Soft output detector for parity bit selected CDMA system

Abstract

This paper investigates the performance of the Parity Bit selected Code Division Multiple Access (PB-CDMA) system when a soft output detector based on Log Likelihood Ratio (LLR) calculation is employed. In this system, information bits are convolutionally encoded prior to being used in the parity bit selected Direct Sequence Spread Spectrum (DS-SS) system. Reliability of the detected bits in the receiver is calculated in the form of LLRs and then used as the input of a soft input Viterbi decoder. The performance of the proposed system is compared to conventional coded spread spectrum systems where no parity bit selected spreading sequence is used. Numerical results show that the proposed system can be used to improve the capacity of a Code Division Multiple Access (CDMA) system in terms of the number of simultaneously transmitting users.Index Terms-Spread spectrum communication, CDMA, error control coding, log likelihood ratio.