Self-interference Cancellation Performance Research Articles

Multi-domain blind source separation in-band full-duplex technique considering RF impairments

In-band full-duplex (IBFD) technology can potentially improve spectrum efficiency compared to half-duplex technology. The cancellation of the self-interference (SI) signal is the key to implementing the IBFD system to achieve accurate detection of the signal of interest. In this work, we fully account for the effect of IQ imbalance, phase noise (PN), additional Gaussian white noise (AWGN), and the nonlinear caused by the RF impairments of power amplifier, and adopt the method of canceling SI signal combined with the propagation, analog, and digital domain to cancel the SI signal, but the residual SI signal is still mixed with the signal of interest in the same time and frequency domain. In addition, a novel multi-domain blind source separation (MDBSS) self-interference cancellation method implemented by using the off-the-shelf components is proposed in this paper, which can realize the separation of residual self-interference signals from the signal of interest (SOI). Additionally, the multi-domain blind source separation (MDBSS) self-interference cancellation technique is used to achieve 75 dB of self-interference cancellation performance, which is proven through numerical simulation analysis, and the SOI is successfully separated in the receiver. Moreover, the multi-domain blind source separation (MDBSS) method in this paper can be applied to radar, jammer, and communication systems in the future.

Microwave photonic dispersion immune self-interference cancellation scheme for a radio over fiber based in-band full duplex communication link

A microwave photonic scheme for multi-functional radio over fiber based in-band full duplex communication links to realize deep self-interference cancellation, long range transmission, and efficient signal recovery is proposed. In the proposed scheme, double sideband modulation helps avoid applying electrical couplers or optical filters and is beneficial to improve the frequency conversion efficiency. By virtue of the intermediate frequency signal amplitude regulation mechanism induced by fiber dispersion, the conditions of dispersion induced power fading compensation, amplitude matching, and phase reversion between self-interference and reference signals can be satisfied through joint DC bias tuning. The high precision delay matching in the optical domain is advantageous to the deep self-interference cancellation performance in a wide frequency range compared to its electrical counterparts. Furthermore, the deterioration of signal quality and self-interference cancellation performance caused by fiber dispersion is avoided. Experimental analyses show that a single tone signal in a wide frequency range of 2–20 GHz can be cancelled over 60 dB. A broadband signal with bandwidth of 50 MHz in the C-band, X-band, and K-band can be cancelled over 35 dB. Furthermore, the recovery performance of the signal of interest with different modulation formats and different signal to interference ratios is investigated, showing high quality signal transmission and efficient signal recovery. The capability of dispersion induced power fading compensation is also verified, and the spurious-free dynamic range is measured to be 88.47dB⋅Hz2/3.

Theoretical Investigation and Optimization of Power Amplifier Nonlinearity for In-Band Full-Duplex Radios

Although in-band full-duplex (IBFD) communication can theoretically double the spectral efficiency compared with conventional wireless communication technologies, it cannot be realized without reducing a high-power self-interference (SI) signal. Since the SI signal is affected by nonlinear distortion of radio-frequency (RF) circuits of its own terminal, cancellation systems capable of removing the nonlinear distorted SI have been actively developed. Detailed theoretical analysis based on the authors’ past literature shows that for IBFD using a nonlinear self-interference canceller, the symbol error rate is improved when the power amplifier has certain distortion characteristics rather than being ideally linearized. However, it is not yet known what amplifier characteristics are best for the IBFD system. In this study, we theoretically analyze the self-interference signal to clarify the optimum amplifier characteristics for the IBFD system. An optimization problem to obtain an optimum transfer function from theoretical analysis is formulated as an Rayleigh quotient with constraints, and we introduce a numerical technique to solve the optimization problem. By numerical experiments, we show that the SI cancellation performance, signal-to-interference-distortion-plus-noise ratio (SIDNR), and amplifier’s power efficiency are improved by the optimization.

Full-Duplex UAC Receiver With Two-Sensor Transducer

In this brief we present a design of a full-duplex (FD) underwater acoustic communication (UAC) receiver with a two-sensor transducer. The transducer consists of two piezo-ceramic cylinders cast together in polyurethane resin; one acts as a hydrophone and the other a projector. As the distance between the projector and hydrophone is extremely short, a high level of self-interference (SI) from the projector is received at the hydrophone. This SI must be eliminated to enable far-end data demodulation. In this brief, we adopt a digital SI cancellation scheme based on adaptive filtering. We investigate the SI cancellation and far-end data demodulation performance with the two-sensor transducer in tank and lake experiments. The Rake receiver is used for the far-end data demodulation. As demonstrated by the experimental results, the far-end SNR loss in the FD experiment is less than 3 dB compared to the half-duplex experiment when there is only far-end transmission.

On the Impacts of I/Q Imbalance in Analog Least Mean Square Adaptive Filter for Self-Interference Cancellation in Full-Duplex Radios

Frequency-dependent in-phase/quadrature-phase imbalance (IQI) is a critical hardware impairment, which affects the performance of the self-interference (SI) cancellation structure in full-duplex systems. A novel frequency-dependent IQI model is proposed in this paper, providing a theoretical tool to evaluate the impacts of this impairment in the analog domain. The model is then applied to investigate the performance of the analog least mean square (ALMS) loop under both frequency-selective and frequency-independent IQI. The degradation in the SI cancellation performance of the ALMS loop is revealed and its upper bound is derived. Numerical results show that the ALMS loop is resilient to both types of IQI with the maximum level of only 3.5 dB performance degradation even when the image rejection ratio is just 15 dB.

Frequency-Domain Successive Cancellation of Nonlinear Self-Interference With Reduced Complexity for Full-Duplex Radios

Nonlinear self-interference (SI) cancellation is crucial for eliminating the impact of the transmitter-side nonlinearity on the comprehensive SI cancellation performance in full-duplex radios. However, in the propagation environment with rich reflection paths of signals, the number of the delayed taps required for time-domain nonlinear SI cancellation booms exponentially with the increase of the multi-path number, leading to unacceptable complexity. In this paper, a novel frequency-domain nonlinear SI canceller based on successive interference cancellation (SIC) technique is proposed for orthogonal frequency division multiplexing (OFDM) modulated full-duplex systems subjected to both transmitter nonlinearity and receiver nonlinearity. The proposed frequency-domain nonlinear SIC (FD-NSIC) cancels nonlinear components one by one via separately estimating the channel frequency response suffered by each order nonlinear component, which significantly relaxes the computational cost. Besides, to avoid the influence of the strong correlation between nonlinear components, orthogonalization is operated before SIC to white basis functions, which accelerates the convergence speed of the proposed FD-NSIC. Simulations are explicitly performed, showing that compared with the conventional frequency-domain nonlinear canceller based on recursive least squares (RLS) channel estimation, the proposed FD-NSIC is capable of canceling the SI to the noise floor with only 52% computational cost under 50 dB interference-to-noise ratio.

Self-Interference Channel Training for Full-Duplex Massive MIMO Systems.

Full-duplex (FD) is a promising technology for increasing the spectral efficiency of next-generation wireless communication systems. A major technical challenge in enabling FD in a real network is to remove the self-interference (SI) caused by simultaneous transmission and reception at the transceiver, and the SI cancellation performance depends significantly on the estimation accuracy of the SI channel. In this study, we proposed a novel partial SI channel training method for minimizing the residual SI power for FD massive multiple-input multiple-output (MIMO) systems. Based on an SI channel training framework under a limited training overhead, using the proposed scheme, the BS estimates only a part of the SI channel vectors, while skipping the channel training for the other remaining SI channel vectors by using their last estimates. With this partial training framework, the proposed scheme finds the optimal partial SI channel training strategy for pilot allocation to minimize the expected residual SI power, considering the time-varying Rician fading channel model for the SI channel. Therefore, the proposed scheme can improve the sum-rate performance compared with other simple partial training schemes for FD massive MIMO systems under a limited training overhead. Numerical results confirm the effectiveness of the proposed scheme for FD massive MIMO systems compared with the full training scheme, as well as other partial training schemes.

Energy efficiency of full-duplex cognitive radio in low-power regimes under imperfect spectrum sensing

This paper investigates the energy efficiency of a full duplex (FD) cognitive radio (CR) system in the low-power regime under a practical self-interference cancellation performance and imperfect spectrum sensing. We consider an opportunistic spectrum access network in which the secondary user (SU) is capable of self-interference suppression (SIS). The SIS ability enables the SU to work in simultaneous transmit and sense (TS) mode in order to increase the quality of channel sensing. Towards this goal, we first study the sensing performance, i.e., false-alarm and miss-detection probabilities, of the FD CR networks using TS, and compare the results with traditional half duplex (HD) CR systems using transmit only mode (TO). In the next step, we show that due to imperfect spectrum sensing, the secondary network channel is best characterized as a Gaussian-Mixture (GM) channel, which is widely used to capture the asynchronism in heterogeneous cellular networks. We then analytically characterize the low-signal-to-noise-ratio (low-SNR) metrics of the minimum energy per bit and wideband slope of the spectral-efficiency curve, and obtain these fundamental limits in closed-form. Furthermore, the characterization of these fundamental metrics allows us to identify practical signaling strategies that are optimally efficient in the low-SNR regime for the considered FD CR system. The benefits in terms of energy efficiency offered by FD CR over HD CR are also clearly demonstrated.

Robust Adaptive Filtering Algorithm for Self-Interference Cancellation with Impulsive Noise

Self-interference (SI) is usually generated by the simultaneous transmission and reception in the same system, and the variable SI channel and impulsive noise make it difficult to eliminate. Therefore, this paper proposes an adaptive digital SI cancellation algorithm, which is an improved normalized sub-band adaptive filtering (NSAF) algorithm based on the sparsity of the SI channel and the arctangent cost function. The weight vector is hardly updated when the impulsive noise occurs, and the iteration error resulting from impulsive noise is significantly reduced. Another major factor affecting the performance of SI cancellation is the variable SI channel. To solve this problem, the sparsity of the SI channel is estimated with the estimation of the weight vector at each iteration, and it is used to adjust the weight vector. Then, the convergence performance and calculation complexity are analyzed theoretically. Simulation results indicate that the proposed algorithm has better performance than the referenced algorithms.

Hardware Implementation of Neural Self-Interference Cancellation

In-band full-duplex systems can transmit and receive information simultaneously and on the same frequency band. However, due to the strong self-interference caused by the transmitter to its own receiver, the use of non-linear digital self-interference cancellation is essential. In this work, we describe a hardware architecture for a neural network-based non-linear self-interference (SI) canceller and we compare it with our own hardware implementation of a conventional polynomial based SI canceller. Our results show that, for the same SI cancellation performance, the neural network canceller has an $8.1\times $ smaller area and requires $7.7\times $ less power than the polynomial canceller. Moreover, the neural network canceller can achieve 7 dB more SI cancellation while still being $1.2\times $ smaller than the polynomial canceller and only requiring $1.3\times $ more power. These results show that NN-based methods applied to communications are not only useful from a performance perspective, but can also lead to order-of-magnitude implementation complexity reductions.

Adaptive over-the-air RF self-interference cancellation using a signal-of-interest driven regular triangle algorithm.

An optically-enabled radio frquency (RF) self-interference cancellation system is demonstrated for over-the-air in-band full duplex transmission, based on a signal-of-interest (SOI) driven regular triangle algorithm. Since the goal of a self-interference cancellation system is to retrieve the SOI that is masked by the in-band interference signal, using the SOI quality as the driven parameter for optimizing the self-interference cancellation performance is a natural and effective way to allow the system to adapt to changes and obtain the best cancellation performance. Since regular triangle algorithm has short iteration time, bursts of pseudo-random binary sequence would be used between real data transmission for optimizing the self-interference cancellation performance. The adaptive regular triangle algorithm optimizes the cancellation setting such that the in-band interference can be cancelled to a minimum, i.e., down to the noise floor. During the over-the-air experiment, 22 dB of cancellation depth is obtained over a 300 MHz bandwidth at 18.35 GHz without the need of digital self-interference cancellation.

통신] 자기간섭제거 성능의 개선을 위한 확장형 BFN의 설계와 특성 평가

5G 이동통신 기술은 기존 기술 대비 3배의 스펙트럼 효율 향상이 요구된다. 이를 맞추기 위해 스펙트럼 효율 향상을 위한 연구가 활발히 이루어지고 있다. IBFD(In-Band Full Duplex)를 사용하게 되면 이론상 스펙트럼 효율을 약 2배 향상시킬 수 있다. 그러나 IBFD에서는 SI(Self-Interference)가 발생하기 때문에 이를 줄이는 SIC(Self-Interference Cancellation) 성능이 매우 중요하다. 본 논문에서는 자기간섭제거 성능의 개선을 위한 확장형 BFN(Balanced Feed Network)를 설계하고 특성을 평가한다. 본 논문에서는 SIC 성능을 평가하기 위해서 중심 주파수를 2.45GHz를 기준으로 성능을 평가하였다. 시뮬레이션 결과 SIC 성능은 –77dB로 기존의 Double-BFN 대비 약 3dB의 성능 향상이 있는 것을 확인하였다. 다만 기존 시스템 대비 송신전력이 3dB 감소하는 것을 확인하였다.

Self-Interference Cancellation Enabling High-Throughput Short-Reach Wireless Full-Duplex Communication

In-band full-duplex (FD) wireless communication allows the simultaneous transmission and reception of data at the same frequency band, effectively doubling the spectral efficiency and data rate while reducing the latency. Previously published designs mostly target the self-interference (SI) cancellation in conventional wireless systems. In this paper, we focus on real-time SI cancellation for short-reach wireless FD systems. The superior signal quality of a point-to-point short-reach wireless system, allows the utilization of wideband communications to achieve a high throughput. Besides, in such wireless systems, the impacts of phase noise and nonlinear distortions are largely reduced, easing the SI cancellation. Moreover, the degradation of signal reception quality due to FD operation is experimentally evaluated in different environments. Experimental results of a prototype implementation show that a combination of antenna isolation and digital cancellation can already achieve an overall SI cancellation performance of 72.5 dB over a bandwidth of 123 MHz. This prototype can support a high-data-rate FD communication link of close to 1 Gbps up to 300 cm with an error vector magnitude lower than −26 dB in a typical indoor environment.

Digital Self-Interference Cancellation With Variable Fractional Delay FIR Filter for Full-Duplex Radios

In full-duplex radios, delay alignment errors result in random mismatch between the self-interference (SI) signal and the reconstructed cancellation signal, leading to the performance degradation of the SI cancellation. In this letter, a variable fractional delay (VFD) FIR filter with more precise delay alignment is exploited to suppress the SI in digital domain. The closed-form expressions of the VFD FIR filter coefficients are reformulated by minimizing the residual SI power. Based on the obtained SI cancellation capability, the influence of the filter order on the SI cancellation is then characterized. Simulations show that the deployment of the VFD FIR filter significantly enhances the SI cancellation performance. Specifically, when the SI signal bandwidths are 0.5 and 10 MHz, the cancellation capability of employing the VFD FIR filter based on the proposed coefficients outperforms the VFD FIR filter under the LS criterion by 6.5 and 3.7 dB, respectively, which presents a potential contribution to enhance the SI cancellation performance.

An Architecture for Capturing the Nonlinear Distortion of Analog Self-Interference Cancellers in Full-Duplex Radios

Analog self-interference (SI) cancellation is a key process to enable full-duplex wireless communications, which generates a cancelling signal (a copy of the SI) at the local receiver and subtracts from the received signal to cancel the SI imposed by the local transmitter. However, due to the inherent nonlinearities of analog canceller (AC), the SI cancellation performance is often restricted by introducing extra nonlinear distortion to the receiver. In this letter, an observation architecture is proposed to capture the AC nonlinear distortion by deploying an auxiliary chain identical to the AC and a linear cancellation approach. The captured nonlinear distortion could be used as a reference in the digital domain to mitigate the AC-induced nonlinear interference at the receiver. Experiments verify that the proposed architecture is capable of capturing the weak AC-induced distortion and suppressing it to close the receiver noise floor.

Novel multi-tap analog self-interference cancellation architecture with shared phase-shifter for full-duplex communications

Multi-tap analog self-interference (SI) cancellation structures adopt parallel taps to reconstruct and then cancel SI in full-duplex radios. Each tap is usually comprised of one fixed delay line, one variable attenuator, and one optional variable phase shifter. To balance the quantity of the variable phase shifters and the achievable SI cancellation (SIC) performance, this paper proposes a novel analog SIC cancellation structure, called shared-phase-shifter constrained multi-tap structure (SMTS). In the proposed architecture, all taps share one phase shifter to emulate the dominated phase offset of the SI channel, which reduces the complexity of the implementation of the multi-tap analog SIC structure and avoids the SIC performance degradation. Then, the proposed SMTS and the existing structures are compared in terms of SIC performance and power dissipation. Finally, extensive simulations show that SMTS provides the close-to-optimal SIC performance as well as the lowest power dissipation relative to the existing multi-tap structures.

Subspace-based self-interference cancellation for full-duplex MIMO transceivers

This paper addresses the self-interference (SI) cancellation at baseband for full-duplex MIMO communication systems in consideration of practical transmitter imperfections. In particular, we develop a subspace-based algorithm to jointly estimate the SI and intended channels and the nonlinear distortions. By exploiting the covariance and pseudo-covariance of the received signal, we can increase the dimension of the received signal subspace while keeping the dimension of the signal subspace constant, and hence, the proposed algorithm can be applied to most of full-duplex MIMO configurations with arbitrary numbers of transmit and receive antennas. The channel coefficients are estimated, up to an ambiguity term, without any knowledge of the intended signal. A joint detection and ambiguity identification scheme is proposed. Simulation results show that the proposed algorithm can properly estimate the channel with only one pilot symbol and offers superior SI cancellation performance.

Compact Inband Full-Duplex Relays With Beyond 100 dB Self-Interference Suppression: Enabling Techniques and Field Measurements

In this communication, the self-interference (SI) channel and the novel enabling techniques for a compact inband full-duplex relay are described and characterized in different operating environments. The full-duplex operation is based on a novel antenna design that uses wavetraps to provide passive isolation of up to 70 dB between the transmit and receive antenna ports. The passive isolation is complemented with novel active RF and digital cancellation stages that further suppress the residual SI to the receiver noise floor. Measurement results of a complete prototype implementation show that the proposed design can achieve an overall SI cancellation performance of over 100 dB even with an ambitious instantaneous bandwidth of 80 MHz. Similar results are obtained both in an anechoic chamber as well as in realistic multipath indoor environments.