【作者】 高敬伯；

【导师】 周曼丽；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2006， 博士

【摘要】 从上个世纪90年代中期,低密度校验(Low-Density Parity-Check, LDPC)码重新获得人们的重视以来,LDPC码由于其优异的性能、线性解码复杂度等优点成为了业界的研究热点之一。与此同时,新一代无线移动通信系统需要在频率选择性衰落信道下提供很高的数据传输速率,并且要能够支持多个用户同时进行通信,从而需要提供强纠错能力的纠错码用以提高系统对抗信道噪声和衰落的性能。因此,在无线通信中的纠错码技术得到了人们的广泛关注。目前,新一代无线移动通信系统的关键技术包括:正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)、多天线(Multi-Input Multi-Output, MIMO)、自适应编码调制、多用户检测等。本文研究了LDPC码在无线通信中如何与这些关键技术相融合以进一步改善系统的性能。本文首先研究了采用删余LDPC码的比特交织编码调制(Bit-Interleaved Coded Modulation, BICM)技术,从系统的角度提出了采用删余LDPC码作为成员码的BICM系统方案。其中,删余模式的选择分别采用:通过优化算法确定的LDPC码的优化删余分布,和随机删余两种方案。本文对于系统在采用这两种删余方案时的性能进行了分析。由于实际的物理环境和信道的时变性,使得MIMO信道可能出现空时相关性。为了消除空时相关性对于级联LDPC码与空时分组码系统所造成的性能损失,本文提出一种新的空时解码器设计方案。该方案首先对于预处理信号进行干扰抑制,然后将LDPC码解码硬判决的调制符号作为发射符号的估计值,采用干扰消除算法以去除由于信道的时变特性所产生符号间串扰的影响。新解码器也由于LDPC码提供的分集而弥补了由于发射天线阵列和接收天线阵列的空间相关性所导致的性能损失。因此,新解码器对于信道的时变特性和天线阵列的空间相关性具有较强的鲁棒性。MIMO OFDMA是一种非常有竞争力的宽带无线多用户方案。本文针对频率选择性衰落信道提出了一种采用LDPC码的MIMO OFDMA系统方案。为了能够通过子载波在用户之间进行自适应分配以获得多用户分集从而进一步改善系统性能,本文对于该系统,在满足最大发射功率、用户误码率等约束条件下,以系统的数据传输速率最大为目标函数,推导出系统自适应子载波的分配准则。在此基础上,以及保证用户服务质量(Quality of Service, QoS)要求的前提下,提出一种新的子载波分配方案。对于该方案,首先按照得到的子载波分配准则在用户之间对于子载波进行初步分配,然后按照用户之间的公平性原则和代价最小原则提出了子载波在用户之间调整算法。新子载波分配方案不但能够满足用户的QoS要求,而且在子载波的分配过程中兼顾到了用户之间的公平性、系统资源的利用效率以及如何降低由于保证用户QoS而导致的性能损失。针对规则LDPC码,本文提出了两步比特翻转迭代解码算法。由于最终的翻转比特是通过两次筛选得到的,因此有效地降低了误翻的概率。同时,在第一步的翻转比特的初选过程中,本文综合码字比特的对数似然比和纠错伴随式,提出了新的翻转比特选择标准。仿真结果表明,新解码算法有着良好的性能,在解码运算复杂度和纠错性能之间提供了一个折中。更多还原

【Abstract】 Since the rediscovery of low-density parity-check (LDPC) codes in the middle of 1990’s, LDPC codes are one of the hottest topics in the domain of forward error correction codes because of their excellent performance and linear decoding complexity. At the same time, the next generation wireless mobile communication system should reliably provide very high data transmission rate while supporting multiuser to communicate simultaneously. Because of this, powerful error correction codes must be employed to overcome noise and fading. Therefore, using error-correction codes for wireless communication to improve system performance has drawn great attention for researchers. Nowadays, many techniques have been extensively studied as key techniques for the next generation wireless mobile communication system, for example, orthogonal frequency division multiplexing (OFDM), multi-input multi-output (MIMO), adaptive modulation and coding (AMC), multiuser detection. This dissertation concentrates on the joint study of LDPC codes and these key techniques to further improve the performance of wireless communication systems.The first objective of this research is to design a new bit-interleaved coded modulation (BICM) scheme based on rate-compatible punctured LDPC codes. Two puncturing methods are employed in the proposed system: intentional puncturing designed by linear programming, and random puncturing. Then the performance of the proposed system is studied by simulation and the results are analyzed.In MIMO channels, spatial correlation and time-selective fading exist because of real physical environments and Doppler shifts. In order to overcome performance loss in concatenated LDPC and space-time block coding (STBC) system, a new decoder which joints space-time decoding and LDPC decoding is proposed. In the new decoder, we first suppress ISI in the output symbols from pre-processing. Then, we regard the modulation symbols from hard decisions of LDPC decoding as the estimated value of the transmitted symbols to further eliminate the influence of ISI (Inter-Symbol Interference). Moreover, the new decoder exploits diversity provided by LDPC code to compensate performance loss from spatial correlation between transmitter and receiver antennas. Therefore, the decoder is robust to performance loss caused by spatial and time-selective fading.MIMO OFDMA is a promising transmission scheme for wideband wireless mobile communication. In this dissertation, a LDPC-coded MIMO OFDMA system is proposed to cope with frequency-selective multipath fading for multiuser/multirate communication. In order to further improve performance by exploiting the multiuser diversity resulting from adaptive subcarrier allocation among users, we propose a new allocation criterion, which aims for the maximization of sum-rate of all users under the constraints of transmission signal power and bit error ratio. Furthermore, in order to guarantee quality of service (QoS) of each user, we propose a new low-complexity subcarrier allocation algorithm. In the proposed algorithm, an initial subchannel allocation is first performed according to the proposed subcarrier allocation criterion. A reallocation algorithm then guarantees each user’s QoS requirement while maintaining fairness among the users and efficiency of system resource utilization. The performance penalty from QoS guarantees is also minimized.The last section of this dissertation presents a two-stage bit-flipping decoding algorithm for regular LDPC codes. Since the ultimate flipping bits are got through two selection processes, the probability of doing wrong flipping is greatly reduced. Moreover, a new bit-selection criterion which incorporates the syndrome and the absolute value of the log-likelihood ratio of the bit vector is proposed during the first stage of decoding. Simulation results show that our proposed decoding algorithm has excellent performance, and therefore it provides another kind of tradeoff between error-correcting performance and decoding complexity.更多还原