Uplink and Downlink MC-DS-CDMA Synchronization Sensitivity

Abstract

In this paper, we consider the effect of synchronization errors on the performance of the multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) system and compare the results for downlink and uplink transmission. To evaluate the effect of small synchronization errors on the BER performance of the MC-DS-CDMA system, we derive simple analytical expressions for the BER degradation that are based upon truncated Taylor series expansions. We point out that a constant carrier phase offset or a constant timing offset do not give rise to performance degradation, for neither uplink nor downlink MC-DS-CDMA. The MC-DS-CDMA system is strongly degraded in the presence of a camer frequency offset or a clock frequency offset. This degradation is proportional to the squares of the frequency offset and the number of carriers. Further, the degradation in the uplink is a factor N s 2 (N s is the spreading factor) larger than in the downlink, because the former suffers from a higher level of multi-user interference. The degradation caused by carrier phase jitter or timing jitter is the same in the uplink and the downlink, when the spectrum of the jitter is the same for all users. Further, the degradation is independent of the spectral contents of the jitter, the spreading factor and the number of carriers, but only depends on the jitter variance.