The conventional two-way relaying (TWR) protocol requires that the transmitter (receiver) in one direction must be the receiver (transmitter) in the other direction, an limitation precluding the application of the TWR for sophisticated realworld wireless networks. In this paper, we study a more general multi-cell system consisting of a downlink (DL) traffic in one cell and an uplink (UL) traffic in an adjacent cell, with a multi-antenna relay located in the cell-edge and shared by both cells. For the coexistence of DL and UL transmissions, we propose to exploit the overheard signals from the adjacent cell (commonly known as the inter-cell interference) to improve the quality of signal reception in both cells.

To reduce the power consumption to suit for green networks, we design the optimum relay precoder to minimize the total power at the relay, yet satisfying the rate constraints for both the DL and UL traffics. The original precoder design is a non-convex problem which is difficult to solve. To make the problem tractable, we transform the non-convex problem to an equivalent quadratically constrained quadratic program, which is then solved by the semidefinite relaxation technique. Finally, simulations validate the effectiveness of our proposed protocol together with the optimized relay precoder.

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