The impact of synchronization on receive beamforming with null steering in OFDM MIMO systems

Receive beamfoming is one of the most prominent techniques to mitigate interference in OFDM MIMO communications systems. This technique is especially important in the uplink reception of cell-edge mobiles which are naturally associated with low signal-to-interference-and-noise-ratio (SINR). In the common case where the interference is more dominant than the thermal noise, classical receive beamforming solutions, such as the minimum Variance distortionless response (MVDR), tend to place spatial nulls on the dominant interferers. It is well known that a receiver with N receive antennas can place up to N - 1 nulls while receiving a single stream desired signal. In this paper we adopt a practical MIMO channel model including delay and angular spread, and consider both synchronized and non synchronized systems. We reveal an intimate relationship between synchronization and null steering. In the case of adequate temporal and frequency synchronization, an OFDM receiver with N receive antennas may mitigate up to N - 1 interfering transmitters. However, in the absence of adequate synchronization, the same OFDM receiver may mitigate up to N - 1 interference paths. Since each transmitter is usually associated with multiple paths, this degradation may be very significant in terms of cell-edge capacity. The results in this paper may be of great practical importance, especially for the deployment of LTE systems, in which inter-cell synchronization is optional, and in comparisons between synchronized and non synchronized systems, such as next generation WiFi.