Effect of fast fading on hybrid detection in VSF-OFCDM systems

This paper presents an analytical study on the variable spreading factor (VSF) orthogonal frequency and code division multiplexing (OFCDM) system over a fast fading channel. Hybrid multicode interference (MCI) cancellation and minimum mean square error (MMSE) detection is proposed in the system as an efficient way to eliminate MCI. It is shown that the optimum power ratio of pilot channel to one data channel is about 0.25 K, where K is the total number of data channels. When the Doppler frequency f_D is less than 200 Hz, the effect of the Doppler on the system performance is negligible. However, increasing f_D from 200 Hz degrades the system performance rapidly. Although the gap in BER between the hybrid and pure MMSE detection decreases as f_D increases, the gap is still large as long as f_D is less than around 800 Hz. For a wide range of Doppler shift, a larger frequency domain spreading factor (N_F) is expected, whereas a smaller time domain spreading factor (N_T) provides better performance when f_D increases.