Increased demand for higher spectrum efficiency, especially in the space-limited chip, base station, and vehicle environments, has spawned the development of full-duplex communications, which enable the transmitting and receiving to occur simultaneously at the same frequency. The key challenge in this full-duplex communication paradigm is to reduce the self-interference as much as possible, ideally, down to the noise floor. This paper provides a comprehensive review of the self-interference cancellation (SIC) techniques for co-located communication systems from a circuits and fields perspective. The self-interference occurs when the transmitting antenna and the receiving antenna are co-located, which significantly degrade the system performance of the receiver, in terms of the receiver desensitization, signal masking, or even damage of hardwares. By introducing the SIC techniques, the self-interference can be suppressed and the weak desired signal from the remote transmitter can be recovered. This, therefore, enables the full-duplex communications to come into the picture. The SIC techniques are classified into two main categories: the traditional circuit-domain SICs and the novel field-domain SICs, according to the method of how to rebuild and subtract the self-interference signal. In this review paper, the field-domain SIC method is systematically summarized for the first time, including the theoretical analysis and the application remarks. Some typical SIC approaches are presented and the future works are outlooked.

Aimed at radio-frequency (RF) interference suppression, this article presents an RF interference adaptive cancellation system (RFIACS) with the down-conversion correlator and proposes the time domain model of the RFIACS with integrated delay mismatch about mixers, RF circuit, and intermediate frequency (IF) circuit. Then, the influence of the integrated delay mismatch and loop gain on the system stability, convergence time, and interference cancellation ratio is analyzed based on the time model. Studies show that the RF and IF delay mismatch each must be matched on their respective frequency bands and the proper arrangement of circuits and loop gain can effectively improve the cancellation performance of the system. Results of simulations and experiments with different parameters show that the theoretical model is valid and accurate. The research result is of much value to the circuit design and delay-matching debugging of the RFIACS.

The analog control circuit of existing RF interference cancellation system is too complex, and the convergence speed cannot be flexibly tuned. This article proposes a digital-to-analog hybrid RF interference cancellation system based on multitap structure (MDARFICS), where the analog control circuit is digitized, so it can be highly integrated and easily tuned. The analytical theory model of the MDARFICS is established for the first time, which can suppress interference with a large power dynamic range and bandwidth. The factors that affect the cancellation performance, i.e., stability, wideband interference cancellation ratio (WICR), and convergence speed, are studied. The stability criterion is obtained, that is, loop gain, sample rate, and tap delay affect the stability cooperatively, and it is found that the sample rate must be selected according to tap delay. Studies show the MDARFICS can effectively suppress wideband interference. The larger the number of taps and control step size, the smaller the bandwidth and delay difference between the tap and interference channel, and the larger the WICR. The convergence speed accelerates as the control step size and filter coefficient increase. Simulations and experiments verify the theoretical analysis. The research results are of much value to the implementation of the MDARFICS.

In space-limited co-located communication systems, wideband transmitters and receivers are deployed for various applications. The strong coupling between transmitters and receivers becomes a major concern. It is crucial to suppress the high power self-interference (SI) to prevent the receiver from working in the nonlinear area, or even saturating. As an effective approach to achieve this, the adaptive interference cancellation (AIC) technology is proposed in the context of a complex high power amplitude modulation radios. In this article, the system-level performance including the interference cancellation ratio requirements and the impact of the AIC on the receiving system is comprehensively studied for the first time. The operating mechanism of a single channel AIC system (AICS) and the configuration of a multichannel AICS are discussed. The performance analysis of the receiving system when implementing the AICS is performed, in terms of the noise figure and the sensitivity. By employing the proposed AICS, very limited effect is observed on the receivers, while the SI is successfully reduced with a high ICR, which fully satisfies the requirements for AM systems. The realization of the multichannel AICS as well as the measured cancellation performance are presented, verifying the proposed concept.

The interference cancellation ratio (ICR) is a key performance indicator of digital-to-analog hybrid radio frequency (RF) interference cancellation systems. Aiming at the low convergence speed of a digital-to-analog hybrid RF interference cancellation system based on a multi-tap structure (MDARFICS), a novel, fast, and robust variable-step-size least-mean-square (LMS) algorithm based on an improved hyperbolic tangent function (IHVSS-LMS) is proposed. The IHVSS-LMS algorithm adopts an improved hyperbolic tangent function and uses adjustable parameters and the iteration number to jointly adjust the step size, which improves the convergence speed and reduces the computational complexity. Moreover, by using the prior information of the input signal, the non-linear relationship between the step size and the input signal power is established, which enhances the robustness and the ability to suppress interference with mutable power. The IHVSS-LMS algorithm is applied to the MDARFICS. Through theoretical derivation, the convergence speed and the steady-state expressions of the interference cancellation ratio of the MDARFICS are obtained. The simulation results show that under the conditions of high and low signal-to-noise ratio (SNR), the robustness, convergence speed, and steady-state error performance of the IHVSS-LMS algorithm are better than the existing variable-step-size algorithm. The experimental results show that using the IHVSS-LMS algorithm, the MDARFICS can not only effectively accelerate the convergence speed by at least three times but can also improve the ICR by more than 3 dB.

Under the background of spatially correlated color noise, the incidence angle of a jamming signal in a high-speed moving platform rapidly changes, which leads to the degradation of the anti-interference performance and the waveform distortion of the adaptive beamformer. In this paper, a projection-constrained null broadening beamforming algorithm based on the Toeplitz matrix structure is proposed. The algorithm first extracts the subspace of the covariance matrix of the steering vector of the pre-determined extended angle interval and constructs the constraint matrix and the projection transformation matrix. The received signal covariance matrix with a Toeplitz structure is then constructed using the correlation number between each array element and the pre-set reference array element. Finally, the constructed covariance matrix is transformed through projection, and the weight of each array element is constrained by the constraint matrix. The theoretical optimal solution of adaptive wide null beamforming in spatially correlated color noise is obtained. The simulation results show that, compared with the existing robust adaptive beamforming algorithms, the proposed algorithm can efficiently improve the distortion of adaptive anti-jamming beams, and can achieve null broadening in the jamming direction under the condition of spatially correlated color noise, which improves the output signal to the interference-plus-noise ratio (SINR) of the adaptive beamformer.