Design and Analysis of Downlink Spectrum Sharing in Two-Tier Cognitive Femto Networks

In two-tier networks consisting of a macrocell overlaid with femtocells in cochannel deployment and closed-access policy, spatial reuse is achieved at the price of severe intratier and cross-tier interference from concurrent transmissions. The interference causes significant performance degradation, particularly when coordination among base stations (BSs) is infeasible. Cognitive radio (CR) is a promising technique for interference mitigation, where femto-BSs with cognitive information accomplish concurrent transmissions while meeting a per-tier outage constraint. This paper studies the role of information sensed at femto-BSs on the transmission capacity. By exploiting different cognitive information, we propose spectrum-sharing schemes between macrocell and femtocell, as well as among femtocells, to improve spatial reuse gain. Bounds on the maximum intensity of simultaneously transmitting femtocells that satisfy a given per-tier outage constraint in these schemes are theoretically derived via a stochastic geometry model. We conduct simulations to evaluate the performance of the proposed schemes in terms of transmission capacity. The results confirm that, when femto-BSs acquire the knowledge of user channel statistics or user location information, significant spatial reuse gain can be achieved by exploiting the avoidance region and multiuser diversity.