Code Detection and Acquisition Techniques in the DS/CDMA Communications

Abstract

In the direct sequence- code division multiple access (DS/CDMA) communications, one of the primary functions of the receiver is to despread the received PN code. This is accomplished by generating a local replica of the PN code in the receiver and then synchronizing it to the one superimposed on the received waveform. In general, this synchronization process is accomplished in two steps are; code acquisition, which is a coarse alignment process bringing the two PN sequences within one chip interval, and code tracking, which is a fine tuning and synchronization-maintaining process. However, in many situations, such as communications intelligence, the PN code utilized to spread the received signal is unknown to the intercepting receiver. In this paper, we focus on the fundamental concepts of both code estimation and code acquisition techniques. An algorithm based on the higher order statistics (HOS) for code estimation is provided. This HOS algorithm is complex and requires high signal to noise ratio, SNR for operating correctly. For that reason a much simpler algorithm based on the structure of the PN code itself and short observation of the received signal is proposed. The proposed algorithm, based on short observation, performs faster and better than the HOS algorithm, especially in dense environments. Finally we compare the serial search, the parallel search and the Z- search methods, in terms of the average delay time to acquire code acquisition for different spreading code lengths.