Separation of cochannel signals under imperfect timing and carrier synchronization

We propose a complete receiver for separating cochannel signals in a time-division multiple-access type system when the timing and carrier synchronization are imperfect or only nominally accomplished. The receiver is all digital, data aided, assumes the presence of multiple antennas at the receiver, and has three functional components. The first part computes the timing offsets of the different cochannel users through a maximum likelihood search. The second part uses a bank of filters on an oversampled version of the baseband signals to obtain a new symbol rate sampled signal in which the different users are all synchronous but are still spatially mixed and rotated with unknown carrier offsets. The third part uses an extended Kalman-filter-based algorithm to estimate the parameters of this model and to separate and decode the transmitted signals of the different users. The performance of this receiver is demonstrated with simulations. This work is significant in the context of smart antenna technology, considered as a potentially significant improvement to wireless systems, since it is a complete solution encompassing flat fading, timing error, and frequency offsets for multiple cochannel users