OCDMA/OCDMA Overloading Scheme for Cellular DS-CDMA Using Orthogonal Gold Codes and Complex Scrambling

Abstract

Overloading is a method to extend capacity limitation of multiple access techniques. The system becomes overloaded, when the number of users exceeds the signal dimensions. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O). In this paper, the BER performance of a new overloading scheme using scrambled orthogonal Gold code (OG/OG) sets is evaluated with soft decision interference cancellation (SDIC) receiver. When complex scrambling is not used, it is shown that OG/OG 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 1e-5. We have evaluated the overloading performance, when two set are scrambled with set specific deterministic or random complex scrambling sequence. It is shown that the amount of overloading increases significantly from 25% to 63% (40 extra users) by using random complex scrambling for N=64. For deterministic (periodic) scrambling, the overloading percentage increases considerably to 78. On a Rayleigh fading channel, an overloading of 40% is obtained without scrambling at a BER of 5e-4 with near single user performance. With complex scrambling overloading % increases considerably to 100%.

Performance Analysis of Uplink Cognitive Radio Transmission based on Overloaded MC-DS-CDMA

“…A variety of overloading schemes have been proposed [16][17][18][19][20][21]. In one study [16], two sets of orthogonal codes were generated using Walsh-Hadamard (WH) codes and the codes were orthogonal within the sets.…”

“…An overloading scheme using a set of orthogonal Gold codes was proposed [17]. The same authors evaluated the BER performance using orthogonal Gold code (OG/OG) sets with an iterative multistage detector (IMSD) based on soft decision interference cancellation (SDIC) [18]. An assortment of receiver structures for overloaded CDMA systems were also proposed [19][20][21].…”