Exact and asymptotic BEP of cooperative DS-CDMA systems using decode and forward relaying in the presence of multipath propagation

In this paper, we derive the bit error probability (BEP) of cooperative DS-CDMA systems using decode and forward (DF) relaying. We consider a conventional DF protocol where all the relays that have correctly decoded transmit and a selective DF (S-DF) protocol where we activate only the relay that offers the best instantaneous signal to noise ratio (SNR) of the relay to destination link among the relays that have correctly decoded. The derived results are valid for any multipath intensity profile of the channel, any path delays and take into account the correlation of the multipath gains.     

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.     

多天线中继网络中的终端性能和多样性分析

The performance of multi-antenna multi- relay cooperative system is investigated in this paper. Two relaying strategies, i.e., reactive and proactive strategies are analyzed with the Amplifyand- Forward (AF) and Decode-and-Forward (DF) protocols. We derive the Cumulative Distribution Function (CDF) of the received Signal-to-Noise Ratio (SNR) at the destination, which is used to calculate the exact outage probability, for both AF and DF protocols. According to these results, we conclude that a cooperative network which composes K relays each equipped with nr antennas can achieve maximal order-(2nrK+1) diversity gain, by proper processing at relays and destination. Furthermore, the performance comparison is given, in terms of outage probability. These two strategies outperform each other in different scenarios in AF protocol, whilst proactive strategy is always better than its counterpart in DF protocol. According to these results, the optimal power allocation schemes among relay nodes are also presented, with reasonable power constraint.     

An amplify‐and‐forward and decode‐and‐forward mixed relay communication system and its performance analysis

A kind of amplify‐and‐forward (AF) and decode‐and‐forward (DF) mixed relay communication system is proposed in this letter. The source broadcasts the signal to all the relays. Relays that can decode the signal adopt DF scheme to retransmit the signal, while the rest adopt AF scheme for retransmission. The destination employs maximum ratio combining technique to maximize the received signal‐to‐noise ratio. Another situation concerned in this letter is that when the relay cannot decode the source signal, it may retransmit the interference signal with AF scheme. Closed‐form expressions of outage probability are derived. Simulation results show that the analytical curves agree with the simulated ones very well, and the AF‐DF mixed relay system can improve the availability of the relays. Copyright © 2013 John Wiley & Sons, Ltd.     

Cooperative HARQ protocol and diversity analysis of systems with multiple incremental relays

Wireless networks contain an inherent distributed spatial diversity that can be exploited by relays. Relay networks can take advantage of the broadcast-oriented nature of wireless transmission, but require more radio resource to transmit data for their multi-hop traits. Fortunately, incremental relaying technique, which can choose direct or multi-hop transmission adaptively, can efficiently utilize resource. In this article, the incremental transmission with amplify-and-forward (AF) relays is focused on. A practical hybrid-automatic retransmission request (HARQ) protocol is designed, and the related optimal relay selection strategy is proposed. To analyze the cooperative diversity of system with the proposed protocol, the capacity lower bound is deduced. Simulation and analytical results indicate that by adopting the optimal relay selection strategy, the system with the proposed HARQ protocol can achieve an order of cooperative diversity that equals the aggregated number of the relay and source nodes.     

Performance Analysis of TDMA Relay Protocols Over Nakagami- $m$ Fading

Several time-division multiple-access (TDMA) cooperative wireless relay protocols and their performances have recently been developed by Nabar, Bolcskei, and Kneubuhler. Their work, however, is limited to an upper bound-based performance analysis for Rayleigh fading. We thus provide an exact analysis of two of their protocols in single-relay and multiple-relay networks over independent identically distributed (i.i.d.) Nakagami- $m$ fading channels. Our analysis is focused on an Alamouti-coded system with two-stage protocols, fixed-gain amplify-and-forward (AF) relays, and maximal ratio combiner (MRC) reception. The performance metrics are the capacity, the diversity order, and the symbol error rate (SER). The closed-form moment-generating function (MGF) of the total end-to-end signal-to-noise ratio (SNR) is derived. The MGF is then used to derive the diversity order and the SER of $M$-ary phase-shift keying ($M$-PSK) and $M$-ary quadrature amplitude modulation ($M$ -QAM). It is found that the end-to-end SNR for relaying with orthogonal channels is higher than that of nonorthogonal relay channels. The diversity order of a multiple-relay network ( $n$ relays) in a Nakagami-$m$ environment is shown to be $(n + 1)m$. The closed-form SER expressions for relay–destination links with high SNRs and static relay–destination links are derived. Numerical and simulation results are provided to verify the analysis.      

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     

Multi-antenna Decode and Forward Relay Selection over Nakagami-m Fading Channels

This paper analyzes the performance of opportunistic relay under aggregate power constraint in Decode-and-Forward (DF) relay networks over independent, non-identical, Nakagami-m fading channels, assuming multiple antennas are available at the relay node. According to whether instantaneous Signal-to-Noise Ratio (SNR) or average SNR can be exploited for relay selection, two opportunistic relay schemes, opportunistic multi-antenna relay selection (OMRS) and average best relay selection (ABRS) are proposed. The closed form expressions of outage probability and error performance for binary phase shift keying (BPSK) modulation of OMRS and ABRS are determined using the moment generating function (MGF) of the total signal-to-noise ratio (SNR) at the destination. Simulations are provided to verify the correctness of theoretical analysis. It is observed that OMRS is outage-optimal among multi-antenna relay selection schemes and approaches the Beamforming (BF) scheme known as theoretical outage-optimal very closely. Compared with previous single-antenna Opportunistic Relaying (OR) scheme, OMRS brings remarkable performance improvement obtained from maximum ratio combining (MRC) and beamforming, which proves that multiple antennas at the relays could provide more array gain and diversity order. It also shows that the performance of ABRS in asymmetric channels is close to OMRS in the low and median SNR range.     

Diversity Analysis of Smart Relaying

It has been known that relaying can provide spatial diversity while satisfying the size-limited constraint of the users' devices in wireless communications systems. For a practically attractive decode-and-forward (DF) relaying system with maximal-ratio combining at the destination, spatial diversity is lost, except when the source–relay link is reliable. To deal with the problem and inspired by the work of Wang , this paper considers and analyzes an adaptive transmission scheme, which is referred to as smart relaying, when only the average relay–destination (R–D) signal-to-noise ratio (SNR) is available at the relay. In the system under consideration, the source continuously transmits the information to the destination. The relay adaptively scales its transmitted power to changes in the channel condition but never exceeds the total power that the conventional relaying used. Performance analysis proves that a diversity order of 2 is always obtained for binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK). A diversity order of 2 is also observed for higher order rectangular quadratic-amplitude modulation (QAM) constellations through numerical results. The result on diversity order does not depend on how perfect the R–D feedback channel and how exact the quantization of the power scaling are. All the corresponding results of Wang are subsumed in our analysis.      

协作通信中的解码转发中继策略优化

在单中继协作通信系统模型下,研究了直接传输、固定解码转发（ DF）中继、选择DF中继和增量中继协议。为了提高协作通信系统的频谱效率和中断性能,通过比较源-目的端和中继-目的端的瞬时信道增益来判定是否需要采用协同通信模式,提出一种改进的解码转发方式（ IDF）。通过将选择DF中继和增量中继的优点结合,提出了一种增量选择中继（ ISR）策略,并给出了2种改进协议任意信噪比的中断概率表达式。仿真结果表明,与传统的单中继解码转发协议相比, IDF和ISR均具有更低的中断概率, ISR具有最低的中断概率。     

协作通信系统中基于频谱效率优化的跨层设计

为改善协作分集系统的频谱效率,提出一种改进的跨层设计方案,对协作通信系统物理层的自适应调制编码（AMC）技术和链路层的混合自动重传(HARQ)协议进行联合优化设计。该方案利用少量比特的反馈,使得仅当目的节点通过直接信道不能正确译码分组时或者当直接信道处于深度衰落时触发中继节点转发协作伙伴数据。给出了所提方案基于瑞利衰落信道、解码转发（DF）条件下频谱效率的表达式,然后搜索在任意信噪比条件下使频谱效率最大化的调制与编码方案。通过计算机仿真对理论分析进行了验证。理论分析和仿真表明该跨层设计在中低信噪比(SNR)可进一步提升协作通信系统的频谱效率。      

A Novel Adaptive and Cooperative Scheme to Save Energy for Wireless Sensor Network

In this paper, we evaluate the performance of a novel cooperative adaptive relaying scheme called ISDF (Incremental Selective Decode and Forward). We compare the energy efficiency of this underlying scheme with direct transmission (DT), Fixed and adaptive relaying schemes in Wireless Sensor Network (WSN). The system is constrained by a minimum value of Signal-to- Noise Ratio (SNR) and the end-to-end throughput. We consider a three-node cooperative system. The relay is equidistant to the source and destination, and then we study different positions of the latter. The destination receives two copies of the message: one from the source and the other from the relay. Then it combines them by using maximal ratio combining (MRC). The proposed scheme differs from other schemes by the combination of the selective decode and forward scheme with the incremental scheme which requires the presence of a feed-back from the destination. The analysis reveals that the proposed ISDF provides good energy efficiency compared to DT and other cooperative schemes. It also shows that DF cooperative scheme (decode and forward , either with feedback or no, is outperformed by AF (amplify and forward) scheme especially when the relay is placed near the destination.

     

Cooperative Hybrid‐ARQ Protocols: Unified Frameworks for Protocol Analysis

Cooperative hybrid‐automatic repeat request (HARQ) protocols, which can exploit the spatial and temporal diversities, have been widely studied. The efficiency of cooperative HARQ protocols is higher than that of cooperative protocols because retransmissions are only performed when necessary. We classify cooperative HARQ protocols as three decode‐and‐forward‐based HARQ (DF‐HARQ) protocols and two amplified‐and‐forward‐based HARQ (AF‐HARQ) protocols. To compare these protocols and obtain the optimum parameters, two unified frameworks are developed for protocol analysis. Using the frameworks, we can evaluate and compare the maximum throughput and outage probabilities according to the SNR, the relay location, and the delay constraint. From the analysis we can see that the maximum achievable throughput of the DF‐HARQ protocols can be much greater than that of the AF‐HARQ protocols due to the incremental redundancy transmission at the relay.     

基于矩生成函数的协同中继传输误符号率性能分析

多中继协同分集技术在慢衰落无线环境下可以提供巨大的性能增益。该文从接收端平均误符号率的角度,分析了独立的瑞利衰落信道下,采用放大转发的多中继协同最大比合并检测方案和多节点侦听,单中继转发的机会中继方案的性能;并基于矩生成函数推导出两种方案下M-PSK和M-QAM调制信号平均误符号率的闭合表达式。经蒙特卡罗仿真验证,该表达式在中高信噪比下与仿真的实际结果非常吻合。同时也可以看出机会中继的分集增益与协同侦听的节点数成正比,误符号率性能优于使用最大比合并检测的多中继协同方案。     

过时信道状态信息下的机会式中继选择系统的安全性能分析

在 Nakagami m衰落信道下,目的端和窃听者采用最大比合并策略,本文研究了在机会式自适应解码转发中继选择安全协作系统中的安全性能。由于实际信道中的反馈延迟,最优的合法中继选择基于合法信道反馈的过时信道状态信息。为了评价机会式中继选择在改善安全性能上的表现,分别推导了准确的正安全容量概率和准确的安全中断概率闭合表达式。此外,针对两种不同情况, 推导了形式简单的渐近表达式,并明确给出安全分集阶数和安全阵列增益。理论分析和数值仿真表明,增加中继个数和目的节点的天线数能够改善安全中断概率的性能表现,且在信道状态信息过时的条件下,系统的安全分集阶数与中继数无关。      

放大转发方式下中继选择策略的性能分析

研究了几种主要的协同中继选择策略;通过仿真对比了在放大转发方式（Amplifier-and-forward AF）下全部协同、部分协同、机会中继的中断概率性能;最后归纳总结了中继节点数目、信噪比及目标信息速率对上述中继选择策略中断概率性能的影响情况。     

Outage analysis of opportunistic multi-antenna relay selection in decode-and-forward relay networks

This article studies the closed-form expressions of outage performance for opportunistic relay under aggregate power constraint in decode-and-forward(DF)relay networks over Rayleigh fading channels,assuming that multiple antennas are available at the relay node.According to whether instantaneous signal-to-noise ratio(SNR)or average SNR can be utilized for relay selection,two opportunistic relay schemes,opportunistic multi-antenna relay selection(OMRS)and average best relay selection(ABRS)are proposed.The performances of both two schemes are evaluated by means of theoretical analysis and simulation.It is observed that OMRS is outage-optimal among multi-antenna relay selection schemes and closely approaches the beamforming(BF)scheme known as theoretical outage-optimal.Compared with previous single-antenna opportunistic relaying(OR)scheme,OMRS brings remarkable performance improvement,which is obtained from maximum ratio combining(MRC)and beamforming techniques.It is also shown that the performance of ABRS in asymmetric channels is close to OMRS in the low and median SNR range.     

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

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

Cooperative 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.     

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).