A New Overloading Scheme for Cellular DS-CDMA using Orthogonal Gold Codes

Overloading is a scheme to accommodate more users than the spreading factor N. This is a bandwidth efficient scheme to increase the number of 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% (16 extra users) 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. We have also evaluated the overloading performance, when the two set are scrambled with set specific complex scrambling sequence. It is shown that the amount of overloading increases significantly from 25% to 63% (40 extra users) by using complex scrambling for N=64.