Relay Terminals Research Articles

Successive-Relaying-Aided Decode-and-Forward Coherent Versus Noncoherent Cooperative Multicarrier Space–Time Shift Keying

Successive-relaying-aided (SR) cooperative multicarrier (MC) space-time shift keying (STSK) is proposed for frequency-selective channels. We invoke SR to mitigate the typical 50% throughput loss of conventional half-duplex relaying schemes and MC code-division multiple access (MC-CDMA) to circumvent the dispersive effects of wireless channels and to reduce the SR-induced interference. The distributed relay terminals form two virtual antenna arrays (VAAs), and the source node (SN) successively transmits frequency-domain (FD) spread signals to one of the VAAs, in addition to directly transmitting to the destination node (DN). The constituent relay nodes (RNs) of each VAA activate cyclic-redundancy-checking-based (CRC) selective decode-and-forward (DF) relaying. The DN can jointly detect the signals received via the SN-to-DN and VAA-to-DN links using a low-complexity single-stream-based joint maximum-likelihood (ML) detector. We also propose a differentially encoded cooperative MC-CDMA STSK scheme to facilitate communications over hostile dispersive channels without requiring channel estimation (CE). Dispensing with CE is important since the relays cannot be expected to altruistically estimate the SN-to-RN links for simply supporting the source. Furthermore, we propose soft-decision-aided serially concatenated recursive systematic convolutional (RSC) and unity-rate-coded (URC) cooperative MC STSK and investigate its performance in both coherent and noncoherent scenarios.

Enhancement the Spectral Efficiency of Half-Duplex Single Antenna Multi-Relay System

In this paper, we study two-hop communication protocols where several relay terminals assist in the communication between two terminals. The terminals operate in halfduplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiencydue to the per-log factor one-half in corresponding capacity expressions. In this paper, we propose to apply two-path and two-way protocols to multi-relay networks. These relay protocols avoid the pre-log factor one-half. The communication channels between source and relays and between the relays and destination are assumed to be Rayleigh flat fading. Theperformance of the network is investigated for perfect and estimated channel and for decodeand- forward scheme. The performance is measured in terms of bit error rate. Moreover the outage probability is evaluated when one relay is used and is compared with that one for multi-relay system. Channel estimation is performed using least square algorithm. Theperformance of the channel estimation algorithm is measured in terms of the mean square error of estimation.

Reliable Joint Source-Channel Cooperative Transmission Over Relay Networks

Reliable transmission of a discrete memoryless source to multiple destinations over a relay network is considered. Motivated by sensor network applications, it is assumed that the relays and the destinations all have access to side information correlated with the underlying source signal. Joint source-channel cooperative transmission is studied in which the terminals in the network help the transmission of the source signal to the destinations by using their overheard signals, as in the classical channel cooperation scenario, as well as the available correlated side information. Decode-and-forward-based cooperative transmission is studied in a network of multiple relay terminals and two different achievability schemes are proposed: 1) a regular encoding and sliding-window decoding scheme without explicit source binning at the encoder; and 2) a semiregular encoding and backward decoding scheme with binning based on the side information statistics. It is shown that both of these schemes lead to the same source-channel code rate, which is shown to be the source-channel capacity in the case of 1) a physically degraded relay network with a single destination in which the side information signals are degraded in the same order as the channel; and 2) a relay network with multiple destinations, in which all the terminals want to reconstruct the source reliably, while at most one of them can act as a relay.

Max–min relay selection in bidirectional cooperative networks with imperfect channel estimation

The authors study the performance of wireless bidirectional relay-assisted networks in the presence of imperfect channel state information, where two end-source terminals S1 and S2 communicate with the assistance of M relay terminals Rj's. The max - min relay selection criterion is used to select the best relay that maximises the minimum signal-to-noise ratio of the links S1 �R jS 2 and S2 �R jS 1 over all relay terminals. The authors investigate the impact of imperfect channel estimation on the outage probability Pout of the system by means of the correlation coefficient rSi of the estimated channel gains and their actual values. Furthermore, the authors show that in a bidirectional relay-assisted network neither of the links S1 �R jS 2 and S2 �R jS 1 dominates the performance of the system. Instead, the performance is determined by the average performance of the two links, based on that the authors then discuss the power allocation in such networks. The authors demonstrate that in order to minimise Pout of the entire system, increasing the transmission power of the link with better estimation cannot compensate for the effect of the worse link and therefore the optimum power allocation with the least complexity is to transmit at each source terminals S1 and S2 with equal powers. Numerical results are also presented to corroborate the analytical expressions.

Dual-hop MIMO relaying with OSTBC over doubly-correlated Nakagami-m fading channels

In this article, we investigate the performance of dual-hop amplify-and-forward multiple-input multiple-output relaying system with orthogonal space-time block code transmissions over doubly-correlated Nakagami-m fading channel, where the source, relay, and destination terminals are all equipped with multiple antennas. For two different CSI-assisted relaying schemes, which could be encompassed by a unified model, we provide the compact closed-form expressions for cumulative distribution function, probability density function, moment generating function, and generalized moment (GM) of the instantaneous end-to-end SNR. Besides, the exact analytical expressions for the outage probability (OP) and symbol error rate (SER) and approximate expression for ergodic capacity are also derived. Furthermore, we present the asymptotic expressions for OP and SER in the high SNR regime, from which we gain an insight into the system performance and derive the achievable diversity order and array gain. The analytical expressions are validated by Monte-Carlo simulations.

Energy- and Cost-Efficient Mobile Communication Using Multi-Cell MIMO and Relaying

In this paper, relaying and multi-cell MIMO transmission are investigated as approaches for improving resource reuse and more flexible organization of cellular networks. The analysis focuses on approaches for future cellular systems, which jointly exploit relaying and multi-cell MIMO transmission. Possible candidate approaches are identified, simplified for practical applications and evaluated using a system-level model from the European research project WINNER. Their achievable throughput is analyzed under practical constraints using three different normalization approaches: cost-normalization, energy-normalization, and joint cost-energy-normalization. It can be shown that the combined approach of relaying and multi-cell MIMO provides significant gains for the uplink communication. The selected approach exploits cooperative multi-cell MIMO processing between base stations and relay nodes, and uses a resource coordination technique on the links between relay nodes and user terminals. If a relay-based deployment is subject to a cost- and energy-normalization, multi-cell MIMO outperforms relaying with respect to achievable downlink throughput.

Robust Transmitter Design for Amplify-and-Forward MIMO Relay Systems Exploiting Only Channel Statistics

In this paper, we address statistically robust transmit design problems that maximize the average mutual information of amplify-and-forward (AF) multiple-input multiple-output (MIMO) two-hop relay channels having perfect channel state information at the receiver (CSIR) and statistical channel state information at the transmitter (CSIT). In the selected scenario, the source, relay, and destination terminals are equipped with correlated antennas where a direct link between the source and the destination terminals can be found. Moreover, the statistical CSIT consists of the channel means and covariance matrices of various links. The design problem is taken into account for regimes in large systems because there are no closed-form expressions of the average mutual information in the MIMO relay channel. The contribution of this study includes the derivation of asymptotic mutual information expressions for the MIMO relay channel in the large system limit. Additionally, an efficient optimization algorithm based on the asymptotic mutual information is proposed in order to find the asymptotically optimal source covariance matrix and the relay amplifying matrix. Numerical simulation results show that the new approach achieves indistinguishable performance compared to those of the maximization approaches in finite-dimensional systems, even for a small number of antennas at each link. The impact of the CSI of various links on the throughput of the MIMO relay channel is studied using the new approach.

Improving Energy Efficiency and Network Lifetime of Manets Through Ec-Mac Protocol

A MANET is an ad hoc network that can change locations and configure itself, and use wireless connections to connect various networks. The performance of Mobile Ad Hoc Networks (MANETs), is improved using Energy Conservation-MAC Protocol along with DEL-CMAC Protocol in terms of energy efficiency and network lifetime. This can be achieved through LZW (Lempel,Ziv and Welch) compression technique. The best relay selection strategy is incorporated, which selects the best relay based on residual energy and location information. To improve the spatial reuse and an innovative NAV (network allocation vector) setting is provided to deal with the varying transmitting power of the source and relay terminals. It provides the best selection algorithm to select the cooperative node with better channel condition, higher transmission rate, best relay and more balanced energy consumption.

Effect of Feedback Delay on the Performance of Cooperative Networks with Relay Selection

In this paper, we analyze the effect of feedback delay and channel estimation errors on the performance of a decode-and-forward (DF) cooperative transmission scenario with relay selection. In our relay selection scheme, only one relay with the best relay-to-destination (R → D) channel quality is selected among the set of relays that decode the source information correctly. Specifically, the destination terminal first estimates the channel state information (CSI) of all active R → D links and then sends the index of the best relay to the relay terminals via a delayed feedback link. Due to the time varying nature of the fading channels, selection is performed based on the old version of the channel estimate. Closed-form expressions for the outage probability, average capacity and average symbol error rate (ASER) are derived. Through asymptotic diversity order analysis, we show that the presence of feedback delay reduces the asymptotic diversity order to one, while the effect of channel estimation errors reduces it to zero. Finally, simulation results are presented to corroborate the analytical results.

Average capacity performance of opportunistic relay selection with outdated CSI

The authors investigate the effect of feedback delay on the average capacity of a decode-and-forward cooperative network with relay selection. In particular, a multi-relay cooperative scenario is considered, where the best relay is selected from a subset of relays that are able to decode the source information correctly. In this selection scenario, the authors assume that the destination terminal estimates the relay-to-destination ( R→D ) channel-state-information perfectly and sends the index of the best relay to the relay terminals via a delayed feedback link. The authors investigate the performance of the considered scenario in terms of average capacity. Simulation results are presented to corroborate the analytical results.

Precoder Design for MIMO Relay Networks with Direct Link and Decision Feedback Equalisation

In this letter we derive the source and relay precoders for a multiple-input multiple-output (MIMO) relay system with zero-forcing (ZF) decision feedback equalisation (DFE) when a direct link exists between the source and destination. Assuming full channel state information (CSI) is available at each node in the network, the processors are optimised for a large class of objective functions subject to the ZF condition and average power constraints at both the source and relay terminals. Simulation results show that the proposed design outperforms existing techniques in terms of bit error ratio (BER).

DMT Optimal Cooperative Protocols with Destination-Based Selection of the Best Relay

We design a cooperative protocol in the context of wireless mesh networks in order to increase the reliability of wireless links. Destination terminals ask for cooperation when they fail in decoding data frames transmitted by source terminals. In that case, each destination terminal D calls a specific relay terminal B with a signaling frame to help its transmission with source terminal S. To select appropriate relays, destination terminals maintain tables of relay terminals, one for each possible source address. These tables are constituted by passively overhearing ongoing transmissions. Hence, when cooperation is needed between S and D, and when a relay B is found by terminal D in the relay table associated with terminal S, the destination terminal sends a negative acknowledgment frame that contains the address of B. When the best relay B has successfully decoded the source message, it sends a copy of the data frame to D using a selective decode-and-forward transmission scheme. The on-demand approach allows maximization of the spatial multiplexing gain and the cooperation of the best relay allows maximization of the spatial diversity order. Hence, the proposed protocol achieves optimal diversity-multiplexing trade-off performance. Moreover, this performance is achieved through a collision-free selection process.

Capacity Performance of Relay Beamformings for MIMO Multirelay Networks With Imperfect ${\cal R}$-${\cal D}$ CSI at Relays

In this paper, we consider a dual-hop multiple-input-multiple-output (MIMO) wireless relay network in the presence of imperfect channel state information (CSI), in which a source-destination pair, both equipped with multiple antennas, communicates through a large number of half-duplex amplify-and-forward (AF) relay terminals. We investigate the performance of three linear beamforming schemes when the CSI of the relay-to-destination ( ℜ- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> ) link is not perfect at the relay nodes. The three efficient linear beamforming schemes are based on the matched-filter (MF), zero-forcing (ZF) precoding, and regularized ZF (RZF) precoding techniques, which utilize the CSI of both the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -ℜ channel and the ℜ- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> channel at the relay nodes. By modeling the ℜ- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> CSI error at the relay nodes as independent complex Gaussian random variables, we derive the ergodic capacities of the three beamformers in terms of instantaneous signal-to-noise ratio. Using the Law of Large Number, we obtain the asymptotic capacities, upon which the optimized MF-RZF is derived. Simulation results show that the asymptotic capacities match the respective ergodic capacities very well. Analysis and simulation results demonstrate that the optimized MF-RZF outperforms MF and MF-ZF for any power of the ℜ- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> CSI error.

Optimization scheme with pre-processing for cooperative relay multicast networks in cellular system

For cooperative relay multicast networks, the general cross-layer optimization approaches converge to the global optimal value slowly because of the large quantity of relay terminals. However, the mobility of relay terminals requires quick converging optimization strategies to refresh the relay links frequently. Based on the capacity analysis of multiple relay channels, an improved cross-layer optimization scheme is proposed to resolve this problem, in which the bound of the relay selecting region is determined as a pre-processing. Utilizing the primal-dual algorithm, a cross-layer framework with pre-processing optimizes both the relay terminal selection and power allocation with quick convergence. The simulation results prove the effectiveness of the proposed algorithm.

Opportunistic Relaying for MIMO Wireless Communication: Relay Selection and Capacity Scaling Laws

We propose new low complexity opportunistic relaying strategies for multiple-antenna relay networks. Assuming that a source communicates with a destination, both equipped with M antennas, assisted by K single-antenna relay terminals using an amplify-and-forward half-duplex protocol, we propose a new transmission strategy which employs linear zero-forcing transmission and reception. Integrated with this transmission strategy, we propose two low complexity opportunistic relay selection algorithms, referred to as the Maximum Sum Rate Relay Selection (MSR) and Greedy Semi-Orthogonal Relay Selection (GSO) algorithms, which select only a few very important relays to share the total power. For the GSO algorithm, we present a theoretical analysis of the sum capacity as K grows large, which is shown to be M/2 log log K + O(1). This result is also shown to coincide with a fundamental cut-set upper bound on the sum capacity of MIMO networks with opportunistic relaying which we derive; thereby establishing a new exact scaling law for such networks, as well as demonstrating the asymptotic optimality of our proposed low complexity approach. Our numerical studies also indicate that our proposed opportunistic relaying techniques yield significant capacity benefits over the conventional approach without opportunistic selection, even when the number of relays is not large.

On the Asymptotic Properties of Amplify-and-Forward MIMO Relay Channels

This paper studies the asymptotic properties of amplify-and-forward (AF) multiple-input multiple-output (MIMO) two-hop relay channels, where the source terminal (ST), relay terminal (RT) and destination terminal (DT) are equipped with a number of correlated antennas and there is a direct link between the ST and DT. The analysis is based on the Kronecker correlated fading model where the correlation properties of the MIMO channel are modeled at the transmitter and receiver ends separately. Our first main contribution is the derivation of the asymptotic mutual information expressions for the MIMO-AF relay channel for any given signaling input distributions (not necessarily Gaussian) in the large-system regime, which is particularly relevant if practical modulation schemes are used. In addition to the formula that uses joint optimal decoding (JOD) at the DT, we also obtain the asymptotic result for a more practical system which performs separate detection and decoding (SDD) (i.e., spatial detection followed by a temporal error-correction decoder). Another major contribution of this paper is an efficient algorithm that can compute the optimal source covariance matrix and the relay precoding matrix to maximize the asymptotic mutual information of the MIMO-AF relay system if channel covariance information (CCI) is known at the ST and RT.

Adaptive DF Relaying Scheme Based on CDD for Multi-Hop Networks with Multiple Relay Terminals

In multi-hop wireless networks, the transmitted signal is generally forwarded over several relay terminals. So, the quality of communications is degraded due to these re-transmissions over fading channels in relay terminals. Basically, the conventional cooperative relaying scheme based on cyclic delay diversity (CDD) reduces this degradation because this scheme could have numerous relay terminals which simultaneously transmit the signals. However, we cannot obtain the maximum diversity gain because of the re-transmission without considering relay channel environments. In this letter, to overcome the decreased performance, we propose an adaptive decode-and-forward (DF) relaying scheme based on CDD which uses cyclic redundancy check (CRC) code. Simulation results show that the proposed scheme provides more improvement in error performance than the conventional schemes in multi-hop networks.

Performance Analysis of Relay Selection With Feedback Delay and Channel Estimation Errors

In this letter, we investigate the effect of feedback delay and channel estimation errors in a decode-and-forward (DF) cooperative network with relay selection. In particular, we consider a multirelay cooperative scenario, where the best relay is selected from a subset of relays that are able to decode the source information correctly. In the selection scenario, the destination terminal estimates the relay-to-destination (R → D) channel state information (CSI) and sends the index of the best relay to the relay terminals via a delayed feedback link. We investigate the performance of the considered scenario in terms of average symbol error rate (ASER) and asymptotic diversity order. Simulation results are presented to corroborate the analytical results.

Optimal Beamforming in Two-Way Relay Networks with Cognitive Radio Capabilities

In this paper, we present a cognitive relay network with two primary transceivers that communicate via several distributed relay terminals. Spectrum sensing is deployed at the relays to sense the absence/presence of the primary transceivers based on energy detection. The primary network utilizes a two-step two-way amplify-and-forward (AF) scheme by using the cognitive radio (CR) terminals as its relay nodes when the primary network is not in operation, in contrast, the CRs communicate with their own base station (BS). In the first relaying step, the primary transceivers send their signal to the CRs/relays. Distributed beamforming is then performed in the second relaying step. Our aim is to set the beamforming weights so as to minimize the total power dissipated in the relay network while satisfying a target signal-to-noise ratio (SNR) at the primary transceivers and at the cognitive BS. This is achieved by solving an optimization problem that we formulate as a nonconvex quadratically constrained quadratic program (QCQP). This problem is solved efficiently by semidefinite relaxation (SDR) and Lagrangian duality. Simulation results are provided to demonstrate the superiority of our proposed technique, compared to classical beamforming techniques, in terms of power reduction.

On the Sum-Rate of Uplink MIMO Cellular Systems with Amplify-and-Forward Relaying and Collaborative Base Stations

Cooperation and relaying are emerging technologies as flexible solutions for improving the capacity of cellular systems. Motivated by this, we investigate the asymptotic achievable sum-rate for multiple-input multiple-output (MIMO) multiple-access cellular systems in the uplink employing cooperative base stations (BSs) and amplify-and-forward (AF) relaying. In our model, the antennas at the user equipments (UEs), relay terminals, and BSs are correlated and there are direct links between UEs and BSs. Using the replica method, we derive the large-system sum-rate for the uplink system using joint optimal decoding (JOD) as well as separate detection and decoding (SDD) and show that each UE's performance can be characterized by a vector Gaussian channel with an appropriate vector input, an effective channel gain, and a vector Gaussian noise. Based on the asymptotic result, we also devise an efficient algorithm to determine the asymptotic optimum transmit matrices for the UEs and relays that can maximize the sum-rate of the MIMO multiple-access cellular network if the spatial channel covariance information (CCI) is available and exploited at the UEs and the relay terminals.