Delay Estimation Effects on the Coding-Spreading Trade-Off in CDMA Systems

This paper studies the trade-off between coding and direct-sequence spreading in a code-division multiple-access (CDMA) system, in the presence of errors in the delay estimate. One extreme is when all spreading is done by forward error correcting codes , referred to as code-spread CDMA (CS-CDMA) systems. CS-CDMA systems have been shown to achieve high capacity, assuming perfect channel knowledge. However, estimating channel parameters, is harder in a CS-CDMA system than in a conventional direct-sequence CDMA system. Another effect that pushes the trade-off toward increased spreading, is that, after a certain threshold in the code rate, there will be very little further coding gain by moving to an even lower rate. The optimum trade-off for several sets of system parameters are found by simulations.