Relay transmit power allocation for distributed-space–time block code cognitive small cell networks

Small cell networks enhance spectrum efficiency by handling the indoor traffic of mobile networks on a frequency-reuse operation. Since most of the current mobile traffic happen in indoor, the authors introduce a prioritisation shift by imposing a limitation on the outage generated by the outdoor mobile system to the indoor small cells. The prioritisation shift consists of setting an outage probability threshold limitation on the macrocell system which is classically considered as a primary system. In this study, they consider downlink spectrum sharing between a system consisting of small cells and a macrocell with relays. The relays constitute a distributed-space-time block code (STBC) relaying for the outdoor system terminals. New closed-form expressions are derived to validate the proposed bit error rate (BER) function used in their optimisation algorithm considering STBC with unequal branch SNR. They propose a joint transmit antenna selection and power allocation which minimises the proposed BER. The proposed optimisation yields a dynamic selection of the relays with power control pertaining to the outdoor mobile terminal performance. The simulation results show that their proposal exhibits a better BER compared with the conventional Alamouti STBC with equal power allocation.