Coexistence in two-tier femtocell networks: Cognition and optimization

Two-tier networks which comprise of macrocells coexisting with femtocells have emerged as a promising solution to improve in-building coverage and capacity by spatially reusing the spectrum resources. However, the lack of direct coordination between macrocells and femtocells as well as the severe cross-tier interference pose challenges to the successful deployment of femtocells. In this paper, we propose the concept of cognitive femtocell in the uplink, where the femtocell access point (FAP) first senses for the absence/presence of the macrocell user (MU) after which the FAP adapts its action accordingly. Specifically, when the MU is sensed to be absent, the femtocell user (FU) transmits at maximum power. Otherwise, the FAP optimizes the transmit power of the FU. The proposed method designs the optimal spectrum sensing and channel training time by maximizing the femtocell throughput subject to outage constraint of the MU.