Performance of multirate techniques in W-CDMA

We analyze and compare the error performances of two multirate transmission schemes in W-CDMA: multicode (MCD) and variable spreading gain (VSG). First, we show that in an AWGN channel, the VSG data user performs better than the MCD user if the number of in-cell voice users are low and the background noise is weak. However, as the noise power or the number of voice subscribers increases, the performances of the two schemes become similar. We then show that in a multipath fading channel, the two schemes achieve very similar error performances as long as their receivers are able to track all resolvable signal paths. However, when the receiver can only track multipath components within the current symbol interval, the VSG user yields a much worse error probability than the MCD user. This is because the VSG data user has a much shorter symbol interval. Simulation results are illustrated to support the analysis