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1、南京航空航天大学 硕士学位论文 基于LDPC码的协作通信系统及联合迭代译码性能的研究 姓名：陈婧文 申请学位级别：硕士 专业：通信与信息系统 指导教师：仰枫帆 20090201 南京航空航天大学硕士学位论文 I 摘 要 随着无线通信的发展，协作通信技术的研究引起广泛关注。协作通信的概念是在中继信道 模型的基础上， 受多输入多输出(MIMO)技术的启发而提出的。 这种新型物理层技术的基本思想 是在多用户环境中，使具有单根天线的移动终端可以按照一定的方式来共享彼此的天线从而产 生一个虚拟 MIMO 系统。 协作通信主要有三种模式：放大转发、检测转发、编码协作。其中，编码协作模式的突出 特点是通过信
2、道编码设计来实现协作，降低了协作中错误扩散的可能性，提高了接收端的纠错 能力，具有比前两者更为优异的性能。可见，如何将日趋成熟的各种信道编码方式与协作通信 的理念相结合，设计出有效的协作通信方案有很高的学术研究价值和现实意义。 本文的研究工作主要包括以下几方面： 1. 在充分理解 Shannon 有噪信道编码定理的基础上，认识到 Shannon 限是衡量不同编码方案 性能的标尺，从而明确编码学术界的研究目标。 2. 研究了 LDPC 码和 Turbo 码的编译码方式及其性能，并在图像传输的应用中比较和分析了 这两种编码方式的性能差异。 3. 提出了两种加速迭代的改进 BP 译码算法，使 LDP
3、C 码的每一次迭代更多地利用在该次迭 代中已得到更新的外信息。理论分析和模拟结果都显示，在加性高斯白噪声信道下，这两 种改进算法在译码复杂度不变或基本不变的情况下，有效加速了迭代的收敛，得到了优于 传统 BP 算法的译码性能。 4. 提出了中继信道下基于正规 LDPC 码的协作通信方案。该方案采用联合译码。在译码初期， 两个译码器分别计算并输出对各自输入序列的估计；再进行联合译码，将这两组软信息合 并，即得到对信息序列的最终估计。利用正规 LDPC 码的编码增益和协作通信带来的分集 增益，该系统获得了较好的性能。 5. 在中继信道下首次提出了新的基于非正规 LDPC 系统码的协作通信方案及联合
4、迭代译码。 首先，构造这种系统形式的非正规 LDPC 码，并将其与相同条件下的正规 LDPC 码进行性 能比较。在该协作通信方案的编码阶段，源节点与中继节点选用相同参数的的不同 LDPC 码进行编码，降低了两组校验序列的相关性，其作用类似于 Turbo 码中的交织器。在相应 的译码阶段采用联合迭代译码。将两个 LDPC 译码器并行级联，建立联合 Tanner 图，在每 一次迭代译码内部将协作双方的外信息沿联合 Tanner 图的边双向传递。这种联合迭代译码 的原理，类似于 Turbo 码译码器中的串行级联和反馈机制，在译码的每一次迭代中都完成 基于 LDPC 码的协作通信系统及联合迭代译码性能
5、的研究 II 了两个子译码器的信息传递，用更少的迭代次数实现了更显著的性能提高。虽然非正规 LDPC 码本身的性能劣于正规 LDPC 码，但通过协作通信及其联合迭代译码，编码增益和 分集增益被充分利用，显著地提高了系统的性能，充分体现了协作通信的分集优势。相比 前一种方案，本方案提高了总码率和联合译码的效率。 关键词关键词：协作通信，中继信道，编码协作，LDPC 码，BP 译码算法，联合迭代译码 南京航空航天大学硕士学位论文 III Abstract With the rapid development of wireless communications, the research of c
6、ooperative communication technology arouses more and more peoples attention. The concept of the so-called cooperative communications was originally proposed based on the theory of relay channels and MIMO technique.) In this new technique, the cooperative users can achieve the benefits from diversity
7、 through a signaling scheme that allows them to send their information using both of their antennas, just like a virtual MIMO system. There are basically three methods for user cooperation strategy: amplify-and-forward, detect-and-forward and coded cooperations, respectively. More specifically, in t
8、he coded cooperation, the users employ channel coding to avoid transmitting erroneous data for their partners. As a result, it achieves much better performance than the other two signaling schemes. This thesis is organized as follows: 1. Understand the importance of the phenomenal Shannon limit for
9、capacity of noisy channel and revisit the 60-year (1948-2008) great discoveries in Shannon theory since its birth. 2. Study the encoding and decoding principles of turbo and LDPC codes. Then apply the two coding schemes to static image transmission system over an AWGN channel. The simulation results
10、 and BER performances are given to evaluate the quality of the image transmissions. 3. Initially propose two modified BP algorithms related to vertical and horizontal process in order to accelerate iterative LDPC decoding over an AWGN channel, where the newly updated extrinsic information is immedia
11、tely used in the current decoding round. Theoretical analysis and simulation results demonstrate that both modified approaches provide significant performance improvements over the traditional BP algorithm with almost no additional decoding complexity. The proposed algorithm with modified horizontal
12、 process offers even better performance than another algorithm with modified horizontal process. Two modified BP algorithms are very promising in practical communications since both can achieve an excellent trade-off between the performance and decoding complexity. 4. Propose a cooperative communica
13、tion scheme based on regular LDPC codes and joint decoding over the relay channel. In the first stage of decoding, two decoders at the destination node decode their own received symbols, and then output the corresponding soft detection information separately. In the process of joint decoding, two se
14、ts of soft information from the respective different decoders, which are complementary with each other, are incorporated into one step of signal estimation. This 基于 LDPC 码的协作通信系统及联合迭代译码性能的研究 IV joint decoding achieves the coding gains from the employed LDPC codes and the diversity gains from the coo
15、perative strategy. 5. Initially, we propose another cooperative communication scheme based on this newly designed irregular LDPC code over the relay channel. Firstly, a new kind of systematic irregular LDPC coded is constructed, and then compare its performance with that of a regular LDPC code under
16、 basically the same conditions. In the encoding stage of the cooperation, two different irregular LDPC codes with the same structural parameters are applied to the source and relay, respectively. The different codes employed imply the resulted two parity-check sequences with lower correlation, which is quite similar as a turbo code with the strategy of interleaving. In the decoding stage of the cooperation, the joint iterative de
