Emilio Antonio-Rodríguez

Recent advances in antenna design and interference cancellation algorithms for in-band full duplex relays

In-band full-duplex relays transmit and receive simultaneously at the same center frequency, hence offering enhanced spectral efficiency for relay deployment. In order to deploy such full-duplex relays, it is necessary to efficiently mitigate the inherent self-interference stemming from the strong transmit signal coupling to the sensitive receive chain. In this article, we present novel state-of-the-art antenna solutions as well as digital self-interference cancellation algorithms for compact MIMO full-duplex relays, specifically targeted for reduced-cost deployments in local area networks. The presented antenna design builds on resonant wavetraps and is shown to provide passive isolations on the order of 60-70 dB. We also discuss and present advanced digital cancellation solutions, beyond classical linear processing, specifically tailored against nonlinear distortion of the power amplifier when operating close to saturation. Measured results from a complete demonstrator system, integrating antennas, RF cancellation, and nonlinear digital cancellation, are also presented, evidencing close to 100 dB of overall self-interference suppression. The reported results indicate that building and deploying compact full-duplex MIMO relays is already technologically feasible.

An Adaptive Feedback Canceller for Full-Duplex Relays Based on Spectrum Shaping

Although full-duplex relaying schemes are appealing in order to improve spectral efficiency, simultaneous reception and transmission in the same frequency results in self-interference, distorting the retransmitted signal and making the relay prone to oscillation. Current feedback cancellation techniques by means of adaptive filters are hampered by the fact that the useful and interference signals are highly correlated. We present a new adaptive algorithm which effectively and blindly restores the spectral shape of the desired signal. In contrast with previous schemes, the novel adaptive feedback canceller has low complexity, does not introduce additional delay in the relay station, and partly compensates for multipath propagation.

Modeling and efficient cancellation of nonlinear self-interference in MIMO full-duplex transceivers

This paper addresses the modeling and digital cancellation of self-interference in in-band full-duplex (FD) transceivers with multiple transmit and receive antennas. The self-interference modeling and the proposed nonlinear spatio-temporal digital canceller structure takes into account, by design, the effects of I/Q modulator imbalances and power amplifier (PA) nonlinearities with memory, in addition to the multipath self-interference propagation channels and the analog RF cancellation stage. The proposed solution is the first cancellation technique in the literature which can handle such a self-interference scenario. It is shown by comprehensive simulations with realistic RF component parameters and with two different PA models to clearly outperform the current state-of-the-art digital self-interference cancellers, and to clearly extend the usable transmit power range.

SINR optimization in wideband full-duplex MIMO relays under limited dynamic range

We develop a method for self-interference mitigation in wideband full-duplex multiple-input multiple-output regenerative relays, taking into account transmitter impairments and limited receiver dynamic range. The method combines a cancellation and suppression architecture by incorporating feedforward filters at both sides of the relay. The design criterion is to maximize the signal-to-interference-plus-noise ratio (SINR) at the relay input. On the transmit side, linear constraints are imposed in order to control the effect on the information signal at destination. Simulation results show the effectiveness of the proposed method both in terms of self-interference reduction and SINR improvement.

Adaptive self-interference cancellation in wideband full-duplex decode-and-forward MIMO relays

Full-duplex relays suffer from self-interference due to simultaneous reception and transmission in the same frequency. This relay-induced interference signal can be of significant power and considerably distort the information-bearing signal, leading to unacceptable performance degradation. In this paper we present a method to solve the problem of self-interference mitigation for full-duplex decode-and-forward multiple-input multiple-output (MIMO) relays that is based on an adaptive gradient-descent algorithm. The method makes use of signals available at the relay to estimate the self-interference channel in order to cancel it. We analyze the behavior of the algorithm in terms of its stationary points and mean convergence. Under a low SNR scenario case and convergence time constraints, simulations show that the proposed method can provide several tens of dBs of mitigation.

Autocorrelation-based adaptation rule for feedback equalization in wideband full-duplex amplify-and-forward MIMO relays

Simultaneous reception and transmission in the same frequency, so-called full-duplex operation, causes an infinite feedback loop in an amplify-and-forward relay. The unwanted echoes may result in oscillation at the relay, making it unstable, and distorting the spectrum. This paper presents an adaptive MIMO filtering method for full-duplex amplify-and-forward relays that aims at solving the joint problem of self-interference mitigation and equalization of the source-relay channel. The scheme exploits the knowledge of the autocorrelation of the transmitted signal as the only side information while allowing the relay, in the best case, to implement precoding as if there was not any self-interference or frequency selectivity in the source-relay channel. Finally, the proposed adaptation algorithm is investigated by determining its stationary points and by performing simulations in a MIMO-OFDM framework.

Adaptive self-interference suppression for full-duplex relays with multiple receive antennas

In full-duplex relays, simultaneous reception and transmission in the same frequency results in self-interference which distorts the retransmitted signal and makes the relay prone to oscillation. We present an adaptive feedback canceller for multiple-input single-output (MISO) relays, efficiently combining spatial and temporal processing. The receive array is adaptively steered towards the minimum variance distortionless response (MVDR), whereas the temporal filter update is based on a novel low-complexity spectrum shaping scheme which avoids introducing additional delay in the relay station.

Wideband full-duplex MIMO relays with blind adaptive self-interference cancellation

We develop adaptive self-interference cancellation algorithms for both filter-and-forward and decode-and-forward multiple-input multiple-output relays. The algorithms are blind in the sense that they only exploit the spectral properties of the transmitted signal to identify the self-interference channel, while dealing with frequency-selective channels and arbitrary signal spectra. Our approach is non-intrusive in the sense that the algorithms can successfully identify, track, and cancel the self-interference distortion while the relay is operating in its normal mode. We study the stationary points of the algorithms and analyze under which conditions they achieve perfect cancellation of the self-interference. Simulation results show that the algorithms provide residual self-interference levels below the noise floor by using the time samples of only a few OFDM symbols. HighlightsWe solve the problem of self-interference mitigation in full-duplex MIMO relays.Blind adaptive time-domain algorithms based on recursive least-squares are developed.Simulation results show residual self-interference levels below the noise floor.

Subspace-constrained SINR optimization in MIMO full-duplex relays under limited dynamic range

In this paper, we propose a method for maximizing the signal-to-interference-plus-noise ratio (SINR) in a wideband full-duplex MIMO regenerative relay that accounts limited dynamic range of the receiver and transmitter impairments. Transmit and receive filters are designed at the relay, by means of an alternating minimization algorithm, to minimize the interference at the decoder input in the destination node. We impose channel shortening and subspace constraints to ensure that the received signal at the destination is not compromised. Simulation results show that the presented method significantly outperforms other constrained approaches.

Cancelling self-interference in full-duplex relays without angle-of-arrival information

With full-duplex relays, simultaneous reception and transmission in the same frequency produce self-interference distortion and relay oscillation, hampering reception at the receiver end unless properly mitigated. Previous adaptive methods for self-interference cancellation using multiple receive antennas require knowledge of the direction of arrival of the source signal. We present a novel architecture and adaptive scheme for Amplify-and-Forward relays allowing cancellation without such knowledge. The temporal filter is updated under the property restoral paradigm in order to match the power spectrum of the retransmitted signal to that of the source signal.