Performance of OFDM-Based Dual-Hop Amplify-and-Forward Relaying

In this letter, the end-to-end BER performance of a dual-hop OFDM-based relay system is investigated. The relay terminal operates in the amplify-and-forward (AF) mode. We derive the bit error rate (BER) of AF relaying with average power scaling (APS) constraint valid for source-relay link high signal- to-noise ratio conditions. We also study a frequency diversity technique that improves the performance of single antenna AF-APS relaying. The theoretical BER expressions are verified by computer simulations for a 4-QAM modulated OFDM system.     

End-to-end BER analysis for dual-hop OSTBC transmissions over Rayleigh fading channels

In this letter, a BER study is presented for the end- to-end performance of dual-hop wireless communication systems employing transmit diversity with orthogonal space-time block codes (OSTBCs), where a nonregenerative or regenerative relay is equipped with a single antenna operating over flat Rayleigh fading channels. More specifically, we provide probability density functions (PDFs) and moment generating functions (MGFs) for the end-to-end SNR of the dual-hop OSTBC transmissions and then present its BER performance over M-ary QAM and PSK modulations, respectively. Numerical investigation shows that the analytic BER provided in the letter makes an exact match with the simulation result in various multiple-antenna transmission scenarios. The result also shows how the number of antennas equipped at the source and destination affects the end-to-end performance.     

认知中继网络中的信道分配方法研究

认知中继网络中,对信道进行分配可以有效地提高端到端吞吐量。对三节点认知中继网络下的信道分配进行了研究。中继节点采用解码转发协议时,提出了一种次优的分配方法,将信道按信道增益排序,然后逐个地分配中继信道。中继节点采用放大转发协议时,给出了最优的信道分配方法,提出了一种次优的信道分配方法。次优方法逐个地比较中继信道采用传统协作方式传输时的端到端吞吐量、分配为双跳信道S-R和R-D链路时的端到端吞吐量完成分配。和最优方法相比,两种次优方法以较小的性能损失换取了计算复杂度的降低。给出了数值仿真,比较了两种传输方式下的端到端吞吐量性能,验证了以上方法的有效性。通过对比仿真时间,比较了最优方法和次优方法的计算复杂度。给定信道总数,对次优方法下信道分配后的比例进行了仿真,发现待分配中继信道以1/3的比例分配为直传信道;而在放大转发下,待分配中继信道几乎不被分配为双跳信道。      

Outage Probability Analysis of Dual-Hop Decode-and-Forward Relaying Over Mixed Rayleigh and Generalized Gamma Fading Channels

In this letter, outage probability of dual-hop decode-and-forward (DF) relaying scheme is analyzed over mixed Rayleigh and generalized Gamma fading channels. Cooperation model considered in this work consists of a source, a relay and a destination. It is assumed that source-relay and relay-destination channels experience Rayleigh fading and generalized Gamma fading, respectively. Exact outage probability expression is derived and outage performance is illustrated for both direct transmission and DF relaying scheme.     

Beamforming in Dual-Hop AF Relaying with Imperfect CSI and Co-channel Interference

This letter studies the joint effects of imperfect channel state information (CSI) and co-channel interference (CCI) on a dual-hop variable gain amplify-and-forward (AF) relay network with beamforming. The source and destination are both equipped with multiple antenna, while the relay has a single antenna. Specifically, the analytical expressions for the outage probability and average symbol error rate of the considered AF relaying are derived. Moreover, the asymptotic analysis at high SNR is also presented to reveal the diversity order and array gain of the relay system. Finally, computer simulations are given to confirm the validity of the proposed theoretical analysis and quantify the combined influence of imperfect CSI and CCI on the system performance.     

MGF based performance analysis of dual-hop regenerative relaying IR-UWB systems

In this paper, a dual-hop regenerative cooperative relaying UWB communication system with detect-and-forward (DTF) and decode-and-forward (DF) in multipath channels is studied. The average system bit error rate (BER) and the system performance with DTF and DF relaying schemes are provided. The DF relaying with convolutional coding is employed in this cooperative relaying UWB system. Gaussian quadrature numerical method is used to obtain the moment-generating function (MGF) of the received energy. Our simulation results show the advantages of the DF based cooperative IR-UWB system over IEEE 802.15.3a channels.     

Semi-blind amplify-and-forward in two-way relaying networks

This paper addresses the analysis of a two-way semi-blind amplify-and-forward (AF) relay network, in which the relay node requires partial instantaneous channel state information (CSI) to amplify the received signals. First, we derive the expression of end-to-end signal-to-noise ratio in dual-hop transmissions over independent and not necessarily identically distributed Rayleigh fading channels. Based on the opportunistic AF selection relaying, we define bounds of some metrics such as average sum-rate and outage probability. Furthermore, we provide exact and approximate expressions for the average symbol error rate. The obtained results show that the performance of the proposed system decreases slightly while the processing complexity is reduced significantly compared to those of CSI-assisted AF relay schemes. In order to prove the exactness of the proposed analysis, a selection of numerical results is provided.     

Exact analysis of the multihop multibranch hybrid AF/DF relaying networks

In this paper, the exact analysis of a multihop multibranch (MHMB) relaying network is investigated wherein each relay can operate in amplify‐and‐forward (AF) or decode‐and‐forward (DF) modes depending upon the decoding result of its received signal. If a relay decodes the received signal correctly, it works in DF mode; otherwise, the relay operates in AF mode. Therefore, we name such relaying network as hybrid amplify‐and‐forward and decode‐and‐forward (hybrid AF/DF) relaying network. We first investigate the signal transmission from source to destination node via n number of relays in a hybrid AF/DF MHMB mode. Then, we obtained the statistical features and analyze the end‐to‐end signal‐to‐noise ratio (SNR). Finally, a comprehensive performance analysis is conducted by using maximal ratio combining (MRC) scheme at the destination node. For comparison, we also obtained the results using selection combining (SC) scheme at the destination node. To the best of our knowledge, very few works in the literature have considered a general system model of MHMB relaying network wherein each relay can operate in AF or DF modes, that is, a hybrid AF/DF relaying network. Accordingly, the analysis of our system model is not only novel and exact, but also is comprehensive and can be employed in the future works.     

Performance Analysis of Relay-Assisted Cooperative Systems in Correlated Fading Channels

In this paper, the effect of fading correlation at the receiving elements of a linear antenna array in a relay-assisted cooperative single-input multiple-output (SIMO) system has been investigated over flat Rayleigh fading channels. Considering a model of local scatterer around the source and relay nodes and uniform angle of arrival (AOA) distribution, the spatial cross-correlation analysis is presented and extensive simulation studies are performed to determine the envelope correlation as a function of antenna spacing, mean AOA and angular spread. Error performance analysis for coherent binary and quaternary modulations are presented for fixed-gain amplify-and-forward (AF) and fixed decode-and-forward (DF) relaying with dual diversity maximal-ratio combiner (MRC) receiver array. Error probability in terms of bit error probability (BEP) and symbol error probability (SEP) is evaluated to study the effect of relevant parameters on the correlation performance. The presented results establish the fact that fixed DF relay network performs relatively well than fixed-gain AF relaying. Superior performance of dual-hop cooperative SIMO system over conventional single-hop SIMO system has also been observed.     

同信道干扰条件下的多天线放大转发中继中断概率分析

该文研究了基于波束形成技术的双跳多输入多输出(MIMO)放大转发(AF)中继系统的中断概率,该系统在发射端、中继端和接收端都配置了多根天线。假设每条链路的发射端采用最大比传输(MRT)技术,接收端采用最大合并比(MRC)技术,该文得出了中继端受到同信道干扰时的信干噪比(SINR),推导了基于固定增益中继方案的中断概率(OP)闭合表达式。计算机仿真结果不仅验证了性能分析的有效性,而且还分析了关键参数对系统性能的影响以及配置多天线带来的好处。     

Particle swarm optimization–based power allocation for Alamouti amplify and forward relaying protocol

The performance of wireless communication systems is improved over flat fading channel by using Alamouti coding scheme, which provides the quality of diversity gain. In this paper, performance analysis of symbol error rate (SER) and particle swarm optimization (PSO)–based power allocation (PA) for Alamouti amplify and forward (AF) relaying protocol using maximum ratio combining (MRC) technique is presented. Analytical expression of SER upper bound and SER approximation is derived for Alamouti AF relaying protocol with quadrature phase shift keying (QPSK) modulation over Rayleigh fading channel and Rician fading channel. In addition, PSO‐based optimum PA factor is calculated on the basis of the minimum SER of proposed method. PSO‐based optimum PA gives 0.5 dB of improved signal‐to‐noise ratio (SNR) compared with the equal power allocation (EPA). The theoretical approximate SER result is compared with the simulated SER. The proposed protocol provides full diversity gain and reduces SER compared with the existing AF and decode and forward (DF) relaying protocols over Rayleigh fading channel and Rician fading channel.     

Delay Analysis of Cooperative Truncated HARQ With Opportunistic Relaying

In this paper, we evaluate the delay experienced by Poisson arriving packets for cooperative truncated hybrid automatic repeat request (HARQ) with opportunistic relaying. We derive the theoretical expressions of the expected waiting time and the packet's sojourn time in the queue of truncated opportunistic cooperative ARQ, HARQ I with and without packet combining (PC), and HARQ II with code combining (CC). The analysis is valid for block Nakagami- $m$ fading channels and any number of relays for both amplify-and-forward (AF) and decode-and-forward (DF) relaying. In opportunistic AF cooperative HARQ, packet retransmission is done by the relay offering the highest instantaneous signal-to-noise ratio (SNR) of the relaying link (source–relay–destination). In opportunistic DF cooperative HARQ, retransmission is done by the relay offering the highest instantaneous SNR of the relay–destination link among the relays that have correctly decoded the transmitted packet by the source. If no relay has correctly decoded, the retransmission is made by the source. Simulation results are also provided to verify the tightness of the derived expressions. The results indicate a significant improvement of packet delivery delays when compared with the absence of cooperation.      

Probability of SNR Gain by Dual-Hop Relaying over Single-Hop Transmission in SISO Rayleigh Fading Channels

In this letter, we address the end-to-end signal-to-noise ratio (SNR) gain achievable by dual-hop relaying over the single-hop transmission in single-input single-output (SISO) flat Rayleigh fading channels. We deal with both nonregenerative and regenerative relaying schemes. To quantify the SNR gain, a closed-form expression of the probability of SNR gain is developed. The probability of SNR gain results in an explicit function of an average power ratio of the per-hop channel in dual-hop relaying to the channel in the single-hop transmission. We investigate impacts of the average power ratio as well as the relaying location on the SNR gain.     

Spectral efficient protocols for half-duplex fading relay channels

We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex 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 efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss     

OFDMA uplink frequency offset estimation via cooperative relaying

Frequency offset estimation for an orthogonal frequency-division multiple access (OFDMA) uplink for amplify-and-forward (AF) relays and a new type of relay (R) called decode-and-compensate-and-forward (DcF) relays are studied. Multiple relays are considered, and the relay with the best S rarr R channel is chosen to perform re-transmission, where S and R represent the source and relay nodes, respectively. Frequency offsets due to the mismatches between the transmitter and receiver oscillators are considered, and without considering the effect of Doppler shift, both S rarr D and S rarr R rarr D links have the same frequency offset, where D represents the destination. Thus, by using these two transmissions, D generates two frequency offset estimates, which are combined to minimize the mean square error (MSE). Power allocation between S and R can be adaptively adjusted to optimize the cooperative scheme in terms of frequency offset error variance. When channel state information (CSI) is available at each mobile node, a scheme where the relays adaptively switch between the cooperative and conventional (no relaying) transmissions is proposed to optimize the frequency offset estimation. Although the frequency offset estimation accuracy in the DcF mode is somewhat worse than the AF mode, both modes outperform the conventional transmission. However, DcF (or decode-and-forward (DF)) relays outperform AF relays in terms of channel capacity and bit error rate (BER).     

Cooperative quadrature physical layer network coding in wireless relay networks

This paper proposes a cooperative quadrature physical layer network coding (CQPNC) scheme for a dual‐hop cooperative relay network, which consists of two source nodes, one relay node and one destination node. All nodes in the network have one antenna, and the two source nodes transmit their signals modulated with quadrature carriers. In this paper, a cooperative quadrature physical layer network coded decode‐and‐forward (DF) relay protocol (CQPNC‐DF) is proposed to transmit the composite information from the two source nodes via the relay node to the destination node simultaneously to reduce the number of time slots required for a transmission. The proposed CQPNC‐DF relay protocol is compared with time‐division multiple‐access amplify‐and‐forward (TDMA‐AF), TDMA‐DF, cooperative network coded DF (CNC‐DF) and cooperative analog network coded AF (CANC‐AF) relay protocols to demonstrate its effectiveness in terms of bit error rate (BER) and system throughput under different propagation conditions. The simulation results reveal that the proposed CQPNC‐DF relay protocol can significantly improve the network performance. Compared with two TDMA schemes and CNC‐DF, the proposal can provide up to 100% and 50% throughput gains, respectively. Moreover, no matter what the scene, the proposed scheme always has the lowest BER in the low SNR region. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimal relay functionality for SNR maximization in memoryless relay networks

We explore the SNR-optimal relay functionality in a mernoryless relay network, i.e. a network where, during each channel use, the signal transmitted by a relay depends only on the last received symbol at that relay. We develop a generalized notion of SNR for the class of memoryless relay functions. The solution to the generalized SNR optimization problem leads to the novel concept of minimum mean squared uncorrelated error (MMSUE) estimation. For the elemental case of a single relay, we show that MMSUE estimate is a scaled version of the MMSE estimate. This scheme, that we call estimate and forward (EF), performs better than the best of amplify and forward (AF) and demodulate and forward (DF) in both parallel and serial relay networks. We determine that AF is near-optimal at low transmit power in a parallel network, while DF is near-optimal at high transmit power in a serial network. For hybrid networks that contain both serial and parallel elements, the advantage of EF over the best of AF and DF is found to be significant. Error probabilities are provided to substantiate the performance gain obtained through SNR optimality. We also show that, for Gaussian inputs, AF, DF and EF are identical     

On optimum selection relaying protocols in cooperative wireless networks

In this letter, the outage probabilities of selection relaying protocols are analyzed and compared for cooperative wireless networks. It is assumed that both source and relay use equal allocated time in transmission. Depending on the quality of the source-relay channel, the relay may choose either Decode-and-Forward (DF), Amplify-and-Forward (AF), or Direct-Transmission (DT) to forward signals. It turns out that in terms of outage probability, two selection relaying schemes are better than others: selecting between DF and AF protocols (DF-AF) or selecting between DF and DT protocols (DF-DT). It is shown that with an equal power allocation, both of the DF-AF and DF-DT selection relaying protocols have the same asymptotic outage probability. However, with an optimum power allocation strategy, the DF-AF selection scheme is in general better than the DF-DT selection scheme. Note that the optimum power allocations depend on channel variances, not on instantaneous channel gains. When the quality of the relay-destination link is much better than that of the source-relay link, observed from simulation, the outage probability of the DF-AF selection protocol with its optimum power allocation is 1.5dB better than that of the DF-DT selection with its own optimum power allocation. Extensive simulations are presented to validate the analytical results.     

散射通信的协作分集技术研究

对协作分集进行简单介绍,提出了将协作分集应用到散射通信中的方案,分析了协作分集在散射通信中的应用可行性。并对其中的放大转发和译码转发在只有一个中继节点的情况下进行仿真,仿真结果证明在大信噪比时译码转发的性能优于放大转发。且协作分集应用到散射通信中在相同的信噪比下,误码率更低,提高了系统性能。利用协作分集技术对散射通信进行组网,较直接组网设备复杂度低,易于实现。     

An Efficient Adaptive Distributed Space-Time Coding Scheme for Cooperative Relaying

A non-regenerative dual-hop wireless system based on distributed Alamouti space-time coding is considered. It is assumed that each relay retransmits an appropriately scaled space-time coded version of its received signal. The main goal of this paper is to find a scaling function for each relay to minimize the outage probability. In the high signal-to-noise ratio (SNR) regime for the relay-destination link, it is shown that a threshold-based scaling function (i.e., the relay remains silent if its channel gain with the source is less than its predetermined threshold) is optimum from the outage probability point of view. Numerical results demonstrate a dramatic performance improvement as compared to the case that the relay stations forward their received signals with full power even for finite SNR scenarios.