Return link code acquisition for DS-CDMA for high capacity multiuser systems

Acquisition of code timing in a direct-sequence code-division multiple-access system must take place before signal detection and decoding is possible. Code acquisition under severe multiple access interference conditions with time varying codes makes the task even more difficult. Inefficient designs cause large numbers of false alarms and/or missed detections. The paper details a powerful code acquisition technique for the uplink of DS-CDMA systems under high load situations, where the number of users is equal to the spreading factor. Under high multiple access interference conditions, the DS-CDMA acquisition problem becomes very difficult and conventional search methods simply fail. The method discussed utilises soft data from the multiuser detector to reduce the interference received by the acquisition unit. Analytical performance in terms of the number of users, processing gain, cancellation factor and noise variance is developed; this is compared to simulation results.