Investigation of Inter-Carrier Interference due to Doppler Spread in OFCDM Broadband Packet Wireless Access

摘要

This paper investigates the impact of inter-carrier interference (ICI) due to Doppler spread on the packet error rate (PER) performance in Orthogonal Frequency and Code Division Multiplexing (OFCDM) packet wireless access employing turbo coding in a multipath fading channel, and describes the optimization of the sub-carrier spacing, Δ f, i.e., the number of sub-carriers, N_c, with an approximate 50-100 MHz bandwidth. Simulation results show that although the uncoded OFCDM in a 1-path flat Rayleigh fading channel is affected by the ICI caused by the Doppler spread when the maximum Doppler frequency, f_D, becomes more than 5% of Δ f, OFCDM employing turbo coding in a 24-path Rayleigh fading channel is robust against Doppler spread and the degradation is not apparent until f_D reaches more than 10% of Δ f. This is because the turbo coding gain and the frequency diversity effect compensate for the degradation due to ICI. Meanwhile, the PER performance with a larger N_c is degraded, since the effect of the error correction capability becomes smaller due to the larger variance of the de-spread OFCDM symbols associated with the narrower spreading bandwidth in the frequency domain. Consequently, along with the packet frame efficiency for accommodating the guard interval to compensate for the maximum multipath delay time of 1 μsec, we clarify that the optimum number of sub-carriers is approximately 512-1024 (the corresponding Δ f becomes 156.3-78.1 kHz) for broadband OFCDM packet wireless access assuming a 50-100 MHz bandwidth.