Trellis-based power control for direct sequence CDMA in a Rayleigh fading environment

A mobile user´s signal strength fluctuates significantly and rapidly due to Rayleigh fading. Unless the time gap in adjusting the transmitting power to an ideal level is eliminated, either the signal-to-interference ratio is too low to meet the minimum requirement, or excessive interference to other users occurs. In the paper, the time gap is reduced by devising a trellis-based prediction method that adjusts the transmitting power strength based on the similarity of the power control states, such that the signal strengths of all mobile users at the base station are nearly identical. In the performance evaluation, the power control events are modelled by a sequence of Os and Is, which can be either periodic or aperiodic. The number of users that can be supported in the system using the proposed power control method is computed for both the average case and the worst case. The analytical results, verified by simulation runs, show that the drop in capacity is small compared to that of a transmission environment with ideal power control, and that the proposed scheme increases capacity over that of the fixed step method, by as much as 50%