Opportunistic spectrum access for cooperative unlicensed femtocells in two-tier heterogeneous networks utilizing HARQ feedback

Abstract

The co-channel deployment of femtocells can lead to severe cross-tier interference to macrocell users which is resolved through the use of cognitive radio technology in which the femtocells, acting as unlicensed users, opportunistically access the spectrum of the primary macrocell in a cognitive way. This research proposes a cooperative scheme to improve the spectrum access opportunity for unlicensed femtocell in a two-tier heterogeneous network by exploiting the hybrid-automatic repeat request (HARQ) feedback information of macrocell transmission assuming the macrocell spectrum is considerably underutilized. This HARQ based scheme is very useful to get rid of the ‘hidden node problem’ and ‘exposed node problem’ encountered in conventional ‘listen-before-talk’ strategy used by the cognitive users. The outage performance of macrocell user is also improved due to cooperation from the femtocell. Performance analysis of the scheme is presented in Nakagami fading environment for a single femtocell scenario and compared with the conventional non-cooperative spectrum access via numerical examples. Appreciable improvements in terms of macrocell outage, macrocell and femtocell throughput over the conventional non-cooperative spectrum access have been found by considering a fixed macrocell traffic arrival rate. Moreover the dependence of femtocell location on the performance of the proposed scheme is further explored.