This paper investigates hybrid-field channel estimation for extremely large-scale MIMO (XL-MIMO) systems. Different from the previous algorithms based on sparsity, the proposed algorithm considers both the low-rank structure and the sparsity of the channel, leading to a more accurate estimation performance. Particularly, we formulate the estimation problem of the hybrid-field channel as a sparse matrix recovery problem with a fixed-rank constraint. However, this problem encounters two challenges: the non-smooth$\ell_{1}-\mathbf{norm}$term and the non-convex fixed-rank matrix constraint. To address these challenges, we employ the proximal operator to handle the non-smooth$\ell_{1}-\mathbf{norm}$term and consider the set of matrices with a fixed rank as the fixed-rank matrix manifold. Then a fixed-rank matrix manifold-based proximal gradient (FRM-PG) algorithm is proposed to smoothly search the solution on the fixed-rank matrix manifold, where the descent direction is derived in each iteration. Simulation results demonstrate that the proposed algorithm outperforms the classical channel estimation algorithms.

Abstract Simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR‐RIS) is capable of achieving full coverage area compared with conventional reflection‐only reconfigurable intelligent surface (RIS). Motivated by this observation, the secrecy performance of STAR‐RIS‐assisted non‐orthogonal multiple access (NOMA) systems over Rayleigh fading channels is investigated, where the incident signals sent by base station are reflected and transmitted to a pair of NOMA users in the presence of two eavesdroppers. To evaluate the secrecy performance of STAR‐RIS‐NOMA systems, the exact and asymptotic expressions of secrecy outage probability (SOP) and secrecy diversity order are derived, in which the correlation of the channels between the elements of each column on STAR‐RIS is taken into consideration. As a benchmark, the secrecy performance of the STAR‐RIS strategy for orthogonal multiple access (OMA) system is also analysed. Numerical results are provided to verify the effectiveness of the analytical results and demonstrate that: The SOP of STAR‐RIS‐NOMA outperforms that of STAR‐RIS‐assisted OMA and conventional cooperative communication systems. Furthermore, the scheme proposed here can obtain full order of secrecy diversity.

Abstract This paper considers a relay‐assisted massive multiple‐input multiple‐output (MIMO) downlink system in which a base station (BS) communicates with all terminals via a relay node enabled simultaneous wireless information and power transfer (SWIPT). Both time‐switching (TS) and energy‐splitting (PS) protocols are respectively considered at the relay node for coordinating energy harvesting and information decoding. The quality‐of‐service (QoS) fairness problem among terminals is investigated in the considered systems. In detail, the power allocation coefficients and energy harvesting time duration for TS based SWIPT, and the power allocation coefficients and PS ratio for PS based SWIPT are jointly optimised, aiming at seeking to maximise the minimum achievable rate over all terminals. As the formulated problems are non‐convex and non‐linear, and hard to solve directly, suboptimal heuristic algorithms are proposed for solving them. Additionally, the convergence and complexity of the proposed algorithms are also analysed. Finally, the performance of the proposed algorithms is demonstrated by extensive computer simulations. Results manifest that the proposed algorithms can provide uniformly great QoS for every terminal by resource allocation.

Abstract The paper investigates simultaneous wireless information and power transfer (SWIPT) in massive distributed antenna systems, where the remote radio heads (RRH) equipped with a large number of antennas are arbitrarily distributed over the coverage area and each terminal operates via a power‐splitting (PS) device. Utilising the harvested energy, each terminal transmits its pilot signals for channel estimation. A closed‐form lower bound expression on the downlink ergodic capacity is derived for each terminal and then downlink ergodic sum capacity maximisation problem with respect to RRH‐terminal association, channel estimation duration, PS ratios and power allocation (PA) is designed. The designed problem is proved to be a mixed‐inter nonlinear programming with combinatorial variables, which is generally hard to solve due to its NP‐hardness. To this end, a hierarchical iterative algorithm is developed by utilising decomposition technique. Moreover, the convergence and computational complexity of this algorithm are analysed as well. Numerical results manifest the feasibility and effectiveness of this algorithm, and also show that it can achieve the maximum downlink ergodic sum capacity for massive distributed antenna systems with SWIPT protocol.

Abstract Both cooperative non‐orthogonal multiple access (NOMA) and physical layer security (PLS) are promising techniques for future communication systems. This paper investigates the PLS performance of a cooperative NOMA system where multiple sources transmit a superposed message to two users by utilizing either amplify‐and‐forward (AF) or decode‐and‐forward (DF) relays in both non‐colluding and colluding eavesdropper scenarios. A source‐relay selection scheme is proposed for this system aiming at improving the secrecy performance. Additionally, the closed‐form expression of the secrecy outage probability (SOP) is derived for the proposed source‐relay selection schemes. It is demonstrated that in the proposed schemes, the theoretical results match well with the simulation results. In addition, the proposed schemes outperform the conventional OMA system in terms of SOP.

In this study, the authors propose a simultaneous wireless information and power transfer scheme based on the power‐splitting (PS) protocol for a massive multiple‐input multiple‐output (MIMO) network. On the downlink, the base station transmits the information and energy simultaneously to each terminal and each terminal divides the received signals by its PS device between information decoder and energy harvester (EH). On the uplink, a portion of the energy harvested by EH is drawn for uplink pilot transmission in the coming frame and then the other is used for uplink data transmission in the current frame. Based on this scheme, the closed‐form lower bound expressions are obtained for the achievable downlink rate and uplink rate of each terminal and then are utilised to formulate an optimisation problem that is to maximise achievable sum rate of the whole network while ensuring the minimum required uplink and downlink rates of each terminal. However, as the original optimisation problem is non‐convex and non‐linear and very hard to solve directly, an algorithm is proposed by utilising successive approximation method based on geometric program. Numerical results verify the correctness and feasibility of the proposed algorithm.

Millimeter wave (mmWave) massive MIMO (Multiple-Input Multiple-Output) systems will enable gigabits per second transmitting data rate owing to the large bandwidth available in mmWave spectrum. It is difficult to utilize conventional lower frequency precoding techniques in mmWave massive MIMO systems because of the hardware constraints. Fortunately, the hybrid precoding can be utilized to extremely reduce the power consumption and high cost. In this paper, we propose the hybrid precoding scheme for downlink multi-user mmWave massive MIMO systems. Our scheme is based on the thought that the inter-user interference can be nullified at the analog step and the transmitting data rate is further maximized at digital step. The analog precoding matrix is derived based on the Equal Gain Transmission (EGT) method. Waterfilling technique is directly employed in the digital precoding matrix based on the almost orthogonal effective channel. Simulation results show that the proposed the hybrid EGT/waterfilling (HEW) scheme which has low complexity outperforms the other existing algorithms in low signal-to-noise ratio (SNR) scenarios.

A new joint best relay and jammer selection (JBRJS) scheme is conceived for enhancing the physical layer security (PLS) of cooperative networks relying on multiple energy harvesting (EH) aided intermediate nodes, which accumulate energy based on the power splitting (PS) protocol. Specifically, we select the best intermediate node as the relay, whilst exploiting all the remaining nodes as friendly jammers. Furthermore, we investigate the joint optimization of the PS ratio and the relay-aided beamforming for maximizing the system's secrecy rate and present a full channel state information (CSI) based joint PS and beamforming (fJPSB) scheme as the optimal solution by converting the optimization problem formulated into a singlevariable optimization problem. We also propose a partial-CSI based JPSB (pJPSB) method for the scenario where only the main link's CSI is available. Our numerical results show that the proposed JBRJS scheme beneficially enhances the PLS compared to the joint random relay and jammer selection (JRRJS) and to the pure best relay selection (PBRS) schemes. Moreover, the secrecy rate of the proposed fJPSB and pJPSB schemes is obviously higher than that of the fixed PS and beamforming (FPSB) method, and it further increases with the number of the relay's transmit antennas.

In this paper, we investigate physical-layer security for a heterogeneous cellular network consisting of a macro cell and a small cell in the presence of a common eavesdropper that intends to tap both the macro cell and small cell. The orthogonal spectrum sharing (OSS) and non-orthogonal spectrum sharing (NOSS) are considered for the heterogeneous cellular network. The OSS allows the macro cell and small cell to access a given spectrum band in an orthogonal manner, whereas the NOSS enables them to access the same spectrum simultaneously and mutual interference exists. We present two NOSS schemes, namely, the interference-limited NOSS (IL-NOSS) and interference-canceled NOSS (IC-NOSS), where the mutual interference is constrained below a tolerable level through power control in the IL-NOSS, and the IC-NOSS exploits a specially designed signal for canceling out the interference received a legitimate user while confusing the eavesdropper. We derive closed-form expressions for an overall secrecy outage probability of the OSS, IL-NOSS, and IC-NOSS schemes by jointly considering the secrecy of both the macro cell and small cell. Furthermore, we characterize the secrecy diversity of the OSS, IL-NOSS, and IC-NOSS schemes through the asymptotic secrecy outage analysis in the high signal-to-noise ratio (SNR) region. It is proved that the OSS and IL-NOSS schemes obtain the same secrecy diversity gain of zero only, and a higher secrecy diversity gain of one is achieved by the IC-NOSS scheme. In addition, numerical results demonstrate that secrecy outage probabilities of the IL-NOSS and IC-NOSS can be minimized through an optimization of the ratio of the SNR of small cell to the SNR of macro cell, called the small-to-macro ratio (SMR). Moreover, with an optimized SMR, the proposed IC-NOSS scheme significantly outperforms the OSS and IL-NOSS schemes in terms of individual secrecy outage probabilities of both the macro cell and small cell.

The decode‐and‐forward multi‐relay system with the presence of an eavesdropper is researched in this study. Two widely used transmission schemes are considered, namely, the best‐relay selection scheme and the all‐relay‐based beamforming scheme. To be specific, the former scheme adopts only the single best relay among all the candidate relays that have succeeded in decoding the source signal to assist the transmission, whereas the all‐relay‐based beamforming scheme allows all the candidates to assist the source simultaneously in a beamforming manner. The authors reveal that in both schemes, if the total transmit power of the two hops is constrained, contradiction exists between the two means of improving physical‐layer security: (i) letting more relays succeed in decoding the source signal; (ii) increasing the transmit power of the selected relay(s). The authors thus analyse the secrecy outage behaviour of the two schemes and derive expressions of secrecy outage probability (SOP), based on which, the optimal power allocation solutions are obtained. It is demonstrated that in both schemes, the analysis results match well with the simulation results. Moreover, the proposed power allocation solutions surpass the conventional average power solution in SOP in both schemes.