Performance Effects of the Uplink Asynchronism in a Spread Spectrum Multi-Carrier Multiple Access System

In this paper we investigate the effects of the asynchronism of user signals in the uplink of a spread spectrum multi-carrier multiple access (SS-MC-MA) system. Different propagation delays due to different distances between the mobile transmitters and their base station cause asynchronous arrivals of the uplink signals in addition to multipath propagation. The proposed uplink SS-MC-MA scheme uses for synchronization only the frame structure received on the synchronous downlink and requires no additional synchronization measures. A guard interval that is smaller than the sum of the maximum time offset between the users and the maximum excess delay of the frequency selective multipath channel is used. It minimizes the loss in bandwidth efficiency by allowing residual interference. The residual interference is minimized by proper positioning of the detection interval in the receiver. It is shown that with this approach the guard interval can be reduced by about 50%. The performance of the proposed uplink scheme is compared to an uplink SS-MC-MA scheme with perfect synchronization.