n this thesis we investigate the effect of Carrier Frequency Offset (CFO) on the performance of downlink Variable Spreading Factor (VSF) OFCDM systems when subcarrier grouping is used. An analytic expression of the SINR is derived for downlink VSF-OFCDM with CFO for the case of maximal ratio combining receiver. Numerical results show that, when the total spreading factor is fixed, the VSF-OFCDM system with higher frequency domain spreading factor is more sensitive to CFO than that with lower frequency domain spreading factor. Due to the adverse impact of the CFO on VSF-OFCDM systems, we propose a correction scheme based on the maximum likelihood principle.We derive the likelihood function for VSF-OFCDM system with CFO and use a gradient algorithm to estimate and minimize the effect of CFO in a tracking mode. Our results show that the BER performance in the low SNR environment can be improved significantly with few number of iterations for different spreading factors. We also propose a threshold-based group-adaptive modulation algorithm used with an adaptive subcarrier allocation technique for downlink VSF-OFCDM to increase the spectral efficiency for a given target BER. The proposed algorithm provides an increase in spectral efficiency without increasing the total transmit power for different spreading factors with and without coding.