Downlink Transmission of Broadband OFCDM Systems—Part I: Hybrid Detection

The broadband orthogonal frequency- and code-division multiplexing (OFCDM) sytem with two-dimensional (2-D) spreading (time- and frequency-domain spreading) is becoming a very attractive technique for high-rate data transmission in future wireless communication systems. In this paper, a quasi-analytical study is presented on the downlink performance of the OFCDM system with hybrid multicode interference (MCI) cancellation and minimum mean-square error (MMSE) detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. The effects of channel-estimation errors and subcarrier correlation are also studied. It is shown that the hybrid detection scheme performs much better than pure MMSE when good channel estimation is guaranteed. The power ratio between the pilot channel and all data channels should be set to 0.25, which is a near-optimum value for the 2-D spreading system with time-domain spreading factor of four and eight. On the other hand, in a slow-fading channel, a large value of the channel-estimation window size , where is an odd integer, is expected. However, is large enough for the system with , while is more desirable for the system with . Although performance of the hybrid detection degrades in the presence of the subcarrier correlation, the hybrid detection still works well; even the correlation coefficient is as high as 0.7. Finally, given , although performance improves when the frequency-domain spreading factor (NF) increases, the frequency diversity gain is almost saturated for a large value of (i.e., ).