Multiuser chip timing recovery in asynchronous CDMA systems using a transpose domain filter

Asynchronous CDMA systems require that the base station synchronizes with each user's length N spreading code to despread and recover the modulated data. This problem is usually solved by a brute-force method of correlating the incoming signal with the spreading code. In this paper we present a method of obtaining chip-level synchronization for all K users of the system simultaneously. The approach derives from the transpose domain filter (TDF) method, which has been used to obtain bit timing of synchronous CDMA users. We propose a modified TDF to obtain chip timing for asynchronous users, whose solution is implemented efficiently using a parallel bank of multistage Wiener filters (MWFs) for all K users simultaneously. We demonstrate the effectiveness of the technique in giving near 100% success in timing estimation for heavily loaded systems when E_b/N₀ ≥ 0 dB and enough bits, M, e.g. M ≥ 16N, is used to obtain the estimate.