In uplink OFDMA, timing offsets (TO) and/or carrier frequency offsets (CFO) of other users with respect to a desired user can cause multiuser interference (MUI). In practical uplink OFDMA systems, effect of this MUI is made acceptably small by requiring that timing/frequency alignment be achieved at the base station (BS) receiver with high precision. Alternatively, an ability to handle large TOs and CFOs using interference cancellation techniques at the BS receiver can offer the benefits of i) allowing low-cost transmit carrier frequency oscillators to be used at all the mobiles in an open-loop mode which can reduce the mobile cost and complexity, and ii) dropping the cyclic prefix (CP), which increases the throughput. In this paper, we first analytically characterize the degradation in the average output signal-to-interference ratio (SIR) due to the combined effect of both TOs as well as CFOs in uplink OFDMA without CP, and subsequently propose a parallel interference canceller (PIC) to mitigate the effect of interferences due to no CP and large TO/CFO. We illustrate the bit error performance of the proposed PIC for the cases of with and without CP. Simulation results show that performance close to that of 'with CP' can be achieved 'without CP' using increased number of PIC stages.
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