On the Impact of Phase Noise on Active Cancellation in Wireless Full-Duplex (1212.5462v1)

Abstract: Recent experimental results have shown that full-duplex communication is possible for short-range communications. However, extending full-duplex to long-range communication remains a challenge, primarily due to residual self-interference even with a combination of passive suppression and active cancellation methods. In this paper, we investigate the root cause of performance bottlenecks in current full-duplex systems. We first classify all known full-duplex architectures based on how they compute their cancelling signal and where the cancelling signal is injected to cancel self-interference. Based on the classification, we analytically explain several published experimental results. The key bottleneck in current systems turns out to be the phase noise in the local oscillators in the transmit and receive chain of the full-duplex node. As a key by-product of our analysis, we propose signal models for wideband and MIMO full-duplex systems, capturing all the salient design parameters, and thus allowing future analytical development of advanced coding and signal design for full-duplex systems.

• The paper demonstrates that phase noise in local oscillators critically limits pre-mixer and post-mixer analog cancellation performance.
• It reveals that cascading active analog with digital cancellation is hindered by residual phase noise, necessitating improved noise management for effective suppression.
• Experimental validations using off-the-shelf and precision hardware confirm that even with enhanced channel estimation, phase noise prevents complete self-interference cancellation.

Analysis of Phase Noise Effects on Active Cancellation in Wireless Full-Duplex Systems

The paper presented by Sahai et al. investigates the influence of phase noise on the efficacy of active cancellation techniques in wireless full-duplex communication systems. Despite the promise of full-duplex systems to simultaneously transmit and receive on the same frequency band, practical deployments face significant challenges due to self-interference. This paper meticulously dissects the limitations posed by residual self-interference, even after employing a combination of passive suppression and active cancellation strategies.

Core Contributions and Methodologies

The paper structures its investigation around three primary questions that seek to elucidate the challenges in achieving effective active analog and digital cancellation in existing full-duplex architectures:

1. Constraints on Active Analog Cancellation:

Active analog cancellation mechanisms can be categorized into pre-mixer and post-mixer cancellers based on the phase in which the cancellation occurs. The primary impediment identified is the phase noise in the local oscillators, which fundamentally limits the self-interference cancellation capability. Specifically, the paper finds that the amount of cancellation in pre-mixer cancellers is significantly hindered by the variance of the phase noise. The authors suggest that matching local oscillators could mitigate these phase noise effects to some extent.

2. Interdependence of Analog and Digital Cancellation:

The cascading of active analog with digital cancellation brings attention to the interdependence of the two. The paper highlights that while active analog cancellation cannot entirely eliminate phase noise-induced residuals, digital cancellation cannot effectively operate in the presence of unconstrained phase noise. The implication is that better phase noise management is vital for improving the cumulative cancellation efficacy across cascaded stages.

3. Influence of Passive Suppression:

While passive suppression—like antenna separation—can theoretically lower self-interference, it indirectly influences the potential for active analog cancellation. Increased passive suppression minimizes line-of-sight interference, thereby demanding less from active analog methods. However, there's a trade-off, as the increased passive suppression does not always lead to linear improvement in overall self-interference reduction when combined with active methods.

Signal Modelling and Practical Insights

One significant contribution of this work is the proposal of specific signal models for SISO, MIMO, and wideband full-duplex systems that incorporate phase noise characteristics. The models provide insights into the practical limits of full-duplex communications by encapsulating the dominant noise sources—including phase noise variance, the quality of self-interference channel estimation, and environmental thermal noise.

Empirical Evidence

The paper extends its theoretical analysis with experimental insights, particularly utilizing both off-the-shelf and precision hardware for validation. Noteworthy is the demonstration that despite channel estimation improvements, the presence of significant phase noise prohibits full self-interference cancellation. This empirical facet validates the critical bottleneck that phase noise imposes, aligning with theoretical signal predictions.

Implications and Future Directions

The findings in Sahai et al.'s research illustrate potent pathways to enhancing full-duplex communication. Practically, the paper underlines the necessity for advancements in local oscillator technology to diminish phase noise. Theoretically, it paves the way for developing more sophisticated models that account for a wider array of non-idealities in hardware design. Future research might explore innovative antenna and circuit design strategies that fortify transceiver robustness against phase noise and refine interference suppression algorithms for enhanced predictive accuracy and adaptability in variable radio conditions.

This paper serves as a critical piece in understanding the limitations and potential of full-duplex systems, offering a foundation upon which subsequent innovation and development can build.