BER Analysis of Hard Decision Interference Cancellation Receiver for Overloaded DS-CDMA System

Overloaded Code Division Multiple Access (CDMA) system accommodates more number of users than the spreading factor, N. One of the efficient scheme 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 technique is used to cancel interference between the two sets of users. In this paper, we have presented the analysis of HDIC receiver for O/O overloaded DS-CDMA system. A closed form expression for the average Bit Error Rate (BER) is obtained, for each stage of HDIC receiver, without considering the correlation between the multiple access interference and Gaussian noise. The analytical results as obtained from the closed form expression are compared with simulation results for quasi orthogonal sequence (QOS) O/O overloading scheme proposed for cdma2000. It is observed that at lower overloadings, analytical result matches closely to that obtained from simulation. But there is a difference between two at higher overloadings. The reason for such a difference is the inaccuracy in variance calculation due to the invalid assumption that the interference from different users and Gaussian noise are mutually uncorrelated. By taking the correlation into account and employing Price's theorem, an improved BER expression is obtained.