Radio Frequency Interference Cancellation Research Articles

Simulation of RFI Cancellation Using Subspace Projection Algorithm for PAF Receiver

The simulation of radio frequency interference (RFI) cancellation by applying a spatial filtering technique for phased array feed (PAF) is presented. In order to better reflect the characteristics of PAF, a new signal model is to add the coupling coefficient among elements of PAF to the conventional array signal model. Then the subspace projection (SP) algorithm is used to cancel RFI from the correlation matrix of the signal, and finally, the 2D power image is drawn. The power variation of signal-of-interest direction and RFI direction before and after using the SP algorithm is analyzed. The new signal model and simulation strategy can be used to test and verify the beamformer.

Wideband Interference Cancellation System Based on a Fast and Robust LMS Algorithm.

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.

Collaboration With Cellular Networks for RFI Cancellation at Radio Telescope

The growing need for electromagnetic spectrum to support the next generation (xG) communication networks increasingly generate unwanted radio frequency interference (RFI) in protected bands for radio astronomy. RFI is commonly mitigated at the Radio Telescope without any active collaboration with the interfering sources. In this work, we provide a method of signal characterization and its use in subsequent cancellation, that uses Eigenspaces derived from the telescope and the transmitter signals. This is different from conventional time-frequency domain analysis, which is limited to fixed characterizations (e.g., complex exponential in Fourier methods) that cannot adapt to the changing statistics (e.g., autocorrelation) of the RFI, typically observed in communication systems. We have presented effectiveness of this collaborative method using realworld astronomical signals and practical simulated LTE signals (downlink and uplink) as source of RFI along with propagation conditions based on preset benchmarks and standards. Through our analysis and simulation using these signals, we are able to remove 89.04% of the RFI from cellular networks, which reduces excision at the Telescope and is capable of significantly improving throughput as corrupted time-frequency bins of data become usable.

Wideband Radio Frequency Interference Cancellation for High-Sensitivity Phased Array Receivers With True Time Delays and Truncated Hadamard Projection

Radio frequency interference (RFI) is a significant challenge for high-sensitivity phased array instruments. RFI can be suppressed using digital signal processing, but to improve dynamic range for wideband RFI, it can be desirable to remove interference in the analog domain before sampling. In previous work, it has been shown that analog true time delay (TTD) stages with a truncated Hadamard transform can place a wide-band spatial null on RFI from a given direction of arrival. We show that TTD and Hadamard projection is mathematically equivalent to a bank of classical narrow-band subspace projection beamformers, but with a structure that allows efficient implementation in either analog circuitry or digital hardware. We analyze how loss in the TTD blocks and time delay errors affect beamformer performance and propose methods for calibrating time delays. Simulation results show that ideal TTD and Hadamard projection matches the bank of subspace projection beamformers and places deep nulls over wideband RFI signals while achieving SNR performance comparable to the maximum signal-to-interference and -noise ratio beamformer.

A Higher-Order Singular Value Decomposition-Based Radio Frequency Interference Mitigation Method on High-Frequency Surface Wave Radar

Recently, high-frequency surface wave radar (HFSWR) has been widely applied in ocean surface dynamic parameter measurement. However, the radar echoes backscattered from ocean surface tend to be contaminated by the external radio frequency interference (RFI), and the mitigation of RFI becomes an intractable problem, especially for wide beam HFSWR. The HFSWR measured data are multichannel. The higher-order singular value decomposition (HOSVD) is an effective method to improve the accuracy of subspace estimation by exploiting this multidimensional structure. In this article, we develop an RFI mitigation method based on the HOSVD algorithm and orthogonal subspace projection. Simulations indicate that, compared with the previous orthogonal subspace projection RFI cancellation schemes, the proposed method has significant advantages in keeping the desired signals while suppressing the interference. The proposed method is applied to the experimental data of the HFSWR with severe RFI. The ocean surface currents inverted from the RFI-mitigated data agree reasonably with the tidal features in the radar detection area. After RFI mitigation, the performance of the HFSWR system is significantly improved on the effective current detection range and precision.

A Digital-Domain Controlled Nonlinear RF Interference Cancellation Scheme for Co-Site Wideband Radios

In this paper, a digital-domain reconstruction based radio-frequency (RF) interference cancellation scheme is proposed to cancel the nonlinear wideband self-interference (SI) signal in co-site radio systems. First, an auxiliary cancellation chain is employed to reconstruct and suppress the SI signal received at the receiver chain. Then, in order to generate the SI replica more accurately, we derive two nonlinear models to predicate the coupling characteristics for the nonlinear wideband SI signal. The proposed models combine the transmit chain and wireless SI channel characteristics, and employ both the Hammerstein model and Wiener model for the transmit chain, respectively. Next, the coefficients of each linear and nonlinear component in the SI chain are integrated into a coefficient vector, which is linear with respect to the output samples, with the proposed models. Based on the linearity, a dynamic least-squares (LS) type algorithm is developed to efficiently estimate and update the coefficients of the proposed models. Finally, experimental results obtained from a FPGA-based testbed have validated that the proposed scheme has a superior interference cancellation performance for nonlinear wideband SI.

RFI Cancellation for an Array Radio Astronomy System With Receiver Nonlinearity

Radio frequency interference (RFI) is an important problem for a radio astronomy system (RAS) due to the very weak radio astronomical signals. Rapid expansions of active wireless systems (AWSs), as well as more demands for spectrum access by both AWS and RAS will result in scenarios with strong RFI. This will cause RAS's low-noise amplifier (LNA) to operate in a nonlinear region and impose a more challenging nonlinear RFI issue. We address this issue for RAS with the antenna array. We develop a new signal model at the output of RAS array processing when the LNA at each RAS receiver behaves nonlinearly. We show that the two existing methods to address RFI in RAS array systems fail to deal with the nonlinear RFI. Next, we propose two modified methods to address the nonlinear RFI. Analysis of the squared error of the RAS signal correlation estimate is also presented. The numerical results show the severe effects of nonlinear RFI and the capability of the proposed methods to keep the squared error almost as low as that of a linear system.

Impact of Nonlinear RFI and Countermeasure for Radio Astronomy Receivers

Radio frequency interference (RFI) has become a critical issue to radio astronomy system (RAS) due to fast-expanding active wireless systems. A recent solution based on a coexistence paradigm offers some RFI-free durations for RAS but further expansion of RAS observation needs to cope with strong RFI. To enhance spectrum access opportunities for both wireless communications and RAS, we explore a new approach where both the systems simultaneously access the spectrum for some durations and RAS exploits RFI cancellation. The main challenge is the nonlinear distortion caused by strong RFI. We develop a signal model incorporating the nonlinearity effect and study its impact on RAS observation performance. We propose an auxiliary receiver assisted nonlinear-RFI cancellation scheme and a modified RAS signal detection scheme. Furthermore, we develop two channel estimators (unscented Kalman filter-based approach and nonlinear-to-linear mapping approach) for nonlinear systems which we apply in our RFI cancellation scheme. Analytical and numerical performance evaluation results show substantial performance advantage of our proposed methods and illustrate importance of incorporating nonlinearity in the channel estimation, RFI cancellation, and RAS signal detection.

Radio frequency interference cancellation for sea-state remote sensing by high-frequency radar

High-frequency surface wave radar (HFSWR) has been widely adopted as a useful remote-sensing tool for sea-state monitoring. There are limitations to the performance because of various unwanted clutters and interferences that contaminate radar signals. Among all types of unwanted signals, radio frequency interference (RFI) is dominant since the frequency bands 3-30-MHz, the operation range of HFSWR, are shared by many radio services. If RFI occupies the areas in the Doppler spectrum that are essential for the estimation of oceanographic parameters, for example, wind speed, current speed, wave height etc., it will significantly reduce the quality of the data. The characteristics of RFI in time, space and Doppler domains are analysed for sea-state monitoring, and a robust and general signal processing method consisting of image recognition, segmentation processing and subspace projection for cancellation of RFI is proposed. The effectiveness of this approach is validated using data obtained with the Pisces HF radar, which is a high-performance radar developed for long-range wave measurement, operating in the lower half of the HF band (5-10-MHz).

Alternative Adaptive Filter Structures for Improved Radio Frequency Interference Cancellation in Radio Astronomy

In radio astronomy, reference signals from auxiliary antennas that receive only the radio frequency interference (RFI) can be modified to model the RFI environment at the astronomy receivers. The RFI can then be canceled from the astronomy signal paths. However, astronomers typically only require signal statistics. If the RFI statistics are changing slowly, the cancellation can be applied to the signal correlations at a much lower rate than is required for standard adaptive filters. In this paper we describe five canceler setups; precorrelation and postcorrelation cancelers that use one or two reference signals in different ways. The theoretical residual RFI and added noise levels are examined and are demonstrated using microwave television RFI at the Australia Telescope Compact Array. The RFI is attenuated to below the system noise, a reduction of at least 20 dB. While dual-reference cancelers add more reference noise than single-reference cancelers, this noise is zero-mean and only adds to the system noise, decreasing the sensitivity. The residual RFI that remains in the output of single-reference cancelers (but not dual-reference cancelers) sets a nonzero noise floor that does not act like random system noise and may limit the achievable sensitivity. Thus, dual-reference cancelers often result in superior cancellation. Dual-reference precorrelation cancelers require a double-canceler setup to be useful and to give equivalent results to dual-reference postcorrelation cancelers.

Overview of technical approaches to radio frequency interference mitigation

This overview provides an interface between lines of thought on radio frequency interference (RFI) mitigation in the fields of radio astronomy and signal processing. The goal is to explore the commonality of different approaches to help researchers in both fields interpret each other's concepts and jargon. The paper elaborates on the astronomers' concept of gain closure relations and how they may be used in a “self‐calibrating” system of RFI cancellation. Further discussion of the eigen‐decomposition method in terms of RFI power and antenna gains introduces adaptive nulling and RFI cancellation through matrix partitioning. While multipath scattering appears at first glance to be fatal to methods of RFI cancellation, its effect is easily incorporated in frequency‐dependent gain coefficients under many circumstances.

Removing Radio Interference from Contaminated Astronomical Spectra Using an Independent Reference Signal and Closure Relations

The growing level of radio frequency interference (RFI) is a recognized problem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscopy where time-averages are recorded. An RFI signal, is constructed from the cross power spectrum of the signals from the two polarizations of a reference horn pointed at the source of the RFI signal. The RFI signal paths obey simple phase and amplitude closure relations, which allows computation of the RFI contamination in the astronomical data and the corrections to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, clean copies of the RFI signal are not required. The method could be generalized (1) to interferometer arrays, (2) to correct for scattered solar radiation that causes spectral standing waves in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a digital correlator plays an important role in broadband pulse dedispersion. Future large radio telescopes, such as the proposed LOFAR and SKA arrays, will require a high degree of RFI suppression and could implement the technique proposed here with the benefit of faster electronics, greater digital precision and higher data rates.