BER Performance Evaluation of Overloaded DS-CDMA System with Timing Synchronization Error

Overloading is a technique to accommodate more number of users than the spreading factor N. This is a bandwidth efficient scheme to increase the number users in a fixed bandwidth. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O). The first set is assigned to the N users and the second set is assigned to the additional users. An iterative multistage detection (IMSD) technique is used to cancel interference between the two sets of users. The interference cancellation receiver uses hard decisions (HDIC) or soft decisions (SDIC) to estimate the interference. In this paper, the BER performance of a new overloading scheme using orthogonal Gold code (OG/OG) sets is evaluated with SDIC receiver. It is observed that this scheme provides 25% channel overloading for synchronous DS-CDMA system in an AWGN channel, assuming perfect timing synchronization in the uplink. The SNR degradation is less than 0.35 dB as compared to single user bound at a BER of 10^-5 for N = 64. But in practical systems, perfect chip timing is very difficult to maintain. We have shown that the overloading performance reduces to 11% for a timing synchronization error of 0.02 Tc for a BER of 10^-5 and N = 64.