Interference analysis and mitigation for time-asynchronous OFDM CoMP systems

One of the major challenges, which prevents the coordinated multipoint (CoMP) communications concept from being widely deployed in new cellular systems, is timing synchronization. In order to achieve the gains promised by CoMP systems, the user equipment's signals in uplink (UL) or the base stations' signals in downlink should be synchronized, such that the time difference of arrivals do not exceed the cyclic prefix length of the transmitted signals; otherwise, the system suffers from unavoidable integer time offsets. These offsets lead to asynchronous interference in terms of inter-carrier, inter-block, and inter-symbol interference. This limits the percentage of area within a cell which can be covered by cooperation and imposes an upper bound on this area. In this paper, we investigate this problem in the UL, and by using geometrical and semi-analytical approaches, we define this upper bound. Also, by characterizing an accurate mathematical model for the asynchronous interference in Rayleigh fading channels, we were able to employ a standard MMSE-based receiver that mitigates this interference. Furthermore, a typical joint channel and delay estimation block is incorporated into the receiver to examine its performance with estimation errors.