BER evaluation of overloaded DS-CDMA system with timing synchronization error

Cell overloading is a bandwidth efficient scheme to increase the number of users in a fixed bandwidth for mobile applications. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O) [2]. An iterative soft decision interference cancellation receiver (SDIC) scheme is used to cancel interference between the two sets of users. In this paper, the BER performance of a new overloading scheme using orthogonal Gold code (OG/OG) sets is evaluated with SDIC receiver. This scheme provides about 25% channel overloading for DS-CDMA 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 0.02Tc for a BER of 10^-5 . On a Rayleigh flat fading channel, SNR degradation is about 1 dB at a BER of 5.10^-4 with a timing error of 0.01Tc at an overloading of 25%. Overloading increases to 40% without any timing error.