Two-way MIMO DF relaying for non-simultaneous traffic in cellular systems

Two-way relaying (TWR) improves the spectral efficiency of cellular systems by enabling data exchange between two nodes via a relay station (RS) in two channel uses. It is assumed in TWR that a user has data to send and receive from the base station (BS) at the same time. This paper considers a non-simultaneous traffic scenario observed in cellular systems, where one user sends data to the BS and another user receives data from the BS. We consider a novel multiple-input multiple-output (MIMO) TWR protocol to enable communication between two users and the BS in two channel uses. By designing a linear precoder at the decode-and-forward RS, we maximize the sum-rate of this protocol by formulating a sequence of semidefinite programs. The sum-rate performance of this protocol is evaluated in two coverage-limited scenarios in a cellular framework by extensive numerical simulations. It is shown that the protocol results in a dramatic performance gain over conventional protocols.