A new spreading scheme for convolutionally coded-CDMA communication in Rayleigh fading channel

We find that the asymptotic bit error probability, P_b, of the convolutionally coded code division multiple access (CDMA) scheme on a Rayleigh fading channel depends on the length of the shortest error event path and the product of symbol distances along that path. Based on this observation, we propose a new spreading scheme for convolutionally coded CDMA system that maximizes the length of the shortest error event path. For a 4-state code, a reduction of 2.0 dB in E_b/N₀ can be attained at P_b=10^-5 over the conventional convolutionally coded CDMA scheme, and even a higher gain in E_b/N₀ can be achieved as the required bit error rate is decreased and/or the number of states is increased