Subcarrier Availability in Downlink OFDM Systems with Imperfect Carrier Synchronization in Deep Fading Noisy Doppler Channels

Multicarrier systems such as orthogonal frequency division (OFDM) are considered as a promising candidate for wireless networks that support high data rate communication. In this article, we investigate the performance of a multiuser OFDM system under imperfect synchronization. Analytical results indicate that the SNR degrades as the average power of the channel impairments such as AWGN, carrier frequency offset due to Doppler frequency and fading gain is increased. The SNR degradation leads to imperfect synchronization and hence decreases the total number of subcarriers available for allocation. Monte Carlo analysis shows up to 22% loss in the number of allocatable subcarriers can be expected under a specific imperfect synchronization condition as compared to perfect synchronization. We utilize empirical modelling to characterize the available number of subcarriers as a Poisson random variable. In addition, we determine the percentage decrease in the total number of allocatable subcarriers under varying channel parameters. The results indicate 19% decrease in the number of available subcarriers as average AWGN power is increased by 10dB; 44% decrease as the Doppler frequency is varied from 10Hz to 100Hz; and 56% decrease as the fading gain is varied from 0dB to -30dB.