CW Interference Research Articles

Envelope Detection by Shannon Energy Calculation in DCT Domain and DFS-Based Notch Filter for Interference Mitigation in GNSS Receivers

A new pre-correlation technique to enhance the Global Navigation Satellite Systems (GNSS) receivers, against the continuous wave interferences (CWI), is presented. Accordingly, the detection and the localisation procedure are constituted of many steps. First, the discrete cosine transform (DCT) is applied on the contaminated signal. Next, the Shannon-energy envelope detector, in the DCT-domain, is accomplished. Then, all envelope magnitudes above a predefined threshold, representing the interference components, are localized in the frequency domain. The following step consists in the use of the discrete Fourier series (DFS) technique to calculate the corresponding contributing harmonics of the CW interferences. Finally, the interference is reduced efficiently by a subtraction of its approximated version from the original contaminated signal. The results provided from the simulation prove that the DFS-based notch filter in terms of signal quality restoration, for both single-tone and multi-tone, is of superior performance compared to the classical notch filtering.

Multi-Frequency Interference Detection and Mitigation Using Multiple Adaptive IIR Notch Filter with Lattice Structure

Radio Frequency Interferences (RFI), such as strong Continuous Wave Interferences (CWI), can influence the Quality of Service (QoS) of communications, increasing the Bit Error Rate (BER) and decreasing the Signal-to-Noise Ratio (SNR) in any wireless transmission, including in a Digital Video Broadcasting (DVB-S2) receiver. Therefore, this paper presents an algorithm for detecting and mitigating a Multi-tone Continuous Wave Interference (MCWI) using a Multiple Adaptive Notch Filter (MANF), based on the lattice form structure. The Adaptive Notch Filter (ANF) is constructed using the second-order IIR NF. The approach consists in developing a robust low-complexity algorithm for removing unknown MCWI. The MANF model is a multistage model, with each stage consisting of two ANFs: the adaptive IIR notch filter Hl(z) and the adaptive IIR notch filter HN(z), which can detect and mitigate CWI. In this model, the ANF is used for estimating the Jamming-to-Signal Ratio (JSR) and the frequency of the interference (w(0)) by using an LMS-based algorithm. The depth of the notch is then adjusted based on the estimation of the JSR. In contrast, the ANF HN(z) is used to mitigate the CW interference. Simulation results show that the proposed ANF is an effective method for eliminating/reducing the effects of MCWI, and yields better system performance than full suppression (kN=1) for low JSR values, and mostly the same performance for high JSR values. Moreover, the proposed can detect low and high JSR and track hopping frequency interference and provides better Bit error ratio (BER) performance compared to the case without an IIR notch filter.

Performance Comparison of Code Discriminators in the Presence of CW Interference

This paper considers the effect of continuous wave interference (CWI) on code tracking loops for global navigation satellite system signal. Based on the integrate-dump model analysis, the overall correlation results in the presence of CWI are theoretically analyzed incorporating the sum frequency interference terms which were regarded as negligible quantities in previous studies. Using the obtained correlation results, the closed-form expressions of the code tracking errors caused by CWI for three types of code discriminators (coherent, noncoherent, and barycenter) are derived and formulated as the solutions of quadratic equations using reasonable assumptions of trigonometric functions. Simulations are performed to demonstrate the accuracy of the theoretical correlation results. Also, the simulated code tracking errors are also presented to corroborate the effectiveness of the closed-form code tracking errors in terms of interference parameters (frequency and phase). It is indicated that the barycenter discriminator has the superior performance and could be preferred for mitigating CWI in tracking stage. These results can be applied for performance comparison, signal design, receiver design, and interference mitigation techniques in the presence of CWI.

New decision variables for GNSS acquisition in the presence of CW interference

This paper investigates the problem of Global Navigation Satellite System signal acquisition using a consumer-grade receiver in the presence of continuous wave interference. By examining in detail the impact of interference on the acquisition search space, a set decision variables are developed. These decision variables leverage the systematic properties of the interference observed in the search space, manipulating or differencing portions of it to achieve improved detection performance. Strategies based on these variables are examined under a selection of operating scenarios including acquisition in the presence of interference when the receiver has some, or has no, a priori information regarding the interference. It is shown that a receiver employing these new schemes can tolerate up to 10 dB more interference power than a traditional receiver.

Design of an Enhanced TDOA Method for Swept CW Interferences

Recently, devices like Personal Privacy Devices (PPD) are being employed to avoid the detection of one's location by GPS, and most PPD transmits swept CW signals. However, signals transmitted from PPD may interfere a precise location system based on GPS. Accordingly, in order to reduce interferences by PPD, a technique to locate an interferer is needed. In order to locate an interferer AOA method and TDOA method are generally used, TDOA method is known to be more accurate than AOA method. Unfortunately, TDOA method has a problem of ambiguity in obtaining measurements of swept CW interference. Thus, this paper design a localization algorithm based on TDOA method that can accurately locate an interferer transmitting swept CW signals by resolving problem of ambiguity. In addition, feasibility of the designed algorithm has been verified by simulation results.

CW Interference Effects on Tracking Performance of GNSS Receivers

Analytical expressions are suggested for GNSS receiver performance such as the effective <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , code tracking (DLL) error and carrier phase tracking (PLL) error for a receiver affected by CW interference signals. The navigation signal model is applied to GNSS power spectral density (PSD) to interpret spectral line effects within receiver tracking low pass filter, limited by navigation data bits. A scaled envelope model is also introduced to estimate average susceptibility of signal against CW interferences. Numerical results show how the spectral lines and correlator model can affect the tracking performances, and the sensitivity of GNSS signals against the CW interferer is investigated by varying data bit duration or integration time.

Switchable Beam Steering/Null Steering Algorithm for CW Interference Mitigation in GPS C/A Code Receivers

CW interference can be a severe threat to Global Positioning System coarse acquisition (GPS C/A) code receivers because it can mix with one of the spectrum lines of the GPS C/A code and affect the signal quality even with a very low power. Spatial domain processing techniques in CW interference mitigation as alternatives to frequency and time domain processing are investigated. Using a multi-antenna system some common adaptive filtering criteria are examined in the presence of CW interference. No single criterion performs best when the relative frequency of the CW interference and the satellite signal changes. An algorithm is proposed---the switchable steering algorithm---which combines both beam steering and null steering criteria and is influenced by the frequency difference between the CW interference and the satellite Doppler. Mathematical analyses and simulation results demonstrate that this algorithm can outperform either single criterion.

A Statistical Inference Technique for GPS Interference Detection

Narrowband interference can severely degrade the performance of GPS receivers. Detecting the interference and then characterizing it can lead to its removal. Knowledge that can be useful is the location of the source or direction of arrival of the interference (spatial-directional characteristics) and/or the time specification, frequency, and power of the interference (temporal-spectral characteristics). This work focuses on the second type. In previous work power spectral density fluctuation detection technique has been introduced [1], and statistical inference has been used to detect narrowband CW interference in the GPS signal band after being captured by the RF front end. In this technique, an assessment window of data is first divided into blocks and then analyzed to obtain the statistical parameters of the signal without Then presence of interference is tested by comparing these parameters as a null hypothesis with the parameters extracted from a later window. The effect of the assessment window size on the level of confidence of this test has been already analyzed also in [1]. In this paper, using properties of the fast Fourier transform (FFT) and the effects of the number of FFT points on the test statistic mean and variance, we show that an appropriate data block size (for a specific window size) can be selected to achieve an optimum solution to detect minimum power detectable interference. The algorithm to find the optimal solution is introduced. The theoretical result is supported by real experiments. Finally this algorithm is employed to detect a real GPS interference signal generated by a TV transmitter in Sydney, Australia.

Two-Pole and Multi-Pole Notch Filters: A Computationally Effective Solution for GNSS Interference Detection and Mitigation

In a Global Navigation Satellite System (GNSS) receiver the presence of detection and mitigation units, capable of reducing the impact of disturbing signals, can extremely enhance the position accuracy. However the presence of such units is usually limited to professional receivers that dispose of additional computational power that can be used for interference detection and mitigation. In this paper the two-pole notch filter, the natural extension of the one-pole notch filter analyzed in [1], is proposed as computationally effective solution for interference detection and mitigation. The notch filter structure and the adaptive algorithm employed for tracking the disturbing signal are analyzed, and an interference detection unit, based on the adaptive algorithm convergence, is proposed. The two-pole notch filter coupled with the detection unit is used as elementary block for the design of a multi-pole notch filter that can efficiently mitigate more than one CW interference. Theoretical and simulative analyses show the feasibility and the good performance of the proposed method.

A preventative approach to mitigating CW interference in GPS receivers

A preventative approach to mitigating CW interference in GPS receivers

Low cost architecture of direct conversion digital receiver

The paper presents a low-cost architecture for a direct conversion digital receiver that uses six-port technology. An experimental prototype has been constructed and tested. Analogue carrier recovery and decision circuits are investigated as a means to provide future system integration and high data rate capacity. The designed receiver is operated at an ISM frequency of 2.45 GHz and uses a QPSK modulation format. Results on data rate limitations are given for QPSK signals up to 52 Mb/s. Test results related to adjacent channel, co-channel and CW interferences are presented for a data rate of 40 Mb/s. Phase offset between carrier and reference signals, along with carrier frequency deviation performances, are also presented for a rate of 40 Mb/s. A frequency hopping spread spectrum technique is discussed on the basis of the proposed architecture.

Performance of M-ary/spread spectrum communication system in the presence of continuous wave interference

This paper describes an evaluation method and performance improvement for an M-ary/spread spectrum (SS) communication system in the presence of continuous wave interference. First, we introduce a theoretical equation for the average bit error rate of M-ary/SS receiver in a transmission line with AWGN and single CW interference. Second, we define the required jamming margin for M-ary/SS receiver to satisfy a certain level of average bit error rate. Then we analyze the theoretical value of the required jamming margin and evaluate the performance of receiver by a simulation. From this study, we found that the jamming margin relates to the spectrum spreading factor of the total system and does not relate to code length of pseudo-random noise sequence. To improve performance against CW interference, we first examine the effect of improvement when we use error correcting code as a spreading code. However, we cannot find any effect of the improvement, even if the jamming margin relates to spectrum spreading factor and even if there is a coding gain by the error correcting code. Second, we studied the performance improvement when whitening filter based on LMS algorithm is added to the receiver to reduce CW jamming. For a spreading factor between 7 and 25 dB, the jamming margin can be improved by 15 dB to 25 dB. © 1998 Scripta Technica. Electron Comm Jpn Pt 3, 81(5): 32–40, 1998

Preprocessing of GNSS signals subject to interference

Increasingly, interference effects on GNSS receivers are becoming important as more safety-critical applications such as civil aviation are employing the system. The inherent interference rejection capability of the GNSS spread spectrum modulating technique is now no longer considered adequate to remove the effects of very strong jammers. These can be generated intentionally, as in jamming situations, or unintentionally by systems such as mobile satellite systems. Continuous satellite tracking through this interference is essential for high-integrity applications. This study describes a new hardware-based GNSS interference mitigation scheme which is particularly effective against CW and pulsed CW interference. Interferers with some degree of frequency or amplitude modulation can also be reduced. The core hardware can remove two CW interferers from anywhere within the GPS P code bandwidth or the GLONASS frequency spectrum and further interferers can be removed by cascading modules. Simulation and actual data are shown. Owing to the wideband nature of the signal processing, the module introduces very little additional phase distortion and thus group delay to the GLONASS band, which has been shown to be advantageous to differential GLONASS processing. © 1997 John Wiley & Sons, Ltd.

Rejection of frequency sweeping signal in DS spread spectrum systems using complex adaptive filters

It is already known that the performance of narrowband interference adaptive filtering in DS spread spectrum systems is additionally degraded in the presence of a frequency hopped jamming signal compared to the case with CW interference. Even so, the adaptive filter can be rather efficient because during the hopping interval, while the jamming signal parameters are constant, the filter adjusts its weights; and if the hopping rate is not too high, there are time periods when the interfering signal is suppressed. In order to have a better insight into the performance of these systems, we analyze the DS spread spectrum receiver behavior, when the jamming signal frequency is continuously changing (chirp signal). We find that this is a reasonable step for a jammer that is trying to be more efficient. In such a case, a two-sided filter structure shows worse overall performance than a prediction error structure. As the jamming frequency sweeping rate or bandwidth is increased, the system performance is degraded. The analysis provides an insight into how much the system performance will be degraded as a function of the interfering (frequency sweeping) signal parameters. >

Letter: Cycle slip statistics of a first‐order phase‐locked loop in the presence of additive noise and cochannel interference

AbstractNumerical results on the mean cycle slip rate of a first‐order phase‐locked loop due to additive Gaussian noise and cochannel CW interference reported in [1] are corrected. In addition, the mean first slip time is given under the same situation. It is shown that both the mean cycle slip rate and the mean first slip time increase with growing I/S ratio. The analysis is supported by extensive computer simulation.

Multiple cochannel interference effects in a first‐order phase‐locked loop

AbstractThe phase error statistics of a first‐order phase‐locked loop in the presence of multiple cochannel CW interference and additive noise is derived. An accurate numerical solution based on the nonlinear theory is outlined. Approximate analytical formulae are obtained from the linear theory for low interference‐to‐signal ratios. The linear analysis shows that the phase error statistics in the linear approximation depends only on the interference‐to‐signal ratio independent of the number of interferers and the interference power distribution. Errors appeared in some of the numerical results reported by Yoon and Lindsey [1] in the single interferer case are identified and corrected. Numerical examples for N = 2, 3, and 6 interferers of equal power are given. The effect of the interference power distribution is investigated.

Author's reply: "On the phase error variance of a first-order phase-locked loop in the presence of cochannel CW interference and additive noise"

The author of the original article (ibid., vol.42, p.2672, no.9, 1994) is concerned with the use of the averaged conditional phase-error variance, as a performance measure for a PLL operating in the presence of CW interferers. He correctly points out that the variance of the random variable /spl phi/ can be obtained using the equation given; however, the physics of the problem suggests that the average of the conditional variance used by Lindsey and Yoon (1982) is perhaps the more meaningful performance measure for use in loop design and performance evaluation. The author discusses this issue further in his reply. >

On the phase error variance of a first-order phase-locked loop in the presence of cochannel CW interference and additive noise

The qualitatively different behavior of the PLL phase error variance reported by Yoon and Lindsey (1982) and Sarkar (1990) under the same assumptions is examined. An error which appeared in Yoon and Lindsey is identified and corrected. >

Increasing the up‐link CW interference immunity of non‐coherent direct sequence pseudonoise (DSPN) reception with on‐board processing

AbstractThe adaptive threshold A/D converter is by now a familiar technique for increasing the CW interference immunity of coherently detected direct sequence pseudonoise (DSPN) spread‐spectrum signals, especially where the frequency of the CW interferer may be changing so rapidly and over such a large portion of the signalling bandwidth that well known spectrum excision techniques prove ineffective. The present paper extends the theory to the case of non‐coherent reception of direct sequence pseudonoise (DSPN) signals. The A/D converter is found to perform well in both CW and Gaussian interference. Conversion gain is seen to persist even if the interference is intentionally pulsed at an optimally reduced duty factor (from the interferer's point of view) in the attempt to confuse the adaptive threshold. The specific case of 32‐chip data symbols is treated, wherein the adaptation is applied strictly to individual symbols. In some cases these restrictions will actually improve performance.

A data-derived reference signal technique for adaptive arrays

A new technique is presented for reference signal generation in adaptive arrays, and it is shown how it can be used in a binary FSK communication system. This method consists of encoding the message bit stream at the transmitter to convert it to a first-order Markov source with a transition probability of p>1/2. Then, in the receiver, each detected bit is used to predict the next bit, and the predicted bits are used to generate a reference signal. The performance of an FSK system based on this concept is analyzed, and an experimental system is described. This experimental system demonstrated that the reference signal generation concept is feasible and showed that this technique could be used to protect an FSK communication system from a CW interference signal. >