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In this paper, we investigate the performance analysis of the buffer-aided energy harvesting device-to-device (D2D) communication system. A joint data and energy resource scheduling problem is proposed considering the data packet and energy packet Poisson arrival but interactive departure. Considering the coupling relationship between data and energy, the joint resource scheduling problem is modeled as a data and energy coupled queuing model. We raise the expressions of carrier-to-interference and noise-ratio (CINR) interval division, which contains the dynamic interferences from cellular users and the other D2D users. To compromise between data delay and energy consumption, we propose a novel adaptive power transfer (APT) strategy that allows more energy consumption to improve data delay. Studying user mobility and APT strategies will lead to complex boundary behavior of the system transition state matrix. By random geometry theory and quasi-birth and death method to obtain steady-state transition probability, the expressions of throughput, delay and packet drop rate for both data queue and energy queue are derived. Simulations are demonstrated to verify the accuracy of the theoretical derivation results.

In this paper, we examine the physical layer security of a cooperative relay network where two source-destination pairs communicate through a decode-and-forward (DF) relay in the presence of multiple eavesdroppers. To safeguard the legitimate communications against eavesdropping, we propose a novel two-stage secure relay selection (TSSRS) with a non-orthogonal multiple access (NOMA) scheme to maximize the capacity of one source-destination pair, while guaranteeing the successful communication of the other source-destination pair. To explicitly reveal the benefits of our proposed scheme, we derive the exact and asymptotic expressions for its secrecy outage probability. As a benchmark, we also analyze the secrecy performance of the TSSRS strategy with an orthogonal multiple access (OMA) scheme. Both theoretical analysis and simulation results demonstrate that our proposed TSSRS-NOMA scheme significantly outperforms the TSSRS-OMA scheme when the transmit power at the source and relay is in the low and medium regimes. In addition, we show that the advantage of the TSSRS-NOMA scheme over the TSSRS-OMA scheme becomes obvious when the two source-destination pairs have profoundly different secrecy requirements.

In this paper, we first propose a joint user and relay selection (JURS) scheme to enhance physical layer security in a multi-user multi-relay network, where the best pair of the user and relay that maximizes the user-to-destination signal-to-interference-to-noise ratio (SINR) is jointly selected. We analytically examine the secrecy outage probability (SOP) of this scheme, based on which the optimal power allocation between the useful signal and artificial noise at the users is determined in order to minimize the corresponding SOP. To avoid the high complexity of the joint selection, we also propose a separate user and relay selection (SURS) scheme, where one relay is firstly selected to maximize the relay-to-destination SINR and a user is then selected to maximize the SINR from the user to the selected relay. We also derive the SOP of the SURS scheme and determine the corresponding optimal power allocation. As expected, the JURS scheme can outperform the SURS scheme. However, our examination also indicates that the low-complex SURS scheme can achieve similar secrecy performance as the high-complex JURS scheme under some specific conditions, e.g., when the number of users is much larger than that of the relays or when the average signal-to-noise ratio of user-to-relay channel is much higher than that of the relay-to-destination channel.

This paper investigates the physical layer security of non-orthogonal multiple access (NOMA) with decode-and-forward (DF) relays in underlay cognitive radio (CR) network. Secondary transmitter communicates with legitimate users via the aid of relays, while the eavesdroppers try to intercept the message. To safeguard legitimate communications against eavesdropping, the joint of artificial-noise (AN) and relay selection (RS) is applied. We derive the secrecy outage probability (SOP) and the secrecy throughput to evaluate the secrecy performance. In addition, the asymptotic floor of SOP is derived. Numerical results verify that cooperative NOMA in underlay CR (CRNOMA) significantly outperforms the conventional orthogonal multiple access (OMA) in underlay CR network (CROMA) in terms of the secrecy outage performance. Furthermore, the effect of some critical parameters on the secrecy performance has been studied.

Non-orthogonal multiple access (NOMA) is one of the promising radio access techniques in 5G communication networks. In this paper, a cooperative NOMA relaying scheme with two-phase superposition coding is proposed to enhance the performance of NOMA relaying systems. In the proposed scheme, the source simultaneously transmits two symbols using superposition coding in the first time slot, while the relay decodes the symbols by employing the successive interference cancellation. In the second time slot, the relay forwards the two symbols with a new superposition coding to the destination, which jointly decodes them by employing maximum-ratio combining technology over the received signals from both the source and relay. In order to reveal the benefits of the proposed scheme, its achieved ergodic sum rate, outage probability, and throughput are analyzed with imperfect channel state information taken into account. Our examination shows that the proposed scheme can significantly outperform existing schemes in terms of achieving a higher ergodic sum rate, a lower outage probability, or higher throughput at the cost of a slightly increased complexity.

In this paper, a transmit antenna selection (TAS)-based incremental hybrid decode–amplify–forward (IHDAF) scheme is proposed to enhance physical layer security in cooperative relay networks. Specifically, TAS is adopted at the source in order to reduce the feedback overhead. In the proposed TAS-based IHDAF scheme, the network transmits signals to the destination adaptive select direction transmission (DT) mode, AF mode, or DF mode depending on the capacity of the source-relay link and source-relay link. In order to fully examine the benefits of the proposed TAS-based IHDAF scheme, we first derive its secrecy outage probability (SOP) in a closed-form expression. We then conduct asymptotic analysis on the SOP, which reveals the secrecy performance floor of the proposed TAS-based IHDAF scheme when no channel state information is available at the source. Theoretical analysis and simulation results demonstrate that the proposed TAS-based IHDAF scheme outperforms the selective decode-and-forward, the incremental decode-and-forward, and the noncooperative DT schemes in terms of the SOP and effective secrecy throughout, especially when the relay is close to the destination. Furthermore, the proposed TAS-based IHDAF scheme offer a good tradeoff between complexity and performance compared with using all antennas at the source.

In this paper, we investigate cooperative secure beamforming for simultaneous wireless information and power transfer (SWIPT) in amplify-and-forward (AF) relay networks. We propose a joint cooperative beamforming (CB) and energy signal (CB-ES) scheme for providing both secure communication and efficient wireless energy transfer. By considering colluding eavesdroppers with imperfect channel state information (CSI), we formulate an optimization problem for maximizing the secrecy rate between the source and the legitimate information receiver (IR) under both the power constraints at the relays and the wireless power transfer constraint at the energy-harvesting receiver (ER). Since such a problem is nonconvex and hard to tackle, we propose a two-level optimization approach that involves a 1-D search and the semidefinite relaxation (SDR) technique to solve this problem. The proposed robust scheme is compared with some other nonrobust schemes, such as a CB and artificial noise (CB-AN) scheme and a perfect scheme. Simulation results show that the proposed robust scheme achieves better worst-case secrecy rate performance than the other nonrobust schemes and the CB-AN scheme, while it approaches the perfect scheme.

This paper proposes a backscatter-assisted wireless powered communication network that includes a hybrid access point and multiple users. In conventional wireless powered communication networks with only harvest-then-transmit (HTT) mode, urgent data transmission is not possible since users need to first harvest sufficient energy before transmitting information. Backscatter communication depends on instantaneous excitation energy such that the dedicated time for harvesting energy first is not required. To improve the system performance, both HTT and backscatter modes are employed at the users in the proposed model. An optimization problem is formulated to maximize the sum-throughput by finding the optimal transmission policy, including the optimal users’ working mode permutation and time allocation. Simulation results demonstrate the superiority of the proposed model.

In this work, we propose a novel beamforming design to enhance physical layer security of a non-orthogonal multiple access (NOMA) system with the aid of artificial noise (AN). The proposed design uses two scalars to balance the useful signal strength and interference at the strong and weak users, which is a generalized version of the existing beamforming designs in the context of physical layer security for NOMA. We determine the optimal power allocation among useful signals and AN together with the two optimal beamforming scalars in order to maximize the secrecy sum rate (SSR). Our asymptotic analysis in the high signal-to-noise ratio regime provides an efficient and near-optimal solution to optimizing the beamforming scalars and power allocation coefficients. Our analysis indicates that it is not optimal to form a beam towards either the strong user or the weak user in NOMA systems for security enhancement. In addition, the asymptotically optimal power allocation informs that, as the transmit power increases, more power should be allocated to the weak user or AN signals, while the power allocated to the strong user keeps constant. Our examination shows that the proposed novel beamforming design can significantly outperform two benchmark schemes.

In this paper, we investigate an artificial-noise (AN) aided secure transmission for non-orthogonal multiple access (NOMA) full-duplex (FD) relay network. We propose a novel joint NOMA and AN-aided full-duplex relay (NOMA-ANFDR) scheme to enhance the physical security. In this scheme, the optimal power allocation between the information and the AN signal is determined such that the capacity of the two end-to-end (i.e., two source-relay-destination pairs) channel are maximized to ensure the highest quality of cooperative transmission. To fully examine the benefits of the NOMA-ANFDR scheme, we derive a new closed-form expression for the secrecy outage probability.We show that the NOMA-ANFDR scheme significantly outperforms the joint NOMA and AN in half-duplex relay (NOMA-ANHDR) scheme as well as the NOMA-HDR scheme in terms of minimum secrecy outage probability and effect secrecy throughout. This result indicates that adopting the joint of FD and AN technique at relays can effectively enhance the physical layer secrecy performance in the NOMA cooperative network.

This paper considers a backscatter communication (BackCom) system including a reader and N tags, where each tag receives excitation signals transmitted by the reader and concurrently backscatters information to the reader in time-division-multiple-access (TDMA) mode. In this system, we aim to maximize the total system goodput by jointly optimizing reader transmission power, time allocation, and reflection ratio for the cases of passive and semi-passive tags. For each case, an optimization problem is formulated, which is non-convex and can be solved by being decomposed into at most N feasible sub-problems based on the priority of allocated reader transmission power. First, for the passive tags case, by solving the convex sub-problems sequentially and comparing their maximum total goodput, we derive the optimal resource allocation policy. Then, for the semi-passive tags case, we find a close-to-optimal solution, since each sub-problem can be reformulated as a biconvex problem, which is solved by a proposed block coordinate descent (BCD)-based optimization algorithm. Finally, simulation results demonstrate the superiority of the proposed resource allocation policies.