LDPC码在相干光通信下的性能分析与研究

对于相干光通信系统,相位噪声成为影响系统性能的重要因素之一,并严重影响LDPC码(低密度奇偶校验码)软译码的性能。仿真结果表明,在16QAM调制方式下,500kHz线宽的激光器相位噪声将导致1.5dB的性能损失。为了降低相位噪声的影响,提出了一种基于相位噪声估计值修正LLR(对数似然比信息)的方法。经仿真验证,该LLR修正方法能够挽回0.5dB的译码性能损失。     

大气激光通信自适应系统中LDPC的识别

从大气激光通信信道 模型出发,结合大气激光通信特殊的调制方式,利用接收解调软判决序列的后验概率信息 ,导出大气激光通信 自适应系统中低密度奇偶校验码(LDPC)的识别算法公式。仿真结果表明, 当识别正确率为 0.8时, 在强湍流条件下, 算法公式比传统算法性能提高约1.2dB;在弱湍流条件下,算法公 式 比传统算法性能提高约1.5dB;并 且随着码率的降低和码长的增加,识别成功率均有提高。因此,所提算法公式能够进一步抑 制大气湍流影响,提高系统性能。     

LDPC编码的MIMO大气激光通信系统研究

通过将信道编码LDPC与分集技术相结合,分析了级联空时码在对抗大气湍流效应的作用。首先建立了Gamma-Gamma大气湍流信道模型,并且设计了LDPC编码的MIMO自由空间光通信系统,仿真了弱湍流和强湍流两种情况下,未编码和已编码的MIMO系统的性能。结果表明,在弱湍流和强湍流情况下,LDPC编码的MIMO系统较未编码的MIMO系统的误码率得到了很好的改善,性能得到了极大的提升。     

多元LDPC码串行译码算法在光纤通信中的应用

将串行BP译码算法用在多元LDPC码中,降低了在光纤传输系统中的译码延时.详细介绍了在多元LDPC码中的串行BP译码算法和光纤通信系统的仿真模型.给出了在采用串行BP算法的LDPC译码器中,译码最大迭代数量对译码性能的影响,比较了采用传统的BP算法扣串行BP算法时LDPC译码器的性能.结果表明,采用串行BP算法确实能够提升LDPC译码器的收敛速度.     

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。     

高速相干光通信系统中水印辅助的低密度奇偶校验方案

为了研究水印辅助的低密度奇偶校验(LDPC)方 案在高速相干接收光通信系统 中的译码性能,提出了一种适用于相干光通信系统的水印符号辅助的LDPC前向纠错方案 ,与传统LDPC相比,可以大大提高相干接 收机的抗噪声能力。在100Gbit/s 16进制正 交幅度高制(16QAM)相干接收光通信系统上的仿真实验结果表明, 在误码率(BER)为10－9时, 所提出的前向纠错方案与传统LDPC相比 较,信噪比(SNR)增益提高了0.45dB ;同时,在相同BER下,水印符号辅助的LDPC方 案的译码迭代次数有很明显的降低。     

光通信中一种准循环非二进制LDPC码的新颖构造方法

在对目前普遍采用的非二进制低密度奇偶校验(NB -LDPC)码校验矩阵的准循环构造方法进行深入研究的基础上,提出了一种基于有限域的NB -LDPC码的立体构造方法,在构建基于有限域的基础矩阵后,运用立体扩展的方式构成循环 子矩阵,最终构造出具备准循环特性的非二进制校验矩阵。 通过对采用立体构造法构造的NB-LDPC码的性能仿真发现,与基于GF(2⁹)的 RS(511,5)相比,本文 构造的NB-LDPC码在误比特率(BER)为10－7时可 以增加3.3 dB的净编码增益(NCG);在BER为 10－6时,本文构造的LDPC码与采用传统准循环方式构造的二 进制LDPC码、随机构造 的二进制LDPC码、基于有限域构造的32进制准循环LDPC码和基于欧式 几何构造的64进制的循 环LDPC码比较,分别多获得了0.56、0.56、0.03dB的NCG。通过对本文 构造的NB-LDPC码性能仿真发现,这类具有高度结构化的NB-LDPC码不仅具备 准循环特性,有利于硬件实现,同时在中短码长情况时展现出较好的纠错性能。     

短波宽带通信系统中LDPC码的应用分析

针对短波宽带信道时变衰落色散的特性,为减小多径效应以及空间噪声、干扰等不利因素的影响,改善短波通信质量,采用高效纠错编码LDPC码,结合扩频OFDM技术,设计了一种适用于短波宽带信道的编码调制系统结构,仿真结果显示：系统能够显著提高短波宽带通信系统的误码性能,为短波宽带通信提供了一种可靠、有效的设计方案。     

Gamma-Gamma大气湍流中部分相干光通信系统性能研究

部分相干光在湍流大气中传输时,可以有效抑制湍流所引起的光强闪烁效应,从而改善通信链路性能。针对Gamma-Gamma大气湍流信道模型和部分相干光的光束特性,得到了采用OOK调制方式下部分相干光通信系统的平均误码率、中断概率和平均信道容量三个性能指标的解析表达式;在此基础上,分析了光束的空间相干长度和通信距离对通信链路的性能影响。计算结果表明,在相同的大气湍流条件和传输距离下,随着部分相干光的空间相干长度的减小,系统的误码率和中断概率逐步降低,在平均信噪比为30 dB时,系统的误码率可以达到10-5,中断概率低于10-6;另外,系统的平均信道容量会随着光束相干长度的减小而增加,在信噪比等于12 dB时,平均信道容量达到3.8 b/sHz-1。分析结果为部分相干光在湍流大气中实现可靠通信提供了理论依据。     

A new LDPC decoding scheme for PDM-8QAM BICM coherent optical communication system

A new log-likelihood ratio (LLR) message estimation method is proposed for polarization-division multiplexing eight quadrature amplitude modulation (PDM-8QAM) bit-interleaved coded modulation (BICM) optical communication system. The formulation of the posterior probability is theoretically analyzed, and the way to reduce the pre-decoding bit error rate (BER) of the low density parity check (LDPC) decoder for PDM-8QAM constellations is presented. Simulation results show that it outperforms the traditional scheme, i.e., the new post-decoding BER is decreased down to 50% of that of the traditional post-decoding algorithm.     

自由空间光通信中相干圆偏振调制系统性能研究

自由空间光通信（FSO）具有高带宽、不需要铺设光纤、部署迅速、低功率损耗等优点,得到了广泛的关注。在Gamma-Gamma大气湍流信道下,分别考虑弱、中、强三种大气湍流强度,结合相位噪声,研究了相干探测的圆偏振调制系统的性能,推导出了误码率和中断概率的闭合表达式。在不同的大气湍流强度和相位噪声的情况下,仿真分析了相干探测的圆偏振调制系统的性能,并与直接探测系统的误码性能进行比较,结果表明,采用相干探测系统的误码性能优于直接探测系统。因此,在圆偏振调制系统中,采用相干探测方式可减小大气湍流的影响,大大降低了误码率,提高了系统的通信性能。     

Space coherent optical communication systems--An introduction

Critical technology and system issues for space optical heteroydyne communication systems using GaAlAs lasers are discussed. Heterodyne systems offers 10-20 dB better receiver sensitivities and thus smaller telescope sizes for the same laser power than direct detection systems. Based on present state-of-the-art technologies, it is concluded that an efficient optical intersatellite link can be realized with a few years of development.     

多元LDPC码与二元LDPC码的性能分析

为了比较多元LDPC码与二元LDPC码的性能,文章从校验矩阵、Tanner图、BP译码算法等方面将两者进行有效的分析,并结合具体的Monte Carlo仿真实验,得出多元LDPC码的性能确实优于等长度码长的二元LDPC码.     

反馈电压偏置型Hopfield 网络电域盲检测相干光通信信号

为解决无线分集相干光接收机的自适应盲检测问题,提出了一种新的离散时间连续状态的网络输出反馈偏置型的复Hopfield 神经网络用以解决多值QAM 信号的盲检测问题。反馈电压偏置的引入即不脱离传统Hopfield 模型,又能有效满足多值信号检测时所需的搜索空间变大的特殊要求。全文完成多值信号盲检测的优化问题构造和能量函数的映射,给出能量函数的证明、分析和它的约束条件,给出适用该问题的激活函数的基本特征,正确盲检测信号的权矩阵的配置方法。最后,通过详细的仿真结果展示和与其他算法性能对比进一步验证算法的有效性和优越性并指出算法所存在的问题和下一步的研究方向。     

Joint Source-Channel Decoding of Huffman codes with LDPC codes

In this paper, we present a Joint Source-Channel Decoding algorithm （JSCD） for Low-Density Parity Check （LDPC） codes by modifying the Sum-Product Algorithm （SPA） to account for the source redundancy, which results from the neighbouring Huffman coded bits. Simulations demonstrate that in the presence of source redundancy, the proposed algorithm gives better performance than the Separate Source and Channel Decoding algorithm （SSCD）.     

利用自适应逆模拟提高大气光通信性能

大气信道的随机变化导致光通信系统误码率的增大,这里提出了一种利用自适应逆模拟补偿大气信道的自适应方法,并给出了算法设计,这有利于提高大气光通信系统的性能。     

Recognition of spectral amplitude codes by frequency swept coherent detection for flexible optical label switching

We propose a new method of recognizing spectral amplitude codes by using optical coherent detection with a frequency swept local light source oscillator. Our proposed method offer a substantial simplification in terms of required components to built optical label processing units with enhanced flexibility to accommodate for wavelength tuneability and a large number of labels. We present a performance analysis, comparison with conventional spectral codes recognition methods, based on computer simulation results. We consider a payload bit rate of 40 Gb/s.     

Leveraging coherent space-time codes for noncoherent communication via training

Training codes are introduced for the multiple-antenna, noncoherent, multiple block-Rayleigh-fading channel in which the fading coefficients, which are constant over a fixed number of dimensions (coherence interval) for each block and then change independently to a new realization, are known neither at the transmitter nor at the receiver. Each codeword of a training code consists of a part known to the receiver-used to form a minimum mean-squared error (MMSE) estimate of the channel-and a part that contains codeword(s) of a space-time block or trellis code designed for the coherent channel (in which the receiver has perfect knowledge of the channel). The channel estimate is used as if it were error-free for decoding the information-bearing part of the training codeword. Training codes are hence easily designed to have high rate and low decoding complexity by choosing the underlying coherent code to have high rate and to be efficiently decodable. Conditions for which the estimator-detector (E-D) receiver is equivalent to the optimal noncoherent receiver are established. A key performance analysis result of this paper is that the training codes when decoded with the E-D receiver achieve a diversity order of the error probability that is equal to the diversity order of the underlying coherent code. In some cases, the performance of training codes can be measured relative to coherent reception via "training efficiency," which is then optimized over the energy allocation between the training and data phases. In the limit of increasing block lengths, training codes always achieve the performance of coherent reception. The examples of training codes provided in this work have polynomial complexity in rate but an error rate comparable to the best performing unitary designs available, even though the latter require exponential decoding complexity.     

Performance analysis of WS-EWC coded optical CDMA networks with/without LDPC codes

One extended Welch-Costas (EWC) code family for the wavelength-division-multiplexing/spectral-amplitude coding (WDM/SAC; WS) optical code-division multiple-access (OCDMA) networks is proposed. This system has a superior performance as compared to the previous modified quadratic congruence (MQC) coded OCDMA networks. However, since the performance of such a network is unsatisfactory when the data bit rate is higher, one class of quasi-cyclic low-density parity-check (QC-LDPC) code is adopted to improve that. Simulation results show that the performance of the high-speed WS-EWC coded OCDMA network can be greatly improved by using the LDPC codes.