Signal-space-alignment-based opportunistic two-way communication via relay selection

Multiuser two-way communication suffers from both self and inter-user interference. Conventional interference management scheme based on signal space alignment requires transmit channel-state-information (CSI) to precode the transmitted signals to achieve the alignment, which could be difficult to accomplish in practical communication systems. This paper considers a multiuser two-way communication system where one base station (BS) exchanges information with multiple user equipments (UE), through a relay station (RS) selected from a collection of available RSs. The selection procedure is based on a single real-valued report from each RS. We show that even though neither the BS nor the UEs have the transmit CSI, optimal degrees of freedom (DoF) can still be achieved as long as there are sufficient number of available RSs. We derive a sufficient condition for the number of RSs to achieve the optimal DoF. Numerical results also verify our theoretical analysis.