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作为一种接近香农极限的信道编码,近年来LDPC码被广泛采用,例如欧洲第二代数字视频广播卫星标准DVB-S2和空间数据咨询委员会(CCSDS)遥测信道编码标准就分别采用了LDPC码。文章将讨论LDPC码在卫星导航系统中的应用,内容涵盖星上编码与接收机解码。GPS联合项目办公室在接口规范(Draft IS-GPS-800,Navstar GPS Space Segment/User Segment L1C Interfaces,19 April 2006)中展现了L1电文结构中的LDPC码。子帧2和子帧3将分别用一个独立的24bit循环冗余检验(CRC)算法。加入CRC比特之后每一子帧将进一步被长度为1200/548码率为1/2的LDPC码编码。文章将给出同等长度同等码率的不同类型的LDPC码。它们表现出了优异的译码性能、简单的编码结构和较低的译码复杂度。另外一个优点是其校验矩阵无需用一系列的冗余表格来表示。这种LDPC码的构造方法可以被扩展到其它卫星导航系统,比如欧洲的伽俐略(Galileo)系统和中国的北斗(Compass)系统。
As a near-Shannon limit channel coding, LDPC codes have been widely adopted in recent years. For example, DVB-S2 and CCSDS telemetry channel coding standards of the European second generation digital video broadcasting satellite are respectively adopted LDPC codes. This article will discuss the application of LDPC codes in satellite navigation systems, covering both on-chip coding and receiver decoding. The GPS Joint Project Office presents the LDPC code in the L1 message structure in the interface specification (Draft IS-GPS-800, Navstar GPS Space Segment / User Segment L1C Interfaces, 19 April 2006). Subframes 2 and 3 will each use an independent 24bit Cyclic Redundancy Check (CRC) algorithm. After adding the CRC bits, each subframe will be further encoded by an LDPC code with a length of 1200/548 code rate of 1/2. The article will give different types of LDPC codes with the same code rate of the same length. They show excellent decoding performance, simple coding structure and low decoding complexity. Another advantage is that its check matrix does not need to be represented by a series of redundant tables. The construction of this LDPC code can be extended to other satellite navigation systems, such as the Galileo system in Europe and the Compass system in China.