A Study on Synchronous Acquisition Methods with Code-Orthogonalizing Filters in Asynchronous Direct Sequence/Code Division Multiple Access (DS/CDMA) Communication Systems

近年来，同步直接序列/码分多址(DS/CDMA)通信系统作为一种数字通信系统因其具有高容量、抗干扰能力强等优点而被研究。然而，在大的多址干扰(MAI)环境下，由于互相关函数的大峰值周期性出现，滑动相关器很难实现序列捕获。针对这一问题，提出了一种基于码正交滤波(COF)的捕获方法(直接检测法)，该方法可以在不知道其他用户的伪随机序列和信号时延的情况下降低多址干扰，但平均捕获时间很长，本文提出了一种改进的COF捕获方法，该方法利用部分相关的正交化和滑动自身序列的预估计。首先，给出了基于COF的两种捕获方法(直接检测法和预估法)的模型和原理，并给出了捕获时间的减少效果。其次，提出了一种适用于多径衰落信道的复数LMS算法。其次，在直接检测法中，为了缩短训练时间，对LMS算法的步长参数进行了检验。最后，提出了一种考虑虚警惩罚的多驻留检测COF捕获方法。因此，例如，在多径衰落信道中，当码长为127、Eb/No=24.0[db]、用户数为24、信干比(SIR)为-6.0[db]时，直接检测方法的平均捕获时间为1.32×10^6T(T为数据持续时间)，而对于直接检测方法和使用修正步长参数的预估方法的组合，其平均捕获时间可以提高到7.94×10^4T，而对于使用多驻留检测(即，约为以前方法的1/20)。

Recently, an synchronous direct sequence / code division multiple access (DS/CDMA) communication system has been studied as a digital communication system because of its high capacity, robustness against interference, and so on. However, in large multiple access interference (MAI) environment, it is difficult to achieve the sequence acquisition by the sliding correlator, because large peaks of cross-correlation functions appear periodically. In order to overcome this problem, a novel acquisition method (direct detection method) with code-orthogonalizing filter (COF), which can reduce MAI without the knowledge of other user's PN sequence and signal delay, have been proposed previously, however, the average acquisition time becomes very long.This paper proposes a modified acquisition method with COF (pre-estimation method), which uses the orthogonalization of partial correlations and pre-estimation by sliding the own sequence. First, a model and principle of the two acquisition methods using COF (direct detection method and pre-estimation method) are described and the reduction effect of the acquisition time is shown. Second, a complex style LMS algorithm over multipath fading channels is presented. Next, in the direct detection method, the step size parameter of LMS algorithm is examined to shorten training time. Finally, an acquisition method with COF by multiple-dwell detection considering false alarm penalty is proposed. As a result, for example, over multipath fading channels, when code length is 127, Eb/No=24.0[dB], the number of users is 24, and signal to interference ratio (SIR) is -6.0[dB], the average acquisition time 1.32×10^6T (T is data duration) is necessary for the direct detection method, however, it can be improved to 7.94 × 10^4T for the combination of direct detection method and pre-estimation method using the modified step size parameter, further, 5.96 × 10^4T for the method using multiple-dwell delection (i.e., about 1/20 of the previous method).