Co-channel interference cancellation using precoding for bi-direction multiple hops MIMO relay systems

In order to increase performance of a system while saving power consumption, multiple hop MIMO relay systems (MHMRSs) have been considered. There are many researches on the physical (PHY) layer, medium access control (MAC) layer, and MAC-PHY cross-layer to avoid a co-channel interference, such as the matched/orthogonal weight algorithm, TDMA-based smart antenna and so on. The matched/orthogonal algorithm is available for bi-directional communications; however, it requests at least three antenna elements at every relay. Consequently, it is unavailable for small wireless devices that are being demanded. In this paper, we propose an algorithm to cancel the co-channel interference of bi-directional communication MHMRSs by signal processing at receivers using precoding at transmitters. The proposal algorithm can perform with only two antenna elements while maintaining character of MIMO. The proposal algorithm is compared to the TDMA-based smart antenna algorithm which also can perform with two antenna elements, in both perfect and imperfect channel state information (CSI) schemes. Numerical calculation results indicate that a channel capacity of proposal algorithm is much higher than that of TDMA-based smart antenna algorithm in the perfect CSI scheme due to complete cancellation of co-channel interference and viability of bi-directional communications of proposal algorithm. In the imperfect CSI scheme, the channel capacity of the TDMA-based smart antenna algorithm is more robust when channel estimation errors increase; however, the channel capacity of proposal algorithm is still higher when the channel estimation error is large.