Variable Spreading Factor-OFCDM with Two Dimensional Spreading that Prioritizes Time Domain Spreading for Forward Link Broadband Wireless Access(<Special Section>Multi-carrier Signal Processing Techniques for Next Generation Mobile Communications-Part 2)

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This paper proposes the optimum design for adaptively controlling the spreading factor in Orthogonal Frequency and Code Division Multiplexing (OFCDM) with two-dimensional spreading according to the cell configuration, channel load, and propagation channel conditions, assuming the adaptive modulation and channel coding (AMC) scheme employing QPSK and 16QAM data modulation. Furthermore, we propose a two-dimensional orthogonal channelization code assignment scheme to achieve skillfully orthogonal multiplexing of multiple physical channels. We first demonstrate the reduction effect of inter-code interference by the proposed two-dimensional orthogonal channelization code assignment. Then, computer simulation results show that in time domain spreading, the optimum spreading factor, except for an extremely high mobility case such as for the fading maximum Doppler frequency of f_D=1500Hz, becomes SF_<Time>=16. Furthermore, it should be decreased to SF_<Time>=8 for such a very fast fading environment using 16QAM data modulation. We also clarify when the channel load is light such as C_<mux>/SF=0.25 (C_<mux> and SF denote the number of multiplexed codes and total spreading factor, respectively), the required average received signal energy per symbol-to-noise power spectrum density ratio (E_s/N_0) is reduced as the spreading factor in the frequency domain is increased up to say SF_<Freq>=32 for QPSK and 16QAM data modulation. When the channel load is close to full such as when C_<mux>/SF=0.94, the optimum spreading factor in the frequency domain is SF_<Freq>=1 for 16QAM data modulation and SF_<Freq>=1 to 8 for QPSK data modulation according to the delay spread. Consequently, by setting several combinations of spreading factors in the time and frequency domains, the near maximum link capacity is achieved both in cellular and hotspot cell configurations assuming various channel conditions.
2005-02-01

Variable Spreading Factor-OFCDM with Two Dimensional Spreading that Prioritizes Time Domain Spreading for Forward Link Broadband Wireless Access(<Special Section>Multi-carrier Signal Processing Techniques for Next Generation Mobile Communications-Part 2)

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