Cognitive cooperation for the downlink of frequency reuse small cells

We develop and analyze a space time coded cooperative diversity protocol that achieves improved quality of service for mobile users in the downlink of small cells. The protocol, called cooperative frequency reuse (CFR), leverages the cellular frequency reuse concept to create space and frequency diversity among pairs of adjacent base stations. The CFR protocol, which is consistent with the half-duplex mode assumption, consists of two phases. During the first phase, each base station transmits symbols to it own user on its dedicated frequency band, while listening to the symbols intended to the users of the other base station on another frequency band. Cognitive cooperation is implemented in the second phase. Each base station transmits on the two frequency bands to the scheduled users in both base stations, by means of an appropriately chosen space time code. We construct a full rate distributed space time code, based on the Golden code, for cooperation between the two adjacent base stations. We discuss the performance of the proposed protocol in terms of bit error rate and probability of outage. Simulation results show that the proposed CFR protocol yields considerable improvement over traditional direct transmission frequency reuse strategies.