论文标题:SCDMA系统G.729声码器的实现与保护
Implementation and Protection of G.729 Vocoder in SCDMA System
论文作者 李天新
论文导师 张翠芳,论文学位 硕士,论文专业 通信与信息系统
论文单位 西南交通大学,点击次数 126,论文页数 73页File Size2983k
2003-06-01论文网 http://www.lw23.com/lunwen_68134792/ SCDMA;G．729声码器;C54x;RS码;PESQ
SCDMA,G.729 Vocoder,C54x,Reed-Solomon coding,PESQ
SCDMA(同步码分多址)系统是使用智能天线、同步码分多址和SWAP~(TM)(同步无线接入信令规范)信令、用软件无线电实现的无线通信系统。受多种因素影响，SCDMA系统的语音质量一直不理想，有时表现还很严重，这大大阻碍了该系统的商用进度，也有悖于SCDMA系统为用户提供高品质服务的初衷。 本文从更新声码器及增强其抗噪声能力入手解决语音质量问题，经过分析比较，选择了ITU-T G.729的简化算法G.729A实现声码器，以代替目前性能不稳定的基于QCELP算法的声码器。论文的主要任务是在TMS320C54x上实时实现G.729A算法，并设计和实现有效保护该声码器的方案以增强其噪声鲁棒性。 ITU-T提供了可供16位定点DSP实现G.729A算法的C源码和标准测试向量，由于运算量很大，直接编译该源码不能满足在C54x实时实现的要求。本文从改写C源码为C54x汇编代码着手，对实现过程中的变量存储区分配、参数的传递、寻址方式的选择、循环的嵌套与控制、辅助寄存器的使用、流水线等方面的问题进行了设计和优化，在C54x上实时实现了G.729A声码器。第三章末给出了对汇编实现代码的性能测试。与改写前相比，编码器各模块的代码效率平均提高了七十多倍。 增强G.729声码器在无线信道中的噪声鲁棒性对提高SCDMA系统语音质量有着重要的现实意义。在PESQ客观语音质量评价工具分析G.729码元主观敏感度的基础上，提出了如何利用SCDMA系统1.6kbps剩余业务信道带宽对主观最敏感比特进行保护的问题。结合G.729自身保护机制、缩短RS编码和SCDMA信道的特点，提出了基于RS(7，5)码带CRC检错及帧内交织的保护方案。并在第四章末采用PESQ算法验证了该保护方案的有效性。
Synchronous CDMA ( SCDMA ) system is a wireless communication system using smart antenna, synchronous CDMA, SWAP? ( Synchronous wireless access protocol) and it is implemented on software radio architecture. Due to many practical problems, the voice quality of the system is not so ideal as what it had been thought to be, it works unsteadily usually when channel error occurs, which absolutely hindered the marketing process of the system.To improve the voice quality, this thesis start with replacing the present QCELP vocoder with a new one. After analysis and comparison among candidate vocoders that can be used in SCDMA, ITU-T G729A, which is the simplified version of G.729 algorithm, proved itself the optimum one. Therefore, the main purpose of this thesis is to implement G729A algorithm on TMS320C54x in real time and design an effective protect scheme to enhance its error robustness.ITU-T provided a C source code for G729A implementation on DSPs of 16-bit fixed point. However, owing to the heavy load of computation, such C code can not work in real time on C54x after compilation. Chapter 3 focuses on translating the C code into assembly code. The implementation and optimization involves issues of assignment of variables and memories, pass of function parameters, addressing modes, control of nested repetition, use of AR registers, pipeline conflicts, and so on. A performance test for the assembly code is given at the end of chapter 3 and the result shows that the assembly code is tens of times efficient in comparison with the original C code.It is highly important to enhance the error robustness of G.729 vocoder. On the subjective sensitivity analysis of G.729 vocoder bits using PESQ algorithm, the problem is concentrated on how to protect those subjectively important bits by using the additional 1.6kbps channel bandwidth in SCDMA. Referring to the self-guard mechanism of G.729, characteristics of RS coding and SCDMA channel, a scheme using RS(7,5) coding with CRC and block interleaving is put forward and its effectiveness is verified by both PESQ algorithm and subjective test at the end of chapter 4.
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