大气弱湍流信道下松弛极化码译码方法

针对自由空间光通信中存在的大气湍流降低通信系统误码性能的问题,提出了基于松弛极化码的分段式极化码-循环冗余校验-串行抵消列表(PC-CRC-SCL)译码方法,并将其用于自由空间光通信系统中,在大气弱湍流强度下进行了仿真分析.仿真结果表明:与完全极化码的分段式CRC-SCL译码方法相比,在误码率为10-4时,基于松弛极化...     

一种适用于大气弱湍流信道的极化纠错编码调制方案

针对服从对数正态分布的大气弱湍流信道模型,基于高斯近似的方法计算分析信道的极化现象,提出了一种适用于大气弱湍流信道的极化编码调制方案,并对其性能进行了仿真分析.不同湍流强度下采用串行抵消译码算法时的仿真结果表明,相比于递归构造方案,文中所提方案在达到1e-5误码率时可获得大约1.0dB到1.4dB的编码增益提升.对极化码分别采用开关键控（OOK）调制与脉冲位置调制（PPM）时的仿真结果表明,采用2-PPM的极化编码调制方案可有效提升无线光通信链路在弱湍流条件下的译码纠错性能.     

基于极化码的无线光通信副载波误码性能分析

提出了一种基于极化码的无线光信道副载波调制方法,给出了极化编译码的具体算法过程,并将其应用于无线光通信系统。在不同大气湍流强度下,对系统的差错性能进行了仿真分析,其重对基于极化码的二相相移键控和四相相移键控两种调制方法的误码率进行了对比,结果表明,采用极化码的副载波二相相移键控调制系统的性能优于四相相移键控系统。最后,实验比较分析了极化编码前后副载波调制系统的误比特率,结果表明,在无线光通信大气湍流信道模型下,采用副载波极化码编码调制技术可使误码率性能改善一个量级。     

改进Min-sum的LDPC译码算法研究

为了弥补Min-Sum译码算法相对于LLR-BP算法的性能缺陷,降低LLR-BP算法的实现复杂度,提出一种改进型Min-Sum译码算法,将Normalized BP-Based和Offset BP-Based 2种算法有效地结合,在计算校验节点消息时,同时引进校正因子和偏移因子,并通过最小均方差准则来选择参数。仿真结果表明,在误码率相同的条件下,改进型Min-Sum译码算法比Min-Sum算法、Normalized BP-Based算法和Offset BP-Based算法具有更好的译码性能,译码性能逼近于LLR-BP译码算法。     

自由空间光通信中极化码译码方法研究

针对自由空间光通信中极化码译码复杂度高的问题,提出了一种适用于大气湍流信道的具有较低译码复杂度的分段CRC辅助SCL剪枝译码算法。将剪枝算法引入大气湍流信道下的极化码译码中,采用分段循环冗余校验作为检错单元,剪枝算法减少SCL译码的译码路径,进而降低极化码译码复杂度。仿真结果表明,在不同湍流强度下,该译码算法可以获得优于SCL译码算法的误码性能,并可通过改变性能损失值,获得较低的译码复杂度。     

卫星高性能LDPC译码算法研究

卫星通信网络与地面通信网络的融合是未来通信网络的发展趋势,但目前卫星通信的发展相对于地面网络处于滞后的状态,此外,星地信息传输还面临着传输距离远易受干扰、卫星快速移动导致多普勒频移以及信道快速变化等挑战,使得信息传输的可靠性大大降低;信道编码是降低数据传输误码率的有效方式,目前,低密度奇偶校验(Low-Density Parity Check, LDPC)码因其优异的性能得到广泛认可并应用于各个领域;针对LDPC码译码算法进行优化,结合归一化最小和算法(Normalized Min-Sum Algorithm, NMSA)与偏移最小和算法(Offset Min-Sum Algorithm, OMSA)译码方案,提出在获取NMSA最优归一化因子后将其带入OMSA校验节点更新公式中,再获取最优偏移因子,实现两次优化的过程,最终通过仿真显示,译码性能得到进一步的提升。     

自由空间光通信中极化码的应用研究

【目的】在自由空间光（FSO）通信中,大气湍流效应会导致通信链路性能下降。【方法】文章提出了一种极化码信道编译码方案,使用蒙特卡洛算法构造了针对FSO通信中常见的Gamma-Gamma分布大气湍流信道的极化码。比较了文章提出的极化码编译码方案与未编码以及相近码长的低密度奇偶校验（LDPC）码在Gamma-Gamma分布大气湍流信道下的误码率（BER）。【结果】仿真结果表明,所提极化码与未编码比较时,在弱湍流强度下,当信噪比>6.7dB时,极化码方案性能相较于未编码更优;在中湍流强度下,当信噪比>10.3dB时,极化码方案性能相较于未编码更优;在强湍流强度下,当信噪比>11.5dB时,极化码方案性能相较于未编码更优。所提极化码与LDPC码比较时,在弱湍流情况下,当信噪比>7.1dB时,极化码整体性能优于LDPC码;在中湍流条件下,当信噪比>10.6dB时,极化码整体性能优于LDPC码;在强湍流条件下,当信噪比>12dB时,极化码整体性能优于LDPC码。此外,在码长短、码率低和译码宽度小时,文章所提出的极化码编译码方案具有更优的性能。【结论】使用所提极化...     

海洋湍流下波长分集无线光通信系统性能分析

由于海水的吸收、散射衰减以及海洋湍流效应会引起水下无线光通信（Underwater wireless optical communication,UWOC）系统接收端光信号的闪烁,导致UWOC系统传输性能下降。基于Gamma-gamma分布的海洋湍流信道模型,根据海洋湍流参数和各向异性因子表示的等效结构参数,推导出波长分集UWOC系统中断概率（Outage probability,OP）与平均误码率（Bit error rate,BER）封闭表达式。研究分析随着各向异性因子的增加,具有不同波长分集阶的水下无线光通信系统中断概率与平均误码率的变化,比较接收端使用最佳组合（Optimal combining,OC）与等增益组合（Equal gain combining,EGC）技术的水下无线光通信系统平均误码率,并仿真不同海洋湍流参数、传输距离对波长分集UWOC系统性能的影响。数值结果表明,随着各向异性因子的增加,海洋湍流对水下无线光通信系统产生的影响逐渐减弱,使用波长分集技术的UWOC系统比无波长分集技术的UWOC系统中断概率与平均误码率明显改善。     

无线光开关键控调制在Turbo码下的差错特性分析

目前无线光通信工程主要采用开关键控调制,该调制简单、易实现,但误码率高,需要引入纠错编码技术.在分析Turbo码译码结构和开关键控迭代译码算法的基础上,分析仿真了迭代次数、帧长以及约束长度对开关键控调制误码率的影响.结果表明,引入Turbo码编译码技术能够有效改善无线光开关键控调制系统的误码性能.     

基于FSO系统极化码的改进译码方案研究

针对服从对数正态分布的大气弱湍流信道模型,选用极化码来改善FSO系统传输性能。引入一种基于3×3核心矩阵的置信传播(BP)译码方法,对核心矩阵、码字长度、以及基本单元计算式进行了改进。对比采用新型BP译码方法与传统BP译码方法的FSO系统误码率性能。仿真实验结果表明,改进型BP译码方法与传统BP译码方法相比有着更低的误码率。由于系统构造的原因,改进型BP译码占用的存储空间较少、译码延迟时间短,在码长较长的情况下,对FSO系统传输性能有较大的提升。     

A high‐performance belief propagation decoding algorithm for codes with short cycles

The simplicity of decoding is one of the most important characteristics of the low density parity check (LDPC) codes. Belief propagation (BP) decoding algorithm is a well‐known decoding algorithm for LDPC codes. Most LDPC codes with long lengths have short cycles in their Tanner graphs, which reduce the performance of the BP algorithm. In this paper, we present 2 methods to improve the BP decoding algorithm for LDPC codes. In these methods, the calculation of the variable nodes is controlled by using “multiplicative correction factor” and “additive correction factor.” These factors are obtained for 2 separate channels, namely additive white Gaussian noise (AWGN) and binary symmetric channel (BSC), as 2 functions of code and channel parameters. Moreover, we use the BP‐based method in the calculation of the check nodes, which reduces the required resources. Simulation results show the proposed algorithm has better performance and lower decoding error as compared to BP and similar methods like normalized‐BP and offset‐BP algorithms.     

Regular Low-Density Parity-Check codes based on shifted identity matrices

This paper extends the class of Low-Density Parity-Check （LDPC） codes that can be constructed from shifted identity matrices. To construct regular LDPC codes, a new method is proposed. Two simple inequations are adopted to avoid the short cycles in Tanner graph, which makes the girth of Tanner graphs at least 8. Because their parity-check matrices are made up of circulant matrices, the new codes are quasi-cyclic codes. They perform well with iterative decoding.     

Pseudocodewords of Tanner Graphs

This paper presents a detailed analysis of pseudocodewords of Tanner graphs. Pseudocodewords arising on the iterative decoder's computation tree are distinguished from pseudocodewords arising on finite degree lifts. Lower bounds on the minimum pseudocodeword weight are presented for the BEC, BSC, and AWGN channel. Some structural properties of pseudocodewords are examined, and pseudocodewords and graph properties that are potentially problematic with min-sum iterative decoding are identified. An upper bound on the minimum degree lift needed to realize a particular irreducible lift-realizable pseudocodeword is given in terms of its maximal component, and it is shown that all irreducible lift-realizable pseudocodewords have components upper bounded by a finite value t that is dependent on the graph structure. Examples and different Tanner graph representations of individual codes are examined and the resulting pseudocodeword distributions and iterative decoding performances are analyzed. The results obtained provide some insights in relating the structure of the Tanner graph to the pseudocodeword distribution and suggest ways of designing Tanner graphs with good minimum pseudocodeword weight.     

LP Decoding Corrects a Constant Fraction of Errors

We show that for low-density parity-check (LDPC) codes whose Tanner graphs have sufficient expansion, the linear programming (LP) decoder of Feldman, Karger, and Wainwright can correct a constant fraction of errors. A random graph will have sufficient expansion with high probability, and recent work shows that such graphs can be constructed efficiently. A key element of our method is the use of a dual witness: a zero-valued dual solution to the decoding linear program whose existence proves decoding success. We show that as long as no more than a certain constant fraction of the bits are flipped by the channel, we can find a dual witness. This new method can be used for proving bounds on the performance of any LP decoder, even in a probabilistic setting. Our result implies that the word error rate of the LP decoder decreases exponentially in the code length under the binary-symmetric channel (BSC). This is the first such error bound for LDPC codes using an analysis based on "pseudocodewords." Recent work by Koetter and Vontobel shows that LP decoding and min-sum decoding of LDPC codes are closely related by the "graph cover" structure of their pseudocodewords; in their terminology, our result implies that that there exist families of LDPC codes where the minimum BSC pseudoweight grows linearly in the block length     

一种改进的无线光通信LDPC码译码算法

针对无线光通信中低密度奇偶校验码(LDPC)置信传播(BP)译码算法复杂度高及置信度振荡造成译码错误等缺点,基于对数BP算法提出了一种改进的译码算法。改进的译码算法在校验节点运算时,判断输入到校验节点消息的最小值与某个门限的大小,根据比较结果,分别用消息最小值或若干个最小值进行运算,在损失很少性能的情况下降低了运算复杂度;同时在比特节点采用振荡抵消处理运算,提高了算法的性能增益。最后在对数正态分布湍流信道模型下,分别对比特充分交织和交织深度为16的情况进行了仿真实验。仿真结果表明,改进的译码算法与BP算法相比,大幅度降低了计算复杂度,而且译码性能有一定的优势,收敛速度损失很少;而相对于最小和算法,改进的算法虽然译码复杂度有所增加,但误码率性能有明显的优势,并且收敛速度也优于最小和算法。因此,改进的译码算法是无线光通信中LDPC码译码算法复杂度和性能之间一个较好的折中处理方案。     

LDPC码及其应用

先在阐述几种LDPC编码的基础上分析了多进制LDPC编码优点;接着介绍作为解释LDPC码和积译码算法的因子图;最后分析了LDPC码的优势以及它在通信领域中应用的可行性。     

LDPC-Coded Cooperation with Receive Multi-Antenna and Unknown CSI in the Destination

This paper studies a coded cooperation scheme with receive multi-antenna in the destination. A kind of simple-encoding irregular systematic low-density-parity-check (LDPC) codes is applied to the scheme. For Rayleigh block fading channels, in the unknown channel state information scenario, space-based adaptive least-mean square (LMS) transversal filters are adopted in the destination to realize receive diversity gain. The filtered signals from the source and relay are decoded by a joint “Min-Sum” iterative decoding algorithm. This algorithm agrees with the bilayer Tanner graph that can be used to fully characterize two distinct irregular systematic LDPC codes employed by the source and relay. Simulation results verify the effectiveness of the space-based adaptive LMS transversal filters in the coded cooperation scheme. Meanwhile, theoretical analysis and numerical simulation show that the proposed coded cooperation scheme can well combine cooperation diversity, multi-receive diversity and channel coding gains, and clearly outperforms the coded non-cooperation one under the same conditions.     

Combinatorial Interleavers for Systematic Regular Repeat-Accumulate Codes [Transactions Letters]

This paper proposes novel interleaver and accumulator structures for systematic, regular repeat-accumulate (RA) codes. It is well known that such codes are amenable to iterative (sum-product) decoding on the Tanner graph of the code, yet are as readily encodable as turbo codes. In this paper, interleavers for RA codes are designed using combinatorial techniques as a basis for deterministic interleaver constructions, yielding RA codes whose Tanner graphs are free of 4-cycles. Further, a generalized RA code accumulator structure is proposed, leading to codes, termed w3RA codes, whose parity-check matrices have many fewer weight-2 columns than conventional RA codes. The w3RA codes retain the low-complexity encoding of conventional RA codes and exhibit improved error-floor performance.     

基于随机置换展开与停止集的LT码联合编译码算法

针对短码长LT码(码长在103以下)的随机编码方式,需要较高的编码冗余才能保证一定的译码性能的问题。设计了一种基于限制Tanner图连接边随机关系的随机置换展开编码算法,可以实现在较小编码冗余开销下提高短码长LT码的可译码概率。在此基础上,针对短码长LT码的传统BP译码算法效率不高的问题,充分利用BP算法译码失败的停止集剩余信息,设计了采用上述编码算法的短码长LT码的停止集高斯译码算法,可以获得接近最大似然译码的性能。仿真结果表明,所提出的编译码联合算法有效降低了短码长LT码满足10 4译码失败概率时所需的编码冗余开销。