Half-duplex Relay Research Articles

Intercept Probability Analysis of the Energy Harvesting Enabled Multisource Half-Duplex Relaying Network with Receiver Diversity Techniques

Intercept Probability Analysis of the Energy Harvesting Enabled Multisource Half-Duplex Relaying Network with Receiver Diversity Techniques

Intelligent deep reinforcement learning-based scheduling in relay-based HetNets

We consider a fundamental file dissemination problem in a two-hop relay-based heterogeneous network consisting of a macro base station, a half-duplex relay station, and multiple users. To minimize the dissemination delay, rateless code is employed at the base station. Our goal is to find an efficient channel-aware scheduling policy at the half-duplex relay station, i.e., either fetch a packet from the base station or broadcast a packet to the users at each time slot, such that the file dissemination delay is minimized. We formulate the scheduling problem as a Markov decision process and propose an intelligent deep reinforcement learning-based scheduling algorithm. We also extend the proposed algorithm to adapt to dynamic network conditions. Simulation results demonstrate that the proposed algorithm performs very close to a lower bound on the dissemination delay and significantly outperforms baseline schemes.

On the diversity-multiplexing trade-off of hybrid diamond relay channels

The diversity-multiplexing trade-off (DMT) of a diamond relay channel in a quasi-static slow Rayleigh fading environment is studied, where one of the relays operates in half-duplex mode while the other one is full-duplex; hence, the term hybrid is used to refer to this channel. DMT is a known powerful tool that captures the inherent tension between the reliability and the transmission rate in a wireless communication network at high signal-to-noise ratios (SNR). In this study, we have assumed that the average SNRs of different links vary at different rates, each represented by an exponent at high SNR. These average SNR exponents are categorized into two different classes: In Class I, both of the relays are placed either in close proximity to the source or in close proximity to the destination, whereas in Class II one of the relays is closer to the source while the other one is closer to the destination. Subsequently, for Class I and for a special symmetric case of Class II, an explicit expression of the DMT of the static quantize-map-and-forward (SQMF) strategy and the optimal static schedule in the half-duplex relay, are derived. The non-symmetric case of Class II can numerically be studied using the optimization problem provided in the paper for any given parameters. As a corollary, our analysis shows that for Class I, the SQMF strategy in the proposed hybrid diamond relay channel reaches the ideal full-duplex DMT upper bound if the degrees of freedom provided by the path through the full-duplex relay is greater than or equal to that provided by the path through the half-duplex relay, or if the degrees of freedom provided by the former is less than that provided by the latter, but still large enough to support the desired multiplexing gain.

Cooperative Non-Orthogonal Multiple Access Transmission Through Full-Duplex and Half-Duplex Relays

We investigate the beamforming for cooperative non-orthogonal multiple access (NOMA) transmission through a decode-and-forward relay either with full-duplex (FD) mode or with half-duplex (HD) mode. The beamforming vectors at the source and at the relay are optimized to maximize the end-to-end sum rates of the NOMA transmission. The optimal beamforming for the second-hop of the NOMA transmission is present, from which the beamforming algorithms for the two relay modes are derived. For the FD case, the removal of self interference from the feedback loop is important and a two dimensional (2-D) search algorithm based on semi definite relaxation (SDR) is provided, where the 2-D search range is narrowed down as the algorithm iterates. For the HD mode, no iteration is necessary and only one implementation of SDR is required since the SI suppression is not needed. Simulation results show the performance advantage of the proposed FD mode system over that of HD mode in various parameter settings. This justifies the cost and complex hardware circuitry of FD mode.

Improving the performance of wireless half-duplex and full-duplex relaying networks with intelligent reflecting surface

In this paper, an intelligent reflecting surface (IRS), which consists of L reflecting elements (REs), is utilized to improve the coverage and reliability of half-duplex relaying (HDR) and full-duplex relaying (FDR) wireless communication systems. We provide a rigorous analysis on the system performance, which is represented by the exact form expressions of outage probability (OP), symbol error probability (SEP), and ergodic capacity (EC), of the HDR-IRS and FDR-IRS systems over Rayleigh fading channels. All derived expressions are validated by Monte-Carlo simulations. It is shown that IRS can significantly improve the performance of HDR and especially FDR networks, in which IRS can suppress the impact of the loopback interference at the relay.

Spectrally Efficient Uplink Cooperative NOMA With Joint Decoding for Relay-Assisted IoT Networks

We propose a spectrally efficient uplink cooperative relaying protocol with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> multiple half-duplex relay stations (RNs) for wireless Internet of Things (IoT) networks, where two IoT devices (IDs) and an access point (AP) cannot directly communicate with each other. The two IDs simultaneously transmit signals over the same spectrum resource using the nonorthogonal multiple access (NOMA) technique. In the proposed technique, at a certain time slot, a single RN is selected to send the received packets from two IDs in the previous time slot, while the remaining <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K-1$ </tex-math></inline-formula> RNs receive new packets from two IDs. Thus, the proposed relaying protocol can send <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> packets from an ID to the AP during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N+1$ </tex-math></inline-formula> time slots without a duty-cycle loss in conventional half-duplex relaying schemes. We call the proposed protocol a spectrally efficient cooperative NOMA. The main highlight of our results is that we mathematically derive a closed-form outage probability of the proposed protocol by considering inter-relay interference, assuming an optimal joint decoding technique instead of successive interference cancelation (SIC) decoding at each RN. To the best of our knowledge, this is the first theoretical result in the literature. Extensive computer simulations are used to validate the mathematical results. Furthermore, it is shown that the proposed protocol significantly outperforms the existing two-hop half-duplex cooperative NOMA technique and the SIC decoding scheme in terms of outage probability.

Joint Beamforming Design for Multiuser MISO Downlink Aided by a Reconfigurable Intelligent Surface and a Relay

Reconfigurable intelligent surfaces (RISs) have drawn considerable attention due to their ability to direct electromagnetic waves into desirable directions. Although RISs share some similarities with relays, the two have fundamental differences impacting their performance. To harness the benefits of both, we propose a downlink system wherein a relay and an RIS improve performance in terms of energy-efficiency. Using singular value decomposition (SVD), semidefinite programming (SDP), and function approximations, we propose different solutions for optimizing the beamforming matrices at the base-station (BS), the relay, and the phase shifts at the RIS to minimize the total power under quality-of-service (QoS) constraints. The problem is solved when the relay operates in half-duplex and full-duplex modes and when the reflecting elements have continuous and discrete phase shifts. Simulation results compare the performance of the system with and without the RIS or the relay, under different optimization solutions. The results show that the system with full-duplex relay and RIS outperforms the other scenarios, and the contribution of full-duplex relay is higher than that of the RIS. However, an RIS outperforms a half-duplex relay when the required QoS is high. The results also show that increasing the number of reflecting elements improves the performance better in the presence of a relay than in its absence.

Relay-Assisted Cooperative Federated Learning

Federated learning (FL) has recently emerged as a promising technology to enable artificial intelligence (AI) at the network edge, where distributed mobile devices collaboratively train a shared AI model under the coordination of an edge server. To significantly improve the communication efficiency of FL, over-the-air computation allows a large number of mobile devices to concurrently upload their local models by exploiting the superposition property of wireless multi-access channels. Due to wireless channel fading, the model aggregation error at the edge server is dominated by the weakest channel among all devices, causing severe straggler issues. In this paper, we propose a relay-assisted cooperative FL scheme to effectively address the straggler issue. In particular, we deploy multiple half-duplex relays to cooperatively assist the devices in uploading the local model updates to the edge server. The nature of the over-the-air computation poses system objectives and constraints that are distinct from those in traditional relay communication systems. Moreover, the strong coupling between the design variables renders the optimization of such a system challenging. To tackle the issue, we propose an alternating-optimization-based algorithm to optimize the transceiver and relay operation with low complexity. Then, we analyze the model aggregation error in a single-relay case and show that our relay-assisted scheme achieves a smaller error than the one without relays provided that the relay transmit power and the relay channel gains are sufficiently large. The analysis provides critical insights on relay deployment in the implementation of cooperative FL. Extensive numerical results show that our design achieves faster convergence compared with state-of-the-art schemes.

NOMA-based cognitive radio network with hybrid FD/HD relay in industry 5.0

The continued growth of the industrial Internet of Things (IoT) paves the way for the fifth industrial revolution (Industry 5.0), which is expected to further enhance the wireless connectivity and human machine collaboration. Nonetheless, Industry 5.0 will inevitably face the great challenges such as large scale connectivity and explosive data traffic because of the proliferation of IoT devices. To meet these challenges, we propose a novel non-orthogonal multiple access (NOMA)-based overlay cognitive radio (OCR) network for Industry 5.0. Specifically, the hybrid full-duplex (FD)/ half-duplex (HD) mode at the secondary user (SR) is adopted and in-phase and quadrature phase imbalance (IQI) is introduced to better comply with the practical scenarios. We investigate the outage probability (OP) and ergodic sum rate (ESR) of the considered network with the Nakagami-m channels. Extensive simulation experiments are performed to verify the accuracy of the analysis. From the simulation results, we can conclude: i) Increasing the signal-to-noise ratio (SNR), OP decreases but ESR increases and tends to a fixed value at high SNR regimes, indicating that the existence of IQI leads to a ceiling for the ESR; ii) The primary system has higher reliable performance at low SNR regions when SR is in FD mode, and for ergodicity, the system with FD relay is superior to HD relay; iii) The reliable performance of OMA system for primary users will be better since there is no interference at the relay.

New Unequal Error Protection Strategy for Image Transmission Based on Bilayer-Lengthened PLDPC Code in Half-Duplex Relay System

To reduce the waste of energy in communications, unequal error protection (UEP) is used to provide asymmetric protection for messages with different levels of importance. This paper proposes new efficient strategies of UEP based on bilayer protograph-based low-density parity check (PLDPC) codes in decoding-and-forward (DF) relay systems. In particular, we jointly utilize source coding and channel coding to design UEP strategies and then save transmission energy. According to the different levels of importance of discrete cosine transform (DCT) coefficients of image and variance statistical characteristics of image sub-blocks, bilayer-lengthened PLDPC codes are exploited to protect the transmitted image information with different importance levels at the half-duplex relay system. In the end, the simulation result shows that the proposed UEP schemes achieve excellent performance gains compared to conventional equal error protection (EEP) scheme. Additionally, the complexity analysis of the UEP strategies is given.

Comparative Analysis of Full Duplex and Half Duplex Relay for High-Speed Vehicular Scenario

Comparative Analysis of Full Duplex and Half Duplex Relay for High-Speed Vehicular Scenario

A simple Encoding Scheme to Achieve the Capacity of Half-Duplex Relay Channel

In this paper, the Half-Duplex Relay Channel (HDRC) is thoroughly investigated. Even though this channel model is widely studied, the capacity is not yet fully understood and particularly has not been tightly expressed. In this work, a new capacity expression of the discrete memoryless HDRC is explicitly established. In particular, a new expression of the achievable rate is derived by taking advantage of the well-known capacity results of both the degraded broadcast channel and the multiple access channel. Specifically, in order to obtain the achievable rate, the transmission from the source to destination is operated over two phases. In the first phase, the broadcast phase, the source broadcasts to both relay and destination. In the second phase, both source and relay transmit to destination to form multiple access channel. Then, we prove that the new achievable rate meets the cut-set outer bound such that the capacity of the discrete memoryless HDRC is attained. Next, the new derived capacity result is extended to the case of additive Gaussian channel. Further, the attained capacity is analytically and then numerically shown to encompass all well-known available findings in the literature. Additional numerical examples are also shown to present the cases in which the relay is beneficial and how the achievable capacity varies with the source-relay and relay-destination channel gains.

Optimality of Energy-Efficient Scheduling and Relaying for Half-Duplex Relay Networks

This paper considers a single-source single-destination half-duplex <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -relay network with arbitrary topology, where the source communicates with the destination through a direct link and with the help of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> half-duplex relays. The focus is on the linear deterministic approximation of the Gaussian noise network model. First, sufficient conditions under which operating the network in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n+1$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">energy-efficient</i> states (out of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{ n}$ </tex-math></inline-formula> possible states) is sufficient to achieve the approximate capacity (that is, an additive gap approximation of the Shannon capacity) are characterized. Specifically, these <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n+ 1$ </tex-math></inline-formula> energy-efficient states are those in which at most one relay is in transmit mode while the rest of the relays are in receive mode. Under such sufficient network conditions, closed-form expressions for the scheduling and the approximate capacity are provided. Then, a time-block relaying scheme, where at each point in time at most one relay is in transmit mode, is designed. In particular, the designed relaying scheme leverages information flow preservation at each relay to explicitly provide the information that each relay is exclusively responsible to store and forward to the destination. Furthermore, the destination can decode the information bits sent by the source in block <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B$ </tex-math></inline-formula> by the end of block <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B+1$ </tex-math></inline-formula> , and the proposed scheme is shown to achieve the approximate capacity whenever the sufficient conditions are satisfied. Such features make the designed scheme relevant for practical use.

Effective capacity maximization of two-way full-duplex and half-duplex relays with finite block length packets transmission

In order to satisfy the delay requirements of telecommunication systems, in this paper, we present a cooperative network with the short packet transmission in the Rayleigh fading channel. The desired relay can be implemented as a two-way half-duplex (HD) or a two-way full-duplex (FD). Also, for more accurate satisfaction and reduction of communication delays, sending and receiving with short packets is considered. Effective capacity appropriately measures the transmission rate under the delay constraint. Therefore, it is considered as a performance evaluation criterion here. With a two-way relay, two nodes exchange data with each other using a relay simultaneously. The priorities and requirements of the two nodes are not necessarily the same. Therefore, to increase performance, the system is modeled and solved as a multi-objective problem. In this way, the available power in the network is divided between the relay and two nodes, and the effective capacity of the two nodes is maximized. Depending on the different conditions, the optimal amount of allocated power to relay and nodes is calculated. However, due to the complexity and time consuming calculations, an approximate method which speeds up the calculation is presented. The approximated solution has a very close performance to the optimal allocated power. Finally, various comparisons have been made in different conditions between the performance of two-way HD and two-way FD relays. The improvement of multi-objective power allocation has been shown, especially when the relay is not located in the middle of two nodes.

Toward a Green Secure Relay System for Mission-Critical IoT: Hybrid Duplex Relay Selection and Resource Allocation in the Finite Block Length Regime

In this paper, we investigate the physical layer security of a relay system toward secure and green mission-critical Internet of Things, in the finite block length regime, which was hardly considered in the literature. Closed-form expressions for statistical secrecy throughput of full-duplex (FD) and half-duplex (HD) relay systems are respectively derived and verified by simulation results. To maximize the secrecy throughput derived, joint optimization of block length and transmit powers at source and relay is conducted, and a hybrid duplex relay selection (HyDRS) scheme is also proposed, where a relay out of multiple candidates and its duplex mode is selected. Simulation results demonstrate the following findings. a) Together with the HyD relaying scheme, the proposed joint power allocation and block length adaptation scheme achieves much higher secrecy throughput than the conventional sole FD/HD mode relay systems. b) With the proposed HyDRS scheme, a higher degree of freedom in transmission is achieved than the single relay candidate scenario, leading to higher secrecy throughput and energy efficiency. c) It is revealed that increasing block length or transmit power may not always make a positive contribution to secrecy throughput and energy efficiency.

BER performance of unmanned aerial vehicle assisted spatial modulation system in Rician channels

Unmanned aerial vehicle (UAV) is an emerging technology with high mobility and flexible reconfiguration. In this paper, an UAV-assisted spatial modulation (SM) system with amplify-and-forward (AF) protocol is presented, and the corresponding bit error rate (BER) performance is investigated over Rician fading channels, where the UAV is employed as a half-duplex relay and remains stationary over a given area, and the path loss exponent and Rician factor depend on the position of the UAV. Based on the imperfect channel state information (CSI), the effective signal-to-noise ratio (SNR) of the system is firstly derived. Then, with this result, we analyze the BER performance of the system, and deduce an upper bound of average BER for evaluation. On these bases, the asymptotic performance under large SNR is analyzed, and asymptotic expression of BER bound is derived to attain the diversity gain which is zero for imperfect CSI and one for perfect CSI. Simulation results demonstrate the effectiveness of the presented performance analysis, and reveal that the degradation arisen from estimation error and multiple antennas at the transmitter in BER performance is serious while the improvement brought by spatial diversity at the receiver is relatively modest. Specifically, the increase of 0.04 in estimation error variance results in the diminution of 12.5dB in performance gain, and the use of eight transmit antennas is 8dB worse than two transmit antennas. At the receiver, the increase in performance gain offered by going from ND=1 to ND=6 is about 4dB. Furthermore, according to the simulation, the optimal BER performance can be reached by adjusting the UAV position and power allocation factor. At last, the open issues and challenges focusing on both the deployment and architecture of this system are highlighted.

Outage Trade-Offs Between Full/Half-Duplex Relaying for NOMA Aided Multicarrier Cooperative D2D Communications System

ABSTRACT Non-orthogonal multiple access (NOMA) and full-duplex (FD) have recently emerged as a potential candidate for the fifth-generation and beyond (B5G) cellular networks with the target of increasing spectral efficiency and guaranteeing the quality-of-services (QoS). This paper proposes an orthogonal frequency division multiple access (OFDMA) based FD cooperative device-to-device (C-D2D) communications system, wherein a D2D transmitter (DT) acts as an FD relay to support simultaneous bidirectional communication between a base station (BS) and a cellular user (CU). In addition, a power splitting protocol using NOMA is incorporated for the FD C-D2D system to share the resources between the cellular and D2D user. Further, the successive interference cancellation (SIC) is used to decode the signals, while self-interference suppression (SIS) is employed at DT to mitigate self-interference (SI). The analytical expressions for achievable rates of the cellular and D2D links and their outage probabilities for the FD and half-duplex (HD) systems are derived. Outage trade-offs between FD/HD relaying have been analyzed. Results show that the proposed model enhances the overall outage performance of the cellular and D2D users. Moreover, for high cellular outage constraints, DT as an FD relay outperforms the conventional HD C-D2D communications system wherein DT acts as an HD relay.

Efficient Beam Scheduling for Half-Duplex mmWave Relay Networks

Millimeter wave (mmWave) communication is expected to play a central role in next generation mobile systems (5G) and beyond, by providing multi-Gbps data rates. However, the severe pathloss and sensitivity to blockages at mmWave frequencies significantly challenge practical implementations. One effective way to mitigate these effects and to increase the communication range is beamforming in combination with relaying. In this paper, we study the beam scheduling problem for mmWave half-duplex (HD) relay networks, where the relay topology can be arbitrary. Based on theoretically optimal scheduling results, we first implement a network simplification procedure to reduce the network topology complexity, and then propose two practically relevant beam scheduling schemes: the deterministic edge coloring (EC) scheduler and the adaptive backpressure (BP) scheduler. The former consists of a very simple one-time computation of the sequence of scheduling states, which is then repeated periodically. The one-time computation depends on the underlying network topology, and therefore it must be repeated when such topology changes. As such, this approach is more suited to quasi-static scenarios. The latter is an “online” approach which updates scheduling weights and solves at each time slots a weighted sum rate maximization. Hence, it's computational complexity may be significantly higher than that of EC, but it is better suited to dynamic time-varying scenarios. With the aid of computer simulations, we show that both the proposed schedulers guarantee network stability within the network capacity. Particularly, in comparison with two baseline schemes, the proposed schedulers achieve much smaller queuing backlogs, much smaller backlog fluctuations, and much lower packet end-to-end delays.

On the performance of physical layer security in virtual full-duplex non-orthogonal multiple access system

This paper investigates physical layer security analysis of cooperative non-orthogonal multiple access (NOMA) communication system. A virtual full-duplex (VFD) relaying scheme with an untrusted amplify-and-forward (AF) half-duplex (HD) relay and a trusted decode-and-forward (DF) HD relay is used in this system to improve the spectral efficiency. In order to prevent the untrusted relay from eavesdropping, a simple and practical cooperative jamming scheme is designed to confuse the untrusted relay. The exact expressions of effective secrecy throughput (EST) for NOMA users and approximate expression of EST for non-NOMA user are derived. All theoretical results are validated by numerical simulations which demonstrate that the proposed VFD-NOMA scheme is superior to existing HD-NOMA scheme in cooperative system and jamming plays an important role for obtaining acceptable EST. In addition, simulation results shows that the best secrecy performance highly depends on the system parameters such as transmit powers and jamming signal power.

Cooperative Relaying for Multi-User MIMO Wireless Backhaul Networks

With the increasing deployment of more short-range small cells using mmWave technologies, backhaul becomes a bottleneck and wireless backhaul is relatively attractive compared to wired backhaul due to lower cost and efficient operation. As such, cooperative relaying for multi-user multiple-input multiple-output (MU-MIMO) wireless backhaul networks is studied, in which the source sends independent messages to $K$ destinations with the help of $N$ relays where the relays operate in full- or half-duplex mode. Herein, we focus on a line-of-sight dominant environment in which each of the channel matrices between nodes becomes rank-deficient due to the lack of multi-paths. In such cases, the multiplexing gain from using multiple antennas is severely limited, and the capacity cannot be improved with the number of antennas. To overcome the rank deficiency, we develop a novel cooperative relaying scheme that utilizes relays as active reflectors. In the proposed scheme, the source distributes the streams over the relays as uniformly as possible, and the relays extract the streams of each destination based on their received signals and forward them to their corresponding destinations, while any inter-user and inter-relay interference is also properly nulled out. We analyze the achievable sum degrees of freedoms (DoFs) and sum rates with full-duplex and half-duplex relays, and derive an upper bound on the sum DoF, which is tight when the relays operate in full-duplex mode. The results demonstrate that our scheme offers significant improvement to system throughput in comparison with direct transmission without cooperative relaying.