Transmission capacity of spectrum sharing ad-hoc networks with multiple antennas

Two coexisting ad-hoc networks, one primary and other cognitive, are considered, where each node of the primary network has a single antenna, while each node of the cognitive network is equipped with multiple antennas. Using multiple antennas, each cognitive transmitter uses some of its spatial transmit degrees of freedom (STDOF) to null its interference towards the primary receivers, while each cognitive receiver employs interference cancelation using some of its spatial receive degrees of freedom (SRDOF). This paper derives the optimal STDOF for nulling, and SRDOF for interference cancelation, that maximize the scaling of the transmission capacity of the cognitive network with respect to the number of antennas, when the cognitive network operates under an outage constraint at the primary receivers. With a single receive antenna, using a fraction of the total STDOF for nulling at each cognitive transmitter maximizes the transmission capacity of the cognitive network. With multiple transmit and receive antennas and fixing all but one STDOF for nulling, using a fraction of the total SRDOF to cancel the nearest interferers maximizes the transmission capacity of the cognitive network.