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

Multicarrier system such as orthogonal frequency division multiplexing (OFDM) is considered as a promising candidate for wireless networks that support high data rate communication. In this paper, we investigate the performance of a multi-user OFDM system under imperfect synchronization which is caused due to noise, Doppler shift and frequency selective fades in the channel. Monte Carlo analysis indicates an average of 22% loss in the number of allocatable subcarriers under imperfect synchronization as compared to perfect synchronization. As the number of users is increased, the average number of available subcarriers remains moderately constant, but the minimum and maximum number of allocatable subcarriers varies significantly. Based on empirical modelling, we 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 10 dB; 44% decrease as the Doppler frequency is varied from 10 Hz to 100 Hz; and 56% decrease as the fading gain is varied from 0 dB to -30 dB. Hence, determining the availability of subcarriers under imperfect synchronization is essential in optimization process of radio resource allocation for multiuser systems.