BER of Overloaded Cellular DS-CDMA System with Imperfect Power Control

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 or data rate in a fixed bandwidth. These schemes can be used to avoid the orthogonal code shortage problem to provide broadband wireless access in cellular communication. 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. In this paper, the BER performance of a new overloading scheme using orthogonal Gold code (OG/OG) sets is evaluated with soft decision interference cancellation scheme (SDIC). It is observed that this scheme provides 25% channel overloading for synchronous DS-CDMA system in an AWGN channel for spreading factor length N = 64, assuming perfect power control. The SNR degradation is about 0.35 dB as compared to single user bound at a BER of 1e-5. With imperfect power control, the BER performance degrades considerably and we can only overload up to 11% with an SNR degradation of about 1.25 dB as compared to perfect power control single user bound at a BER of 1e-5.