Delay tracking for direct sequence spread spectrum systems in multipath fading channels

This paper considers the problem of multipath delay tracking in direct sequence spread spectrum systems operating in multipath fading channels. Due to the pulse shaping of the PN code chips, tracking multipath delays is a challenging problem, for which we introduce three novel techniques. In the first technique, maximum likelihood estimation, we search all possible combinations of delays and select the set of delays that minimize a metric derived from the error between the received signal and an estimated signal based on these postulated delays. To reduce complexity, we introduce the ordered maximum likelihood technique, in which the above mentioned metric is minimized iteratively assuming that the channel has one path, then two paths, ... etc. At each iteration, the delay estimates derived from previous iterations are fixed. Therefore, in each iteration only one delay estimate is produced. Another technique presented in this paper is envelope tracking with subtraction. In this technique, we select peaks of the correlation function between the received direct sequence spread spectrum signal and the local replica. After selecting each peak, the contribution due to the corresponding channel path is subtracted from the correlation function.