系统分布式LT码的构造

LT码是一类可现实的喷泉码,喷泉码由于其本身的特性可以克服深空通信中反馈难以实现,距离远,变时延的困难。依次介绍了在有中继传输的深空通信中适用的分布式LT码编码以及相对普通LT码编译码复杂度更低,系统开销更小的系统LT码。在此基础上提出系统的两信源分布式LT码的编码方法,并通过仿真结果验证其性能,为喷泉码在深空通信中的应用提供了一类性能优良的编码方案和建议。     

分布式无标度LT码

LT码是喷泉码的一种,在删除信道中性能优越,分布式喷泉码多信源多中继的特性适合用于深空通信中。无标度网络（SF network）具有平均路径（APL）长最小的特性,非常适合喷泉码的度分布设计需求。首先介绍了LT码的编译码算法,然后对无标度网络和基于无标度网络的SF-LT码度分布设计进行了详细分析和优化,最后在删除信道条件下,选取码长较短、删除概率较小的情形对分布式二信源SF-LT码进行仿真分析,仿真结果表明,与LT码相比,分布式SF-LT码具有更好的性能。     

短长度分布式喷泉码的性能分析

LT码是喷泉码的一种,由于深空通信不能实现反馈,距离远,变时延的传输特性,LT码适合用于深空通信中,由于传统的单信源通信已经不能满足当前的通信需求,多信源多中继通信系统成为了研究的重点。介绍了短长度分布式二、四信源的通信模型,以及各自的度分布,并选取了码长较短、删除概率较小这几种情形通过仿真验证了各自的性能。仿真结果表明,二、四信源分布式LT码比LT码有更好的性能。     

双LT码防窃听方案设计

针对传统LT码在窃听信道中窃听者截获效率较高的问题,提出一种双LT码编码的防窃听方案。该方案在传统LT编码的基础上,将第一次LT编码得到的部分码字作为第二次LT编码的信源再次进行LT编码,推迟接收者译码开始时间,致使合法接收者先于窃听者译码概率增加,窃听者无法继续接收LT码并对所接收到的LT符号进行译码,进而使其截获效率降低。实验结果表明,双LT码防窃听方案与其它防窃听LT码相比,仅增加少量译码开销,但是却使窃听者的截获效率大幅度降低。     

一种适用于LT码的选择编码算法

无速率码的出现为自适应数据传输提供了新途径。作为第一种实用的无速率码,LT码在高斯信道中的性能不佳,存在较高的误码平台。针对此问题,提出一种选择编码(SE)算法。SE算法按照度数值大小将信息节点分类成若干个集合,并通过控制编码过程使每个校验节点优先从小度数值节点集合中选取与之相连接的信息节点,从而消除了小度数值的信息节点,降低了LT码的误码平台。通过蒙特卡洛仿真得到不同条件下SE算法中信息节点的度分布,并利用外信息传递图法对SE算法及传统编码方法的收敛性进行对比分析,结果显示SE算法能够进一步拓宽译码通道,使误比特率更快趋近于0。此外,SE算法在给定范围信噪比及码率值条件下均能降低误码平台,当误比特率为10-5时,码长为512 bit的LT码可以得到近5 dB的性能改善。     

基于可译集大小的LT码编码算法的改进

在中、高码率情况下,使用传统随机图法(RG)构造中短码长LT码,其生成矩阵并非一定满秩,所以存在着一定的译码失败率(信源不完全可译).针对随机图法的这种缺点,提出一种改进的累积边增加法(IPEG),此方法通过控制可译集大小,保证全部信源符号的可译性.实验结果表明,与传统方法相比,经过改进的LT码性能得到了显著提高,而且每帧中错误的码字数量也更加稳定,以这种稳定的LT码作为内码,Raptor码的性能也能得到极大提升.     

基于规则变量节点度和扩展窗喷泉码的不等差错保护算法

该文提出了一种可适用于加性高斯白噪声(AWGN)信道的融合扩展窗喷泉码(Expanding Window Fountain, EWF)和规则变量节点度LT码(Regularized variable-node Luby Transform, RLT)策略的不等差错保护(UEP)算法,称为EWF-RLT编码算法。首先利用扩展窗口技术给不同重要等级的数据加窗,编码时让较高重要等级数据以更高的概率参与编码;同时,结合规则变量节点度算法,改变传统LT 码编码过程中随机选取邻居节点的编码方式,使较高重要等级的数据具有较大的最小变量节点度,改善错误平层现象。分析和仿真结果表明,该文提出的EWF-RLT算法与传统算法相比,能对较高重要等级数据进行更强的保护,提升网络传输质量;在UEP方案设计中,加入RLT码编码参数,使得该文方案更加灵活与适用。     

空间信息网络中基于LT码的分布式存储策略

针对空间信息网络(Space Information Network, SIN)节点存储资源严重受限及存储可靠性问题,该文提出一种基于LT(Luby Transform)码的分布式存储策略(Distributed Storage Strategy based on LT codes, DSSLT)。采用定向随机漫步机制,使得源数据包能够更快地遍历整个网络。在信息估计阶段利用基于ID的估计方法进行网络全局信息估计,使所有节点快速获得网络全局信息。合理的数据包选择机制使得最终编码度分布趋于期望的度分布。分析和仿真结果表明,与具有代表性的分布式存储策略相比,该方法大幅度减少了数据包传输时的随机漫步步长,同时提高了译码性能,简单易行。     

基于改进高斯近似方法的LT码度数分布设计

高斯近似是一种用来分析基于Tanner图的信道编译码性能和进行度数分布设计的有效方法,但是在对LT码进行分析时,不能准确计算译码过程中的迭代消息.在此方法的基础上,结合LT码译码的特点,提出迭代盒的概念,对LT码的BP解码中的迭代消息密度函数进化过程进行分析.在LT码消息迭代模型基础上,运用微分演化方法对LT码的输出度数分布进行优化.仿真表明,经过度数分布优化的LT码的抗误码性能有了明显的提高.     

协同通信中的分布式线性疏散码

该文提出一种分布式线性疏散码解决无线网络中协同节点不能确定时的空时编码问题。参与协同的节点采用独立的疏散矢量进行编码,推导协同节点减少时分布式线性疏散码的互信息损失,并以此建立一种分布式疏散码的编码准则。仿真结果验证了这种分布式线性疏散码的误比特性能及编码准则的可行性。     

LT码的性能分析及仿真

LT码（Luby Transform Codes）属于喷泉码的其中一种,它是一类基于图的线性纠删码,其应用的信道环境是删除信道（BEC,Binary Erasure Channel）。这里主要介绍在二元删除信道模型下,LT码的4种度分布设计方法及,分析了短码长和长码长情况下的LT码在不同信道删除概率情况下的性能及比较。研究结果表明：在删除概率非常大的情况下,只要译码端接收到足够多的包都能以高概率译出原始数据包,只是产生的冗余度大,这里的结果将为系统选择不同参数的LT码提供参考。     

LT码和q-LDPC码级联方案在深空通信中的应用

该文针对深空通信对长纠删码的需求,提出了LT (Luby Transform)码和q-LDPC码的级联方案。在综合考虑性能和复杂度的情况下,选取8-LDPC码和8PSK的级联作为等效的删除信道,长度选择灵活、编译码简单的LT码实现纠删功能。文中设计了两种短8-LDPC码,并对整个级联系统的纠错性能进行了仿真。仿真结果表明8-LDPC码的性能优于信源信息速率和码率相同的二进制LDPC码,级联系统在等效包删除概率不超过0.1时,系统误比特率以概率1趋于0。     

Decomposed LT Codes for Cooperative Communications

In order to reduce computation complexity and latency in cooperative communication system based on fountain code, a decomposed LT codes (DLT) based cooperative transmission scheme was proposed. The scheme comprises of two layers of random encoding but only a single layer of decoding. A general decomposition technique for the decomposed LT codes construction is developed. The transmission latency for the proposed scheme was analyzed. Simulation results show that the total time consumed under the proposed scheme can be significantly reduced compared to direct transmission and conventional cooperative transmission based on LT codes in the medium to high packet erasure rate regimes. Moreover, With the increase of packet erasure rate, the advantage of the proposed scheme is more evident.     

Decoding of LT‐Like Codes in the Absence of Degree‐One Code Symbols

Luby transform (LT) codes were the first practical rateless erasure codes proposed in the literature. The performances of these codes, which are iteratively decoded using belief propagation algorithms, depend on the degree distribution used to generate the coded symbols. The existence of degree‐one coded symbols is essential for the starting and continuation of the decoding process. The absence of a degree‐one coded symbol at any instant of an iterative decoding operation results in decoding failure. To alleviate this problem, we proposed a method used in the absence of a degree‐one code symbol to overcome a stuck decoding operation and its continuation. The simulation results show that the proposed approach provides a better performance than a conventional LT code and memory‐based robust soliton distributed LT code, as well as that of a Gaussian elimination assisted LT code, particularly for short data lengths.     

Distributed Joint Source-Channel Coding of Video Using Raptor Codes

Extending recent works on distributed source coding, this paper considers distributed source-channel coding and targets at the important application of scalable video transmission over wireless networks. The idea is to use a single channel code for both video compression (via Slepian-Wolf coding) and packet loss protection. First, we provide a theoretical code design framework for distributed joint source-channel coding over erasure channels and then apply it to the targeted video application. The resulting video coder is based on a cross-layer design where video compression and protection are performed jointly. We choose Raptor codes - the best approximation to a digital fountain - and address in detail both encoder and decoder designs. Using the received packets together with a correlated video available at the decoder as side information, we devise a new iterative soft-decision decoder for joint Raptor decoding. Simulation results show that, compared to one separate design using Slepian-Wolf compression plus erasure protection and another based on FGS coding plus erasure protection, the proposed joint design provides better video quality at the same number of transmitted packets. Our work represents the first in capitalizing the latest in distributed source coding and near-capacity channel coding for robust video transmission over erasure channels.     

LT码的性能分析及参数优化规则

As a new class of forward error correcting encoding algorithm, Luby Transform codes are suitable for the erasure channel environment based on the packet communication. The encoding, decoding algorithms and the implementation of LT codes are summarized in the paper. Meanwhile simulations of the ideal soliton distribution and robust soliton distribution are conducted to evaluate the performance of LT codes in terms of successful decoding probability, mean degree and decoding time over the erasure channel. The parameter optimization rules of LT codes are deeply discussed and proposed in the paper. The research results are of great practical importance for improving the real time performance in the erasure correction applications.     

Variable shortened-and-punctured Reed-Solomon codes for packet loss protection

Techniques using Reed-Solomon (RS) codes to recover lost packets in digital video/audio broadcasting and packet switched network communications are reviewed. Usually, different RS codes and their corresponding encoders/decoders are designed and utilized to meet different requirements for different systems and applications. We incorporate these techniques into a variable RS code and present encoding and decoding algorithms suitable for the variable RS code. A mother RS code can be used to produce a variety of RS codes and the same encoder/decoder can be used for all the derivative codes, with adding/detecting zeros, removing some parity symbols and adding erasures. A VLSI implementation for erasure decoding of the variable RS code is described and the achievable performance is quantitatively analyzed. A typical example shows that the signal processing speed is up to 2.5 Gbits/second and the processing delay is less than one millisecond, when integrating the decoder on a single chip. Therefore, the proposed algorithm and the encoder/decoder can universally be utilized for different applications with various requirements, such as transmission data rate, packet length, packet loss protection capacity, as well as layered protection and adaptive redundancy protection in DVB/DAB, Internet and mobile Internet communications.     

Progressive transmission of images over memoryless noisy channels

An embedded source code allows the decoder to reconstruct the source progressively from the prefixes of a single bit stream. It is desirable to design joint source-channel coding schemes which retain the capability of progressive reconstruction in the presence of channel noise or packet loss. Here, we address the problem of joint source-channel coding of images for progressive transmission over memoryless bit error or packet erasure channels. We develop a framework for encoding based on embedded source codes and embedded error correcting and error detecting channel codes. For a target transmission rate, we provide solutions and an algorithm for the design of optimal unequal error/erasure protection. Three performance measures are considered: the average distortion, the average peak signal-to-noise ratio, and the average useful source coding rate. Under the assumption of rate compatibility of the underlying channel codes, we provide necessary conditions for progressive transmission of joint source-channel codes. We also show that the unequal error/erasure protection policies that maximize the average useful source coding rate allow progressive transmission with optimal unequal protection at a number of intermediate rates     

改善LT码性能的预译码集优化方法研究

喷泉码是第一种与码率无关的编码方式.LT码是在删除信道下性能优异的稀疏随机线性喷泉码.与传统编码相比,大大降低了编译码的复杂度.通过分析影响LT码性能的预译码集,提出了优化LT码预译码集的方法.主要在编码端改变初始预译码集大小以及修改度为2的生成矩阵,在译码端利用编码冗余情况来破解译码停止集,同时消除短环.大量的数据仿真结果表明本文方法对译码性能有明显的改善.