Downlink transmission of broadband OFCDM Systems-part I: hybrid detection

The broadband orthogonal frequency and code division multiplexing (OFCDM) system with two-dimensional 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 quasianalytical study is presented on the downlink performance of the OFCDM system with hybrid multi-code 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 sub-carrier 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 two-dimensional spreading system with time domain spreading factor (N_T) of 4 and 8. On the other hand, in a slow fading channel, a large value of the channel estimation window size γN_T, where γ is an odd integer, is expected. However, γ=3 is large enough for the system with N_T=8 while γ=5 is more desirable for the system with N_T=4. Although performance of the hybrid detection degrades in the presence of the sub-carrier correlation, the hybrid detection still works well even the correlation coefficient is as high as 0.7. Finally, given N_T, although performance improves when the frequency domain spreading factor (N_F) increases, the frequency diversity gain is almost saturated for a large value of N_F (i.e., N_F ≥ 32).