Adaptive Self-interference Cancellation with a Lossless N-Tap Transversal Filter

Abstract

Simultaneous transmit and receive (STAR) architectures must continually improve receiver isolation techniques to support more demanding frequency agile and instantaneous RF bandwidth requirements. To ensure adequate lifetime and STAR performance, leakage signals above receive saturation levels must be suppressed to protect sensitive front end components. A commonly used technique to suppress leakage signals, known as selfinterference cancellation (SIC), can provide a sufficient level of analog isolation for full duplex operation. The adaptive self-interference canceller developed in this paper utilizes a novel transversal filter approach that passively replicates the strongly distorted transmitted signal, coupled at the receiver, for true time delay cancellation. Conventional transversal filter implementations introduce significant divider, combiner, and attenuator losses between each tap, severely limiting the synthesizable order of the multiple-path leakage channel, which effectively degrades the cancellation performance. To avoid these fixed losses, the novel transversal filter prototype incorporates a lineup of tunable coupler pairs transversely through delay lines as a theoretically lossless alternative to set amplitude and phase weighting coefficients at each tap. Minimizing transversal filter loss lessens the required coupling factor and improves insertion and power efficiency loss penalties at receive and transmit paths respectively. The loss improvements of the SIC architecture define the maximum interference power that can be cancelled. Therefore, to maximize the dynamic range of a STAR system, the SIC architecture should utilize a matching filter that balances loss, noise, and distortion. We propose a technique that provides a completely passive method to cancel strongly coupled signals up to 25 dBc below the transmitted power. The work outlines an avenue to substitute loss driven amplitude and phase weighting components with tunable reflective discontinuities, see equation (1) for a 2-tap filter expression, which can be extended to N-taps by cascading N-1 tunable coupler pairs. Analysis of the novel lossless transversal filter architecture has shown a cancellation performance of 30 dB across a125 MHz bandwidth centered at 2.4 GHz.