基于IR_HARQ系统的LDPC码改进译码方式研究

【作者】 杨舒雅；

【导师】 周亮；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2008， 硕士

【摘要】 LDPC码的IR_HARQ方式,通过在重传时增加校验比特或对信息比特打孔两种方式降低码率,从而获得纠错能力更强的码。由于重传过程只需传输部分校验比特,相比其他HARQ方式,LDPC码的IR_HARQ方式能够得到更好的吞吐量性能,但它同时要求LDPC码具有码率兼容的特性。目前的研究主要集中在如何构造LDPC码方面,译码解决方案成为被忽视的问题。本文提出了一种基于IR_HARQ系统的LDPC码的改进译码方法。3σ原理指出,在n比特的分组中出现错误比特的个数大于3σ错误区间t 3σ的概率( )P t≥t 3σ1,因此,根据译码前后的误比特率能够计算出译码前后的错误比特数。结果表明,一次译码之后,一帧中绝大部分的错误比特能够被正确译码。如何在重传译码过程中利用已纠正比特所提供的有效信息成为提高重传译码性能的关键所在。本文对改进后译码方法的具体实现进行了研究。改进译码方法由信道的信噪比特性确定上次译码过程所能提供的信息量,从而确定加权系数以实现对上次译码结果的有效利用。由于绝大多数错误比特在一次译码过程中都可以被纠正,在重传过程中使用新的初始软信息相当于对信息传输过程中的失真进行了一定程度的弥补。同时,LDPC码采用软信息迭代译码,信道的信噪比特性是译码时必须的参数,而加权系数与上次译码结果的乘法运算和迭代译码的计算量相比微不足道,因此,改进译码方法在提高重传译码性能、减少迭代次数的同时,并不会增加译码器额外的复杂度。本文选用802.16标准中的(2304, 1920)LDPC码和(2304, 1728)LDPC码作为纠错码,在AWGN信道下,采用BPSK调制和停等协议,对新旧译码方法的吞吐量和迭代次数进行了比较。仿真结果表明,在信噪比相对较低的范围内,新的译码方法可以使吞吐量提高2%～20%,新方法显示了在低信噪比下的性能优越性;而在信噪比相对较高的范围内,新译码方法对吞吐量的提高并不明显,但迭代次数减少了5%～20%。更多还原

【Abstract】 A special type of LDPC codes, named structured Rate Compatible LDPC (RC_LDPC) code with flexible code rate and code sizes is proposed. Based on the essential feature of this kind of LDPC code, its Incremental Redundancy Hybrid ARQ is also presented. It reduces code rate by puncturing message bits or extending check bits. As only a part of check bits are retransmitted during retransmission, this scheme could provide better throughput performance than that of other HARQ schemes. However, this scheme requires that the FEC is Rate Compatible codes. The focus of people’s research has been on how to build the RC codes, and the decoding scheme is the area to be ignored.This paper proposes an improved decoding method based on IR_HARQ scheme that uses rate-compatible LDPC codes as the FEC codes.3σprinciple indicates that if a variable X is normally distributed, 99.7% of the observations fall within 3 standard deviations of the mean, that is, betweenμ? 3σandμ+ 3σ. So it is possible to get the numbers of error bits both before and after the decoding process with the bit-error-rate (BER) correspondingly. And the computing result shows that there are always parts of bits has been decoded correctly during a decoding process despite of whether the decoding is successful. How to make good use of the former decoding results is important to enhance the performance of retransmission decoding.This paper proposes implementation of this improved decoding method. Instead of the initial reliabilities of the first decoding process, the final reliabilities from the previous decoding process could be selected as the initial reliabilities of the previously received bits during the latest decoding process. And the new decoding method calculates the privilege factor by the properties of the channel. Then the former decoding result could be used effectively. As most of error bits have been corrected during last decoding process, the new decoding method gives compensation to the distortion which the channel brings to. Moreover, the proposed scheme will not increase the complexity of the decoder.Choosing both the (2304, 1920) and (2304, 1728) LDPC codes defined in the 802.16 standard as the FEC codes, this paper simulates the performance of new and traditional decoding methods under AWGN channel with BPSK modulation and stop-and-wait scheme. And the results show that nearly 2%~20% enhance of throughput efficiency could be got while SNR is comparatively lower with the new decoding method. And during the area where SNR is comparatively higher, the enhance of throughput efficiency is not apparent, however, the decrease of iterative times reaches to 5%~20%, and that means the same trend of system decoding time-delay.更多还原