Multiple Decode-and-forward Relays Research Articles

Joint optimization scheme for intelligent reflecting surface aided multi‐relay networks

The latest research on hybrid networks of decode-and-forward (DF) relay and intelligent reflecting surface (IRS) is limited to just a single relay. To harvest multiple relay gains, this paper proposes a hybrid communication networks that combines multiple DF relays with an IRS. The authors formulate and address the joint optimization problem to maximize the end-to-end transmission rate by jointly optimizing the reflect beam forming, the best relay selection and the power allocation, subject to total transmit power constraint. Moreover, to better exploit the degrees of freedom, it is proposed that the source also transmits signals in the second time slot. While the optimization problem is difficult to solve, the authors propose some efficient approaches to make the problem tractable. First, power allocation problem is simplified by applying Cauchy-Schwarz inequality and introducing equivalent channel gain, second, non-convex rank-one constraint is overcome by utilizing semidefinite relaxation (SDR) approach, then the slacked non-convex constraint is overcome by applying first-order Taylor expansion approximation. The simulation results demonstrate that our proposed joint optimization scheme for hybrid networks of multiple DF relays with an IRS has significant performance improvement over the other optimization schemes, such as the optimization scheme for hybrid networks of single DF relay and an IRS.

RIS source decode-and-forward relaying network with interference

We consider in this paper the scenario of reconfigurable intelligent surface (RIS) source decode-and-forward (DF) relaying network with interference. The system includes single source of multiple reflecting elements, multiple DF relays, and single destination with interference at the relays and destination. Assuming Rayleigh fading channels, we obtain closed-form accurate approximations for the outage probability. Additionally, we investigate the behavior at high signal-to-noise ratio (SNR) values where the diversity order and the coding gain are obtained. Findings illustrate that the number of reflecting elements N at the source has a small impact on the system coding gain and not on the diversity gain. Furthermore, results show that the number of relays K is affecting the diversity order and is more impactful on the behavior than N. Findings demonstrate that when N=1, the interference value at the relays and destination is determining, which hop dominates the behavior, whereas when N>1 the behavior is always dominated by the second hop regardless of the interference value.

Buffer-aided relay selection for secure two-hop wireless networks with decode-and-forward relays and a diversity-combining eavesdropper

This paper investigates the buffer-aided relay selection issue for achieving physical layer security (PLS) in two-hop wireless networks with Decode-and-Forward (DF) relays. Available works mainly focus on scenarios with independent-decoding eavesdroppers. However, the more hazardous scenarios with diversity-combining eavesdroppers that combine the signals in two hops to decode the packets are largely ignored. This paper considers a two-hop wireless network with multiple DF relays and one diversity-combining eavesdropper under two cases with perfect and partial eavesdropper channel state information (CSI) respectively. We first propose two secure buffer-aided relay selection schemes to suppress the eavesdropper for both eavesdropper CSI cases respectively. We then derive analytical expressions for the end-to-end (E2E) secure transmission probability (STP) and E2E delay of the network. Finally, we provide extensive simulation and numerical results to validate the correctness of the expressions and also to investigate the E2E STP and E2E delay performances. The results showed that the proposed schemes can achieve better E2E STP performance than the well-known Max-ratio buffer-aided relay selection scheme but at the cost of an increased E2E delay.

PL approximation of DBPSK in DF-based cooperative FSO network with pointing error

In this paper, we utilize piecewise linear (PL) approximation to analyze the performance of cooperative free space optical (FSO) network employing differentially modulated binary phase shift keying (DBPSK) data with multiple decode-and-forward (DF) relays. The maximum-likelihood (ML) decoding rule at the destination is approximated by PL approximation which considers the possibility of erroneous relaying and performs very similar to the ML decoder with reduced decoding complexity. The atmospheric fading optical links are modeled by Gamma–Gamma distribution subject to both types of detection techniques, i.e., heterodyne detection and intensity modulation/direct detection (IM/DD) with pointing error. We analytically formulate the probability of error for the multiple-DF relay-based FSO network. However, the novel unified expression of average bit error rate (BER) of PL decoder with single relay and single source to destination pair is derived. Further, we also derive the asymptotic approximate BER of DF-FSO network with multiple relays at high signal-to-noise ratio (SNR) of source to relay links considering heterodyne detection with negligible pointing error. In addition, the unified closed-form expressions of outage probability with single and multiple DF relays are derived in terms of Meijer G function. The expression of outage probability is examined at high SNR in order to obtain analytical diversity order. The impact of different power distribution techniques on outage probability is determined by utilizing power distribution parameters. The derived analytical results are validated through simulation.

Cooperative decode-and-forward quadrature spatial modulation over correlated and imperfect η–μ fading channels

This paper analyzes the performance of quadrature spatial modulation (QSM) multiple-input multiple-output (MIMO) system in cooperative decode and forward (DF) networks over correlated and imperfect $$\eta $$ – $$\mu $$ fading channels. QSM is a recently proposed propitious MIMO technique that promises significant advantages over conventional MIMO schemes including high spectral efficiency with single RF-chain transmitter and very low receiver complexity. In this study, DF cooperative communication system adopting QSM technique is presented and throughly analyzed. Single or multiple DF relays are placed between the source and the destination to cooperate in the transmission process. Only the relays that decode the signal correctly will participate in the retransmission process. The end to end performance of the considered system is analyzed over correlated and imperfect $$\eta $$ – $$\mu $$ fading channels. The $$\eta $$ – $$\mu $$ channel is a general fading distribution that includes some other well-known channels, such as Rayleigh and Nakagami-m, as spacial cases. Monte Carlo simulation results are presented to corroborate the accuracy of the conducted analysis. The impact of spatial correlation, imperfect channel estimation and the fading parameters $$\eta $$ and $$\mu $$ on the overall performance is investigated and exhaustively discussed.

Outage analysis of relay-assisted underwater wireless optical communication systems

In this paper, we theoretically evaluate the outage probabilities of underwater wireless optical communication (UWOC) systems. Our derivations are general as the channel model under consideration takes into account all of the channel degrading effects, namely absorption, scattering, and turbulence-induced fading. We numerically show that the UWOC systems, due to the severe channel impairments, cannot typically support longer link ranges than 100 m. Therefore, in this paper, in order to increase the transmission reliability and hence extend the viable communication range of UWOC systems, we apply decode-and-forward (DF) relay-assisted communications either in the form of multi-hop transmission, where multiple intermediate relays are serially employed between the source and destination, or parallel relaying in which multiple DF relays are distributed among the source-to-destination path to cooperate in the end-to-end transmission. Our numerical results reveal that multi-hop transmission, owing to the distance-dependency of all of the channel degrading effects, can tremendously improve the end-to-end outage probability and increase the accessible link ranges to hundreds of meter. For example, a dual-hop transmission in a 45 m coastal water link can provide up to 41 dB performance improvement at the outage probability of 10−9.

Dual‐hop signal space cooperative systems using multiple DF relays

In a dual-hop relaying system without a direct link between the source and the destination, the source broadcasts information signal to the relay and relay broadcasts it to the destination, thus it uses two phases to transmit one symbol. This study proposes a novel scheme to incorporate signal space diversity into a dual-hop relaying system with multiple decode-and-forward (DF) relays to enhance its spectral efficiency. The proposed dual-hop signal space cooperative relaying scheme transmits two symbols in three phases while the conventional dual-hop DF relaying system uses four phases to transmit the same two symbols. Therefore, the proposed scheme improves the spectral efficiency without additional complexity, bandwidth or transmit power. The proposed scheme is analysed over Rayleigh fading channel and error probability performance is derived. Moreover, an asymptotic approximation for the error probability is obtained to illustrate the impact of different system parameters and diversity gain. In addition, this study discusses the power allocation optimisation, relay position optimisation and the joint optimisation of both. Furthermore, closed-form expression for the average channel capacity is derived. In the end, analytical results are compared and validated through Monte Carlo simulations.

Beamforming with opportunistic relaying for wireless security

The authors present a beamforming scheme with opportunistic relaying (namely opportunistic beamforming) for wireless security under amplify-and-forward (AF) and decode-and-forward (DF) strategies. The relay selection is performed using the method of distributed timers, where all the relays use their timers to estimate own instantaneous channel gains and compete to access the wireless medium according to their own channel conditions. The performance of opportunistic beamforming scheme in terms of secure outage probability is analysed and the beamforming problem as a semidefinite programming problem with its optimisation framework is formulated. The results show that, in an opportunistic beamforming scheme, competition among cooperative relays offers diversity benefits in the direction of destination that enhances secrecy rate (i.e. minimises secure outage probability) and adhere to the ‘opportunistic’ cooperation rule giving priority to the ‘best’ available relay even when they are not chosen to transmit but rather chosen to cooperatively listen. Opportunistic beamforming scheme with DF strategy enhances secrecy rate than the beamforming with multiple DF relays while in the case of AF strategy, it provides significant improvement in the secrecy rate compared to the opportunistic relaying, the equal-power multiple-relay transmission and the single-relay transmission schemes.

Performance Analysis of Spatial Modulation with Multiple Decode and Forward Relays

Spatial modulation (SM) is a multiple-input multiple-output (MIMO) transmission scheme that draws momentous attention lately due to its reported performance enhancements over other MIMO techniques. However, a major drawback of SM system is that the achieved diversity gain is limited to the number of receive antennas and poor performance is reported for small number of receive antennas. Therefore, it is the aim of this paper to study the performance of SM with multiple decode and forward (DF) relays in which the relays that correctly detect the source signal forward the decoded message to the destination in pre-determined orthogonal channels. A closed form expression of the pair wise error probability (PEP) is obtained for a system consisting of two transmit antennas, multiple DF relays and single receive antenna. As well, asymptotic expression for the PEP at high signal to noise ratio (SNR) is derived. It is shown that significant performance enhancements can be achieved and the derived analysis is validated through Monte Carlo simulation results.

Sum Rate Maximized Resource Allocation in Multiple DF Relays Aided OFDM Transmission

In relay-aided wireless transmission systems, one of the key issues is how to manage the energy resource at the source and each individual relay, to optimize a certain performance metric. This paper addresses the sum rate maximized resource allocation (RA) problem in an orthogonal frequency division modulation (OFDM) transmission system assisted by multiple decode-and-forward (DF) relays, subject to the individual sum power constraints of the source and the relays. In particular, the transmission at each subcarrier can be in either the direct mode without any relay assisting, or the relay-aided mode with one or several relays assisting. We propose two RA algorithms which optimize the assignment of transmission mode and source power for every subcarrier, as well as the assisting relays and the power allocation to them for every {relay-aided} subcarrier. First, it is shown that the considered RA problem has zero Lagrangian duality gap when there is a big number of subcarriers. In this case, a duality based algorithm that finds a globally optimum RA is developed. Most interestingly, the sensitivity analysis in convex optimization theory is used to derive a closed-form optimum solution to a related convex optimization problem, for which the method based on the Karush-Kuhn-Tucker (KKT) conditions is not applicable. Second, a coordinate-ascent based iterative algorithm, which finds a suboptimum RA but is always applicable regardless of the duality gap of the RA problem, is developed. The effectiveness of these algorithms has been illustrated by numerical experiments.

WSR Maximized Resource Allocation in Multiple DF Relays Aided OFDMA Downlink Transmission

This paper considers the weighted sum rate (WSR) maximized resource allocation (RA) constrained by a system sum power in an orthogonal frequency division multiple access (OFDMA) downlink transmission system assisted by multiple decode-and-forward (DF) relays. In particular, multiple relays may cooperate with the source for every relay-aided transmission. A two-step algorithm is proposed to find the globally optimum RA. In the first step, the optimum source/relay power and assisting relays that maximize the rate is found for every combination of subcarrier and destination, assuming a sum power is allocated to the transmission at that subcarrier to that destination in the relay-aided transmission mode and the direct mode, respectively. In the second step, a convex-optimization based algorithm is designed to find the globally optimum assignment of destination, transmission mode and sum power for each subcarrier to maximize the WSR. Combining the RAs found in the two steps, the globally optimum RA can be found. In addition, we show that the optimum RA in the second step can readily be derived when the system sum power is very high. The effectiveness of the proposed algorithm is illustrated by numerical experiments.