Source-relay Link Research Articles

Efficient Polar Coded Selective Decode-and-Forward with Cooperative Decision Threshold in Cooperative Multi-Relay Transmissions

In some satellite Internet of Things (IoT) devices with terrain shielding, the qualities of the direct source-destination (S-D) channel are poor, requiring cooperative communications with multi-relays to be employed. In order to solve error propagation of current decode-and-forward (DF) on such occasions, an efficient polar coded selective decode-and-forward (SDF) cooperation method is proposed with a new decision threshold derived from channel state information (CSI). First, the proposed threshold is derived from the CSI by exploiting the channel gain ratio of optimal relay-destination link (R-D) with source-relay (S-R) link. The above R-D link possesses good channel quality among all links in the system. Second, when the channel gain ratio of certain relay links is larger than the aforementioned decision threshold, the source and all these relays cooperatively send messages together to the destination to accomplish perfect SDF transmission. Otherwise, all relays are frozen and the messages are directly transmitted through the S-D link. If it fails anyway, a retransmission is subsequently tried in the next transmission cycle. In addition, a polar code for fading channels is designed and adaptively adjusted to a proper code rate according to channel quality to attain good bit error rate (BER) performance. Simulation results show that the proposed scheme achieves about 0.9 and 0.5 dB gain at BER of 10-4, respectively, in multi-relay cooperative communications with multi-path fading channels compared with those of non-cooperation and existing polar coded cooperation channels. Therefore, the proposed polar coded SDF (PCSDF) scheme can improve both the BER and the outage probability (OP) performance in multi-relay cooperative systems, making it quite suitable for heterogeneous network applications in cooperative satellite IoT systems involving sixth-generation (6G) communications.

A Novel Hybrid Duplex Scheme for Relay Channel: Joint Optimization of Full-Duplex Duty Cycle and Source Power Allocation

Full-duplex (FD) mode has great potential in improving the spectral efficiency. Mitigating the effect of self-interference becomes one key for performance enhancement of FD system. This work proposes a novel hybrid duplex scheme where the relay receives information for a fraction of time and simultaneously transmits and receives information for the rest, following a duty cycle. First, we formulate the achievable rate maximization of the proposed scheme as a joint FD duty cycle and source power allocation optimization problem. The optimal FD duty cycle, the optimal source power allocation, and the maximal achievable rate are explicitly given for some cases and characterized in detail for other cases. Then, the proposed scheme is applied to two-hop relaying systems. Specifically, the optimal source power allocation is proved to be a water-filling solution over the FD phase and the receives-only phase on the source-relay link. By dividing the system as low-, medium-, and high-source power cases, the optimal FD duty cycle and the maximal achievable rate are obtained in (approximate) closed-form case-by-case, where the source power thresholds among cases are clearly expressed. Numerical results validate that the proposed hybrid duplex scheme outperforms other benchmark schemes and can improve the achievable rate significantly.

Design and analysis of novel Ka band NOMA uplink relay system for Lunar farside exploration

This paper proposes a novel Ka band non-orthogonal multiple access (NOMA) uplink relay system for Lunar farside exploration, where a satellite relay node with the help of NOMA scheme, amplifies and forwards the signal from the Earth Base Station (EBS) to a Lunar rover and a lander. We assume that the signal undergoes shadowed-Rician fading for the source-relay link and Rayleigh fading for the relay-destination link. Then, the analytical expressions for the outage probability (OP) and ergodic capacity are derived for the satellite relay node equipped with single and multiple antennas, respectively. In addition, we obtain the optimal power allocation coefficients by maximizing the sum ergodic capacity of the system, and we calculate the power consumption of the NOMA uplink relay system to achieve the same OP performance of OMA system and provide some useful guides for the design of the Earth-to-Lunar communication system. Simulations are provided to confirm the reliability of our analytical results and show the impact of various parameters on the system performance.

Relay selection schemes in threshold DF cooperative systems with wireless power transfer

In this work, a dual-hop cooperative wireless energy harvesting system, in which there are a Source-Destination (S−D) pair and several relay candidates is considered. all relay candidates adopt threshold decode-and-forward (DF) relaying scheme to decide whether to aid S−D pair information transmission or not. We assumed that these relay candidates and S are all equipped with single receiving antenna while D is equipped multiple receiving antenna and adopts generalized selection combining scheme to achieve receiving diversity gain. We also consider that relay candidates and S can harvest energy from D with finite energy storage capacity under time switching scheme. Three kinds of best relay selection (RS) schemes based threshold (TH) DF relay scheme, that is, the TH optimal source-relay link, the TH optimal relay-destination link, and the TH optimal source-relay-destination link, are proposed and studied. Closed-form analytical expressions for the outage probability over the three proposed RS schemes are derived and verified by Monte-Carol simulations.

On maximizing information reliability in wireless powered cooperative networks

Unpredictable nature of fading channels and difficulty in tracking channel state information pose major challenge in wireless energy harvesting communication system design. In this work, we address relay selection problem for wireless powered communication networks, where the relays harvest energy from the source radio frequency signals. A single source–destination pair is considered without a direct link. The connecting relay nodes are equipped with storage batteries of infinite size. We assume that the channel state information (CSI) on the source–relay link is available at the relay nodes. Depending on the availability of the CSI on the relay–destination link at the relay node, we propose two relay selection schemes and evaluate their outage probability. Availability of the CSI at the relay node on the relay–destination link considerably improves the performance due to additional flexibility in the relay selection mechanism. Due to absence of CSI throughout the network at the time of transmission path selection, the analysis of the problem is not tractable. Therefore, we relax our assumptions on availability of CSI and closed-form expressions of the outage probability as a function of the amount of the available harvested energy are derived for both CSI availability cases. Finally, we numerically quantify the performance for the proposed schemes and compare the outage probability for fixed and equal number of wireless powered forwarding relays.

Resource Allocation for Wireless-Powered Full-Duplex Relaying Systems With Nonlinear Energy Harvesting Efficiency

In wireless power transfer (WPT)-assisted relaying systems, spectral efficiency (SE) of source-relay link plays a dominant role in system SE performance due to the limited transmission power at the WPT-aided relay. In this paper, we propose a novel protocol for a downlink orthogonal frequency division multiple access (OFDMA) system with a WPT-aided relay operating in full-duplex (FD) decode-and-forward (DF) mode, where the time slot durations of the source-relay and relay-users hops are designed to be dynamic, to enhance the utilization of degrees of freedom and hence the system SE. In particular, a multiple-input and signal-output (MISO) source-relay channel is considered to satisfy the stringent sensitivity of the energy harvesting (EH) circuit at the relay, while a single-input and single-output (SISO) relay-user channel is considered to alleviate the power consumption at the relay node. Taking into account the non-linearity of EH efficiency, a near-optimal iteration-based dynamic WPT-aided FD relaying (A-FR) algorithm is developed by jointly optimizing the time slot durations, subcarriers, and transmission power at the source and the relay. Furthermore, self-interference generated at the relay is utilized as a vital energy source rather than being canceled, which increases substantially the total energy harvested at the FD relay. We also reveal some implicit characteristics of the considered WPT-aided FD relaying system through intensive discussions. Simulation results confirm that the proposed A-FR achieves a significant enhancement in terms of SE with different relay's locations and the number of users, compared to the conventional symmetric WPT-aided FD relaying (S-FR) and the time-switching-based WPT-aided FD relaying (TS-FR) benchmarks.

LDPC-based Joint Source Channel Coding and Decoding Strategies for single relay cooperative communications

This paper investigates new cooperative communication strategies based on Low-Density Parity-Check codes (LDPC) with Joint Source-Channel (JSC) coding and decoding algorithms to efficiently deliver a correlated content to a destination node. The proposed strategies rely on double LDPC codes where the source and the channel coding are similar to standard LDPC operations, and can be mapped on Tanner graphs for message-passing decoding. Three coding strategies are proposed where different LDPC source and channel coding setups are applied at the source node and the relay node. For each transmission strategy, we propose a graph mapping the whole network, over which iterative message-passing decoding is applied, and where the probability of errors in the source–relay link are taken into account. We showed, based on computer simulations, that the proposed strategies can provide substantial improvements compared to equivalent rates point-to-point systems. Simulation results also showed that the system performance depends on the relay position, and that adapting the source and the relay channel coding rates accordingly can enhance the overall system performance for a time-varying network.

Effective User-Pair Association Schemes for Relay-Based Multi-tier Heterogeneous Networks with Physical Layer Security

This paper focuses on relay-assisted multi-tier heterogeneous networks (HetNets) in term of feasible user-pair association (UPA) criterions and physical layer secrecy analysis. In the interesting relay-assisted multi-tier HetNets, the randomly located mobile user-pair communicates with the help of the relay of its associated tier. We model the locations of all network elements as independent Poisson point process. For such relay-assisted HetNets, similar to the nearest relay defined for user association in traditional single-hop HetNets, we define the so-called best relay in each tier for a typical mobile user-pair by using the equivalent end-to-end biased received power (BRP). Then based on the defined best-relay, we first propose the max–min user-pair association (MM-UPA) criterion. Due to the fact that the MM-UPA criterion is dominated by the bottleneck link’s BRP and does not exploit the joint effect of both the source-relay and relay-destination links, we present the maximum harmonic mean user-pair association (MHM-UPA) criterion, again. For the two UPA criterions, by using feasible mathematical analysis, we derive the corresponding UPA probabilities. Finally, as an implement of the two proposed UPA criterions, by using stochastic geometry, we perform the secrecy performance analysis of the considered relay-assisted multi-tier HetNets. The presented numerical analysis first validates our derivations through the comparison analysis with traditional single-hop user association criterion. At the same time, we also present the comparison analysis between the two proposed MM-UPA and MHM-UPA criterions. It is found that when the transmission power $$P_{R(S)}^{k}$$ is small, the MHM-UPA scheme outperforms the MM-UPA one in term of UPA probability. On the contrary, the two schemes achieve approximately the same UPA probability. For the total secrecy probability, we find that when transmit power is small, the MHM-UPA achieves the higher secrecy probabilities. Moreover, the achieved gain by MHM-UPA is increasing with the decrease of transmission power. Contrarily, when the transmission power is large, although the MM-UPA outperforms the MHM-UPA, the achieved gain by MM-UPA cover MHM-UPA is small.

Lossy-Forward Relaying for Lossy Communications: Rate-Distortion and Outage Probability Analyses

This paper presents an in-depth performance analysis of lossy communications in a single-relay system, where the recovered information is not necessarily lossless in both the relay and the destination. In this system, the relay continues transmitting the sequence with source-relay (S-R) link errors to the destination even if errors are detected after decoding, i.e., so-called lossy-forward (LF) strategy. The problem can be decomposed into two parts as follows: a point-to-point coding problem in the S-R link and a lossy source coding problem with an LF relay in the source–destination (S–D) and a relay-destination (R-D) links. To begin with, we derive the admissible rate region of the lossy source coding problem with the LF relay for a specified distortion requirement. Then, we focus on the analysis of outage probability over block Rayleigh fading channels. Finally, a practical encoding/decoding scheme is proposed for the evaluation of system performance by computer simulations. Due to the suboptimal channel coding and incomplete utilization of joint typicality, the theoretical performance cannot be achieved in the simulation; however, the tendency of curves in simulations matches that in theoretical calculation.

CSI Based Multiple Relay Selection and Transmit Power Saving Scheme for Underlay CRNs Using FRBS and Swarm Intelligence

In this article, a multiple relay selection (MRS) scheme for signal-to-noise ratio (SNR) enhancement is proposed for underlay relay-assisted cognitive radio networks (RCRNs). A secondary source-destination pair experiencing deep fading on direct path is assisted by amplify-and-forward (AF) relays in an underlay mode. In this energy-constrained scenario, the aim is to maximize the secondary network's end-to-end SNR through an intelligent power-saving method incorporated with MRS. In contrast to the prior relay selection (RS) schemes, the relay-selection factor is the difference of SNR of the source-relay link and corresponding relay-destination link for each relay along with its corresponding interference channel coefficient. The difference factor aims to achieve the SNR upper bound while performing minimum power amplification, eventually resulting in interference mitigation as well. The proposed algorithm has been implemented using Fuzzy Rule Based System (FRBS), Artificial Bee Colony (ABC) and Particle Swarm Optimization (PSO) and their performance has been compared through simulations.

Relay Selections for Cooperative Underlay CR Systems With Energy Harvesting

In this paper, we consider a cooperative underlay cognitive radio system in which one secondary user ( ${S}$ ) transmits its data to a secondary receiver with the assistance of multiple relays. In the considered underlay spectrum sharing scenario, the transmit power levers at ${S}$ and forwarding relays are adjusted simultaneously according to a given threshold interference power that the primary user can tolerate. We also consider that relays adopt threshold decode-and-forward relay scheme and harvest energy from the primary transmitter with finite energy storage capacity under time switching scheme. Three best relay selection (RS) schemes, optimal source-relay link with wireless energy harvesting (WEH), optimal relay-destination link with WEH and optimal source-relay-destination link with WEH schemes, are proposed and studied. Closed-form analytical expressions for the outage probability of these three best RS schemes are derived, respectively. Finally, our analytical results are verified by Monte-Carlo simulations.

Performance Analysis of Hybrid Protocol Based AF EH Relaying over Asymmetric Fading Channels

Simultaneous wireless information and power transfer is a practicable solution to encourage energy-constrained relay nodes to cooperate with the source to transmit information to the destination. In this paper, we study the outage performance of hybrid protocol based amplify-and-forward (AF) relay networks over asymmetric fading channels, where the source-relay link and the relay-destination link are subjected to Rician fading and Rayleigh fading, respectively. In particular, we derive the lower bound of outage probability and the upper bound of outage capacity based on a high signal-to-noise ratio approximation, respectively. We further investigate the effects of various system parameters, such as the parameters of hybrid protocol, the target rate, and the Rician K-factor, on the investigated network. It is shown that a good selection of parameters of hybrid protocol is of significance to improve system capacity, and that a larger Rician factor is desirable in the investigated network.

Improved Hybrid Relaying Protocol for DF Relaying in the Presence of a Direct Link

This letter proposes an improved hybrid relaying (i-HRe) protocol for a decode-and-forward relay system in the presence of a direct link between the source and the destination. A non-linear energy harvester is equipped at the relay to facilitate simultaneous wireless information and power transfer. We formulate an optimization problem to maximize the system capacity by optimizing the parameters of the proposed i-HRe protocol, i.e., time-switching ratio and power-splitting ratio. As this problem is non-convex, we convert it into two subproblems by means of the inequality relation between the signal noise ratios of source-relay link and relay-destination link to derive the optimal solution. Simulation study verifies the fast convergence of the proposed iterative algorithm. The results show that the i-HRe protocol achieves a higher system capacity than the existing protocols.

Deep Learning Detection in MIMO Decode-Forward Relay Channels

We consider a signal detection problem by using deep learning techniques in a multiple-input multiple-output (MIMO) decode-forward (DF) relay channel. There exist some suboptimal detectors such as the near maximum likelihood (NML) detector and the NML with two-level pair-wise error probability (NMLw2PEP) detector in the channel. However, the NML detectors require an exponentially increasing complexity as the number of transmit antennas increases. More seriously, without the channel state information (CSI) of the source-relay (SR) link, there is no detector that can achieve good performance even at high complexity. In this paper, we propose a deep learning approach to the NML (DL-NML) detector that achieves good performance with low complexity regardless of whether the CSI of the SR link is known or not at the destination. The DL-NML detector can detect signals in changing channels after a single training by using randomly generated channels. Furthermore, we propose a linear detector and a semidefinite relaxation approach to the NML detector to compare with the DL-NML detector in performance and complexity. The complexity analysis and simulation results validate the superiority of the proposed DL-NML detector.

The Wiretapped Diamond-Relay Channel

In this paper, we study a diamond-relay channel where the source is connected to $M$ relays through orthogonal links and the relays transmit to the destination over a wireless multiple-access channel in the presence of an eavesdropper. The eavesdropper not only observes the relay transmissions through another multiple-access channel but also observes a certain number of source-relay links. The legitimate terminals know neither the eavesdropper’s channel state information nor the location of source-relay links revealed to the eavesdropper except the total number of such links. For this wiretapped diamond-relay channel, we establish the optimal secure d.o.f. In the achievability part, our proposed scheme uses the source-relay links to transmit a judiciously constructed combination of message symbols, artificial noise symbols, and fictitious message symbols associated with secure network coding. The relays use a combination of beamforming and interference alignment in their transmission scheme. For the converse part, we take a genie-aided approach assuming that the location of wiretapped links is known.

Min–Max User-Pair Association Criterion and Outage Performance of K-Tier Relay-Based Heterogeneous Networks

This paper focuses on the user-pair association for multi-tier relay-based dual-hop heterogeneous networks (HetNets) and proposes a novel min–max user-pair association (MM-UPA) criterion by integrating the bias factors, which is an evolution of conventional single-hop user association based on receive signal strength (RSS). The core idea of the proposed MM-UPA is that, in each tier the nearest relay to a typical user-pair is characterized by the maximum of the RSS reciprocals $${1 / {P_{SR}^{j} }}$$ and $${1/ {P_{RD}^{j} }}$$ by the source and destination from relay. Then, a typical user-pair is associated to the relay based on minimizing these maximums of each tier. This proposed MM-UPA criterion is fit especially for the relay-based HetNets due to effectively exploiting the source-relay and relay-destination links simultaneously. With the MM-UPA criterion, by using stochastic geometry and homogeneous Poisson point processes, we present the analytical expression of the probability that a typical user-pair is associated with a relay of the kth tier as well as the corresponding statistical description of the distances from the source and destination of a typical user-pair to its associated relay. Finally, the average outage probability of a typical user-pair relay communication link is derived. The presented simulations and numerical results validate the derivations.

Secrecy outage of threshold-based cooperative relay network with and without direct links

In this paper, we investigate the secrecy outage performance of a dual-hop decode-and-forward (DF) threshold-based cooperative relay network, both with and without the direct links between source-eavesdropper and source-destination. Without assuming that all the relays can always perfectly decode, here we consider that only those relays who satisfy predetermined threshold can correctly decode the message. We have investigated the outage probability of optimal relay selection scheme, when either full instantaneous channel state information (ICSI) or statistical channel state information (SCSI) of all the links is available. We have shown that CSI knowledge at the transmitter can improve secrecy, and the amount of improvement for the outage probability is more when the required rate is low and for low operating SNR. Asymptotic and diversity gain analysis of the secrecy outage for both the single relay and multi-relay system is obtained, when average SNRs of source-relay and relay-destination links are equal or unequal. We have shown that the improvement in predetermined threshold, eavesdropper channel quality, direct links, and required secrecy rate significantly affects the secrecy performance of the system.

On Full-Duplex Relaying for Optical Wireless Scattering Communication With On-off Keying Modulation

We address full-duplex relaying for optical wireless communication with stochastic and non-negligible self-interference at the relay node with on-off keying modulation. Assume a relay communication system with a source node, a relaying node, and a destination node, where there is no interference at the source and destination nodes, since they merely transmit or receive. We propose two full-duplex relay protocols, namely, the detect-and-forward relaying protocol and the decode-and-forward relay protocol. For the former, the relay node performs the symbol detection and forwards the detected symbols to the destination node, and for the latter, the relay node performs block decoding and forwards the re-encoded block to the destination node. We prove that for sufficiently high self-interference intensity, the achievable transmission rate of the full-duplex relay system can be lower than that of the half-duplex relay system without self-interference for the detect-and-forward relay protocol. We also prove that for the decode-and-forward relay protocol, the full-duplex relay system always has a larger transmission rate compared with the half-duplex relay system. This can be justified by the fact that, for the decode-and-forward protocol, the source-relay link adopts two codebooks in the scenarios with and without self-interference. The performance of the two proposed protocols is evaluated by the numerical results.

Impact of Relay’s Eavesdropping on Untrusted Amplify-and-Forward Networks Over Nakagami- $m$ Fading

We investigate the impact of eavesdropping on the secrecy outage performance of dual-hop amplify-and-forward networks with a single untrusted relay, in which cooperative jamming technique is employed at the destination to realize positive secrecy rate. Under Nakagami- ${m}$ fading channels, we derive tight closed-form and asymptotic expressions for the lower and upper bounds of the secrecy outage probability (SOP). Our results explicitly show that a secrecy diversity order of ${\min ({m_{1}}, {m_{2}}/{2})}$ in terms of the SOP is achieved, where ${m_{1}}$ and ${m_{2}}$ are the respective integer fading parameters of the source-relay and relay-destination links. Afterwards, we propose an asymptotically optimal power allocation (OPA) scheme that minimizes the SOP. Numerical and simulation results show that the proposed OPA offers significant improvement on the SOP performance, especially in the case of ${m_{1}> m_{2} /2}$ .

Secure source-relay link based threshold DF relaying scheme

In this work, a dual-hop cooperative system, in which there are a Source-Destination (S-D) pair, a relay node (R) and an eavesdropper (E), which attempts to eavesdrop the confidential message sent by S and forwarded by R, is considered. In order to enhance the system performance and save the system resource, we propose an S-R link based threshold decode-and-forward (DF) relaying scheme for R to decide whether to aid S-D pair’s information transmission or not, other than the traditional DF relaying scheme. The secrecy outage performance of the considered system is investigated and the closed-form analytical expression for secrecy outage probability is derived and verified via Monte-Carlo simulations.