Acquisition schemes for burst mode direct sequence spread spectrum packet radio
Desmond Wai, Ming Gao Yan · 1994
Code Division Multiple Access (CDMA) is a promising technology for future digital cellular and personal communicatio~~s. Some of the nice features in CDMA include (1) anti-multipath, (2) higher system capacity than TDMA and FDMA, and (3) its flexibility in providing bandwidth on demand. To despread the pseudo noise (PN) code and thus understand the message sent, PN code synchronization is necessary a priori. As a matter of fact, the throughput of a packet mode CDMA system is directly related to the performance of its synchro~lization algorithm. PN code synchronization is usually performed in two stages: acquisition and tracking. The former aims to find a coarse alignment between the incoming signal and the local PN code replica, while the latter fine adjusts the phase. In this thesis, we will consider the PN code acquisition problem. Particularly, we propose two new acquisition algorithms: the On-Off Keying Assisted Acquisition Scheme and the Differentially Encoded Barker Sequence Acquisition Scheme. The preamble in the first method follows an on-off pattern and the algorithm performs a Markov search. The second method, on the other hand, uses DPSK to encode a Barker sequence as the preamble. The algorithm looks for the Barker sequence to declare acquisition. We have also introduced a windowing technique in both methods which we found effective in overcoming the partial correlation effect. The two proposed schexnes are compared with some existing ones in the literature like the coincidence detector or simple Markov search, and we find that our algorithms outperform the others both under AWGN and frequency selective fading channels.