With the exponential increase in high rate traffic driven by a new generation ofwireless devices, data is expected to overwhelm cellular network capacity in thenear future. Femtocell networks have been recently proposed as an efficient andcost-effective approach to provide unprecedented levels of network capacity andcoverage. However, the dense and random deployment of femtocells and theiruncoordinated operation raise important questions concerning interference pollution and spectrum allocation.
Motivated by the flexible subchannel allocation capabilities of cognitive radio, inthis talk we present a cognitive hybrid division duplex (CHDD) which is suitable for heterogeneous networks in future mobile communication systems. Specifically, this CHDD scheme has a pair of frequency bands to perform frequency division duplex (FDD) on the macrocell while time division duplex (TDD) is simultaneously operated in these bands by the underlaid cognitive femtocells. By doing so, the proposed CHDD scheme exploits the advantages of both the FDD and TDD scheme: operating FDD at the network level manages inter-tier interference while operating TDD at the femtocell level provides femtocells the flexibility of adjusting uplink and downlink rates. Using tools from stochastic geometry, we analyze the network performance and provide a methodology on how to design optimal switching mechanisms for cognitive TDD operation of femtocells. Finally, an open access policy for the CHDD scheme is proposed, which is shown to improve the performance of macrocell transmission.