Harnessing Self-Interference in Full-Duplex Relaying: An Analog Filter-and-Forward Approach

TL;DR

This paper introduces an analog filter-and-forward design for full-duplex relays that avoids quantization errors and achieves rate improvements by jointly optimizing source power and relay filtering, challenging traditional self-interference cancellation methods.

Contribution

It proposes a novel analog filter-and-forward approach that makes the relay's loop-back channel irrelevant to the maximum achievable rate, with an optimal joint source and relay design.

Findings

Achieves rate gains over heuristic designs.

Outperforms conventional self-interference cancellation methods.

Optimal solution obtained via Lagrange duality.

Abstract

This paper studies a full-duplex filter-and-forward (FD-FF) relay system in frequency-selective channels. Conventionally, the loop-back signal at the FD relay is treated as harmful self-interference and needs to be significantly suppressed via both analog- and digital-domain cancellation. However, the performance of the conventional self-interference cancellation approach is fundamentally limited due to the quantization error induced by the analog-to-digital converter (ADC) with limited dynamic range. In this paper, we consider an analog filter-and-forward design to help avoid the quantization error, and surprisingly show that the maximum achievable rate of such an FD-FF relay system is in fact regardless of the loop-back channel at the FD relay. We characterize the maximum achievable rate of this channel by jointly optimizing the transmit power allocation over frequency at the source and the frequency response of the filter at the relay, subject to their individual power constraints. Although this problem is non-convex, we obtain its optimal solution by applying the Lagrange duality method. By simulations it is shown that the proposed joint source and relay optimization achieves rate gains over other heuristic designs, and is also advantageous over the conventional approach by cancelling the relay loop-back signal as self-interference, especially when the residual self-interference after cancellation is still significant.