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

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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).