Mitigation of Loopback Self-Interference in Full-Duplex MIMO Relays

TLDR

Full‑duplex relaying is spectrally more efficient than half‑duplex, but requires minimizing relay self‑interference to be feasible. The study analyzes a range of MIMO self‑interference mitigation schemes and optimizes filters to minimize interference power. The authors evaluate natural isolation, time‑domain cancellation, and spatial suppression—including antenna subset selection, null‑space projection, joint beam selection, and MMSE filtering—solve optimal filters for joint, separate, and independent designs, and compare their performance via simulations. The results confirm that self‑interference can be mitigated effectively even with imperfect side information.

Abstract

Full-duplex relaying is more spectrally efficient than half-duplex relaying as only one channel use is needed per two hops. However, it is crucial to minimize relay self-interference to render full duplex feasible. For this purpose, we analyze a broad range of multiple-input multiple-output (MIMO) mitigation schemes: natural isolation, time-domain cancellation, and spatial suppression. Cancellation subtracts replicated interference signal from the relay input while suppression reserves spatial dimensions for receive and transmit filtering. Spatial suppression can be achieved by antenna subset selection, null-space projection, i.e., receiving and transmitting in orthogonal subspaces, or joint transmit and receive beam selection to support more spatial streams by choosing the minimum eigenmodes for overlapping subspaces. In addition, minimum mean square error (MMSE) filtering can be employed to maintain the desired signal quality, which is inherent for cancellation, and the combination of time- and spatial-domain processing may be better than either alone. Targeting at minimal interference power, we solve optimal filters for each scheme in the cases of joint, separate and independent design. The performance of mitigation schemes is evaluated and compared by simulations. The results confirm that self-interference can be mitigated effectively also in the presence of imperfect side information.