Multipath tracking and combining in direct-sequence spread-spectrum packet-radio communications

Multipath fading in wide-area radio communications motivates the use of RAKE reception in a packet-radio system that employs direct-sequence (DS) spread-spectrum modulation. The acquisition header in each packet is used to derive initial tap delays for multipath combining in the RAKE receiver. However, fast fading can result in differences in the optimal tap delays during the reception of different portions of a given packet. The receiver obtains best performance if it tracks changes in the channel response during reception of a packet and makes corresponding changes to the tap delays, but the improved performance is achieved only at the expense of the additional signal energy and complexity required to implement the tracking. Two strategies for tracking and combining multipath signal components are examined for a DS packet-radio system. Analytical and simulation results are presented for the probability of error for each approach under consideration. In particular, the performance of a simple scheme using fixed tap delays for the duration of the packet is compared with the performance of more complex multipath tracking receivers. The effects of the channel characteristics, the mobile velocity, and the number of taps is considered in the comparison