Spatial loop interference suppression in full-duplex MIMO relays

The main technical problem in full-duplex relaying is to suppress the looping interference signal from relay transmission to relay reception. The earlier literature on the topic is mostly restricted to SISO channels. We take a step further and consider a system where the coverage of a MIMO transmission link is boosted with a two-hop full-duplex MIMO relay. First, we present a MIMO extension of SISO time-domain cancellation techniques based on subtraction of an estimated loop signal. We show how the loop interference can be suppressed in the spatial domain by applying multi-antenna techniques. The solution involves design of linear receive and transmit filters for the relay to improve the quality of the useful signal and to minimize the effect of the loop interference.We propose null space projection and minimum mean square error filters for spatial loop interference suppression as well as discuss shortly how to combine them with time-domain cancellation.