GB20600标准的LDPC码的改进

尽管LDPC码已经被GB20600标准采纳作为信道编码,与其它LDPC码相比,在同样码长和码率的情况下,GB20600 LDPC码误码率性能并非最佳;GB20600标准的LDPC码的码长达7493,存在编码复杂性问题,但是GB20600 LDPC码未采用基于校验矩阵的快速算法,这给GB20600 LDPC编解码器的硬件实现带来较大的困难。本文在现有GB20600 LDPC码的设计框架下,对GB20600中LDPC码的校验矩阵进行了修改,在此基础上提出一种有效的LDPC码的快速迭代算法,使编解码器的硬件易于实现。改进后的LDPC码的编码算法具有较低的实现复杂度。仿真结果表明,改进后的LDPC码的误包率性能优于现GB20600中LDPC码的误包率性能。      

Performance of low density parity check coded continuous phase frequency shift keying (LDPCC‐CPFSK) over fading channels

In this paper, in order to improve bit error performance, bandwidth efficiency and reduction of complexity compared to related schemes such as turbo codes, we combine low density parity check (LDPC) codes and continuous phase frequency shift keying (CPFSK) modulation and introduce a new scheme, called ‘low density parity check coded‐continuous phase frequency shift keying (LDPCC‐CPFSK)’. Since LDPC codes have very large Euclidean distance and use iterative decoding algorithms, they have high error correcting capacity and have very close performances to Shannon limit. In all communication systems, phase discontinuities of modulated signals result extra bandwidth requirements. Continuous phase modulation (CPM) is a powerful solution for this problem. Beside CPM provides good bandwidth efficiency; it also improves bit error performance with its memory unit. In our proposed scheme, LDPC and CPFSK, which is a special type of CPM, are considered together to improve both error performance and bandwidth efficiencies. We also obtain error performance curves of LDPCC‐CPFSK via computer simulations for both regular and irregular LDPC code. Simulation results are drawn for 4‐ary CPFSK, 8‐ary CPFSK and 16‐ary CPFSK over AWGN, Rician and Rayleigh fading channels for maximum 100 iterations, while the frame size is chosen as 504. Copyright © 2006 John Wiley & Sons, Ltd.     

一种基于低码率LDPC码的编码与导频联合辅助载波同步算法

针对深空通信系统中编码和导频辅助载波同步算法分别存在同步范围与精度受限的问题,该文提出一种基于编码与导频联合辅助的载波同步算法。首先按照最优导频放置方式设计了一种基于相关函数和的粗同步算法,其频率估计量的精度接近CRB界;然后分析了导频结构对算法估计精度与范围的影响机理;接着通过加入导频及频率积分器的方式对基于期望最大的细同步算法进行了改进;最后用码率为1/12的低码率LDPC-Hadamard码对算法进行了仿真验证。结果表明新算法可以大幅提高同步参数的估计范围与精度,在一定的导频长度下能达到理想同步。     

Frame synchronization for pulsed jammed satellite telecommand links

A new issue of the satellite telecommand synchronization and channel coding sublayer protocol includes LDPC‐coded communication link transmission units (CLTUs) that contain a 64‐bit start sequence. The novel data structures allow operation at lower signal‐to‐noise ratios than before and offer improved protection against jamming attacks. This paper considers the corresponding CLTU frame synchronization process. We derive practical algorithms to locate the start sequence in the presence of high noise levels and pulsed jamming. The different algorithms are compared in terms of implementation complexity and performance under various jamming conditions. It is shown that among the considered frame synchronizers, those involving a full search over the entire observation window provide the desired accuracy, ie, they guarantee a frame synchronization error probability that is significantly smaller than the codeword error rate, for codeword error rates near a target value of 10^?4. Among these synchronizers, the full‐search hard‐decision–directed correlation‐based algorithm has the lowest complexity.     

Estimation of Bit and Frame Error Rates of Finite-Length Low-Density Parity-Check Codes on Binary Symmetric Channels

A method for estimating the performance of low-density parity-check (LDPC) codes decoded by hard-decision iterative decoding algorithms on binary symmetric channels (BSCs) is proposed. Based on the enumeration of the smallest weight error patterns that cannot be all corrected by the decoder, this method estimates both the frame error rate (FER) and the bit error rate (BER) of a given LDPC code with very good precision for all crossover probabilities of practical interest. Through a number of examples, we show that the proposed method can be effectively applied to both regular and irregular LDPC codes and to a variety of hard-decision iterative decoding algorithms. Compared with the conventional Monte Carlo simulation, the proposed method has a much smaller computational complexity, particularly for lower error rates.     

Channel Estimation and LDPC Code Puncturing Schemes Based on Incremental Pilots for OFDM

In this letter, we propose a channel estimation algorithm based on incremental pilots. These are pilots additionally inserted after puncturing the modulated orthogonal frequency division multiplexing (OFDM) symbols to enhance channel estimation performance without lowering bandwidth efficiency. A low‐density parity‐check code puncturing scheme is also proposed to prevent the performance degradation due to the codeword bit loss caused by punctured OFDM symbols.     

Construction of quasi‐cyclic low‐density parity‐check codes with low encoding complexity

Low encoding complexity is very important for quasi‐cyclic low‐density parity‐check (QC‐LDPC) codes used in wireless communication systems. In this paper, a new scheme is presented to construct QC‐LDPC codes with low encoding complexity. This scheme is called two‐stage particle swarm optimization (TS‐PSO) algorithm, in which both the threshold and girth distribution of QC‐LDPC codes are considered. The proposed scheme is composed of two stages. In the first stage, we construct a binary base matrix of QC‐LDPC code with the best threshold. The matrix is constructed by combining a binary PSO algorithm and the protograph extrinsic information transfer (PEXIT) method. In the second stage, we search an exponent matrix of the QC‐LDPC code with the best girth distribution. This exponent matrix is based on the base matrix obtained in the first stage. Consequently, the parity‐check matrix of the QC‐LDPC code with the best threshold and best girth distribution are constructed. Furthermore, bit error rate performances are compared for the QC‐LDPC codes constructed by proposed scheme, the QC‐LDPC code in 802.16e standard, and the QC‐LDPC code in Tam's study. Simulation results show that the QC‐LDPC codes proposed in this study are superior to both the 802.16e code and the Tam code on the additive white Gaussian noise (AWGN) and Rayleigh channels. Moreover, proposed scheme is easily implemented, and is flexible and effective for constructing QC‐LDPC codes with low encoding complexity. Copyright © 2012 John Wiley & Sons, Ltd.     

Fair and efficient scheduling for UMTS forward link

This paper presents two scheduling algorithms, MWF²Q+ and MDRR, for multiple classes of service over the same spectrum in the forward link of the UMTS network. These scheduling algorithms can allocate bandwidth in proportion to weights of flows sharing the channel, and assign OVSF code to backlogged flows on a frame‐by‐frame basis. The MWF²Q+ algorithm has better fairness properties while the MDRR algorithm requires less computational complexity and space complexity. The fairness properties of these scheduling algorithms are analysed in this paper. Our simulation results show that these two algorithms support multiple traffic sources with heterogeneous rate guarantees while fully utilizing the system bandwidth. The impact of self‐similar traffic is also addressed in our simulations. Copyright © 2005 John Wiley & Sons, Ltd.     

低信噪比下归零Turbo码码长及其帧同步识别

针对目前高斯消元法在归零Turbo码长、帧同步等参数识别过程存在容错性能低且计算复杂度高的缺点,该文提出一种低信噪比(SNR)下基于差分似然差(DLD)的识别算法。首先通过定义差分似然差的概念,利用归零Turbo码帧头两码元差分似然差为正值(“+”)的特性,构建分析矩阵实现码长的识别;其次,提出基于最小错误判决准则下的差分似然差“+”位置门限判决方法,完成帧同步;最后,从工程实际出发,遍历寄存器个数的可能值,实现码率、寄存器个数以及交织长度识别。仿真实验表明:所提算法对于归零Turbo码码长、帧同步等参数识别有效,差分似然差“+”位置分布与分析的数据结构特征一致,判决门限能够有效判断差分似然差“+”位置,同时,算法容错性能较强,在信噪比为–5 dB条件下,码长、帧同步等参数识别率能够达到90%以上,并且算法的复杂度远小于现有算法。     

一种低复杂度的LDPC码迭代译码算法

在LDPC码的译码算法中，和积算法性能最优但复杂性较高，最小和算法实现简单但性能与和积算法相差较多。针对这一性能与复杂度的矛盾，带有修正项的最小和算法成为研究的热点问题。文中基于一种性能与和积算法接近的修正最小和算法进行研究，对修正项的修正方式进行了简化，简化后的算法在性能上与和积算法仍非常接近，实现复杂度却比原修正最小和算法有明显的降低。     

Decoding and Design of LDPC Codes for High-Order Modulations

A concatenated code model is proposed for high-order low-density parity-check (LDPC) coded modulations. A corresponding concatenated-code belief propagation (CCBP) decoding algorithm is derived for our proposed concatenated code. Moreover, the design of LDPC codes under the CCBP decoding is developed using extrinsic information transfer (EXIT) charts. Compared with other algorithms, the CCBP method provides an excellent parallel decoding process, and the EXIT-based design method offers highly accurate LDPC code ensembles. Simulation results show that the performance of the proposed CCBP algorithm is superior to that of the conventional belief propagation decoding within a wide range of modulation orders, and the EXIT-based method can design capacity-approaching LDPC codes for high-order modulations.     

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

Frequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.     

QC LDPC码低复杂度消环算法

QC LDPC (Quasi-才yclic Low-density Parity-check)是一类半结构化的低密度奇偶校验码,其分块的矩阵结构具有超大规模集成电路实现上的便利,同时保持了优异的纠错性能. 本文针对QC LDPC码的基矩阵,提出一种移位因子的搜索方法及其改进版本。通过对基矩阵的扩展矩阵的Tanner图进行树形展开来进行环的检验,避免了传统算法中的复杂算术操作,降低了复杂度。在采用和IEEE 802.16e中码率为0.5的LDPC码方案相同的基矩阵条件下,本文的算法构造出的QC LDPC码具有更优的环长分布,同时纠错性能也有提升。      

Performance of joint multilevel/AES‐LDPCC‐CPFSK schemes over wireless sensor networks

In wireless sensor networks, data encryption and channel coding are considered together for ensuring secure and robust communication. In order to achieve this purpose, we introduce a new joint scheme, namely ‘Multilevel/Advanced Encryption Standard‐Low Density Parity Check Coded‐Continuous Phase Frequency Shift Keying (ML/AES‐LDPCC‐CPFSK)’. AES algorithm is the most powerful and widely used symmetric key cryptography in providing secure data transmission. LDPC codes have very large Euclidean distance and use iterative decoding algorithms. In this study, we have increased error performance employing multilevel structure to AES and LDPC. In all communications systems, phase discontinuities of modulated signals result in extra bandwidth requirements. CPFSK, which is a special type of continuous phase modulation, is a powerful solution for this problem. In this paper, we simulate error performance of ML/AES‐LDPCC‐CPFSK for regular LDPC codes. Simulation results are drawn for 4CPFSK, 8CPFSK and 16CPFSK over wireless cooperative sensor networks. Using this scheme, we are able to improve bit error performance, channel throughput, security level of communication and reduction in complexity compared with related schemes such as various turbo code structures. Copyright © 2009 John Wiley & Sons, Ltd.     

浅海水声信道中原模图LDPC码的设计及性能分析

原模图低密度奇偶校验码相较于传统LDPC码,具有结构简单、迭代译码门限低、易于扩展及线性编译码复杂度等优点。针对强多途、长时延、窄带宽的浅海水声信道,该文研究了PG-LDPC码的设计及性能特征,提出一种码型设计方案,并采用基于原模图度分布的外部信息转移图算法,对所设计PG-LDPC码的纠错性能进行分析及预测。仿真与实验结果表明,与(3,6)随机规则LDPC码相比,所提的PG-LDPC码在低、高信噪比区域均有良好的纠错性能。     

LDPC Decoding by Failed Check Nodes for Serial Concatenated Code

The use of serial concatenated codes is an effective technique for alleviating the error floor phenomenon of low‐density parity‐check (LDPC) codes. An enhanced sum–product algorithm (SPA) for LDPC codes, which is suitable for serial concatenated codes, is proposed in this paper. The proposed algorithm minimizes the number of errors by using the failed check nodes (FCNs) in LDPC decoding. Hence, the error‐correcting capability of the serial concatenated code can be improved. The number of FCNs is simply obtained by the syndrome test, which is performed during the SPA. Hence, the decoding procedure of the proposed algorithm is similar to that of the conventional algorithm. The error performance of the proposed algorithm is analyzed and compared with that of the conventional algorithm. As a result, a gain of 1.4 dB can be obtained by the proposed algorithm at a bit error rate of 10^?8. In addition, the error performance of the proposed algorithm with just 30 iterations is shown to be superior to that of the conventional algorithm with 100 iterations.     

MIMO-OFDM系统中LDPC编码和自适应比特功率分配算法研究

该文提出了将LDPC码与自适应比特功率分配相结合应用于MIMO-OFDM系统中的算法。文中分别对两种不同码率的LDPC码与自适应OFDM,自适应MIMO-OFDM相结合的算法进行了仿真,同时还对未编码的自适应OFDM以及MIMO-OFDM进行了仿真。仿真结果显示,在相同的传输带宽以及相同的传输信息量（即数据净码率）情况下,采用低码率LDPC编码的自适应OFDM系统的性能要比未编码的自适应OFDM以及自适应 MIMO-OFDM系统的性能好;采用高码率LDPC编码的自适应OFDM系统的性能比未编码的自适应OFDM系统性能好,但比未编码的自适应MIMO-OFDM系统性能差。     

改进的离散字母表迭代译码算法研究

为了优化LDPC迭代译码性能和降低算法复杂度,提出了一种改进的基于Gallager A算法的2b离散字母表迭代译码算法。在每一轮迭代中,Tanner图上的校验节点与变量节点之间所传递的消息有1b表示符号值,另1b反映码字结构特性,其中变量节点更新规则是通过查表法来实现的。在二元对称信道下针对列重为3的规则LDPC码做了仿真实验,仿真结果表明该算法性能明显优于原算法,并且具有较低的复杂度。     

Semi‐random LDPC codes for CDMA communication over non‐linear band‐limited satellite channels

This paper considers the application of low‐density parity check (LDPC) error correcting codes to code division multiple access (CDMA) systems over satellite links. The adapted LDPC codes are selected from a special class of semi‐random (SR) constructions characterized by low encoder complexity, and their performance is optimized by removing short cycles from the code bipartite graphs. Relative performance comparisons with turbo product codes (TPC) for rate 1/2 and short‐to‐moderate block sizes show some advantage for SR‐LDPC, both in terms of bit error rate and complexity requirements. CDMA systems using these SR‐LDPC codes and operating over non‐linear, band‐limited satellite links are analysed and their performance is investigated for a number of signal models and codes parameters. The numerical results show that SR‐LDPC codes can offer good capacity improvements in terms of supportable number of users at a given bit error performance. Copyright © 2006 John Wiley & Sons, Ltd.     

Computationally Efficient Lattice Reduction Aided Detection for MIMO‐OFDM Systems under Correlated Fading Channels

We analyze the relationship between channel coherence bandwidth and two complexity‐reduced lattice reduction aided detection (LRAD) algorithms for multiple‐input multiple‐output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in correlated fading channels. In both the adaptive LR algorithm and the fixed interval LR algorithm, we exploit the inherent feature of unimodular transformation matrix P that remains the same for the adjacent highly correlated subcarriers. Complexity simulations demonstrate that the adaptive LR algorithm could eliminate up to approximately 90 percent of the multiplications and 95 percent of the divisions of the brute‐force LR algorithm with large coherence bandwidth. The results also show that the adaptive algorithm with both optimum and globally suboptimum initial interval settings could significantly reduce the LR complexity, compared with the brute‐force LR and fixed interval LR algorithms, while maintaining the system performance.