Impact of frequency offset on the spreading and despreading in broadband MC-CDMA systems [mobile radio]

This paper investigates the impact of frequency offset on the performance of multicarrier CDMA (MC-CDMA) transmission schemes using a variable spreading factor (SF). Different from previous research, which only paid attention to the orthogonality of subcarriers, we also take the influence of the spreading and despreading process into consideration. Due to the frequency offset, the auto-correlation and cross-correlation of the specific Walsh spreading sequences cannot be kept ideal any longer. The inter-carrier interference (ICI), which is decided by the former, is the main source of interference for the case of a single user, while the multiple access interference (MAI), corresponding to the latter, is the source for the case of multiple users. Based on a combination of theoretical analysis and computer simulation, it is observed that the despreading performance presents great sensitivity to the frequency offset. Moreover, it assumes a large divergence for different spreading sequence numbers. The user-specific spreading sequence, with user number close to SF/2, is the most robust at combating the frequency offset.