# An Interference Cancellation Scheme for High Reliability Based on MIMO Systems
## 1 Introduction  
Due to an increasing amount of traffic in recent wireless communication systems, many schemes for high reliability and bandwidth efficiency have been studied. One of the systems to obtain high data rate and bandwidth efficiency is orthogonal frequency division multiplexing (OFDM) [1; 2]. Also, relay systems such as amplifying-and-forward (AF) and decoding-and-forward (DF) have been studied for expanding cell coverage [3]. For practical realization of the relay systems, a half-duplex relay is usually used. Among half-duplex based relay systems, the cooperative communication system is very popular. The cooperative communication system, which is a virtual multiple input multiple output (MIMO) system obtains high diversity order with combined gain at a destination due to independent path of channel state from a source to a destination and a relay to a destination [4; 5]. However, the half-duplex based relay cannot transmit and receive signals at the same time and it results in the degradation of channel capacity compared to direct point-to-point communication systems. So, general relaying system such as the cooperative communication system requires many time slots to transmit several data streams and it causes low data rate for overall systems. To solve this problem, two-path relay scheme is considered in this article. In the two-path relay system, there are one source node, one destination node and two half-duplex relay nodes. In an odd time slot or an even time slot, a source transmits signals alternately to the relay nodes. Since a source can transmit signal to the relays continuously, the main problem for degradation of channel capacity in a single relay is resolved. However, simultaneous transmissions from a source to a relay and from one relay to another relaycause interferences at the relays and these interferences are called as inter-relay interference (IRI) [6; 7; 8]. The IRI severely degrades overall signal-to-interference plus noise ratio (SINR) at a receiver and it makes very hard to decode the received signals at a destination. In [6], full interference cancellation (FIC) is used to remove the IRI at a destination. When the number of transmit data streams is \(N\), \((N+1)\) time slots are required and resultant data rate for the FIC is \(N/(N+1)\) which is close to the full data rate of 1 and has very high bandwidth efficiency. When \(N\) is very large, the resultant data rate for the FIC is approximate to 1. However, the error rate for the conventional FIC is very high because the relays transmit signals which are suffer from the IRI and it causes severe error propagations. Finally, the error performance for the conventional FIC is degraded when a distance between one relay and another relay decreases because the effects of interferences increase and are propagated to a destination. This article proposes a new interference cancellation scheme for high reliability in half-duplex based two-path relay system. Unlike the conventional FIC, the proposed scheme completely cancels the IRI at relay nodes and the effects of error propagations are low at a destination. To remove the IRI at relay nodes, the proposed scheme uses MIMO signal detection [9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. The complexity for the MIMO signal detection is high at a receiver. However, complex signal detection may be applied well when the receiver is a relay which is not usually mobile user. To optimally cancel the IRI, the relays use the low-complexity QR decomposition-\(M\) algorithm (QRD-\(M\)) in [13] which has the same error performance as maximum likelihood (ML). The conventional QRD-\(M\) is reduced complexity version of the ML. However, its complexity is very high as the used modulation order, the number of transmit antennas is increased. As shown in simulation results in Section 5, the used modulation scheme is 256-quadrature amplitude modulation (QAM). So, the low-complexity QRD-\(M\) is used.