A comparison between uplink and downlink MC-DS-CDMA sensitivity to static timing and clock frequency offsets

We study the effect of fixed timing offsets and clock frequency offsets on the performance of multicarrier direct-sequence CDMA (MC-DS-CDMA) for both uplink and downlink communication, assuming orthogonal spreading sequences and a slowly varying multipath channel. We point out that a constant timing offset does not give rise to performance degradation for neither uplink nor downlink MC-DS-CDMA. We derive simple but accurate approximate expressions that allow us to easily quantify the effect of clock frequency offset and the influence of the different system parameters on the receiver performance in practical situations. Further, we show that for both uplink and downlink MC-DS-CDMA, the performance in the presence of a clock frequency offset rapidly degrades with an increasing number of carriers. It turns out that this degradation is larger in the uplink than in the downlink because the former suffers from a higher level of multiuser interference. For a given maximum relative clock frequency offset, enlarging the spreading factor in a fully loaded system does not affect the downlink degradation but strongly increases the uplink degradation.