Optimum design of two-dimensional spreading factor control in VSF-OFCDM with two-dimensional spreading for forward link brodband wireless access

摘要

This paper proposes an optimum design for adaptively controlling the spreading factor in Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) with two-dimensional spreading according to the cell configuration, channel load, and propagation channel conditions. Simulation results elucidate that the optimum spreading factor in the time domain spreading becomes SF{sub}(Time) = 16, except for an extremely high mobility case such as for the fading maximum Doppler frequency of f{sub}D = 1500Hz, and it should be decreased to SF{sub}(Time) = 8 for such a very fast fading environment using 16QAM data modulation. Furthermore, when the channel load is light such as C{sub}(mux)/SF = 0.25 (C{sub}(mux) and SF denote the number of multiplexed codes and total spreading factor, respectively), the required average received signal energy per symbol-to-background noise power spectrum density ratio (E{sub}s/N{sub}0) is reduced as the spreading factor in the frequency domain is increased up to say SF{sub}(Freq) = 32 both for QPSK and 16QAM data modulations. Meanwhile, when the channel load is near lull condition such as when C{sub}(mux)/SF= 0.94, the optimum SF{sub}(Freq) for 16QAM data modulation is 1 and that for QPSK takes from 1 to 8 according to the delay spread. Consequently, by setting optimum combinations of SF{sub}(Time), and SF{sub}(Freq) among the limited number of orthogonal combinations, the near maximum system capacity is achieved both in cellular and hot-spot cell configurations assuming various channel conditions.