The degrees of freedom of MIMO networks with full-duplex receiver cooperation but no CSIT

TL;DR

This paper demonstrates that full-duplex receiver cooperation can significantly enhance the degrees of freedom in two-user MIMO networks without channel state information at the transmitters, through novel interference alignment schemes.

Contribution

It introduces new retro-cooperative interference alignment methods that improve DoF regions in MIMO networks with no CSIT and establishes their optimality.

Findings

Full-duplex receiver cooperation enhances DoF regions.

New interference alignment schemes achieve DoF optimality.

Exact DoF regions are characterized for key MIMO network classes.

Abstract

The question of whether the degrees of freedom (DoF) of multi-user networks can be enhanced even under isotropic fading and no channel state information (or output feedback) at the transmitters (CSIT) is investigated. Toward this end, the two-user MIMO (multiple-input, multiple-output) broadcast and interference channels are studied with no side-information whatsoever at the transmitters and with receivers equipped with full-duplex radios. The full-duplex feature allows for receiver cooperation because each receiver, in addition to receiving the signals sent by the transmitters, can also simultaneously transmit a signal in the same band to the other receiver. Unlike the case of MIMO networks with CSIT and full-duplex receivers, for which DoF are known, it is shown that for MIMO networks with no CSIT, full-duplex receiver cooperation is beneficial to such an extent that even the DoF region is enhanced. Indeed, for important classes of two-user MIMO broadcast and interference channels, defined by certain relationships on numbers of antennas at different terminals, the exact DoF regions are established. The key to achieving DoF-optimal performance for such networks are new retro-cooperative interference alignment schemes. Their optimality is established via the DoF analysis of certain genie-aided or enhanced version of those networks.