【作者】 曹一卿；

【导师】 杨大成；

【作者基本信息】 北京邮电大学 ， 通信与信息系统， 2011， 博士

【摘要】 通信是对抗错误的艺术,差错控制技术是通信核心技术之一。移动通信系统经历了复杂的时变和频率选择性衰落信道,信道状况非常复杂,需要设计高效、可靠的差错控制方案保证通信的可靠性。移动通信系统的差错控制技术包括两个方面：混合自动请求重传技术(HARQ)和信道编码技术。混合自动请求重传技术利用重发错误信息同时提供功率增益和时间分集增益；信道编码技术利用增加冗余为传输的信息提供检错和纠错能力。在下一代移动通信系统中,如何结合MIMO、OFDM等传输技术和中继网络结构,设计高效、可靠的差错控制技术是一类极具研究价值的课题。HARQ技术因其原理简单,易于实现,在移动通信领域得到了广泛的应用。目前使用较多的是递增冗余传输方式,即发送端并不完全重复前次发送信息,而是采用不完全相同(或者完全不同)的冗余版本。而由于编码本身特点和信道传输状况,不同的比特位对解码的影响不同,因此如何结合编码和信道传输特点,设计高效的传输重发比特选择方案,是HARQ技术设计的关键。在近十年问,低密度奇偶校验码(LDPC)无疑信道编码领域最热的技术,LDPC码具有结构简单,编码性能优秀,解码时延低等特点,被广泛应用在了移动广播和通信领域。LDPC码根据校验矩阵的列重分为规则和非规则两种,其中非规则码性能远胜于规则码。对于非规则LDPC码来说,由于列重(度分布)不同,各个编码后的比特对抗错误的能力不尽相同。总体来说：列重(度分布)较大的保护能力强,对解码贡献也较大；列重小的比特容易发生错误,贡献较小。这种不同的对抗错误能力被称为LDPC码的不等保护特性。利用这种特性,优选的设计系统,可以为传输提供较高的功率增益,即发送功率一定,性能更好。本论文以LDPC码和HARQ技术为代表,研究了差错控制技术在下一代移动通信系统中的应用。基于LDPC码的不等保护特性,本论文研究了MIMO、OFDM和中继网络的差错控制技术：优化设计了LDPC编码的HARQ、MIMO、OFDM和中继网络传输方案,并初步研究了中继网络的HARQ重传中继节点选择方法。以提高传输功率效率为目标,结合MIMO系统带来的空间自由度,OFDM系统子载波的灵活分配,HARQ灵活的重发比特选择性以及中继网络带来的分集传输增益,优化的设计了下一代移动通信系统的传输方案。本文主要工作包括：1.提出了一种基于LDPC码度分布的MIMO传输方案(DDB-MIMO)。在该方案中,具有较大的度的重要比特采用传输分集(TD)模式传输,而具有较小的度的比特采用空间复用(SM)模式进行传输。本方案通过为重要比特提供更多的保护,显著的提高了系统的吞吐量增益。2.提出了一种基于LDPC码度分布的OFDM传输方案(DD&RLDPC-COFDM)。该方案同时考虑了初传和重传的码字与子载波映射关系,结合了LDPC码度分布的不等保护特性和OFDM子载波不同的可靠度。本方案中,初次传输时将度分布大的节点映射到接收信噪比高的子载波,接收失败后,优先重传(前次传输时)接收信噪比较低的子载波承载的信息。由于在初传时为重要信息提供更多保护,在重发时优先重发(前次传输)可靠度较的子载波信息,本方案可以提供显著的性能增益。3.基于中继网络特点,分析了基于成对差错概率(PEP)的帧错误概率(FEP)性能,得出结论：为了同时获得编码中继系统的编码增益和分集增益,广播阶段需要好的信道编码,多址阶段冗余版本设计较为重要。结合LDPC码的特点,本文提出一种中继网络的LDPC编码方法：即首先通过搜索得到一个好码,然后依据度分布将LDPC校验矩阵分为平均度分布不同的几个子码块。在传输阶段分别采用不同的子信道编码码块：在广播阶段源节点优先发送性能较好的高度分布子块,保证中继节点可以成功接收并进一步将信息转发,以确保系统的具有分集增益；在多址阶段源节点和中继节点发送不同于广播阶段的子码块,在目的节点形成互补冗余结构,提供较高的编码增益。4.提出了一种基于中继网络的HARQ重发节点选择方案。本方案研究了在保证目的节点接收性能的前提下,最小转发功率中继节点选择的问题。可用于重发的中继节点根据所需转发功率的不同,被分了若干候选节点区域。通过分析,依据本方案提供方案选择的转发节点既可以提供转发的分集增益,同时还可以提供较高的系统功率效率。更多还原

【Abstract】 Communication is the art of error control, error control technologies is the key of the communication. Enduring time-variant and frequency-selection fading channel, the mobile channel is quite complicated and thus a qualified and reliable error control mechanism is required for communication reliability. Error control technologies in mobile communication system mainly include two different kinds of technologies:hybrid automatic retransmission query (HARQ) and channel coding. HARQ achieves both energy and time-diversity gains by retransmission and channel coding offers error detection and correction abilities by introducing redundancy information.In next generation mobile communication system, it’s a highlighted issue how to combine MIMO, OFDM and relay with error control technologies effectively. HARQ has been widely applied due to its simple but effective principle and easy to be implemented. Among all schemes, incremental redundancy is mostly deployed, in which different coding version is repeated. However different repeat packet selection schemes lead different gain due to different code’s property and channel condition. As a result, how to design an effective repeat packet selection method is the key challanege for HARQ design.In last decady, low density parity check (LDPC) code is the hottest topic in channel coding area. With simple coding structure, significant coding gain and low decoding dealy LDPC has been widely implemented in mobile TV and communication systems. Recognized as different column weight (a.k.a. degree) LDPC is divided into two categories:regular and irregular LDPC code. As the column weight is well optimized and different in a check martrix, irregular LDPC code outperforms regular code. Bits in a code block with different degree have different performance and those with large degree perform better due to larger protection. Known as unequal protection (UEP) of LDPC code bits with larger degrees are often used to protect important information. The system is optimized with UEP property and higher power efficiency would thus be achieved.In this paper the performance of HARQ and LDPC codes is studied. Topics including the error control scheme of MIMO, OFDM and relay system are studied based on the UEP property. Besides, the performance of repeat partner selection scheme in relay network is also investigated. By leveraging the space diversity of MIMO, frequency selection scheduling of OFDM, repeat packet selection of HARQ and time-space diversity of relay, transmission schemes for next generation mobile network are designed for high transmission power efficiency.Work in this paper could be summarized as:1. A LDPC coded MIMO scheme (DDB-MIMO) is proposed. In this scheme, important bits (with large degrees) are mapped to the transmission diversity (TD) symbols for better protection and bits with small degrees are mapped to spatial multiplexing (SM) symbols for higher transmission rate. By offering better protection to important bits the scheme achieves significant gain.2. A LDPC coded OFDM scheme (DD&R LDPC-COFDM) is proposed by taking both first transmission and repeat packet selection method into consideration. By combining LDPC UEP property and different reliability of OFDM subcarriers together the important bits are mapped to reliable subcarriers (with large receiving SNR) and for retransmission the bits on less reliable subcarriers are prior to be repeated.3. The frame error probability (FEP) based on pairwise error probability (PEP) for relay network is analyzed and it is found that:in order to achieve both diversity and coding gain a good channel code is required in broadcast (BC) mode and for MAC mode different redundancy version is important. A LDPC based relay transmission scheme is proposed. In the scheme a good LDPC code is required and then divided into several sub code blocks by mean column weight (MCW). In BC mode the sub block with large MCW is employed and the rest blocks are transmitted in MAC mode. The scheme provides reliable transmission in BC mode and thus the diversity gain is guaranteed. In MAC mode by combining different redundancy version the decoding at destination node is more reliable.4. A HARQ repeat partner selection scheme is proposed. In the partner candidate region where every partner could provide enough power for retransmission the partner with less forwarding power is chosen and thus the higher repeat power efficiency is achieved.更多还原