Rate Splitting (RS) is a versatile and powerful technique for multi-antenna transmission. In this paper, we study the precoding optimization for RS, which is critically important for improving the system performance but often challenging to address. We first investigate the non-convex sum rate maximization problem under perfect Channel State Information at the Transmitter (CSIT). By constructing a separable structure for the sum-of-functions-of-ratios problem and jointly leveraging the Convex Concave Procedure (CCP) and the Alternating Direction Method of Multipliers (ADMM), we obtain a fast algorithm that substantially reduces the overall computational time through the parallel computation and explicit closed-form solutions. We then investigate the average sum rate maximization problem under imperfect CSIT, which is known as a more challenging non-convex stochastic problem. To obtain a fast algorithm for practical use, we carefully approximate the non-convex stochastic problem to a non-convex deterministic one with acceptable performance loss and tailor the fast algorithm derived from perfect CSIT for imperfect CSIT with modest changes. Numerical results show that compared to the state-of-the-art algorithms, the proposed algorithms achieve comparable sum rates or average sum rates but short computation times for large problem sizes, owing to the unique parallel computation structures and few matrix inverse operations.

In this paper, we investigate reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) systems for robust physical layer security (PLS) schemes. Traditionally, eavesdroppers (Eves) typically avoid interacting with the base station, making it challenging to obtain their relevant information, which limits the implementation of PLS. Fortunately, the sensing information obtained by ISAC can contribute to the design of PLS. Therefore, leveraging imperfect sensing estimation and employing dedicated radar signals as artificial noise, we formulate an RIS-assisted joint active and passive beamforming design problem to maximize the sum secrecy rate while satisfying the user’s quality of service constraints, the transmission power constraints, and the sensing signal strength requirements. To make the problem tractable, we first derive the bound for Eve’s channel state information uncertainty region based on security approximations. Subsequently, we employ the$\mathcal {S}$-procedure and the symbolic-deterministic methods to transform the infinite number of inequalities. We then utilize the first-order Taylor expansion, the second-order cone methods, and the successive convex approximation to address the nonconvexity problem, leading to an efficient suboptimal solution obtained by an iterative algorithm. Finally, the simulation results demonstrate the significant potential of the sensing function in enhancing security and the effectiveness of the proposed robust scheme in flexibly balancing communication and sensing quality.

Movable antennas (MAs) enhance flexibility in beamforming gain and interference suppression by adjusting position within certain areas of the transceivers. In this paper, we propose an MA-assisted integrated sensing and communication framework, wherein MAs are deployed for reconfiguring the channel array responses at both the receiver and transmitter of a base station. Then, we develop an optimization framework aimed at maximizing the sensing signal-to-interference-plus-noise-ratio (SINR) by jointly optimizing the receive beamforming vector, the transmit beamforming matrix, and the positions of MAs while meeting the minimum SINR requirement for each user. To address this nonconvex problem involving complex coupled variables, we devise an alternating optimization-based algorithm that incorporates techniques including the Charnes-Cooper transform, second-order Taylor expansion, and successive convex approximation (SCA). Specifically, the closed form of the received vector and the optimal transmit matrix can be first obtained in each iteration. Subsequently, the solutions for the positions of the transmit and receive MAs are obtained using the SCA method based on the second-order Taylor expansion. The simulation results show that the proposed scheme has significant advantages over the other baseline schemes. In particular, the proposed scheme has the ability to match the performance of the fixed position antenna scheme while utilizing fewer resources.

In this paper, we investigate physical layer security (PLS) for terrestrial RIS-assisted multibeam satellite commu-nications with similar channels. Deploying the reconfigurable intelligent surface (RIS) in each beam, and we use inter-beam in-terference and the channel variability from the RIS to the satellite user and eavesdroppers to enhance satellite security. Specifically, considering the constraints of the secrecy rate, we formulate a problem to minimize the total power by jointly optimizing the satellite beamforming and the RIS beamforming. To solve the problem, the original problem is decoupled into two non-convex optimization sub-problems, i.e., the active and the passive beamforming optimization problem. Then, semidefinite relax-ation (SDR) is used to solve the active beamforming optimization problem. The maximum ratio transmission-based beamforming and SDR are employed separately to solve passive beamforming optimization problem in each beam. The alternating optimization (AO) is utilized to solve the original problem and the convergence is proven. We have carried out simulations to evaluate the effectiveness of our proposed approach, the results show that the proposed algorithms have superior system performance.

The integration of nonorthogonal multiple access (NOMA) with integrated sensing and communication (ISAC) heralds a novel and promising paradigm, advancing the frontier of wireless communication technologies. Nevertheless, this synergistic integration may confront security challenges attributable to intrinsic vulnerabilities within the ISAC frameworks, i.e., sensing targets assume the role of potential eavesdroppers. In this article, we exploit the additional degrees of freedom afforded by reconfigurable intelligent surfaces (RISs) to enhance secure communication and achieve target detection within the NOMA-assisted ISAC system. Specifically, the coexisting radar and NOMA signals are concurrently propagated through both the direct and reflected links. Subsequently, we formulate a secure optimization problem by collaboratively designing the beamforming vectors of the base station and the phase shifts of RIS under the constraints of total transmit power, communication quality of service, and sensing quality. Due to nonconvexity, the optimization problem is decomposed into two subproblems and addressed separately, employing the successive convex approximation approach based on the first-order Taylor expansion and the second-order cone. Finally, we obtain the solution to the original problem utilizing alternating optimization. Simulation results demonstrate that our proposed approach exhibits superior capabilities in secure communication and precise target detection.

This study presents a full‐duplex (FD) relaying non‐orthogonal multiple access (NOMA) scheme with self‐interference at relay taken into account, where a sender communicates with multiple terminals simultaneously with the help of an FD amplify‐and‐forward relay. The relay is equipped with two antennas, one for transmission and the other for reception. Moreover, with a direct link as a useful information transmission link, selection combining is applied to merge the information from the sender and the relay. General closed‐form expressions of the outage probability are deduced and the upper bound of the ergodic sum rate is derived, including half‐duplex (HD) mode relay as a special case. Compared with the HD relaying scheme, the authors can observe that the FD relaying scheme can achieve higher ergodic sum rate in the moderate‐ and high‐SNR regions and that the outage behaviour can be improved in most cases (if the self‐interference is not extremely severe). More importantly, the use of NOMA in FD relaying system is shown to outperform conventional orthogonal multiple access (OMA) network in terms of terminal fairness. Extensive simulation results are presented to confirm their conclusions and provide a significant reference for the system design.

In this paper, we present a time-domain turbo equalizer for oversampled single carrier (OSC) system under doubly selective fading channels, which is in a new scenario. The detected information symbols are stacked into a state space vector, and its relationship with the received signals is developed, which contributes to realising a Kalman filter. In the receiver, the adaptive equalizer using Kalman filters cooperates with the soft-input soft-output (SISO) decoder to iteratively generate posteriori estimates for each data symbol. Numerical simulation results demonstrate the proposed scheme outperforms both the extended Kalman filter (EKF) method and the linear Kalman filter (LKF) method significantly. Furthermore, complexity analysis is also presented to illustrate the increasing complexity.

Traditional vector orthogonal frequency division multiplexing (V‐OFDM) can obtain both multi‐path diversity and outage gain by transforming an inter‐symbol interference (ISI) channel into multiple ‘ISI‐free’ channel vectors. To achieve full multi‐path diversity and acquire better outage performance, a low‐complexity precoding scheme combined with simple minimum mean square error (MMSE) equalisation for V‐OFDM system is proposed in this study. First, the upper mutual information bound of the V‐OFDM system is derived with channel state information at transmitter. Based on the upper bound, an optimal precoding scheme for V‐OFDM is presented. Meanwhile, the closed‐form and asymptotic expressions on outage probability of the proposed scheme are derived. Simulations show that this low‐complexity scheme can achieve full multi‐path diversity and result in lower outage probability than the traditional V‐OFDM scheme. Besides, the proposed scheme in ISI fading channel is capable of approaching the upper bound with <0.1 bits/s/Hz performance gap and improving average channel capacity with 2 bits/s/Hz over the traditional V‐OFDM scheme.

Energy harvesting, which is a promising technology to prolong the lifetime of energy-constrained wireless networks, has attracted lots of attention. In this letter, we consider an energy-harvesting multiple-relay multiple-input multiple-output (MIMO) decode-and-forward (DF) cooperation network. A joint relay-and-antenna selection (JRAS) scheme, which jointly determines the best transmit antenna at the source node, the best relay, and the best transmit antenna at the selected relay, is proposed to minimize the system outage probability. Then, we analyze the outage performance of the JRAS scheme, and the exact expression for the outage probability is derived. Finally, simulation results show the correctness of the theoretical analysis and the superiority of the JRAS scheme.

This paper investigates joint relay and antenna selection schemes for two-way amplify-and-forward (AF) relaying system, where a multiantenna source exchanges information with a single-antenna source by exploiting a group of single-antenna relays. In view of performance and complexity tradeoffs, two optimal and one suboptimal joint selection schemes are proposed and compared. In contrast to the optimal selection schemes, the suboptimal counterpart is characterized by its low overhead and implementation complexity. The performance of these selection strategies has been investigated by deriving the outage probability approximations in closed form. Moreover, the achievable diversity orders and the corresponding coding gains are derived through high signal-to-noise ratio (SNR) approximations of the outage probability. Our theoretical and simulation results show that the performance gap between the optimal and suboptimal schemes disappears for specific scenarios, which provides useful design guidelines for practical two-way relaying system.

Two opportunistic transmission schemes, i.e., joint user-relay selection with beamforming (JURS-BF) and joint user-relay selection with antenna selection (JURS-AS), are proposed and analyzed for multi-user multi-relay bidirectional relaying networks (BRNs) composed of one N-antenna base station (BS)/ access point (AP), M single-antenna users, and K single-antenna relays. In particular, by considering the rate asymmetry between the BS/AP and the users, joint user-relay selection is tackled with beamforming and antenna selection to combine the benefits of multi-antenna diversity, multi-relay diversity, and multi-user diversity. To characterize the performance of the proposed JURS-BF and JURS-AS schemes, lower and upper bounds on the system outage probability are derived in closed form under Nakagami-m fading environment, followed by the diversity, coding gain analysis, and some discussions on the relay deployment. Our analytical results are verified through simulations, and it is proved that the proposed suboptimal schemes perform closely to the traditional outage-optimal counterparts while with much lower implementation complexity, which provides valuable insights into practical BRNs.

Summary With the rapid growth of cloud computing services, there is an increasing need to expand applications across data centers. In these paradigms, common information exchange between two data centers can be assisted via one or more substrate nodes, which correspond to bidirectional relaying communication. In this paper, we investigate the performance of time division broadcast (TDBC) protocol in bidirectional cloud networks in the presence of channel estimation errors (CEEs). The tight asymptotic expressions for individual and system outage probabilities are first presented in closed‐form. It is shown that CEE causes outage probability to remain fixed even if the channel is noiseless. Then, the symbol error rate performance for TDBC protocol with M‐ary Phase Shift Keying and M‐ary quadrature amplitude modulation signals is analyzed. Simulation results validate the accuracy of our analytical results. Furthermore, comparison of signal‐to‐noise ratio gap ratio shows that TDBC protocol is less sensitive to the effect of CEE than analog network coding protocol. Copyright © 2014 John Wiley & Sons, Ltd.

Energy harvesting, which is an emerging technology to prolong the lifetime of energy‐constrained wireless networks, has received significant attention. In this study, the authors consider an amplify‐and‐forward‐based asymmetric two‐way relaying system, where two source nodes exchange information through an energy‐constrained relay node. The relay node scavenges energy from the received signals and uses the harvested energy to forward the received signals to the two terminal nodes. The signal power splitting ratio, depicting the trade‐off between the harvesting energy and the forward signals’ power, is a crucial factor for the system outage probability. Therefore, an optimal power allocation (OPA) scheme, which jointly takes into consideration the optimisation of signal power splitting ratio and the transmission power allocation, is proposed to minimise the outage probability. In addition, a close‐form solution for the OPA scheme is obtained. Finally, a joint OPA and relay selection schemes are presented to further improve the system performance. Simulation results highlight the superiority of the author's proposed scheme.

This study analyses the performance of a roadside two‐way relaying system in which a roadside access point (AP) and a vehicle exchange messages with the aid of amplify‐and‐forward mobile relay (MR) based on partial relay selection. It has been shown that AP‐MR‐vehicle communications may experience severer channel fading than conventional cellular communications. Mixed Nakagami ‐m and ‘double’ Nakagami ‐m fading is adopted to provide a realistic description of the involved AP‐MR‐vehicle channels. In this scenario, a tight closed‐form lower bound and high signal‐to‐noise ratio approximate expression for the system outage probability are derived. By applying these results, the authors obtain the diversity and the coding gains and the average symbol error rate for the considered system. In particular, the optimum number of relays is provided, providing valuable guidelines for practical system design. It is shown that when the number of relays is greater than the optimum number, no performance gain would be further achieved. The simulation results highlight the authors theoretical analysis.

Energy efficiency (EE) is among the main considerations in the design of modern wireless networks. In this paper, we investigate the EE enhancement for asymmetric analog network coding (ANC) protocol of a two-way relay system based on statistical channel information. A power allocation problem is formulated as the system EE maximization problem with objective function quantified by Goodbit-per-Energy (GPE). Importantly, the EE optimization problem may not be convex and can be categorized into a nonlinear fractional programming problem. Therefore, to solve the problem, a nonlinear fractional programming based algorithm is proposed and closed-form solution is obtained, providing valuable insights into practical system designs. Simulation results highlight the effect of the proposed power allocation.

This study deals with the performance analysis and power allocation of a two-way amplify-and-forward relay system with channel estimation errors. Exact closed-form expressions for outage probability and average symbol error rate (SER) are first presented. To provide more insights, their closed-form asymptotic expressions are then obtained. It is shown that the presence of channel estimation error causes outage probability and average SER maintain a fixed level even when a noiseless channel is adopted. These results are applied further to study the optimal power allocation problem for each node. From the perspective of service quality, rather than minimise the outage probability as much as possible, the goal is to use the minimum energy consumed to satisfy the traffic requirements and thereby conserve the energy resource. Furthermore, based on the optimal solutions to power allocation, the optimal relay location is investigated, which indicates that the system has higher energy efficiency with the relay located around the middle point of the two end nodes for any asymmetric traffic requirement. The simulation results verify that the derived outage probability and average SER expressions are accurate and highlight the effect of power allocation under various traffic requirements and channel estimation errors.