Effect of Carrier Frequency Offset on the BER Performance of Variable Spreading Factor OFCDM Systems

In this paper, the effect of frequency offset on the performance of downlink OFCDM systems with variable spreading factors is investigated. The bit error rate of downlink VSF-OFCDM is analyzed taking into account the effect of carrier frequency offset when subcarrier grouping is used. An analytic expression of the SINR for downlink OFCDM with frequency offset using BPSK modulation is derived for the case of maximal ratio combining receiver. Numerical results show that, when the total spreading factor is fixed to 32, the VSF-OFCDM system with higher frequency domain spreading factor is more sensitive to frequency offset than that with lower frequency domain spreading factor. Our results also show that, as the number of users increases, the degradation of the BER performance is more pronounced with the higher frequency domain spreading factor since more subcarriers are present in each group.