Return-Link Code Acquisition for 1-D and 2-D With DS-CDMA for High-Capacity Multiuser Systems

Acquisition of the code timing in a direct-sequence code-division multiple-access (DS-CDMA) system at the base station must take place before signal detection and decoding is possible. Code acquisition under severe multiple-access-interference (MAI) conditions with time-varying codes makes the task even more difficult. Inefficient designs lead to a large number of false alarms and/or missed detections. This requirement is needed for conventional single-antenna (1-D) designs, as well as for multielement antenna (2-D) designs. This paper details a powerful code-acquisition technique for the uplink of DS-CDMA systems under high-loaded situations for both 1-D and 2-D schemes, where the number of users is greater than the processing gain. Under this high-MAI condition, the DS-CDMA acquisition problem becomes very difficult, and conventional search methods simply fail. The method discussed utilizes soft data from a multiuser detector to reduce the interference received by the acquisition unit. Analytical performance is compared to simulation results in terms of the number of users, processing gain, interferer-signal power, cancellation factor, antenna configuration, and noise variance. Numerical results validate performance under realistic conditions with amplitude, phase, and frequency impairments.