Performance evaluation of a new overloading scheme for cellular DS-CDMA with iterative multistage interference cancellation receiver

Overloading is a technique to accommodate more number of users than the spreading factor N. This is a bandwidth efficient way 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 IMSD schemes. An effective interference cancellation technique for soft decision IMSD is proposed, to increase the amount of overloading. It is observed that this scheme provides 25% channel overloading for synchronous DS-CDMA system in an AWGN channel, with an SNR degradation of about 0.35 dB as compared to single user bound at a BER of 10^-5.