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

TL;DR

This paper introduces a novel interference alignment method for MIMO cognitive networks that enables secondary links to opportunistically transmit by exploiting unused spatial directions of primary links, maximizing overall spectral efficiency.

Contribution

It proposes a new non-cooperative IA technique allowing secondary MIMO links to coexist with primary links by aligning interference with unused spatial directions, including a power allocation scheme.

Findings

Secondary links can achieve high transmission rates by aligning interference with unused SDs.

The asymptotic analysis shows both links can attain similar transmission rates under certain conditions.

The method is effective in large antenna regimes, enhancing spectrum utilization.

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. Assuming perfect channel knowledge at the primary receiver and transmitter, 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. Here, it is shown 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. The asymptotes of the achievable transmission rates of the opportunistic link are obtained in the regime of large numbers of antennas. Using this result, it is shown that depending on the signal-to-noise ratio and the number of transmit and receive antennas of the primary and opportunistic links, both systems can achieve transmission rates of the same order.