Research on Digitalized Spread Spectrum Communication System Using Real-Valued Sequences

本项目于1998年至1999年进行，目的是为准同步CDMA系统开发无信道间干扰、码间干扰小的实值序列，并为该CDMA系统实现数字化调制解调器的试验，为现有的CDMA系统带来一种新技术。这项研究的目的快要达到了。由于构造的实值序列适用于快速信号处理算法，因此在FPGA上实现了数字快速代码生成和相关处理器。在多址实验中，相关处理的性能基本符合设计值。本研究开发的实值有限长序列可以有效地设计出不受保护间隔干扰的高效数据传输编码，有望开发到未来的具体应用系统中。本项目的成果如下：(1) CDMA序列与快速相关算法的研究进展从元素序列的卷积出发，推导出具有尖锐自相关函数的实值有限长序列的正交集，从而得到快速相关算法。对长度为33的实值位移正交有限长序列的近似整数序列，实现了基于ROM的代码生成器和对应10000个逻辑门的FPGA相关器。8位发生器输出和8位相关器输入的组合工作速率为16Mchips/s，相关脉冲误差为1%。(3) CDMA传输实验基于两个实值有限长序列的光与空间复用传输实验表明，将相关器输入0dB的D/U比提高到输出40dB的D/U比，在允许误差范围内满足正交性。

This project has been performed from 1998 to 1999, for the purpose of developing real-valued sequences for a quasi-synchronous CDMA system with no interchannel interference and with little intersymbol interference and implementing trial degitalized modems for this CDMA system, to bring a novel technique to the current CDMA systems. The aim of this research is nearly attained. Digital fast processors of code generation and correlation are implemented on FPGA, because of construction of the real-valued sequences applicable to fast signal processing algorithm. Performances of correlation processing in the multiple access experiment almost follow those of designed values. The real-valued finite-length sequences developed in this research are effective to design the codes of high efficiency data transmission free from the guard interval to avoid interferences, and so expected to be developed to the future concrete application system.The achievements of this project are the followings.(1) Developments of the sequences for CDMA and the algorithm of fast correlationOrthogonal sets of real-valued finite-length sequences with sharp autocorrelation function are derived from the convolution of element sequences that leads to fast correlation algorithm.(2) Trial production of code generator and correlatorThe code generator using ROM and correlator using FPGA corresponding to 10,000 logic gates are implemented for the aproximated integer sequence from the real-valued shift-orthogonal finite-length sequence of length 33.The combining operation of 8-bit generator output and 8-bit correlator input presents 16Mchips/s rate and correlation pulse of 1% error.(3) Experiment of CDMA transmissionThe experiment of optical and spatial multiplexing transmission based on two real-valued finite-length sequences shows that 0dB D/U ratio at the correlator input is improved to 40dB D/U ratio at its output, satisfying the orthogonality within allowable error.