On the equalization of asynchronous multiuser OFDM signals in fading channels

We study asynchronous multiuser orthogonal frequency division multiplexing (OFDM) systems over fading channels. Multiuser multiplexing is essential to the design of a spectrally efficient communication system. On the other hand, a synchronous mode of communication may not be a practically realizable solution considering the demand for high-speed services with portability. Therefore, we develop the model of asynchronous multiuser OFDM signals, and propose several minimum mean squared error (MMSE) based equalization methods to suppress the asynchronous interference effectively. The proposed space-time domain MMSE filter mitigates the inter-carrier interference (ICI) and the inter-block interference (IBI) induced by the interfering user´s relative delay. Furthermore, additional performance gains can be achieved via a two-step procedure, which applies the space-time MMSE filter twice, first to estimate the interference and secondly to equalize the signal after interference cancellation.