Decode and forward polar coding for Half-duplex two-relay channels based on multilevel construction

Polar codes represent an emerging class of error-correcting codes with power to approach the capacity of the physically degraded relay channel and relevant coding schemes have been proposed in the literature. This paper aims to design new cooperative decode-and-forward (DF) polar coding schemes for half-duplex two-relay channel based on the Plotkin’s construction illustrating their performances gain. In these schemes, we consider the use of time-division process in transmission. The source node transmits its polar-coded information to both relays and the destination nodes during the first time slot. Each relay node receives the data from the source and processes it using the DF protocol. For the second transmission period, each relay first decodes the source signal then after reconstruction of the information reduction matrix based on the multilevel characteristics of polar codes, the extracted data at each relay are recoded using another polar code and transmitted to the destination. At destination node, the signals received from each relay and source nodes are processed by using multi-joint successive cancellation decoding for retrieving the original information bits. We demonstrate via simulation results that by carefully constructed polarisation matrix at each relay node, we can achieve the theoretical capacity in the half-duplex relay channel.     

解码转发双向无线携能全双工中继系统的能效研究

为了优化双向单中继全双工系统的能效,在中继节点引入无线携能技术,提出了一个双向无线携能全双工中继系统。该系统中继节点以功率分割方式进行能量收集,将收集的能量用于解码转发,在保证用户的服务质量需求下,将能效优化问题转化为能量消耗最小的优化问题,依据传输时隙以及各时隙信道容量需求进一步划分为两个子优化问题;然后,以能效最大化为目标,根据各时隙的信道容量求出传输功率,利用解码转发特性求出传输时间和功率分割因子,得出能效最优解。仿真表明,所提出的双向无线携能全双工中继系统的能效是单向无线携能全双工系统的1.3倍、双向全双工放大转发中继系统的2倍和双向全双工解码转发中继系统的2.6倍。      

半双工译码转发中继信道下的空间耦合RA码设计

针对半双工译码转发中继信道,提出了一种可逼近三节点中继信道容量限的空间耦合RA码的设计方法。针对二进制删除信道,源节点分别向中继节点和目的节点发送空间耦合RA码,中继节点先正确恢复出源节点发送的空间耦合RA,然后再次编码产生额外的校验比特并转发给目的节点;目的节点结合中继节点发送的额外校验比特和源节点发送的空间耦合RA码进行译码,正确恢复出源节点的信息。为了评估所设计的空间耦合RA码在三节点中继信道下的渐近性能,推导了密度进化算法用于计算阈值。阈值分析结果表明,所提出的空间耦合RA码能够同时逼近源到中继链路和源到目的链路的容量限。同时,基于半双工二进制删除中继信道,仿真了所设计的空间耦合RA码的误码性能,结果表明,其误码性能与所推导的密度进化算法计算的阈值结果一致,呈现出逼近于容量限的优异性能,且优于采用空间耦合低密度奇偶校验(Low Density Parity Check,LDPC)码的性能。     

协作通信中基于分布式空时码的自适应能量分配方案

基于协作AF(Amplify and Forward)通信模型,提出了一种基于分布武空时分组码的自适应能量分配方案.该方案首先以最小化中断概率为准则,在源节点与中继节点之间根据信道状态信息情况,决定是否放大转发中继节点接收信号.然后,在中继节点采用最优功率分配策略,实现整个系统的最优化传输.仿真显示了所提出的两种自适应方案与传统自适应方案、非自适应方案,以及在不同调制制度下的性能比较.仿真结果表明,与传统非自适应方案相比,所提方案1的误码率性能提高了约5 dB.     

Cooperative Transmission Using Quasi-Orthogonal Space-Time Block Codes with Adaptive Decode-and-Forward Relaying Protocol

In this paper, we propose a cooperative transmission scheme using quasi-orthogonal space-time block codes (QOSTBCs) for multiple-input multiple-output (MIMO) relay networks. Comparing with the conventional cooperative transmission scheme using orthogonal space-time block codes (OSTBCs), the proposed scheme can achieve higher bandwidth efficiency with the same decoding complexity. Moreover, an adaptive decode-and-forward (ADF) relaying protocol is proposed based on one-bit channel state information (CSI) feedback. According to the CSI feedback, a better transmission mode can be selected between the direct transmission and decode-and-forward (DF) cooperative transmission. In addition, the outage performance of the proposed scheme is investigated and a closed-form upper bound on the outage probability is derived. The performance analysis shows that the proposed scheme can achieve a full diversity order, which is higher than that of the direct and DF cooperative transmissions.     

Multiantenna Bidirectional Broadcast Channels— Optimal Transmit Strategies

We consider a three-node network where a relay node establishes a bidirectional communication between the two other nodes using a spectrally efficient decode-and-forward protocol. In the first phase we have the classical multiple-access channel where both nodes transmit a message to the relay node, which then decodes the messages. In the second phase the relay broadcasts a re-encoded composition of them based on the network coding idea. This means that each receiving node uses the same data stream to infer on its intended message. We characterize the optimal transmit strategy for the broadcast phase where either the relay node or the two other nodes are equipped with multiple antennas. Our main result shows that beamforming into the subspace spanned by the channels is always an optimal transmit strategy for the multiple-input single-output bidirectional broadcast channel. Thereby, it shows that correlation between the channels is advantageous. Moreover, this leads to a parametrization of the optimal transmit strategy which specifies the whole capacity region. In retrospect the results are intuitively clear since the single-beam transmit strategy reflects the single stream processing due to the network coding approach.     

Differential modulation for two-way wireless communications: a perspective of differential network coding at the physical layer

This work considers two-way relay channels (TWRC), where two terminals transmit simultaneously to each other with the help of a relay node. For single antenna systems, we propose several new transmission schemes for both amplify-and-forward (AF) protocol and decode-and-forward (DF) protocol where the channel state information is not required. These new schemes are the counterpart of the traditional noncoherent detection or differential detection in point-to-point communications. Differential modulation design for TWRC is challenging because the received signal is a mixture of the signals from both source terminals. We derive maximum likelihood (ML) detectors for both AF and DF protocols, where the latter can be considered as performing differential network coding at the physical layer. As the exact ML detector is prohibitively complex, we propose several suboptimal alternatives including decision feedback detectors and prediction-based detectors. All these strategies work well as evidenced by the simulation results. The proposed protocols are especially useful when the required average data rate is high. In addition, we extend the protocols to the multiple-antenna case and provide the design criterion of the differential unitary space time modulation (DUSTM) for TWRC.     

Low density parity check codes for the relay channel

We propose Low Density Parity Check (LDPC) code designs for the half-duplex relay channel. Our designs are based on the information theoretic random coding scheme for decode-and-forward relaying. The source transmission is decoded with the help of side information in the form of additional parity bits from the relay. We derive the exact relationships that the component LDPC code profiles in the relay coding scheme must satisfy. These relationships act as constraints for the density evolution algorithm which is used to search for good relay code profiles. To speed up optimization, we outline a Gaussian approximation of density evolution for the relay channel. The asymptotic noise thresholds of the discovered relay code profiles are a fraction of a decibel away from the achievable lower bound for decode-and-forward relaying. With random component LDPC codes, the overall relay coding scheme performs within 1.2 dB of the theoretical limit     

Cooperative Strategies and Capacity Theorems for Relay Networks

Coding strategies that exploit node cooperation are developed for relay networks. Two basic schemes are studied: the relays decode-and-forward the source message to the destination, or they compress-and-forward their channel outputs to the destination. The decode-and-forward scheme is a variant of multihopping, but in addition to having the relays successively decode the message, the transmitters cooperate and each receiver uses several or all of its past channel output blocks to decode. For the compress-and-forward scheme, the relays take advantage of the statistical dependence between their channel outputs and the destination's channel output. The strategies are applied to wireless channels, and it is shown that decode-and-forward achieves the ergodic capacity with phase fading if phase information is available only locally, and if the relays are near the source node. The ergodic capacity coincides with the rate of a distributed antenna array with full cooperation even though the transmitting antennas are not colocated. The capacity results generalize broadly, including to multiantenna transmission with Rayleigh fading, single-bounce fading, certain quasi-static fading problems, cases where partial channel knowledge is available at the transmitters, and cases where local user cooperation is permitted. The results further extend to multisource and multidestination networks such as multiaccess and broadcast relay channels.     

解码前传半双工中继信道下协作LDPC码设计

为逼近解码前传半双工中继信道容量,该文提出一种协作LDPC编码结构及度分布优化方法。与双层删除LDPC码不同,该结构将中继校验比特视为协作LDPC码的一部分,目的端利用从信源和中继接收的消息进行联合译码获得信源信息。为了分析协作LDPC码性能,拓展传统外信息转移(EXIT)图,推导了基于消息错误概率的双层EXIT图噪声门限分析方法。在此基础上,提出了协作LDPC码度分布优化方法,采用差分进化算法搜索了一组具有最大噪声门限的协作LDPC码。实验仿真证明,与双层删除LDPC码相比,协作LDPC码的性能得到了不同程度的改善。     

Optimal time-division for bidirectional relaying using superposition encoding

Bidirectional wireless communication between two nodes is enabled by a half-duplex relay node using a two-phase decode-and-forward protocol. In the first phase, the messages of two nodes are transmitted to the relay node, which broadcasts a superposition of the re-encoded messages in the succeeding phase. Individual mean transmit power constraints averaged over both phases are considered so that they are inversely proportional to the time division between the phase. An equivalent characterization of the achievable rate region with optimal time division is presented. Moreover, it is proved that the rate region is convex, which makes efficient algorithmic optimization solutions feasible.     

Outage Performance of Energy Harvesting DF Relaying NOMA Networks

In this paper, we investigate energy harvesting decode-and-forward relaying non-orthogonal multiple access (NOMA) networks. We study two cases of single relay and multiple relays with partial relay selection strategy. Specifically, one source node wishes to transmit two symbols to two respective destinations directly and via the help of one selected intermediate energy constraint relay node, and the NOMA technique is applied in the transmission of both hops (from source to relay and from relay to destinations). For performance evaluation, we derive the closed-form expressions for the outage probability (OP) at D ₁ and D ₂ with both cases of single and multiple relays. Our analysis is substantiated via Monte Carlo simulation. The effect of several parameters, such as power allocation factors in both transmissions in two hops, power splitting ratio, energy harvesting efficiency, and the location of relay nodes to the outage performances at the two destinations is investigated.     

带宽有效传输下多进制LDPC码的不等错误保护

针对频谱有效的多进制低密度奇偶校验（Low-Density Parity-Check,LDPC）码编码调制系统,本文提出了一种在带宽有效传输下的两级不等保护方案,两级不等错误保护分别来自码字特性和高阶调制,充分利用了码字变量节点的度和高阶调制中比特的不等可靠性。编码调制系统采用多进制LDPC码与高阶调制匹配结合,无需信息转换,针对不同误符号率和误比特率的需求,可在符号级和比特级提供不同可靠性达到不等错误保护的目的。仿真结果表明,在AWGN信道下,采用16QAM调制方式的性能优于16PSK调制方式,利用变量节点的度和高阶调制提供的信息,码字的误符号率和误比特率具有明显的不等错误保护区分度,对于重要的比特给予了较强的保护。      

Using Orthogonal and Quasi-Orthogonal Designs in Wireless Relay Networks

Distributed space-time coding was proposed to achieve cooperative diversity in wireless relay networks without channel information at the relays. Using this scheme, antennas of the distributive relays work as transmit antennas of the sender and generate a space-time code at the receiver. It achieves the maximal diversity when the transmit power is infinitely large. This paper is on the design of practical distributed space-time codes (DSTCs). We use orthogonal and quasi-orthogonal designs which are originally used in the design of space-time codes for multiple-antenna systems. It is well known that orthogonal space-time codes have full diversity and linear decoding complexity. They are particularly suitable for transmissions in the network setting using distributed space-time coding since their ldquoscale-freerdquo property leads to good performance. Our simulations show that they achieve lower error rates than the random code. We also compare distributed space-time coding to selection decode-and-forward using the same orthogonal designs. Simulations show that distributed space-time coding achieves higher diversity than selection decode-and-forward (DF) when there is more than one relay. We also generalize the distributed space-time coding scheme to wireless relay networks with channel information at the relays. Although our analysis and simulations show that there is no improvement in the diversity, in some networks, having channel information at the relays saves both the transmission power and the transmission time.     

储能-发送模式的单天线两跳中继系统保密速率的优化

该文研究节点具有能量收集能力的两跳中继系统的物理层安全传输方案。考虑窃听节点与源和中继节点间都有直接链路的情况。每个数据传输时隙分为能量收集和数据传输两个阶段,各节点用收集的能量发送信号。中继采用放大转发方式,目的节点发送人工噪声进行协作干扰,保护在两跳传输中传输的保密信息。以最大化保密速率为目标,采用迭代算法优化能量吸收和数据传输两阶段的时间分配比例系数和协作干扰功率分配因子。仿真结果表明优化算法准确,优化后的协作干扰方案能显著提高系统的保密传输速率。由于考虑了窃听节点在两跳传输中都能接收到信号的可能性,文中方案更贴近实际,并解决了一个复杂的优化问题。     

Differential space–time network coding in MIMO two‐way relay channel network

This paper focuses on noncoherent detection scheme for multiple‐input multiple‐output two‐way relay channel network with two two‐antennas source nodes and one single‐antenna relay node. An orthogonal differential space–time network coding (ODSTNC) scheme based on relay detection and forward protocol is proposed. The proposed scheme combines space–time coding with network coding, and the differential modulation and detection are used in both multiple access stage and broadcast stage. The multiple‐symbol differential detection is employed at the relay. The maximum likelihood decision and its low‐complexity sphere decoding decision are given. The upper and lower bounds on the average symbol error probability for this system under differential binary phase shift keying (DBPSK) are derived, and a diversity order of 2 is confirmed to be achieved. The simulation results show that the ODSTNC scheme has good performance, and it is available for the applications of far distance signal transmission between two terminals where channel state information is unknown. Copyright © 2014 John Wiley & Sons, Ltd.     

半双工中继信道下联合LDPC码设计

为了提高解码前传半双工中继通信系统的编码增益,提出了一种联合LDPC码编码结构及其度分布优化方法。该结构视信源和中继子码为联合LDPC码的一部分,目的端根据从信源和中继接收的消息进行联合译码,同时获得信源和中继的信息。为了分析联合LDPC码的渐进性能,推导了AWGN信道下联合LDPC码的高斯近似密度进化算法。结合译码收敛条件和度分布约束关系,提出联合LDPC码的度分布优化问题。仿真结果表明：联合LDPC码的渐进性能及误码性能优于BE-LDPC码和独立处理（SP）码。     

协作通信技术在恶劣信道条件下的应用研究

为了在信道状况较差时仍能采用编码协作技术,提出了一种有效混合编码协作方法.以两用户为例,在两次发送时隙都采用QPSK调制,并采用LDPC码的混合自动重传(HARQ)方法,当第一时隙传输数据不能被正确译码时,源节点将重新传送数据,中继节点将接收到的数据联合上一次接收的数据进行联合译码.当数据重传后仍不能被正确译码时,中继节点将采用放大转发方式传输第二时隙的数据,此时转为放大转发模式进行协同通信,通过性能分析和仿真结果表明,当协作用户间信道状况较差时,与非协作和普通编码协作方式相比,所提的协作方法仍能获得一定的分集增益.     

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。      

Hybrid Multihop Relay Protocol Using Dynamic Power Allocation for LTE-Advanced System

In this paper, an adaptive relay scheme of hybrid relay system is proposed based on channel state information of the received signal-to-noise ratio (SNR) at the relay node. hybrid relay system is comprised of amplify-and-forward, decode-and-forward and demodulate-and-forward together in cooperative networks. The relay protocols are selected adaptively, on the basis of power allocation (PA) constraint, or the SNR value at previous hop. Thus multihop performance of the relay system can be improved by applying PA allocation scheme. The simulation results show that the performance of the novel hybrid relay protocol can be improved significantly compared to the conventional hybrid system in terms of error performance and link efficiency.