Optimal relay selection for decode-and-forward opportunistic relaying

The optimal relay selection problem in multiuser cooperative wireless networks is considered in this paper. A general discrete time model for such networks is introduced which takes into account the dynamic variations of the channel state as well as stochastic arrival of data packets into the system. The model consists of a set of mobile users, one destination node and R relay nodes which may be either mobile or fixed. The system uses the benefit of cooperative diversity by relaying in decode and forward or amplify and forward mode. We assume that each user either transmits its packets directly to the destination or selects a relay node to cooperatively transmit its packets. It is not however trivial whether a user at each time slot has to cooperate with any relay node or not and if so, which relay node should be selected for cooperation. We will propose a throughput optimal relay selection policy that can stabilize the system for all the arrival rate vectors strictly inside the stability region. Then, we show that the optimal policy is equivalent to finding the maximum weighted matching in a weighted bipartite graph at each time slot. We also use simulations to compare the performance of the throughput optimal relay selection strategy with an instantaneous throughput optimal policy as well as a non-cooperative policy in terms of average queue occupancy (or equivalently, queueing delay).

Buffer-Aided (BA) relaying has shown tremendous performance improvements in terms of throughput and outage probability, although it has been criticized of suffering from long delays that are restrictive for applications, such as video streaming, Web browsing, and file sharing. In this paper, we propose novel relay selection policies aiming at reducing the average delay by incorporating the buffer size of the relays into the decision making of the relay selection process. More specifically, we first propose two new delay-aware policies. One is based on the hybrid relay selection algorithm, where the relay selection takes into account the queue sizes so that the delay is reduced and the diversity is maintained. The other approach is based on the max – link relay selection algorithm. For the max – link algorithm, a delay-aware only approach starves the buffers and increases the outage probability of the system. Thus, for max – link, we propose a delay- and diversity-aware BA relay selection policy targeting the reduction of the average delay, while maintaining the diversity of the transmission. The proposed policies are analyzed by means of Markov Chains and expressions for the outage, throughput, and delay are derived. The asymptotic performance of the policies is also discussed. The improved performance in terms of delays and the use of the proposed algorithms are demonstrated via extensive simulations and comparisons, signifying, at the same time, the need for adaptive mechanisms to handle the interplay between delay and diversity.

Buffer-Aided (BA) relaying has shown tremendous performance improvements in terms of throughput and outage probability, although it has been criticized of suffering from long delays that are restrictive for applications, such as video streaming, Web browsing, and file sharing. In this paper, we propose novel relay selection policies aiming at reducing the average delay by incorporating the buffer size of the relays into the decision making of the relay selection process. More specifically, we first propose two new delay-aware policies. One is based on the hybrid relay selection algorithm, where the relay selection takes into account the queue sizes so that the delay is reduced and the diversity is maintained. The other approach is based on the max - link relay selection algorithm. For the max - link algorithm, a delay-aware only approach starves the buffers and increases the outage probability of the system. Thus, for max - link, we propose a delay- and diversity-aware BA relay selection policy targeting the reduction of the average delay, while maintaining the diversity of the transmission. The proposed policies are analyzed by means of Markov Chains and expressions for the outage, throughput, and delay are derived. The asymptotic performance of the policies is also discussed. The improved performance in terms of delays and the use of the proposed algorithms are demonstrated via extensive simulations and comparisons, signifying, at the same time, the need for adaptive mechanisms to handle the interplay between delay and diversity.

Cooperative relays improve reliability and coverage in wireless networks by providing multiple paths for data transmission. Relaying will play an essential role in vehicular networks at higher frequency bands, where mobility and frequent signal blockages cause link outages. To ensure connectivity in a relay-aided vehicular network, the relay selection policy should be designed to efficiently find unblocked relays. Inspired by recent advances in beam management in mobile millimeter wave (mmWave) networks, this paper addresses the question: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">how can the best relay be selected with minimal overhead from beam management?</i> In this regard, we formulate a sequential decision problem to jointly optimize relay selection and beam management. We propose a joint relay selection and beam management policy based on deep reinforcement learning (DRL) using the Markov property of beam indices and beam measurements. The proposed DRL-based algorithm learns time-varying thresholds that adapt to the dynamic channel conditions and traffic patterns. Numerical experiments demonstrate that the proposed algorithm outperforms baselines without prior channel knowledge. Moreover, the DRL-based algorithm can maintain high spectral efficiency under fast-varying channels.

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.

New applications associated with the Internet of Things (IoT) often involve cooperation and relaying among many autonomous wireless devices. This should be supported by wireless transmission schemes in physical layer. Different from half-duplex (HD) cooperative networks, the relay and destination in full-duplex (FD) cooperative networks suffer from the loop interference and the direct link interference respectively, since all the nodes work on the same frequency band simultaneously. In this paper, we first investigate the relay selection strategies by taking the direct link interference into consideration. Then, we analyze the outage probabilities of relay selection schemes for FD cooperative networks under amplify-and-forward (AF) and decode-and-forward (DF) protocols. Numerical simulations are performed to verify the analytical expressions and make a comparison between the proposed selection schemes and the conventional schemes. It is shown that significant performance gain can be achieved by the proposed schemes compared with the conventional schemes without consideration of the direct link interference, especially in high signal-to-noise ratio (SNR) regime.

Using Moment Generating Function for Performance Analysis in Non-Regenerative Cooperative Relay Networks with Max-Min Relay Selection

This paper focuses on the problem of selecting the optimal common node (CN) in a wireless sensor network to serve as the relay to aid the data transmission between the cluster head (CH) and the data fusion center (DFC). Optimal relay selection with the objective of minimizing the outage probability is a difficult problem which depends on multiple factors. In this paper, an outage-optimal relay selection algorithm using fuzzy comprehensive evaluation is proposed. The algorithm takes into consideration both the channel state information and the cooperation power information of the nodes, and uses fuzzy comprehensive evaluation to select the optimal relay. The optimum fuzzy weight, which guarantees the optimality of the proposed relay selection algorithm in terms of the outage probability, is specified using general-purpose iterative methods. Simulation results show that the proposed relay selection algorithm evidently outperforms the algorithm based only on the channel state information in terms of the outage probability.

This paper investigates the impact of relay selection (RS) on the performance of cooperative non-orthogonal multiple access (NOMA), where relays are capable of working in either full-duplex (FD) or half-duplex (HD) mode. A number of relays (i.e., K relays) are uniformly distributed within the disc. A pair of RS schemes are considered insightfully: 1) single-stage RS (SRS) scheme; and 2) two-stage RS (TRS) scheme. In order to characterize the performance of these two RS schemes, new closed-form expressions for both exact and asymptotic outage probabilities are derived. Based on analytical results, the diversity orders achieved by the pair of RS schemes for FD/HD cooperative NOMA are obtained. Our analytical results reveal that: 1) the FD-based RS schemes obtain a zero diversity order, which is due to the influence of loop interference at the relay; and 2) the HD-based RS schemes are capable of achieving a diversity order of K , which is equal to the number of relays. Finally, simulation results demonstrate that: 1) the FD-based RS schemes have better outage performance than HD-based RS schemes in the low signal-to-noise ratio (SNR) region; 2) as the number of relays increases, the pair of RS schemes considered are capable of achieving the lower outage probability; and 3) the outage behaviors of FD/HD-based NOMA SRS/TRS schemes are superior to that of random RS and orthogonal multiple access based RS schemes.

In this paper, we consider the two-hop MIMO interference network, where T source-destination pairs communicate via T×K relays selected from G alternative relays. Based on optimal relay selection, each source-destination pair selected K relays to transmit data streams. In the first time slot, the source node transmits data steams to the selected relays by adaptive beam forming. In the second time slot, the selected relays forward the data streams by interference alignment technique. Compared the beam forming and interference alignment transmission mechanism based on fairness relay selection, our scheme can achieve higher system capacity.

In this paper, we investigate the energy efficient relay selection scheme for two-way relay network. Two optimal joint relay selection and power minimization schemes are studied. One approach is to minimize the total transmit power, including the relay power and the transceiver power, subject to the signal to noise ratio (SNR) requirements on the two transceiver. And the other focuses on relay power minimization. Closed-form solutions are obtained and only few parameters need to be broadcast to all relays. Based on the close form solution, a new relay selection criterion is proposed, which simply relies on the relay channel coefficients. Simulation results are presented to show the merits of the proposed scheme.

This paper investigates the integration of relay-assisted Internet of Things (IoT) systems, focusing on the use of multiple relays to enhance the system performance. The central metric of interest in this study is system outage probability, evaluated in terms of latency. Our research provides a comprehensive analysis of system outage probability, considering different relay selection criteria to optimize the system’s transmission performance. Three relay selection strategies are employed to enhance the system transmission performance. Specifically, the first strategy, optimal relay selection, aims to identify the relay that minimizes the latency and maximizes the data transmission reliability. The second approach, partial relay selection, focuses on selecting a subset of relays strategically to balance the system resources and achieve the latency reduction. The third strategy, random relay selection, explores the potential of opportunistic relay selection without prior knowledge. Through a rigorous investigation, our paper evaluates the impact of these relay selection criteria on the performance of relay-assisted edge computing systems. By assessing the system outage probability in relation to latency, we provide valuable insights into the trade-offs and advantages associated with each selection strategy. Our findings contribute to the design and optimization of reliable and efficient edge computing systems, with implications for various applications, including the IoT and intelligent data processing.

In the presence of an illegitimate user, we investigate the secrecy outage probability (SOP) of the optimal relay selection (ORS) networks by applying decode-and-forward (DnF) based full-duplex (FD) relaying mode. The closed-form expressions for the allocations of the end-to-end signal-to-interference-plus-noise ratio (SINR) in each wireless network are derived as well as the closed-form expression for the exact SOP of the proposed ORS system is presented under Rayleigh fading schemes. As an important achievement, SOP is also compared between orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) schemes. Our results reveal that the SOP of the suggested scheme can be considerably influenced by several parameters involved, including the number of relays, the average signal-to-noise ratio (SNR) of eavesdropper links, transmit power and the average residual self-interference (SI) enforced on the FD relays.DOI: http://dx.doi.org/10.5755/j01.eie.24.4.21483

Energy and spectrum efficiency for energy management systems based on wireless sensor networks in intelligent structures and powered by ambient energy harvesting (EH) are the main problems in wireless sensor networks. Herein, we consider relay selection methods. To address this issue, we proposed the optimal multiantenna power beacon opportunistic relay selection (OMPB-ORS) protocol, which uses decoding and forward methods, in which the relay wireless sensor nodes and the second source are energy-restricted and can harvest energy from a power beacon (PB) multiantenna to transmit aggregated information data from source to destination. The proposed protocol based on specific s witching t ime r eceiver a rchitecture e nhances e nd-to-end p erformance f or m aximum h ardware impairments and interference for the transceiver. To evaluate the performance, we compared our proposed protocol with best ORS (B-ORS), conventional ORS (C-ORS), and hybrid partial relay selection (H-PRS) protocols. Using the Rayleigh-fading channel, the simulation is driven based on asymptotic and exact form expressions of throughput (TP) and outage probability (OP). Simulation results show that the OMPB-ORS protocol achieves a higher TP and OP than all compared protocols.

In this paper, we study the secure communication of dual-hop cognitive relaying networks. An eavesdropper can combine two received signals from two hops by using the maximum ratio combining technique. The data transmission from the secondary source to the secondary destination is assisted by the best decode-and-forward relay, which is selected from four relay selection schemes. The first scheme, MaSR, is based on the maximum channel gain from the source to the relays. In the second scheme, MaRD, the best relay is selected on the basis of the maximum channel gain from the relays to the destination. In the third scheme, MiRE, the gain to the eavesdroppers is instead minimized. Finally, optimal relay selection is considered as the fourth scheme. For these four schemes, we study the system security performance by deriving the exact analytical secrecy outage probability. These analytical expressions are then verified by comparing them with the results of Monte Carlo simulations. Herein, we evaluate and discuss the outage performance of the schemes while varying important system parameters: the number and locations of the relay nodes, primary user node, and eavesdropper; the transmit power threshold; and the target secure rate.