From Spectrum Pooling to Space Pooling: Opportunistic Interference Alignment in MIMO Cognitive Networks

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Abstract
We describe a non-cooperative interference alignment (IA) technique which allows an opportunistic multiple input multiple output (MIMO) link (secondary) to harmlessly coexist with another MIMO link (primary) in the same frequency band. We assume perfect channel knowledge in the primary receiver and transmitter such that capacity is achieved by transmiting along the spatial directions (SD) associated with the singular values of its channel matrix using a water-filling power allocation (PA) scheme. Often, power limitations lead the primary transmitter to leave some of its SD unused. We show that the opportunistic link can transmit its own data if it is possible to align the interference produced on the primary link with such unused SDs. We provide both a processing scheme to perform IA and a PA scheme which maximizes the transmission rate of the opportunistic link. We determine the asymptotes of the achievable transmission rates of the opportunistic link in the regime of large numbers of antennas to show that depending on the signal-to-noise ratio and the numbers of transmit and receive antennas of the primary and opportunistic links, both systems can achieve transmission rates of the same order.

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