Feasibility of SINR guarantees for downlink transmissions in relay-enabled OFDMA networks

We examine the feasibility of the signal-to-noise-and-interference (SINR) guarantees for downlink transmissions in relay-enhanced OFDMA networks that feature stationary users. The constraints are as follows: (i) the SINR of every user exceeds a certain threshold and (ii) the transmit power for each transmission is less than a certain threshold. We first derive a set of necessary and sufficient feasibility conditions for the specific case in which a user i served by the relay station shares at most one subchannel with a user j served by the base station. These conditions are a function of the target SINR values and the channel gains, and derived using a property of an M-matrix. We then extend these results to the case of networks featuring multiple base stations and multiple relays. Our conditions to check the feasibility can be easily implemented in practice.