Extrinsic information transfer analysis of finite field spreading

Finite field spreading scheme is proposed for a synchronous multiple-access channel with Gaussian noise and equal-power users. For each user, s information bits are spread jointly into a length-sL vector by L multiplications on GF(2^s). Thus, each information bit is dispersed into sL transmitted symbols, and the finite field despreading (FF-DES) of each bit can take advantage of sL independent receiving observations. To show the performance gain of joint spreading during iterative decoding, an extrinsic information transfer (EXIT) function analysis of the FF-DES is given. It shows that the asymptotic slope of this EXIT function increases as s increases. Simulations show that a larger absolute slope of the bit error rate (BER) curve is achieved by joint spreading during the iterative decoding. For s ≥ 2, the BER curve of the finite field spreading has a larger absolute slope than that of the single-user transmission with BPSK modulation.