Relay-destination Link Research Articles

Performance of Fixed Gain Amplify-and-Forward Relay Systems with Partial Channel State Information

For amplify-and-forward (AF) relay systems employing coherent modulation, the receiver at the destination requires the full channel state information (CSI) of the source-relay-destination (SRD) links and the amplified relay-destination (RD) links to perform maximal ratio combining (MRC). To reduce the burden of channel estimation and CSI forwarding in AF relay systems, we propose an MRC receiver with partial CSI which needs the CSI of the SRD links and the variance of the source-relay (SR) links. We analyze the bit-error-rate (BER) of the proposed scheme based on the moment generating function approach. Compared with the full CSI case, the BER performance degradation due to partial CSI depends on the channel variances of the SR and RD links. When the channel variances of the SR links are much larger than those of the RD links, the performance loss due to partial CSI is negligible.

Maximum diversity achieving decoders in MIMO decode-and-forward relay systems with partial CSI

We consider multiple-input multiple-output decode-and-forward relay systems in Rayleigh fading channels under the partial channel state information (CSI) that the channel statistics of the source-relay (SR) link and the instantaneous CSI of the source-destination and relay-destination links are known at the destination. In this paper, we propose a new near maximum likelihood (near-ML) decoder with two-level pairwise error probability (near-ML-2PEP) which uses the average PEP instead of the exact PEP. Then, we theoretically prove that the near-ML and near-ML-2PEP decoders achieve the maximum diversity, which is confirmed by Monte Carlo simulations. Moreover, we show that the near-ML-2PEP decoder can also achieve the maximum diversity by substituting the average PEP with the values that represent the error performance of the SR link.

Improved Spectral Efficiency at Reduced Outage Probability for Cooperative Wireless Networks by Using CSI Directed Estimate and Forward Strategy

Cooperative wireless communication is a communication mechanism to attain diversity through virtual antenna array that is formed by sharing resources among different users. Different strategies of resource utilization such as amplify-and-forward (AF) and decode-and-forward (DF) already exist in cooperative networks. Although the implementation of these strategies is simple, their utilization of the channel state information (CSI) is generally poor. As a result, the outage and bit error rate (BER) performances need much more improvement in order to satisfy the upcoming high data rate demands. For that to happen the spectral efficiency supported by a wireless system at a very low outage probability should be increased. In this paper a new approach, based on the previously existing ones, called CSI directed estimate and forward (CDEF) with a reduced estimation domain is proposed. A closed form solution for the optimal signal estimation at the relay using minimum mean square error (MMSE) as well as a possible set reduction of the estimation domain is given. It will be shown that this new strategy attains better symbol error rate (SER) and outage performance than AF or DF when the source relay link is comparatively better than the relay destination link. Simulation results also show that it has got better spectral efficiency at low outage probability for a given signal to noise ratio (SNR) as well as for a fixed outage probability in any operating SNR range.

Performance Analysis of Full-Duplex Relaying Employing Fiber-Connected Distributed Antennas

Different from the previous studies on distributed-antenna (DA) systems (DASs) that mostly focused on the enhancement of downlink or uplink performance of cellular systems through cooperation of DAs, this paper proposes and investigates the performance of a novel fiber-connected DA relay system (DARS) that employs radio-over-fiber (RoF) techniques to interconnect a DAS to a central processor, which collectively form a relay node. Under the assumption that self-interference can be perfectly canceled in DARS, full-duplex (FD) operation is possible, by which some antennas receive from the source, whereas other antennas simultaneously transmit to the destination. The numbers of transmit and receive antennas can be controlled to achieve a balance between the transmissions of the source-relay (SR) and relay-destination (RD) links. Consequently, higher spectral efficiency can be achieved compared with half-duplex (HD) relaying systems. For Nakagami fading channels with no direct source-destination (SD) link, we show that DARS can obtain the optimal diversity-multiplexing tradeoff (DMT). Moreover, as the primary purpose of FD relaying is to increase the throughput, we also analyze the throughput performance of DARS. Numerical results show that the FD-DARS with a large number of antennas exhibits a much better throughput performance than HD relaying systems and in regions with high signal-to-noise ratios.

Detect-and-Forward Relaying Aided Cooperative Spatial Modulation for Wireless Networks

A novel detect-and-forward (DeF) relaying aided cooperative SM scheme is proposed, which is capable of striking a flexible tradeoff in terms of the achievable bit error ratio (BER), complexity and unequal error protection (UEP). More specifically, SM is invoked at the source node (SN) and the information bit stream is divided into two different sets: the antenna index-bits (AI-bits) as well as the amplitude and phase modulation-bits (APM-bits). By exploiting the different importance of the AI-bits and the APM-bits in SM detection, we propose three low-complexity, yet powerful relay protocols, namely the partial, the hybrid and the hierarchical modulation (HM) based DeF relaying schemes. These schemes determine the most appropriate number of bits to be re-modulated by carefully considering their potential benefits and then assigning a specific modulation scheme for relaying the message. As a further benefit, the employment of multiple radio frequency (RF) chains and the requirement of tight inter-relay synchronization (IRS) can be avoided. Moreover, by exploiting the benefits of our low-complexity relaying protocols and our inter-element interference (IEI) model, a low-complexity maximum-likelihood (ML) detector is proposed for jointly detecting the signal received both via the source-destination (SD) and relay-destination (RD) links. Additionally, an upper bound of the BER is derived for our DeF-SM scheme. Our numerical results show that the bound is asymptotically tight in the high-SNR region and the proposed schemes provide beneficial system performance improvements compared to the conventional MIMO schemes in an identical cooperative scenario.

Distributed Fuzzy Logic-Based Relay Selection Algorithm for Cooperative Wireless Sensor Networks

Relay selection is a technique that can considerably increase the performance of cooperative communications. In this paper, we propose a new relay selection algorithm using fuzzy logic aiming at both network lifetime and end-to-end throughput. The new algorithm operates in a distributed way, running independently at each node, not requiring a central entity for complex coordination. The method considers the channel state of the relay-destination link and the residual energy of its battery. It is shown that the proposed scheme can transmit a larger amount of information during the network lifetime with respect to both the opportunistic (which focuses on throughput) and random (which focuses on lifetime) selection algorithms, increasing the network utility.

Relay Selection for OFDM Wireless Systems under Asymmetric Information: A Contract-Theory Based Approach

User cooperation although improves performance of wireless systems, it requires incentives for the potential cooperating nodes to spend their energy acting as relays. Moreover, these potential relays are better informed than the source about their transmission costs, which depend on the exact channel conditions on their relay-destination links. This results in asymmetry of available information between the source and the relays. In this paper, we use contract theory to tackle the problem of relay selection under asymmetric information in OFDM-based cooperative wireless system that employs decode-and-forward (DF) relaying. We first design incentive compatible offers/contracts, consisting of a menu of payments and desired signal-to-noise-ratios (SNR)s at the destination. The source then broadcasts this menu to nearby mobile nodes. The nearby mobile nodes which are willing to relay, notify back the source with the contracts they agree to accept in each subcarrier. We show that when the source is under a budget constraint, the problem of relay selection in each subcarrier with the goal of maximizing capacity is a nonlinear non-separable knapsack problem. We propose a heuristic relay selection scheme to solve this problem. We compare the performance of our overall mechanism and the heuristic solution with a simple relay selection scheme. Selected numerical results show that our solution performs better and is close to optimal. The benefits of the overall mechanism introduced in this thesis is that it is simple to implement, needs limited interaction with potential relays and hence it requires minimal signalling overhead.

Channel quality estimation for cooperative demodulation in coded OFDM systems with decode‐and‐forward relays

ABSTRACTIn a cooperative communication system, the destination needs to perform cooperative demodulation, which properly combines the two versions of received signals, one through the source–destination link and the other through the source–relay–destination link. Whereas the quality of the source–relay–destination link can be easily estimated at the destination in an amplify‐and‐forward relay system, the channel quality estimation may be more difficult in a decode‐and‐forward relay system, and inaccurate estimates can severely degrade the system performance. This may be especially true for coded orthogonal frequency division multiplexing relay systems, where each subcarrier may have different channel quality and error correction is performed at the relay. This letter proposes a channel quality estimation method for cooperative demodulation technique in coded orthogonal frequency division multiplexing systems with decode‐and‐forward relays. Copyright © 2013 John Wiley & Sons, Ltd.

Multiuser Detection for Uplink DS-CDMA Amplify-and-Forward Relaying Systems

We consider the uplink of a cooperative DS-CDMA communication system, where each user communicates with the base station through a direct link and with the help of relays. A multiuser detection approach for the joint estimation of spatial signatures, channel gains and transmitted symbols of all the users is proposed. Such an approach is derived from a trilinear tensor model for the received signal that combines the source-destination and relay-destination links. The proposed multiuser receiver provides an extension of the traditional trilinear DS-CDMA receiver to cooperative relaying systems, by including the users' relay-assisted links into the trilinear model. Moreover, in contrast to a previous work by the authors, where the relays of a cluster transmit over orthogonal channels, the proposed receiver operates in a more attractive scenario where all the relays simultaneously transmit towards the base station without requiring orthogonal codes. Simulation results illustrate the performance of the proposed receiver in comparison with alternative approaches.

THP Transceiver Design for MIMO Relaying with Direct Link and Partial CSI

In this letter we consider Tomlinson Harashima precoding (THP) for multiple-input multiple-output (MIMO) relay systems with a direct link between the source and destination. It is assumed that the destination can acquire full channel state information (CSI) of all channels, whereas the source and relay have full CSI of the source-relay channel but only acquire statistical knowledge of the source-destination and relay-destination channels. Simulation results demonstrate that, for high signal to noise ratio (SNR) in the relay-destination link, the performance of the proposed design approaches that of non-linear transceivers utilising full CSI.

Noncoherent Amplify-and-Forward MIMO Relaying With OSTBC Over Rayleigh–Rician Fading Channels

This paper investigates orthogonal space-time block-coded (OSTBC) transmission for a multiple-input multiple-output (MIMO) channel with noncoherent amplify-and-forward (AF) relaying in a situation where the source–relay and relay–destination channels undergo Rayleigh fading and Rician fading, respectively. Fading coefficients of the Rician channel are considered to be correlated. Different scenarios are then considered for the Rician channel. We derive exact expressions for the moment-generating function (mgf) and the first and second moments of the instantaneous signal-to-noise ratio (SNR) at the destination to statistically characterize system behavior. We also derive the exact expressions for the probability density function (pdf) and the cumulative distributive function (cdf) to discuss system behavior with the Rician factor. We then analyze system performance by deriving new analytical expressions for the bit error rate (BER), outage probability, amount of fading (AoF), diversity order, and array gain. These performance metrics reveal that strong line-of-sight (LoS) components between a relay–destination link always limit the performance promised by the MIMO scattering environment when both nodes have multiple antennas.

Incremental Relaying for the Gaussian Interference Channel With a Degraded Broadcasting Relay

This paper studies incremental relay strategies for a two-user Gaussian relay-interference channel with an in-band-reception and out-of-band-transmission relay, where the link between the relay and the two receivers is modelled as a degraded broadcast channel. It is shown that generalized hash-and-forward (GHF) can achieve the capacity region of this channel to within a constant number of bits in a certain weak-relay regime, where the transmitter-to-relay link gains are not unboundedly stronger than the interference links between the transmitters and the receivers. The GHF relaying strategy is ideally suited for the broadcasting relay because it can be implemented in an incremental fashion, i.e., the relay message to one receiver is a degraded version of the message to the other receiver. A generalized-degree-of-freedom (GDoF) analysis in the high signal-to-noise ratio (SNR) regime reveals that in the symmetric channel setting, each common relay bit can improve the sum rate roughly by either one bit or two bits asymptotically depending on the operating regime, and the rate gain can be interpreted as coming solely from the improvement of the common messages rate, or alternatively in the very weak interference regime as solely coming from the rate improvement of the private messages. Further, this paper studies an asymmetric case in which the relay has only a single link to one of the destinations. It is shown that with only one relay-destination link, the approximate capacity region can be established for a larger regime of channel parameters. Further, from a GDoF point of view, the sum-capacity gain due to the relay can now be thought as coming from either signal relaying only or interference forwarding only.

Analysis of the Generalized DF-CF for Gaussian Relay Channels: Decode or Compress?

We consider a three-node quasi-static communication system with a full-duplex relay. The goal is to determine the relaying mode that enables rate-efficient communication under given channel conditions. To achieve this goal, we consider a generalized scheme that subsumes the decode-and-forward (DF) and compress-and-forward (CF) schemes as special cases. The generalized scheme is considered when the source and relay signals are synthesized from commonly-used Gaussian codebooks, which are shown to be capacity achieving in two asymptotic cases: perfect relay-destination link and broken source-destination link. Studying the generalized DF-CF scheme, it is shown that, for two non-asymptotic cases in which the signal-to-noise ratios (SNRs) of the links satisfy certain conditions, this scheme reduces to either DF or CF. For another set of non-asymptotic SNRs, the generalized scheme is shown to yield strictly higher rates than both DF and CF. Despite the complexity of the generalized scheme, its rate advantage over DF and CF is shown to be upper bounded by 0.5 bits per channel use. This indicates that the practical benefit of the analysis of this scheme is to enable selecting the relaying mode that suits a given channel realization. Numerical results show that, under Rayleigh fading conditions, this selection yields significant gains over fixed DF and CF.

Joint power allocation and relay selection for decode-and-forward cooperative relay in secure communication

In this paper, we investigate the cooperative strategy with total power constraint in decode-and-forward (DF) relaying scenario, in the presence of an eavesdropper. Due to the difference of channel for each source-relay link, not all relay nodes have constructive impacts on the achievable secrecy rate. Besides, the final achieved secrecy rate depends on both source-relay and relay-destination links in DF relaying scenario. Therefore, the principal question here is how to select cooperative strategy among relays with proper power allocation to maximize the secrecy rate. Three strategies are considered in this paper. First, we investigate the cooperative jamming (CJ) strategy, where one relay with achieved target transmission rate is selected as a conventional relay forwarding signal, and remaining relays generate artificial noise via CJ strategy to disrupt the eavesdropper. Two CJ schemes with closed-form solutions, optimal cooperative jamming (OCJ) and null space cooperative jamming (NSCJ), are proposed. With these solutions, the corresponding power allocation is formulated as a geometric programming (GP) problem and solved efficiently by convex programming technique. Then, to exploit the cooperative diversity, we investigate the cooperative relaying (CR) strategy. An iterative algorithm using semi-definite programming (SDP) and GP together with bisection search method is proposed to optimize the cooperative relaying weight and power allocated to the source and relays. Furthermore, to exploit the advantages of both CR and CJ, we propose an adaptive strategy to enhance the security. Simulation results demonstrate that the efficiency of the proposed cooperative strategies in terms of secrecy rate.

Impact of Primary User Traffic on Adaptive Transmission for Cognitive Radio with Partial Relay Selection

In a cognitive relay system, the secondary user is permitted to transmit data via a relay when licensed frequency bands are detected to be free. Previous studies mainly focus on reducing or limiting the interference of the secondary transmission to the primary users. On the other hand, however, the primary user traffic also affects the data transmission performance of the secondary users. In this paper, we mainly investigate the impact of the primary user traffic on the bit error rate (BER) of the secondary transmission, when the secondary user adopts adaptive transmission with a partially selected relay. In addition, the average collision time and the average collision probability have been used to measure the interference level caused by the secondary transmission to the primary users. Based on some selected numerical results, we can see that the primary user traffic seriously degrades the average BER. The worse-link partial selection can perform almost as well as the global selection when the channel conditions of the source-relay links and the relay-destination links are quite different. Although the relay selection improves the spectral efficiency of the secondary transmission, numerical results show that it only has slight impact on the overall average BER, which implies that the robustness of the system will not be affected by the relay selection.

Outage probability of a relay strategy allowing intra-link errors utilizing Slepian-Wolf theorem

In conventional decode-and-forward (DF) one-way relay systems, a data block received at the relay node is discarded, if the information part is found to have errors after decoding. Such errors are referred to as intra-link errors in this article. However, in a setup where the relay forwards data blocks despite possible intra-link errors, the two data blocks, one from the source node and the other from the relay node, are highly correlated because they were transmitted from the same source. In this article, we focus on the outage probability analysis of such a relay transmission system, where source-destination and relay-destination links, Link 1 and Link 2, respectively, are assumed to suffer from the correlated fading variation due to block Rayleigh fading. The intra-link is assumed to be represented by a simple bit-flipping model, where some of the information bits recovered at the relay node are the flipped version of their corresponding original information bits at the source. The correlated bit streams are encoded separately by the source and relay nodes, and transmitted block-by-block to a common destination using different time slots, where the information sequence transmitted over Link 2 may be a noise-corrupted interleaved version of the original sequence. The joint decoding takes place at the destination by exploiting the correlation knowledge of the intra-link (source-relay link). It is shown that the outage probability of the proposed transmission technique can be expressed by a set of double integrals over the admissible rate range, given by the Slepian-Wolf theorem, with respect to the probability density function (pdf) of the instantaneous signal-to-noise power ratios (SNR) of Link 1 and Link 2. It is found that, with the Slepian-Wolf relay technique, so far as the correlation ρ of the complex fading variation is |ρ|<1, the 2nd order diversity can be achieved only if the two bit streams are fully correlated. This indicates that the diversity order exhibited in the outage curve converges to 1 when the bit streams are not fully correlated. Moreover, the Slepian-Wolf outage probability is proved to be smaller than that of the 2nd order maximum ratio combining (MRC) diversity, if the average SNRs of the two independent links are the same. Exact as well as asymptotic expressions of the outage probability are theoretically derived in the article. In addition, the theoretical outage results are compared with the frame-error-rate (FER) curves, obtained by a series of simulations for the Slepian-Wolf relay system based on bit-interleaved coded modulation with iterative detection (BICM-ID). It is shown that the FER curves exhibit the same tendency as the theoretical results.

Buffer-Aided Relaying With Adaptive Link Selection—Fixed and Mixed Rate Transmission

We consider a simple network consisting of a source, a half-duplex DF relay with a buffer, and a destination. We assume that the direct source-destination link is not available and all links undergo fading. We propose two new buffer-aided relaying schemes. In the first scheme, neither the source nor the relay have CSIT, and consequently, both nodes are forced to transmit with fixed rates. In contrast, in the second scheme, the source does not have CSIT and transmits with fixed rate but the relay has CSIT and adapts its transmission rate accordingly. In the absence of delay constraints, for both fixed rate and mixed rate transmission, we derive the throughput-optimal buffer-aided relaying protocols which select either the source or the relay for transmission based on the instantaneous SNRs of the source-relay and the relay-destination links. In addition, for the delay constrained case, we develop buffer-aided relaying protocols that achieve a predefined average delay. Compared to conventional relaying protocols, which select the transmitting node according to a predefined schedule independent of the link instantaneous SNRs, the proposed buffer-aided protocols with adaptive link selection achieve large performance gains. In particular, for fixed rate transmission, we show that the proposed protocol achieves a diversity gain of two as long as an average delay of more than three time slots can be afforded. Furthermore, for mixed rate transmission with an average delay of $E{T}$ time slots, a multiplexing gain of $r=1-1/(2E{T})$ is achieved. Hence, for mixed rate transmission, for sufficiently large average delays, buffer-aided half-duplex relaying with and without adaptive link selection does not suffer from a multiplexing gain loss compared to full-duplex relaying.

Performance Analysis of Opportunistic Relaying Scheme with Outdated Channel Information

In this paper, we investigate the effects of outdated channel state information (CSI) on an opportunistic relaying scheme (ORS) consisting of relay selection and data transmission phases. The outage probability and average symbol error rate (ASER) of the ORS are derived in a closed-form expression for an arbitrary link signal-to-noise ratio (SNR) when the CSIs in the source-relay and relay-destination links are outdated. We also analyze the diversity order and coding gain in terms of the outage probability and ASER. Analytical and numerical results show that when the CSIs in either the source-relay or relay-destination links are outdated, the diversity order is reduced to one. The overall performance of the ORS is mainly degraded by inaccurate CSI in the relay-destination link. Conversely, CSI accuracy in the source-relay link has a negligible effect on the performance of the ORS unless the CSI is perfect.

Polar Codes for Gaussian Degraded Relay Channels

In this paper we apply polar codes for the Gaussian degraded relay channel. We study the conditions to be satisfied by the codes and provide an efficient method for constructing them. The relay-destination link is special because the noise is the sum of two components: the Gaussian noise and the signals from the source. We study this link and provide the log-likelihood expression to be used by the polar code detector. We perform simulations of the channel and the results show that polar codes of high rate and large codeword length are closer to the theoretical limit than other good codes.

An EM Algorithm-Based Disintegrated Channel Estimator for OFDM AF Cooperative Relaying

The cooperative orthogonal frequency-division multiplexing (OFDM) relaying system is widely regarded as a key design for future broadband mobile cellular systems. This paper focuses on channel estimation in such a system that uses amplify-and-forward (AF) as the relaying strategy. In the cooperative AF relaying, the destination requires the individual (disintegrated) channel state information (CSI) of the source-relay (S-R) and relay-destination (R-D) links for optimum combination of the signals received from source and relay. Traditionally, the disintegrated CSIs are obtained with two channel estimators: one at the relay and the other at the destination. That is, the CSI of the S-R link is estimated at relay and passed to destination, and the CSI of the R-D link is estimated at destination with the help of pilot symbols transmitted by relay. In this paper, a new disintegrated channel estimator is proposed; based on an expectation-maximization (EM) algorithm, the disintegrated CSIs can be estimated solely by the estimator at destination. Therefore, the new method requires neither signaling overhead for passing the CSI of the S-R link to destination nor pilot symbols for the estimation of the R-D link. Computer simulations show that the proposed estimator works well under the signal-to-noise ratios of interest.