation generates high levels of interference either in the form of loopback interference (LI) from the output to the input antenna of a transceiver or in the form of co-channel interference in large-scale multicell networks due to the large number of active links. Using a stochastic geometry model, we investigate how directional antennas can control and mitigate the co-channel interference. Furthermore, we provide a model which characterizes the way directional antennas manage the LI in order to passively suppress it. Our results show that both architectures can benefit significantly by the employment of directional antennas. Finally, we consider the case where both architectures are employed in the network and derive the optimal values for the density fraction of each architecture, which maximize the success probability and the network throughput.

This paper investigates the impact of antenna correlation on wireless powered dual-hop multi-antenna relaying systems with instantaneous channel state information (CSI) or statistical CSI at the relay. Considering the power-splitting architecture, we study the outage probability as well as the achievable diversity order of the system for amplify-and-forward protocol. Our results show that, antenna correlation itself does not affect the achievable diversity order, the availability of CSI at the relay determines the achievable diversity order. Full diversity order can be achieved with instantaneous CSI, while only unit diversity order can be achieved with statistical CSI. In addition, with instantaneous CSI, antenna correlation is detrimental in the moderate and high SNR regime while it results in a better performance when the operating SNR is low. On the contrary, antenna correlation is always beneficial with only statistical CSI.

In this paper, we investigate the sum rate performance of a downlink transmission scheme for massive multiple-input multiple-output (MIMO) systems with a two-dimensional (2D) antenna array at the base station (BS). Assuming Rician fading conditions, we derive some properties of the channel's line-of-sight (LOS) component. Using these properties and under the assumption of only LOS information at the BS, the optimal beamforming vector for each user as well as the main guidelines for user scheduling are derived to maximize an approximation of the ergodic sum rate. Based on the optimal beamforming vector and the guidelines, a statistical three-dimensional (3D) beamforming downlink transmission scheme exploiting only the LOS information of each user is proposed. Numerical results show that the proposed scheme can achieve considerable sum rates while requiring much less channel state information (CSI) overhead at the BS, and is more attractive than the traditional zero-forcing and match-filtering schemes when the CSI at the BS is imperfect.

This paper considers a cloud radio access network (C-RAN) where spatially distributed remote radio heads (RRHs) communicate with a full-duplex user. In order to reflect a realistic scenario, the uplink (UL) and downlink (DL) RRHs are assumed to be equipped with multiple antennas and distributed according to a Poisson point process. We consider all participate and nearest RRH association schemes with distributed beam-forming in the form of maximum ratio combining/maximal ratio transmission (MRC/MRT) and zero-forcing/MRT(ZF/MRT) processing. We derive analytical expressions useful to compare the average sum rate among association schemes as a function of the number of RRHs antennas and density of the UL and DL RRHs. Numerical results show that significant performance improvements can be achieved by using the full-duplex mode as compared to the half-duplex mode, while the choice of the beamforming design as well as the RRH association scheme plays a critical role in determining the full-duplex gains.

This paper investigates the achievable spectral efficiency (SE) of millimeter wave downlink cellular systems accounting for both small and large-scale fading effects. The base station (BS) employs a uniform linear array (ULA) and maximal ratio transmission. We derive the expectation of the squared inner product for different channel links under the assumption that the users are randomly distributed within a circular-shaped cell. Using this result, a new lower bound on the achievable SE, valid for arbitrary numbers of antennas is derived. The analytical results show that the achievable SE increases with the number of BS antennas, whilst it converges to a saturated value in the high signal-to-noise ratio (SNR) regime and larger inter- antenna spacing.

We study the problem of throughput optimization in large multiple-input multiple-output (MIMO) beamforming systems with channel aging, as the number of transmit and receive antennas is increased. We quantify the effective channel coherence time in terms of the intertraining interval, which is the time after which the channel state information needs to be reestimated to avoid loss in performance due to outdated channel estimates. Conventional wisdom suggests that the coherence time of large MIMO systems should decrease with increasing number of antennas, because the system has a large number of independently varying channel coefficients. On the contrary, we show that the effective channel coherence time increases with the number of antennas. We optimize the throughput with respect to the symbol and pilot energies, the number of antennas, frame duration, and target SNR. This analysis brings out the important fact that the effective channel coherence time critically depends on the underlying signaling scheme, and not only on the temporal correlation coefficient of the wireless channel.

In this paper, we investigate a downlink transmission algorithm for single-cell multiuser systems with two-dimensional (2-D) large-scale antenna array at the base station (BS) over Rician fading channels. We first derive some properties of the channel's line-of-sight (LOS) component in the large-scale antenna array scenario. Next, based on these properties and under the assumption of only statistical channel state information (CSI), i.e., the LOS component and Rician $K$-factor, at the BS, we derive the optimal beamforming vector for each user in maximizing an approximation of the ergodic sum rate. The main guidelines for user scheduling are also presented. Then, a three-dimensional (3-D) beamforming downlink transmission algorithm exploiting only the statistical CSI of each user is proposed. For this algorithm, we derive an exact analytical closed-form expression for the achievable ergodic rate and present tractable approximations. Based on our analysis, we gain some valuable insights. The proposed algorithm is shown to perform well in achieving considerable sum rate while requiring much less CSI at the BS. It requires three scalar values at the BS for each user, and can achieve an ergodic sum rate closer to the ergodic sum rate achieved by the matched-filter precoding with perfect CSI at the BS.

We consider a full-duplex (FD) decode-and-forward system in which the time-switching protocol is employed by the multiantenna relay to receive energy from the source and transmit information to the destination. The instantaneous throughput is maximized by optimizing receive and transmit beamformers at the relay and the time-split parameter. We study both optimum and suboptimum schemes. The reformulated problem in the optimum scheme achieves closed-form solutions in terms of transmit beamformer for some scenarios. In other scenarios, the optimization problem is formulated as a semidefinite relaxation problem and a rank-one optimum solution is always guaranteed. In the suboptimum schemes, the beamformers are obtained using maximum ratio combining, zero-forcing, and maximum ratio transmission. When beamformers have closed-form solutions, the achievable instantaneous and delay-constrained throughput are analytically characterized. Our results reveal that beamforming increases both the energy harvesting and loop interference suppression capabilities at the FD relay. Moreover, simulation results demonstrate that the choice of the linear processing scheme as well as the time-split plays a critical role in determining the FD gains.

In this paper, a three-node energy-harvesting (EH) cooperative system with ROle SElection (ROSE) mechanism, where the cooperative role of each node is dynamically adjusted based on the surrounding wireless fading environment is considered. We first study a benchmark scenario with the fixed role configuration, from which a tight lower bound for the asymptotic outage behavior is derived. For this case, the achievable diversity is limited to two and the system outage behavior at high signal-to-noise ratio (SNR) is affected by the easing-off factor log c0together with log(log c0) denoting the source transmit SNR. After that, a dynamic role selection policy is incorporated and the asymptotic analysis shows that the ROSE mechanism can enhance the system diversity order to three, and the decaying law of outage behavior is no longer affected by the c0and (log c0) factors. In addition, the power-splitting receiver achieves the same asymptotic outage performance as that of the ideal EH receiver. Our results reveal that the line-of-sight component is crucial to improve the performance of the EH-based ROSE cooperative system, and the role decision with imperfect channel information has a detrimental effect on the system's diversity gain.

This paper investigates the feasibility of deploying dynamic role selection (ROSE) mechanism in three-node energy harvesting (EH) cooperative relay systems. A benchmark case of fixed role configuration is first studied which shows that full diversity order can be achieved and the system outage probability at high signal-to-noise ratio (SNR) is determined by the scaling laws of log c0/c0^2 together with log(log c0)/c0^2, with c0 denoting the transmit SNR at the source. Then, the ROSE mechanism is merged into the EH cooperative system and it is analytically shown that the ROSE mechanism can enhance the system diversity order to three, and the high-SNR outage behavior is no longer affected by the log c0 and log(log c0) factors. Interestingly different from the conventional EH relay system with fixed role configuration, in the high SNR regime our results show that the energy conversion efficiency and the power-splitting factor at the relay have negligible effect on the outage behavior of ROSE cooperation.

A cooperative network in which confidential messages are conveyed from a source to a legitimate destination with the help of decode-and-forward relays in the presence of a malicious eavesdropper is considered. Tight upper bounds on the ergodic secrecy rate are derived in the cases of i) cooperative beamforming and ii) multi-user selection. Further, a new concept of cooperative diversity gain, namely, adapted cooperative diversity gain (ACDG), is investigated. It is shown that the ACDG can be seen as an effective metric to evaluate the security level of a cooperative wireless network in the presence of eavesdroppers. Also, the ACDG obtained in the cooperative beamforming scenario is equal to the traditional cooperative diversity gain of traditional multiple-input single-output networks, while the ACDG obtained in the multiuser scenario is equal to that of traditional single-input multiple-output networks.

We consider a wireless system with a full-duplex (FD) access point (AP) that transmits to a scheduled user in the downlink (DL) channel, while receiving data from an user in the uplink (UL) channel at the same time on the same frequency. In this system, loopback interference (LI) at the AP and inter user interference between the uplink (UL) user and downlink (DL) user can cause performance degradation. In order to characterize the effects of LI and inter user interference, we derive closed-form expressions for the outage probability and achievable sum rate of the system. In addition an asymptotic analysis that reveals insights into the system behavior and performance degradation is presented. Our results indicate that under certain conditions, FD transmissions yield performance gains over half-duplex (HD) mode of operation.

We consider a dual-hop full-duplex (FD) relaying system, where the energy constrained relay node is powered by radio frequency signals from the source using the time-switching architecture. Both the amplify-and-forward and decode-and-forward relaying protocols are studied. Specifically, we provide an analytical characterization of the achievable throughput of three different communication modes, namely, instantaneous transmission, delay-constrained transmission, and delay tolerant transmission. In addition, the optimal time split is studied for different transmission modes. Our results reveal that, when the time split is optimized, FD relaying could substantially boost the system throughput compared to the conventional half-duplex relaying architecture for all three transmission modes. In addition, it is shown that the instantaneous transmission mode has the highest throughput. However, compared to the delay tolerant transmission mode, the throughput gap is negligible. Unlike the instantaneous time split optimization which requires instantaneous channel state information, the optimal time split in the delay tolerant transmission mode depends only on the statistics of the channel, hence, is attractive for practical implementation.

This paper investigates the impact of the multiple antenna and co-channel interference (CCI) on the outage performance of a dual-hop amplify-and-forward energy harvesting relaying network. The energy constrained relay is powered by radio frequency signals and employs the power splitting receiver architecture. To exploit the benefit of multiple antennas, two different linear processing schemes are investigated, namely, Maximum ratio combining/maximal ratio transmission (MRC/ MRT) and Minimum mean-square error/MRT (MMSE/MRT). For both schemes, a new closed-form outage lower bound and a simple high signal-to-noise ratio outage approximation are derived, respectively. Also, the achievable diversity order is quantified. In addition, we study the optimal power splitting ratio which minimizes the outage probability. Our results show that, by increasing the energy harvesting capability, the implementation of multiple antennas significantly improves the systems performance. Moreover, CCI could be potentially exploited to boost the performance, while how much performance gain can be obtained depends on the choice of the linear processing scheme.

In certain applications, relay terminals can be employed to simultaneously deliver information and energy to a designated receiver and a set of radio frequency (RF) energy harvesters, respectively. In such scenarios, the relay that is preferable for information transmission does not necessarily coincide with the relay that is preferable for energy transfer, since the corresponding channels fade independently. Relay selection thus entails a tradeoff between the efficiency of the information transmission to the receiver and the amount of energy transferred to the energy harvesters. The study of this tradeoff is the subject on which this work mainly focuses. Specifically, we investigate the dependence of the ergodic capacity and the outage probability of the information transmission to the receiver on the amount of energy transferred to the RF energy harvesters. We propose a relay selection policy that yields the optimal tradeoff in a maximum capacity/minimum outage probability sense, for a given energy transfer constraint. We also propose two suboptimal relay selection methods that apply to scenarios with limited availability of channel state information. Additionally, we propose a suboptimal scheme which approximates the optimal scheme for the special case of two relays and facilitates performance analysis. Interesting insights on the aforementioned tradeoffs are unveiled.

A relay network in which a source wishes to convey a confidential message to a legitimate destination with the assistance of trusted relays is considered. In particular, cooperative beamforming and user selection techniques are applied to protect the confidential message. The secrecy rate (SR) and secrecy outage probability (SOP) of the network are investigated first, and a tight upper bound for the SR and an exact formula for the SOP are derived. Next, asymptotic approximations for the SR and SOP in the high signal-to-noise ratio (SNR) regime are derived for two different schemes: 1) cooperative beamforming and 2) multiuser selection. Furthermore, a new concept of cooperative diversity gain, namely, adapted cooperative diversity gain (ACDG), which can be used to evaluate the security level of a cooperative relaying network, is investigated. It is shown that the ACDG of cooperative beamforming is equal to the conventional cooperative diversity gain of traditional multiple-input single-output networks, while the ACDG of the multiuser scenario is equal to that of traditional single-input multiple-output networks.

A full-duplex (FD) multiple antenna access point (AP) communicating with single antenna half-duplex (HD) spatially random users to support simultaneous uplink (UL)/downlink (DL) transmissions is investigated. Since FD nodes are inherently constrained by the loopback interference (LI), we study precoding schemes for the AP based on maximum ratio combining (MRC)/maximal ratio transmission (MRT), zero-forcing, and the optimal scheme for UL and DL sum rate maximization using tools from stochastic geometry. In order to shed insights into the systems performance, simple expressions for single antenna/perfect LI cancellation/negligible internode interference cases are also presented. We show that FD precoding at AP improves the UL/DL sum rate and hence a doubling of the performance of the HD mode is achievable. In particular, our results show that these impressive performance gains remain substantially intact even if the LI cancellation is imperfect. Furthermore, relative performance gap between FD and HD modes increases as the number of transmit/receive antennas becomes large, while with the MRC/MRT scheme, increasing the receive antenna number at FD AP, is more beneficial in terms of sum rate than increasing the transmit antenna number.

In this paper, an energy harvesting dual-hop relaying system without/with the presence of co-channel interference (CCI) is investigated. Specifically, the energy constrained multi-antenna relay node is powered by either the information signal of the source or via the signal receiving from both the source and interferer. In particular, we first study the outage probability and ergodic capacity of an interference free system, and then extend the analysis to an interfering environment. To exploit the benefit of multiple antennas, three different linear processing schemes are investigated, namely, 1) Maximum ratio combining/maximum ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT) and 3) Minimum mean-square error/MRT (MMSE/MRT). For all schemes, both the systems outage probability and ergodic capacity are studied, and the achievable diversity order is also presented. In addition, the optimal power splitting ratio minimizing the outage probability is characterized. Our results show that the implementation of multiple antennas increases the energy harvesting capability, hence, significantly improves the systems performance.

In certain applications, relay terminals can be employed to simultaneously deliver information and energy to a designated receiver and a radio frequency (RF) energy harvester, respectively. In such scenarios, the relay that is preferable for information transmission does not necessarily coincide with the relay with the strongest channel to the energy harvester, since the corresponding channels fade independently. Relay selection thus entails a tradeoff between the efficiency of the information transfer to the receiver and the amount of energy transferred to the energy harvester. The study of this tradeoff is the subject on which this work mainly focuses. We propose a relay selection policy that optimizes the quality of information transmission for a given energy transfer constraint. Additionally, we propose two suboptimal relay selection methods that apply to scenarios with limited availability of channel state information and facilitate closed-form analytical results. We conduct a performance analysis that sheds some light on the tradeoff between ergodic capacity and wireless energy transfer, and we prove that the optimal relay selection policy optimizes also other performance metrics for a given energy transfer, such as the outage probability and the error probability.

We consider a multipair relay channel, where multiple sources communicate with multiple destinations with the help of a full-duplex (FD) relay station (RS). All sources and destinations have a single antenna, while the RS is equipped with massive arrays. We assume that the RS estimates the channels by using training sequences transmitted from sources and destinations. Then, it uses maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To significantly reduce the loop interference (LI) effect, we propose two massive MIMO processing techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the RS. We derive an exact achievable rate in closed-form and evaluate the system spectral efficiency. We show that, by doubling the number of antennas at the RS, the transmit power of each source and of the RS can be reduced by 1.5 dB if the pilot power is equal to the signal power and by 3 dB if the pilot power is kept fixed, while maintaining a given quality-of-service. Furthermore, we compare FD and half-duplex (HD) modes and show that FD improves significantly the performance when the LI level is low.

This paper investigates the impact of different linear processing techniques on the outage performance of dual-hop amplify-and-forward (AF) relaying systems with co-channel interference (CCI) at the multiple antenna relay. Specifically, three heuristic linear precoding schemes are proposed to combat the detrimental effect of CCI, namely, 1) Maximum ratio combining/maximal ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT), 3) Minimum mean-square error/MRT (MMSE/MRT). New exact outage expressions as well as simple high signal-to-noise ratio (SNR) outage approximations are derived for all three schemes. Our results demonstrate that while the MRC/MRT and the MMSE/MRT schemes achieve a full diversity order of N, the ZF/MRT scheme can only achieve a diversity order of N-M, where N is the number of relay antennas and M is the number of interferers. In addition, it is observed that the MMSE/MRT scheme always achieves the best outage performance. The ZF/MRT scheme outperforms the MRC/MRT scheme in the low SNR regime, and exhibits an inferior performance compared to the MRC/MRT scheme in the high SNR regime.

We consider a multipair decode-and-forward relay channel, where multiple sources transmit simultaneously their signals to multiple destinations with the help of a full-duplex relay station. We assume that the relay station is equipped with massive arrays, while all sources and destinations have a single antenna. The relay station uses channel estimates obtained from received pilots and zero-forcing (ZF) or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To significantly reduce the loop interference effect, we propose two techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the relay station. We derive an exact achievable rate expression in closed-form for MRC/MRT processing and an analytical approximation of the achievable rate for ZF processing. This approximation is very tight, particularly for a large number of relay station antennas. These closed-form expressions enable us to determine the regions where the full-duplex mode outperforms the half-duplex mode, as well as to design an optimal power allocation scheme. This optimal power allocation scheme aims to maximize the energy efficiency for a given sum spectral efficiency and under peak power constraints at the relay station and sources. Numerical results verify the effectiveness of the optimal power allocation scheme. Furthermore, we show that, by doubling the number of transmit/receive antennas at the relay station, the transmit power of each source and of the relay station can be reduced by 1.5 dB if the pilot power is equal to the signal power, and by 3 dB if the pilot power is kept fixed, while maintaining a given quality of service.

We consider a dual-hop full-duplex relaying system, where the energy constrained relay node is powered by radio frequency signals from the source using the time-switching architecture, both the amplify-and-forward and decode-and-forward relaying protocols are studied. Specifically, we provide an analytical characterization of the achievable throughput of three different communication modes, namely, instantaneous transmission, delay-constrained transmission, and delay tolerant transmission. In addition, the optimal time split is studied for different transmission modes. Our results reveal that, when the time split is optimized, the full-duplex relaying could substantially boost the system throughput compared to the conventional half-duplex relaying architecture for all three transmission modes. In addition, it is shown that the instantaneous transmission mode attains the highest throughput. However, compared to the delay-constrained transmission mode, the throughput gap is rather small. Unlike the instantaneous time split optimization which requires instantaneous channel state information, the optimal time split in the delay-constrained transmission mode depends only on the statistics of the channel, hence, is suitable for practical implementations.

In this paper, we analyze the ergodic capacity of a dual-hop amplify-and-forward relaying system, where the relay is equipped with multiple antennas and subject to co-channel interference and the additive white Gaussian noise. Specifically, we consider three heuristic precoding schemes, where the relay first applies the: 1) maximal-ratio combining (MRC); 2) zero-forcing (ZF); and 3) minimum mean-squared error (MMSE) principle to combine the signal from the source, and then steers the transformed signal toward the destination with the maximum ratio transmission (MRT) technique. For the MRC/MRT and MMSE/MRT schemes, we present new tight analytical upper and lower bounds for the ergodic capacity, while for the ZF/MRT scheme, we derive a new exact analytical ergodic capacity expression. Moreover, we make a comparison among all three schemes, and our results reveal that, in terms of the ergodic capacity performance, the MMSE/MRT scheme always has the best performance and the ZF/MRT scheme is slightly inferior, while the MRC/MRT scheme is always the worst one. Finally, the asymptotic behavior of ergodic capacity for the three proposed schemes are characterized in large N scenario, where N is the number of relay antennas. Our results reveal that, in the large N regime, both the ZF/MRT and MMSE/MRT schemes have perfect interference cancellation capability, which is not possible with the MRC/MRT scheme.

In this paper, we deal with the deployment of full-duplex relaying in amplify-and-forward (AF) cooperative networks with multiple-antenna terminals. In contrast to previous studies, which focus on the spatial mitigation of the loopback interference (LI) at the relay node, a joint precoding/decoding design that maximizes the end-to-end (e2e) performance is investigated. The proposed precoding incorporates rank-1 zero-forcing (ZF) LI suppression at the relay node and is derived in closed-form by solving appropriate optimization problems. In order to further reduce system complexity, the antenna selection (AS) problem for full-duplex AF cooperative systems is discussed. We investigate different AS schemes to select a single transmit antenna at both the source and the relay, as well as a single receive antenna at both the relay and the destination. To facilitate comparison, exact outage probability expressions and asymptotic approximations of the proposed AS schemes are provided. In order to overcome zero-diversity effects associated with the AS operation, a simple power allocation scheme at the relay node is also investigated and its optimal value is analytically derived. Numerical and simulation results show that the joint ZF-based precoding significantly improves e2e performance, while AS schemes are efficient solutions for scenarios with strict computational constraints.

This paper considers a dual-hop amplify-and-forward (AF) relaying system where the relay is equipped with multiple antennas, while the source and the destination are equipped with a single antenna. Assuming that the relay is subjected to co-channel interference (CCI) and additive white Gaussian noise (AWGN) while the destination is corrupted by AWGN only, we propose three heuristic relay precoding schemes to combat the CCI, namely, 1) Maximum ratio combining/maximal ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT), 3) Minimum mean-square error/MRT (MMSE/MRT). We derive new exact outage expressions as well as simple high signal-to-noise ratio (SNR) outage approximations for all three schemes. Our findings suggest that both the MRC/MRT and the MMSE/MRT schemes achieve a full diversity of N, while the ZF/MRT scheme achieves a diversity order of N-M, where N is the number of relay antennas and M is the number of interferers. In addition, we show that the MMSE/MRT scheme always achieves the best outage performance, and the ZF/MRT scheme outperforms the MRC/MRT scheme in the low SNR regime, while becomes inferior to the MRC/MRT scheme in the high SNR regime. Finally, in the large N regime, we show that both the ZF/MRT and MMSE/MRT schemes are capable of completely eliminating the CCI, while perfect interference cancelation is not possible with the MRC/MRT scheme.

In this paper, a MIMO-Y channel based three-group information exchange problem is considered, where users from each group would like to exchange information with the other users in another two groups. In particular, a Max-Min user selection is proposed, which aims to harvest the multiuser diversity gain for reliable transmission in the MIMO-Y channel. The impacts of user configurations on the multiuser diversity gains are investigated by theoretical bounds as well as numerical results. It is shown that, by adding a few users in only one or two groups, the overall system performance can be improved significantly. This observation reveals an interesting behavior of MIMO-Y channel, i.e., the local configuration has a global impact. However, we prove that such asymmetrical multiuser diversity gain does not scale with the number of users. Finally, we show that multiuser diversity gain scales with the number of users if we add an equal number of users in all three groups.

We consider the problem of antenna selection (AS) in full duplex amplify-and-forward relaying. We assume a basic relay system where the source, the relay and the destination, all equipped with multiple antennas. For this full-duplex relay system, we investigate different AS schemes to select a single transmit antenna at both the source and the relay, respectively, as well as a single receive antenna at both the relay and the destination, respectively. To facilitate comparison, exact outage probability expressions and asymptotic approximations of these AS schemes are derived. In order to eliminate the zero-diversity behavior associated with the full-duplex operation, we also propose a simple power allocation (PA) scheme at the relay. In addition, the optimal value of the PA parameter is analytically derived for the considered AS schemes.

We consider a multi-pair relay channel where multiple sources simultaneously communicate with destinations using a relay. Each source or destination has only a single antenna, while the relay is equipped with a very large antenna array. We investigate the power efficiency of this system when maximum ratio combining/maximal ratio transmission (MRC/MRT) or zero-forcing (ZF) processing is used at the relay. Using a very large array, the transmit power of each source or relay (or both) can be made inversely proportional to the number of relay antennas while maintaining a given quality-of-service. At the same time, the achievable sum rate can be increased by a factor of the number of source-destination pairs. We show that when the number of antennas grows to infinity, the asymptotic achievable rates of MRC/MRT and ZF are the same if we scale the power at the sources. Depending on the large scale fading effect, MRC/MRT can outperform ZF or vice versa if we scale the power at the relay.

In this paper, we propose two new opportunistic interference alignment (IA) schemes to mitigate interference in the K-cell uplink interference channel. Unlike the existing schemes that basically rely on the channel randomness to achieve asymptotical IA for all cells, the proposed schemes employ an active alignment approach to increase the possibility for perfect partial IA for one cell within the network. Specifically, each user adopts the active alignment transmit beamforming such that the user-generated interference is perfectly aligned along the preferred reference interference direction at one of the base-stations (BS) in other cells. With this approach, our schemes increase the chance to obtain degrees-of-freedom gain even with a small number of users. In addition, several new user selection schemes are proposed, and the BS receiver is optimized to enhance performance. Extensive simulations are conducted, and the results show that the proposed schemes outperform the state-of-the-art under the considered scenarios.

This paper presents an analytical investigation on the outage performance of dual-hop multiple antenna amplify-and-forward relaying systems in the presence of interference. For both the fixed-gain and variable-gain relaying schemes, exact analytical expressions for the outage probability of the systems are derived. Moreover, simple outage probability approximations at the high signal-to-noise-ratio regime are provided, and the diversity order achieved by the systems are characterized. Our results suggest that variable-gain relaying systems always outperform the corresponding fixed-gain relaying systems. In addition, the fixed-gain relaying schemes only achieve diversity order of one, while the achievable diversity order of the variable-gain relaying scheme depends on the location of the multiple antennas.

We analyze the impact of antenna correlation on secrecy performance of multiple-input multiple-output wiretap channels where transmitter employs transmit antenna selection while receiver and eavesdropper perform maximal-ratio combining with arbitrary correlation. New closed-form expressions are derived for the exact and asymptotic (high signal-to-noise ratio in transmitter-receiver channel) secrecy outage probability.

In this paper, we analyze the performance of cooperative transmissions in wireless ad hoc networks with random node locations. According to a contention probability for message transmission, each source node can either transmits its own message signal or acts as a potential relay for others. Hence, each destination node can potentially receive two copies of the message signal, one from the direct link and the other from the relay link. Taking the random node locations and interference into account, we derive closed-form expressions for the outage probability with different combining schemes at the destination nodes. In particular, the outage performance of optimal combining, maximum ratio combining, and selection combining strategies are studied and quantified.

We analyze the outage performance of a two-way fixed gain amplify-and-forward (AF) relay system with beam-forming, arbitrary antenna correlation, and co-channel interference (CCI). Assuming CCI at the relay, we derive the exact individual user outage probability in closed-form. Additionally, while neglecting CCI, we also investigate the system outage probability of the considered network, which is declared if any of the two users is in transmission outage. Our results indicate that in this system, the position of the relay plays an important role in determining the user as well as the system outage probability via such parameters as signal-to-noise imbalance, antenna configuration, spatial correlation, and CCI power. To render further insights into the effect of antenna correlation and CCI on the diversity and array gains, an asymptotic expression which tightly converges to exact results is also derived.

This paper focuses on the relay selection problem in amplify-and-forward (AF) cooperative communication with full-duplex (FD) operation. Different relay selection schemes assuming the availability of different instantaneous information are studied. We consider an optimal relay selection that maximizes the instantaneous FD channel capacity and requires global channel state information (CSI) as well as several sub-optimal relay selection policies that utilize partial CSI knowledge such as a) source-relay and relay-destination links b) loop interference c) source-relay links and loop interference. To facilitate comparison, exact outage probability expressions and asymptotic approximations of these policies that show a zero diversity order are derived. In addition, an optimal relay selection that incorporates an hybrid relaying strategy, which dynamically switches between FD and half-duplexing relaying according to the instantaneous CSI, is also investigated.

This paper presents an analytical investigation on the outage performance of dual-hop multiple antenna fixed-gain amplify-and-forward relay systems in the presence of co-channel interference. Exact closed-form analytical expressions for the outage probability of the systems are derived. Moreover, simple outage probability approximations at the high signal to noise ratio regime are provided, and the diversity order achieved by the systems are characterized. Our results suggests that the fixed-gain AF relaying schemes only achieves diversity order of one. Moreover, it is beneficial to put the multiple antennas at the source or destination node rather than at the relay node.

In this paper, the effect of keyhole on the performance of multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks with orthogonal space-time block codes (OSTBCs) transmission is investigated. In particular, we analyze the asymptotic symbol error probability (SEP) performance of a downlink communication system where the amplifying processing at the relay can be implemented by either the linear or squaring approach. Our tractable asymptotic SEP expressions enable us to obtain both diversity and array gains. Our finding reveals that with condition n_S > min(n_R,n_D), the linear approach can provide the full achievable diversity gain of min(n_R,n_D) when only the second hop suffers from the keyhole effect, i.e., single keyhole effect (SKE), where n_S, n_R, and n_D are the number of antennas at source, relay, and destination, respectively. However, for the case that both the source-relay and relay-destination links experience the keyhole effect, i.e., double keyhole effect (DKE), the achievable diversity order is only one regardless of the number of antennas. In contrast, utilizing the squaring approach, the overall diversity gain can be achieved as min(n_R,n_D) for both SKE and DKE. An important observation corroborated by our studies is that for satisfying the tradeoff between performance and complexity, we should use the linear approach for SKE and the squaring approach for DKE.

We study the effect of outdated channel state information on the outage and error rate performance of amplify-and-forward (AF) relay selection, where only one out of the set of available relays is activated. We consider two variations of AF relay selection, namely best relay selection and partial relay selection, when the selection is based upon outdated channel estimates. For both these variations, closed-form expressions for the outage probability are obtained, along with approximate expressions for the symbol error rate in the medium to high signal-to-noise-ratio (SNR) regime. The diversity gain and coding gain of the above schemes are also explicitly derived. Numerical results manifest that the outage performance of AF relay selection is highly dependent on the level of correlation between the actual channel conditions and their corresponding (outdated) estimates. This result has a significant impact on the deployment of relay selection in practical applications, implying that a high feedback rate may be required in practice in order to attain the full benefits of relay selection. It is further shown that it may be preferable, in terms of outage and symbol error probability, not to include links in the relay selection process that experience a sufficiently high maximum Doppler shift, since in those cases partial relay selection outperforms best relay selection.

This paper investigates the impact of using outdated channel estimates for relay selection and signal amplification on the performance of amplify-and-forward (AF) relays under partial relay selection (PRS) and opportunistic relay selection (ORS). In practice, outdated channel state information (CSI) can occur due to feedback or scheduling delay. Both variable gain (VG) AF and fixed gain AF schemes are considered. Outage probability, the average bit error rate (BER) and simplified high signal-to-noise ratio approximations are derived. The effect of parameters such as the number of relays, the rank of chosen relay, and the correlation between the delayed and current channel state information are analyzed. Outdated CSI for computing relay gains in PRS causes about 2 dB loss. In ORS, a 3% reduction in correlation causes up to an order of magnitude increase in the outage probability.

This paper focuses on the relay selection problem in amplify-and-forward (AF) cooperative communication with full-duplex (FD) operation. Different relay selection schemes assuming the availability of different instantaneous information are studied. We consider optimal relay selection that maximizes the instantaneous FD channel capacity and requires global channel state information (CSI) as well as several sub-optimal relay selection policies that utilize partial CSI knowledge such as a) source-relay and relay-destination links b) loop interference c) source-relay links and loop interference. To facilitate comparison, exact outage probability expressions and asymptotic approximations of these policies that show a zero diversity order are derived. In addition, an optimal relay selection procedure that incorporates a hybrid relaying strategy, which dynamically switches between FD and half-duplex relaying according to the instantaneous CSI, is also investigated.

In this paper, we study full-duplex (FD) operation from a diversity perspective and investigate several protocols that extract diversity gains over a block fading channel. The investigated approach introduces a block-by-block transmission and requires data codewords that span several independent realizations of channel fading. This fundamental approach ensures a diversity gain at least equal to one (i.e., it does not suffer from error floors) and is employed to different relaying strategies without loop interference cancellation (LIC) and with imperfect LIC: a) Amplify-and-Forward (AF) without LIC, b) AF with imperfect LIC and c) Decode-and-Forward with imperfect LIC. The proposed protocols are analyzed from a diversity-multiplexing tradeoff (DMT) standpoint and practical universal codes that achieve the DMT of the proposed schemes are also presented. We show that AF without LIC ensures a full time diversity independently on the statistics of the loop interference, and it is introduced as a general FD-based relaying scheme. In addition, we demonstrate that imperfect LIC is not efficient for cases with strong residual loop interference and limits the diversity gain to one.

Multi-keyhole bridges the gap between single keyhole and full-scattering multiple-input multiple-output (MIMO) channels. In this paper, we therefore investigate the multikeyhole effect on the MIMO amplify-and-forward (AF) relay downlink transmission with orthogonal space-time block codes. In particular, we derive the analytical symbol error rate (SER) expression for the considered system with arbitrary number of keyholes. Moreover, SER approximations in the high SNR regime for several important special scenarios of multi-keyhole channels are further derived. These asymptotic results provide important insights into the impact of system parameters on the SER performance. Our analysis is confirmed by comparing with Monte-Carlo simulations.

This paper analyzes the exact average symbol error rate (SER) performance of beamforming in spatially correlated multiple-input single output (MISO) systems with channel estimation error and feedback delay. We derive the joint distribution function of two quadratic forms and employ the result to obtain an expression for the cumulative distribution function (cdf) of the received signal-to-noise ratio (SNR). Using this cdf expression, we investigate the exact SER applicable for a large number of modulation schemes. The results show that the average SER performance is sensitive to feedback delay, channel estimation and spatial correlation at the transmitter. Furthermore, feedback delay causes an error floor and has the most degrading impact on performance. We also present Monte Carlo simulation results as verification of our analytical results.

We investigate the error probability for Alamouti space-time coded orthogonal frequency-division multiplexing (OFDM) systems with carrier frequency offset (CFO) and multipath fading channels. Our main assumptions are: frequency-selective Rayleigh fading, a single receive antenna, and coherent detection. We derive new closed-form results for the (symbol or bit) error probability, for both spatially uncorrelated channels and spatially correlated channels. The proposed analysis shows that the error floor due to the CFO is strongly dependent on the power-delay profile of the multipath channel. Simulation results confirm the accuracy of the proposed theoretical analysis.

In this paper, we investigate the degenerative effects of antenna correlation and keyhole on the performance of multiple-input multiple-output (MIMO) amplify-and-forward (AF) relay networks. In particular, we considered a downlink MIMO AF relay system consisting of an nS-antenna base station S, an nR-antenna relay station R, and an nD-antenna mobile station D, in which the signal propagation originated from the mobile station suffers from keyhole and spatial correlation effects. We have derived an exact expression for the moment generation function of the instantaneous signal-to-noise ratio which enables us to analyze the symbol error probability and outage probability of the considered system. We have shown that although the mobile station is in a poor scattering environment, i.e., keyhole, the relay channel (S → R → D link) still achieves a cooperative diversity order of min(nR, nD) provided that the channel from the source (S → R link) is keyhole-free. This result is important to radio system designers since under such a severe scenario it is unnecessary to deploy a large number of antennas on the relay station (nR) but only requires nR= nDto obtain the maximum achievable diversity gain.

We investigate the outage probability and the average bit error rate (BER) of a dual-hop fixed gain relaying system in the presence of co-channel interference and thermal noise at the relay and the destination. Our analysis assumes Rayleigh fading for the source-relay and relay-destination channels and Rician fading for the interfering channels. We present new closed-form/series expressions for the outage probability and the average BER. The achievable diversity order is also studied. It is further shown through numerical results that the Rician K-factor of the interfering channel has a negative effect on the overall outage probability and BER, yet this effect is very small.

This paper evaluates the impact of using outdated channel estimates due to feedback delay for relay selection and signal amplification on the performance of partial relay selection with amplify-and-forward (AF) relays. Both variable and fixed gain AF schemes are considered. Expressions for the system's outage probability and the average bit error rate, and their high signal-to-noise ratio (SNR) approximations are derived. The effect of parameters such as the rank of relay chosen, SNR imbalance and the correlation between the delayed and current channel state information are studied and verified through simulations.

This paper evaluates the impact of using outdated channel estimates in a multiuser Amplify-and-Forward (AF) relay network, under Nakagami-m fading. Both variable gain AF and fixed gain AF schemes are considered. Expressions for the system's outage probability and the average bit error rate (BER) are derived. Since the expressions are barely tractable, we also present approximations for the high signal-to-noise ratio (SNR) regime. By doing so we characterize the impact of network parameters such as the number of relays, correlation between the delayed and current channel state information, chosen user rank and SNR imbalance on the performance degradation.

This paper analyzes the exact average symbol error rate (SER), outage probability and ergodic capacity performance of beamforming in spatially correlated multiple-input single output systems with channel estimation error and feedback delay. We derive the joint distribution function of two correlated quadratic forms and employ the result to obtain expressions for the cumulative distribution function, probability density function, moment generating function and moments of the signal-to-noise ratio. Using these expressions, we investigate the exact SER applicable for a large number of modulation schemes, outage probability and the ergodic capacity of the system. The results show that the average SER performance is sensitive to both feedback delay, channel estimation and spatial correlation at the transmitter. Furthermore, feedback delay causes an error floor and has the most degrading impact on performance. We also present Monte Carlo simulation results as verification of our analytical results.

Antenna selection schemes offer a good complexity versus performance tradeoff for amplify-and-forward (AF) relay network implementation. In this paper, we consider a dual-hop channel state information assisted AF relay network with a direct source-destination link and investigate the performance of two antenna selection schemes (one optimal and another suboptimal). We derive analytical expressions for the systems' rate outage probabilities and the average bit error rate (BER) that match perfectly with simulation based results in the medium to high signal-to-noise ratio (SNR) regimes. We also investigate the channel capacity of the two schemes by deriving tight upper bounds. In order to gain further insights, simple high SNR approximations for the outage probability and the average BER have also been developed. Our theoretical analysis shows that the performance gap between the optimal and suboptimal schemes largely diminishes in the high SNR regime by increasing the number of antennas at the source. Since the suboptimal scheme has a near optimal performance as well as low signaling overhead compared to the optimal scheme, it seems to be a promising solution for implementing future cellular networks expecting to support relay based communications.

We study the effect of outdated channel estimation on the outage performance of amplify-and-forward (AF) relay selection, where only one out of the set of available relays is activated. In particular, we derive closed-form expressions for the outage probability of two variations of AF relay selection, namely best relay selection and partial relay selection, when the selection is based upon outdated channel estimates. Numerical results manifest that the outage performance of both schemes under consideration is highly dependent on the level of imperfection of the channel estimates. It is further shown that it may be preferable, in terms of outage probability, not to include links in the relay selection process that experience high maximum Doppler shifts.

We examine the distribution of the per virtual-channel received signal-to-noise ratio over a multiple-input multiple-output channel modeled by a rank-2 non-central Wishart matrix. The exact probability density function is derived for all possible non-centrality matrices; and verified through simulation for selected systems. Identities for exact analytic results on the outage probability, the average symbol error rate, and the moment generating function are also derived.

The impact of antenna correlation on the performance of beamforming in a dual-hop fixed gain amplify-and-forward (AF) relay network is analyzed. The source and destination nodes are equipped with multiple antennas correlated in space, whereas the relay has a single antenna. Closed-form expressions for the system's outage probability and the average bit error rate (BER) are derived. We have also presented simple asymptotic outage probability and average BER expressions. When the second link is much stronger than the first link, it is shown that the system's performance at high SNR is only governed by the spatial correlation and number of antennas at the source and the diversity order is equal to the number of antennas at the source. Our analytical results have been verified through comparison with Monte Carlo simulations.

We examine the impact of channel knowledge on the secondary user (SU) in a cognitive radio system. Under a minimum signal-to-interference-and-noise ratio (SINR) constraint for the primary user (PU) receiver, we determine the SU capacity under four channel knowledge scenarios. We derive analytical expressions for the capacity cumulative distribution functions which are verified by means of simulations. We show that the lack of exact knowledge of the PU-PU channel gain by the SU-Tx either prohibits SU transmission or necessitates high interference level at the PU. We also show that the lack of exact knowledge of the SU-Tx to PU-Rx link has little or no impact on SU capacity.

In this paper, the performance of a dual-hop Amplify-and-Forward (AF) multi-antenna relay network, with end-to-end (e2e) best antenna selection, is investigated. To investigate the performance of this system, we first derive the exact outage probability in closed-form. It is then used to obtain expressions for e2e SNR moments and the average symbol/bit error rate (SER/BER) valid for a large class of practical modulation schemes. A simple and accurate BER approximation is also derived to quantify the performance at high SNR. Our analytical results, show that the e2e antenna pair selection scheme achieves the same diversity order as for the case where all antennas are used. To further confirm the validity of our analysis, Monte Carlo simulation results are also presented.

We analyze the impact of outdated channel state information due to feedback delay on the performance of amplify-and-forward relays with the$k$th worst partial relay selection scheme. In our analysis, new expressions for the system's outage probability and the average bit error rate are derived. The effects of the rank of the relay chosen, the average SNR imbalance, and the correlation between the delayed and current signal-to-noise ratio (SNR) on the system performance are investigated. Additionally, simple and accurate outage and average BER approximations are also derived to quantify the performance at high SNR. We also give simulation results to support the theoretical study.

Cognitive radio design aims to increase spectrum utilization by allowing the secondary users (SUs) to coexist with the primary users (PUs) as long as the interference caused by the SUs to each PU is properly regulated. At the SU, channel state information (CSI) between its transmitter and the PU receiver is used to calculate the maximum allowable SU transmit power to limit the interference. We assume that this PU-SU CSI is imperfect as it is the practical case. In addition to a peak received interference power constraint, an upper limit to the SU transmit power constraint is also considered. We derive a closed-form expression for the mean SU capacity under this scenario. Due to imperfect CSI, the SU can not always satisfy the received peak interference power constraint at the PU and has to back off its transmit power. The resulting capacity loss for the SU is quantified using the cumulative distribution function of the interference at the PU. Additionally, we investigate the impact of CSI quantization. Our results are confirmed through comparison with simulations.

In relay networks, partial channel state information at the transmitter due to feedback delay can severely degrade the beamforming performance. In this paper, the decremental effect of transmit beamforming with feedback delay on the downlink performance of a two hop Amplify-and-Forward (AF) relay network over Rayleigh fading channels is investigated. The source is equipped with multiple antennas while the relay and the destination has a single antenna. We have derived closed-form expressions for the outage probability, probability density function of the received signal-to-noise ratio (SNR) at the destination and the average bit error rate (BER). To gain further insights, outage probability and average BER approximations at high SNR are also presented. Numerical results supported by simulations are provided to illustrate the decremental effects feedback delay on the performance of the considered AF relay system.

In this paper, we consider a two hop Amplify and Forward (AF) multiple-input multiple-output (MIMO) relay network with antenna correlation, where beamforming is performed at the source and destination. This network consists of a single relay which is used to amplify and forward the signal from the source to the destination. The source and destination are both equipped with multiple antennas, which are correlated in space, while the relay has a single antenna. In this paper, we derive closed form expressions for the outage probability and probability density function of the received signal-to-noise ratio at the destination. We also present exact symbol error rate expressions for the two hop AF MIMO relay network, and show that the full spatial diversity order can be achieved.

For original paper see X. Zhu et. al., IEEE Commun. Lett., vol.11, pp.304-6, (2007).This paper discusses a number of errors in the Markov model and analysis of the blocking and forced termination probabilities in a cognitive radio network, given in [4]. The correct expressions of forced termination and blocking probabilities are provided. Under the same conditions as in [4], the probabilities and call completion rate are re-evaluated. The results show that there are significant differences both in terms of magnitude and behavior. Furthermore, an expression of aggregate throughput in bits per unit time is also given and evaluated. All the numerical plots are supported by simulation results.

The performance of beamforming with antenna correlation in a two hop amplify and forward (AF) multiple input multiple-output (MIMO) relay network is analyzed. This network consists of a single relay which is used to amplify and forward the signal from the source to the destination. The source and destination are both equipped with multiple antennas, which are correlated in space, while the relay has a single antenna. In this paper, we derive new closed form expressions for the outage probability and probability density function of the received signal-to-noise ratio (SNR) at the destination. We also present exact symbol error rate expressions for the two hop AF MIMO relay network, and show that the full spatial diversity order can be achieved. Our results also indicate that spatial correlation is detrimental to the outage probability and symbol error rate at high SNR, and beneficial at low SNR.

In real wireless communication environments, it is highly likely that different channels associated with a relay network could experience different fading phenomena. In this paper, we investigate the end-to-end performance of a dual-hop fixed gain relaying system when the source-relay and the relay-destination channels experience Rayleigh/Rician and Rician/Rayleigh fading scenarios respectively. Analytical expressions for the cumulative distribution function of the end-to-end signal-to-noise ratio are derived and used to evaluate the outage probability and the average bit error probability of M-QAM modulations. Numerical and simulation results are presented to illustrate the impact of the Rician factor on the end-to-end performance. Furthermore, these results confirm that the system exhibits an improved performance in a Rician/Rayleigh (source-relay link/relay-destination link) environment compared to a Rayleigh/Rician environment.

Cognitive radio technology is an innovative radio design concept which aims to increase spectrum utilization by exploiting unused spectrum in dynamically changing environments. By extending previous results, we investigate the capacity gains achievable with this dynamic spectrum approach in asymmetric fading channels. More specifically, we allow the secondary-to- primary and secondary-to-secondary user channels to undergo Rayleigh or Rician fading, with arbitrary link power. In order to compute the capacity, we derive the distributions of ratios of Rayleigh and Rician variables. Compared to the symmetric fading scenario, our results indicate several interesting features of the capacity behaviour under both average and peak received power constraints. Finally, the impact of multiple primary users on the capacity under asymmetric fading has also been studied.

This communication considers the distribution of the mutual information of frequency-selective spatially uncorrelated Rayleigh fading multiple-input–multiple-output (MIMO) channels. Results are presented for orthogonal frequency-division multiplexing (OFDM)-based spatial multiplexing. New exact closed-form expressions are derived for the variance of the mutual information. In contrast to previous results, our new expressions apply for systems with both arbitrary numbers of antennas and arbitrary-length channels. Simplified expressions are also presented for high and low signal-to-noise ratio (SNR) regimes. The analytical variance results are used to provide accurate analytical approximations for the distribution of the mutual information, and the outage capacity.

In this paper, new timing synchronization methods for optical wireless systems using asymmetrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM) are described. It is well known that OFDM systems are sensitive to timing synchronization errors. Existing synchronization methods cannot be used directly or do not perform well in ACO-OFDM systems because ACO-OFDM signals are unipolar. Therefore, new synchronization methods using a novel training symbol designed for the characteristics of ACO-OFDM are presented. The sensitivity of the new methods to the phase of the receiver sampling clock is considered and it is shown that if only the lower frequency subcarriers in the training symbol are non zero, the sensitivity can be reduced. Analysis and simulation results show that the new timing synchronization methods are effective in ACO-OFDM systems.

We derive new analytic approximations to the capacity distribution of frequency-selective Rayleigh fading MIMO channels. The results apply specifically to OFDM-based spatial multiplexing. In particular, we present a new closed-form Gaussian approximation which yields very high accuracy in many scenarios. For the low SNR regime, we also present a new simpler closed-form Gamma approximation, which we show to be even more accurate than Gaussian in this case. Our approximations are based on new exact closed-form expressions which we derive for the variance of the mutual information which, in contrast to previous results, apply for systems with both arbitrary numbers of antennas and arbitrary-length channel delay profiles.

Orthogonal frequency division multiplexing (OFDM) is used in many wired and wireless broadband communication systems because of its resilience in the presence of signal dispersion or multipath distortion. OFDM has not been used in practical optical communication systems because the bipolar waveform cannot be used in intensity-modulated direct detection (IM/DD) systems. A new unipolar form of OFDM, asymmetrically clipped optical OFDM (ACO-OFDM), has recently been developed. For the case of an AWGN channel, we compare ACO- OFDM and other modulation schemes. It is shown that ACO- OFDM with 4-QAVI subcarrier modulation has the same bandwidth efficiency but requires 2 dB less energy per bit than on-off keying. ACO-OFDM with larger constellation sizes gives higher bandwidth efficiencies and lower optical power than other modulation schemes. Unlike existing methods, the performance of ACO-OFDM is limited by the bandwidth of the transmitter and receiver not the dispersion of the channel.

In this paper we investigate the error probability for OFDM with carrier frequency offset (CFO) in additive white Gaussian noise (AWGN) and Rayleigh fading channels. The statistical nature of the intercarrier interference (ICI) due to CFO and the validity of the usual Gaussian distributed ICI assumption are studied. In AWGN channels, our simulations indicate that the theoretical symbol error rate (SER) obtained by assuming Gaussian characteristics for the ICI is not accurate. The effect of adjacent channel ICI coefficients must be considered. However in a fading channel provided that the channel correlation effects on the useful OFDM signal and the ICI are considered, the theoretical BER results due to Gaussian ICI approach and simulations closely agree. The theoretical SER for MQAM signaling in fading channels is also derived in closed form.

Oscillator phase noise (PN) in orthogonal frequency division multiplexing (OFDM) systems can cause severe performance degradation. In contrary to the assumptions made previously, recent work has discovered that the intercarrier interference (ICI) distribution due to a wide range of PN strengths in fact deviates from Gaussian characteristics. Hence we have compared the accuracy of a theoretical Gaussian symbol error rate (SER) analysis to that of the simulated SER in additive white Gaussian noise (AWGN), frequency flat and frequency selective Rayleigh fading channels. In multipath channels the SER is evaluated jointly considering the fading effects on the OFDM signal and ICI. Our results indicate that modeling ICI due to PN as Gaussian in the AWGN channel can lead to pessimistic theoretical predictions. However in fading channels, the difference between the simulated and the theoretical results is marginal.

Distributed MIMO technology is an effective way of dealing with the inter-cell interference problem in single frequency wireless communication networks. This paper studies the generic channel capacity formula for a distributed MIMO-OFDM system. A practical two-ring STBC distributed MIMO-OFDM transmission scheme is introduced. The cell throughput formula for this distributed scheme is derived. The impact of inner cell radius on cell throughput performance is analyzed for a multi-cell simulation model. From simulation results, we show that the two-ring distributed scheme can effectively mitigate inter-cell interference and provide good radio propagation channel quality, particularly for users at cells boundary. Moreover, we show that, with proper inner cell radius, the two-ring distributed MIMO-OFDM scheme can achieve significant cell throughput gain compared with that of conventional centralized MIMO-OFDM scheme.

It is well known that orthogonal frequency division multiplexing (OFDM) systems are sensitive to carrier frequency offset (CFO). In this paper we present the effects of CFO for OFDM systems where the Alamouti code is used to map adjacent subcarriers (space-frequency) or symbols (space-time) across the space-frequency grid. Simulation results show that space-time-frequency coded OFDM systems are more sensitive to CFO effects than conventional OFDM. We have also presented an analytical error performance analysis for the case of STC-OFDM over frequency flat Rayleigh fading channels. The theoretical results have been validated from the simulations. However the usually assumed signal independent Gaussian ICI analysis provides pessimistic results

This paper investigates the capacity behavior of spatially semi-correlated orthogonal frequency division multiplexing (OFDM) based multiple input multiple output (MIMO) systems. We extend the previous analysis on capacity by considering realistic scattering distributions such as Laplacian and von Mises distributions. Receiver angle spread, antenna spacing and nonuniform antenna configuration are also considered in this paper. Furthermore, we also investigate the accuracy of a Gaussian approximation of the outage capacity of MIMO-OFDM systems. Simulation results indicate that for both the i.i.d and semicorrelated fading cases considered, a Gaussian approximation is very accurate. We also demonstrate that both the delay spread and antenna configuration can increase the capacity of not-so-rich scattering MIMO-OFDM system.

Impulse noise is a significant problem in some OFDM applications, including digital television broadcasting. In this paper, we study a novel decision directed impulse mitigation algorithm, both analytically and with simulations for the DVB-T parameters. In this algorithm, the noise component in each received input sample is estimated based on preliminary decisions on the transmitted data. When the estimate is large enough to indicate that impulse noise is present in the sample, the estimated noise component is subtracted from the input sample before final demodulation. This technique has been shown to be extremely effective in flat fading channels. In this paper, its application in frequency selective fading channels is analyzed. The optimum weighting factors for combining noise estimates from subcarriers subject to different fading are calculated. Simulation results are presented for Rayleigh and Ricean channels which show that the technique can reduce the error rates due to impulsive noise by an order of magnitude.