Downlink Capacity of Two-tier Cognitive Femto Networks

In two-tier networks consisting of a macrocell overlaid with femtocells in co-channel deployment and closed-access policy, spatial reuse is achieved at the price of severe cross-tier interference from concurrent transmissions. The lack of direct coordination between the macro and femtocells makes interference control as a challenging issue. Cognitive radio (CR) becomes a promising solution, where femtocells with cognitive information accomplish concurrent transmissions while meeting a per-tier outage constraint. Several interference-aware allocation approaches are proposed to enhance spatial reuse according to cognitive capabilities of femtocell. By employing stochastic geometry model, bounds on the distribution of aggregated interference from two-tier spatial point processes are successfully analyzed. The maximum number of simultaneously transmitting femtocells and overall downlink capacity of two-tier networks meeting a per-tier outage requirement in each approach are theoretically derived. This paper proves that with stronger cognitive capability (i.e., more knowledge interpreted) at femtocell, more spatial reuse gain can be found.