基于Turbo码的非理想中继协作性能研究

基于Turbo码的非理想中继信道协作方式是在加性高斯白噪声(AWGN)的环境下提出的一种协作通信方式。源节点将编码并调制后的信息分别传送给中继节点和目的节点,中继节点对接收到的码字进行译码、传送,目的节点接收2路不同的信号(一路是来自源节点未经过交织的信息比特和校验比特,另一路是来自中继信道经过交织后来的校验比特)并采用联合迭代的译码方式对其进行译码。仿真结果表明,同等条件下,基于Turbo码的非理想中继信道协作方式在纠错方面的性能远胜于点对点的Turbo码非协作方式。     

网络编码在IMT-Advanced协作中继中的应用

在IMT-Advanced协作中继中引入网络编码技术,它可以进一步提升网络吞吐量、负载均衡、安全性和顽健性。针对多用户的上行多址接入中继信道模型,给出了基于网络编码的传输策略,在此基础上,分析了该策略下的系统可达速率及中断概率,并与传统机制进行了比较。实验结果表明,网络编码可有效提升系统性能并节约无线链路资源,从而表明了网络编码在IMT-Advanced协作中继中应用的巨大潜力和优势。     

面向物联网准静态信道的中继协作密钥生成方法

针对物联网准静态信道下密钥生成速率低的问题,该文提出一种基于中继节点协作的密钥生成方法。首先,通信双方通过信道估计获得直达信道和部分中继信道信息;然后,中继节点采用网络编码技术参与协作,使得通信双方获取全部中继信道信息;最后,通信双方在直达信道上进行密钥协商,利用直达信道信息、中继信道信息与协商信息共同生成相同的密钥。安全性分析表明该方法能够提高可达密钥速率,并且随着信噪比的提高,可达密钥速率呈线性增长,趋于最优值。蒙特卡洛仿真验证了理论分析的结果,并得出了增加中继节点数量、选取信道变化幅度大的中继节点,可以进一步提高可达密钥速率。     

无线多址信道中的协作模型研究

在无线多址信道环境下,为了充分利用协作分集技术提高系统的传输速率和通信质量,引入了多址接入中继信道（MARC）模型。通过加入一个专门的中继节点,MARC模型中的发送节点类似操作在普通的多址信道下,从而使得现有的系统可平滑过渡到协作系统,实用性强。把MARC模型应用在直序扩频CDMA系统中,在Rayleigh信道下对其误码率进行仿真,结果表明,采用MARC模型的协作网络能获得显著的性能增益。     

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.     

一种基于散列邻域搜索网络编码的机会中继重传方法

在无线多播网络中,传统的方法没有考虑某些接收节点与源节点及其他接收节点之间可能具有更好的链路质量.为此提出了一种基于散列邻域搜索网络编码的机会中继重传方法,该方法动态选择数据分组接收情况最好的且信道质量优于源节点的接收节点作为中继,并采用散列邻域搜索网络编码策略进行其他接收节点的丢失分组重传.仿真结果表明,相对于现有的其他网络编码重传方法,该方法能有效减少平均重传次数,提高重传效率,尤其当一些接收节点因受到干扰与源节点之间的信道质量变得很差时,该方法能取得很高的重传增益.     

基于多点协同中继的无线传输技术

多点协同中继的无线传输技术作为LTE-Advanced系统中主要的新技术,对于扩大网络边缘覆盖、保证边缘用户服务质量有着至关重要的作用。文章主要介绍了多址接入、多用户协作通信、无线衰落信道和IDMA这几个多点协同中继无线传输中的关键技术。     

分布式RS码的协作及基于权重分配的联合译码研究

通信系统面临的主要挑战之一是减轻无线信道衰落的影响,衰落会引起接收信号功率的波动,降低系统性能。减少衰落影响的一种常用方法是使用分集技术,这些技术通过多个独立的信道传输预期信号,减轻了衰落的影响。为了研究RS码在分集技术下的性能表现,设计了分布式RS码的中继协作系统,并采用基于权重分配的译码算法。目的节点接收到来自源节点和中继节点的两路幅值信号后,将两路信号的实部与虚部乘以权重,并将实部与虚部独立进行相加之后的结果进行QAM硬判决,之后使用Berlekamp迭代译码算法进行译码。设计比较RS(63,51)码与RS(255,239)码在加性高斯白噪声信道(Additive White Gaussian Noise,AWGN)和快衰落信道(Fast fading channel)下性能表现。对于RS(63,51)码的权重协作方案,在快衰落信道下,误码率为10~(-6)时,相比于非协作方案,大约有5 dBs的性能增益,带来显著的分集增益。     

一种新的编码协作中继选择策略性能仿真

针对多用户的编码协作通信网络,给出了增量中继和机会中继结合起来的增量最佳编码协作中继选择策略并对其进行了计算机仿真。结果表明,与其它编码协作节点选择方法相比,该策略在取得机会中继中断概率性能的基础上,进一步节省了信道资源,提高了系统有效信息速率。     

基于信号空间对齐的网络编码方案

在多用户协同中继系统中,受共信道干扰的影响,系统性能下降严重。提出了一种融合多源信号空间对齐和网络编码的方案,每个用户通过中继向其他的用户发送不同的信息,同时也通过中继接收其他用户的不同信息。整个过程可分为多址接入信道(MAC)和广播信道(BC)两个阶段。在MAC阶段,利用信号空间对齐技术在中继形成较少的信息量、抑制多用户干扰。在BC阶段,通过中继的网络编码,设计简单的预编码算法,有效降低了用户译码的复杂度。整个系统的完成只需要两个时隙,能有效地抑制干扰和提高系统自由度。     

Achievable rate for three-node discrete memoryless relay channel with generalized feedbacks

This paper studies the achievable rate for three-node discrete memoryless relay channel.Specifically in this mode,we explore two generalized feedbacks simultaneously:the source node actively collects feedback signals from the channel;and at the same time,the destination node actively transmits feedback signals to the relay node.These two feedback signals,which are called generalized feedback overheard from the channel that is likely to be noisy,induce that all the three nodes are in full duplex mode.The basic coding strategies of Cover and El Gamal are applied to the relay-source feedback transmission by the source forwarding the compressions of the channel output sequences at the relay node to the destination,and are also applied to the destination-relay feedback transmission to improve the decoding ability at the relay.Based on Cover and El Gamal coding,a new coding scheme adopting rate splitting and four-block Markov superposition encoding is proposed and the corresponding achievable rate is achieved.The proposed scheme is able to exploit two feedbacks simultaneously which can effectively eliminate underlying transmission bottlenecks for the channels.The derived achievable rate result generalizes several previously known results by including them as special cases.     

Cooperative Strategies and Achievable Rate for Tree Networks With Optimal Spatial Reuse

In this paper, a low-complexity cooperative protocol that significantly increases the average throughput of multihop upstream transmissions for wireless tree networks is developed and analyzed. A system in which transmissions are assigned to nodes in a collision free, spatial time division fashion is considered. The suggested protocol exploits the broadcast nature of wireless networks where the communication channel is shared between multiple adjacent nodes within interference range. For any upstream end-to-end flow in the tree, each intermediate node receives information from both one-hop and two-hop neighbors and transmits only sufficient information such that the next upstream one-hop neighbor will be able to decode the packet. This approach can be viewed as the generalization of the classical three node relay channel for end-to-end flows in which each intermediate node becomes successively source, relay and destination. The achievable rate for any regular tree network is derived and an optimal schedule that realizes this rate in most cases is proposed. Our protocol is shown to dramatically outperform the conventional scheme where intermediate nodes simply forward the packets hop by hop. At high signal-to-noise ratio (SNR), it yields approximately 66% throughput gain for practical scenarios.     

异步协同系统频选信道下基于线性预处理的DSTC结构设计

在协同中继系统中,应用分布式空时码(Distributed Space Time Coding, DSTC),可以在有效提高系统效率的同时获得全协同分集。但是,各中继节点的异步传输和节点间的多径衰落会破坏空时码字的结构,使之不能获得全分集。本文针对两中继的异步协同系统,提出了一种频率选择性信道下的基于线性预处理的DSTC传输结构。在此传输结构中,源节点对发送数据块进行预处理后发送给中继节点,中继节点对接收信号进行简单的共轭重排等处理,使得在目的节点形成DSTC的结构。其中,为抵抗异步传输和多径衰落引入的符号间干扰(Inter-symbol Interference, ISI),在源节点处和中继节点处均加入循环前缀(Cyclic Prefix, CP)。于是目的节点对接收到的信号进行DFT处理后,可以运用ML算法对数据信息进行检测。理论分析和仿真表明,当存在定时误差和节点间为频率选择性信道时,目的节点运用ML检测算法该传输结构可获得全空间分集和全多径分集。然后,本文考虑了信道各径延迟为整数倍符号周期的情况,并且证明了该传输结构的分集增益只与节点间信道的有效信道长度有关。      

Towards an information theory of large networks: an achievable rate region

We study communication networks of arbitrary size and topology and communicating over a general vector discrete memoryless channel (DMC). We propose an information-theoretic constructive scheme for obtaining an achievable rate region in such networks. Many well-known capacity-defining achievable rate regions can be derived as special cases of the proposed scheme. A few such examples are the physically degraded and reversely degraded relay channels, the Gaussian multiple-access channel, and the Gaussian broadcast channel. The proposed scheme also leads to inner bounds for the multicast and allcast capacities. Applying the proposed scheme to a specific wireless network of n nodes located in a region of unit area, we show that a transport capacity of /spl Theta/(n) bit-meters per second (bit-meters/s) is feasible in a certain family of networks, as compared to the best possible transport capacity of /spl Theta/(/spl radic/n) bit-meters/s in Gupta et al. (2000), where the receiver capabilities were limited. Even though the improvement is shown for a specific class of networks, a clear implication is that designing and employing more sophisticated multiuser coding schemes can provide sizable gains in at least some large wireless networks.     

基于AF策略的高斯正交不信任中继信道的最优资源分配

本文研究了高斯正交中继窃听信道,中继节点同时作为窃听者的情况,源节点同中继节点和目的节点在信道1上传输信号,中继节点同目的节点在信道2上传输信号,信道1和信道2在频率上是正交的。通过分析,在总的信道资源受限的情况下,通过优化功率及信道带宽获得最大的安全速率。本文给出了在高斯正交信道下,当只考虑功率或带宽时,存在唯一的功率或带宽使得安全速率达到最大,并且通过仿真结果说明了采用最优资源分配可得到比等资源分配更大的安全速率。     

Resource Allocation for Wireless Fading Relay Channels: Max-Min Solution

Resource allocation is investigated for fading relay channels under separate power constraints at the source and relay nodes. As a basic information-theoretic model for fading relay channels, the parallel relay channel is first studied, which consists of multiple independent three-terminal relay channels as subchannels. Lower and upper bounds on the capacity are derived, and are shown to match, and thus establish the capacity for the parallel relay channel with degraded subchannels. This capacity theorem is further demonstrated via the Gaussian parallel relay channel with degraded subchannels, for which the synchronized and asynchronized capacities are obtained. The capacity-achieving power allocation at the source and relay nodes among the subchannels is partially characterized for the synchronized case and fully characterized for the asynchronized case. The fading relay channel is then studied, which is based on the three-terminal relay channel with each communication link being corrupted by a multiplicative fading gain coefficient as well as an additive Gaussian noise term. For each link, the fading state information is assumed to be known at both the transmitter and the receiver. The source and relay nodes are allowed to allocate their power adaptively according to the instantaneous channel state information. The source and relay nodes are assumed to be subject to separate power constraints. For both the full-duplex and half-duplex cases, power allocations that maximize the achievable rates are obtained. In the half-duplex case, the power allocation needs to be jointly optimized with the channel resource (time and bandwidth) allocation between the two orthogonal channels over which the relay node transmits and receives. Capacities are established for fading relay channels that satisfy certain conditions.     

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.     

非对称双向中继系统中的协作多址广播传输方案设计与数据率性能分析

在双向中继系统中,2个节点通过一个半双工中继交换信息,2个方向的数据率都会受到较差链路的制约。当节点位置变化或信道衰落造成,中继两端的信道质量不对称时,这将导致系统的和数据率下降。为了弥补较差链路带来的数据率损失,提出了一种新的三时隙协作多址广播传输方案,使得信道质量较好的源节点与中继节点进行协作传输,并且各节点充分利用各链路所支持的最大数据率进行传输,从而有效提高系统容量及加权和数据率。推导出新方案的容量上界以及解码转发模式下的可达数据率域,并对时隙及功率等资源进行优化分配。仿真结果表明,在加性白高斯和瑞利衰落信道下,新方案在非对称信道下的性能都大大优于已有方案。     

On the capacity of the single source multiple relay single destination mesh network

In this paper, we derive the information theoretic capacity of a special class of mesh networks. A mesh network is a heterogeneous wireless network in which the transmission among power limited nodes is assisted by powerful relays, which use the same wireless medium. We investigate the mesh network when there is one source, one destination, and multiple relays, which we call the single source multiple relay single destination (SSMRSD) mesh network. We derive the asymptotic capacity of the SSMRSD mesh network when the relay powers grow to infinity. Our approach is as follows. We first look at an upper bound on the information theoretic capacity of these networks in a Gaussian setting. We then show that this bound is achievable asymptotically using the compress-and-forward strategy for the multiple relay channel. We also perform numerical computations for the case when the relays have finite powers. We observe that even when the relay power is only a few times larger than the source power, the compress-and-forward rate gets close to the capacity. The results indicate the value of cooperation in wireless mesh networks. The capacity characterization quantifies how the relays can cooperate, using the compress-and-forward strategy, to either conserve node energy or to increase transmission rate.     

Multisource, Multidestination, Multirelay Wireless Networks

Networks with multiple source-destination pairs, involving possibly multicast, and where there are multiple nodes that can serve as potential relay nodes, are considered. A multisource, multirelay coding scheme is developed. In this scheme, each source's information is sent to its destination nodes via a multirelay route, with the multiple multirelay routes operating concurrently even when they intersect with each other, in the same spirit as code-division multiple access (CDMA). It is found that in the generalization to multiple sources, backward decoding achieves higher rates than sliding-window decoding. The routing structure where a joint backward decoding can be performed is characterized. The achievable rate region is found to combine aspects of both multiple relay and multiple access. Potential applications of this coding scheme to sensor networks are discussed. In particular, the exact capacity for the data downloading problem in sensor networks, where there are multiple sensor sources and one sink or collector node, is established for certain geometries when there is phase fading that is unknown to the transmitter.