Alternate relaying and the degrees of freedom of one-way cellular relay networks

In this paper, a cellular relaying network consisting of two source-destination pairs, and four decode-and-forward relays operating in half-duplex mode is considered. Each source is assisted by two relays and all nodes are equipped with N antennas. In order to compensate for the loss of capacity by a factor of half due to the half-duplex mode, an alternate transmission protocol among the two relays is proposed. An outer bound on the degrees of freedom (DoF) of this system is developed. A constructive proof of achievability based on two different schemes is provided. Aligning the inter-relay interference due to alternate transmission and the interference received at the destination nodes is utilized to achieve the maximum DoF and recover the pre-log factor loss. For the K-user case, it is shown that the resulting system of interference alignment equations is proper when the number of streams transmitted by each source is less than or equal to _2(K^3N_;1). Numerical simulations show that for proper systems these DoF are achievable.