Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #1

JOINT GLOBAL POSITIONING SYSTEM COMBAT EFFECTIVENESS (JGPSCE) JOINT TEST AND EVALUATION (JT&E):
J.R. Greenlee, USAF, JGPSCE JTD; M.E. Torres, USAF, JGPSCE/OA; W.C. Kasper, JGPSCE/XO; D.L. Lester, SRC, JGPSCE/TD

On 29 July 1999, the Office of the Secretary of Defense (OSD), Deputy Director, Development Test and Evaluation (DD, DT&E), Strategic and Tactical Systems (S&TS), in cooperation with the Joint Chiefs of Staff and Services, chartered the JGPSCE JT&E. Over a four-year period, the JGPSCE Joint Test Force (JTF) is conducting a series of test events that focus on joint operations where the Global Positioning System (GPS) is denied or degraded by hostile electronic warfare (EW) or friendly electromagnetic interference (EMI). Specifically, the JGPSCE JT&E is addressing the following issues:

Issue 1: What is the impact of GPS vulnerabilities on the effectiveness of joint operational missions that require precision engagement?
Issue 2: What changes in joint tactics, techniques, and procedures (TTPs) or system-level mitigation techniques improve or maintain joint operational mission effectiveness in the event of GPS EW and EMI?
Issue 3: What test methodologies can be employed to characterize GPS vulnerabilities in future acquisition and integration programs?

The JGPSCE JTF conducted its first field-test - GYPSY ALPHA - from 30 Oct 00 through 17 Nov 00 at White Sands Missile Range (WSMR). GYPSY ALPHA test participants included Army special operations force (SOF) and Marine Corps explosive ordnance disposal (EOD) teams; Army UH-60; Navy F/A-18 and EA-6B; and Air Force MH-53 and HH-60. Over the course of the test, ground teams and aircrews executed realistic missions in the context of a small-scale contingency scenario.

JGPSCE plans to conduct three additional field tests during the 2001-2003 timeframe to address issues of interest to the Department of Defense community. These tests will become increasingly complex with the focus shifting from individual tactical systems and missions to integrated operations at both the tactical and operational levels of warfare. The scenarios will also become more robust as the test context shifts from small-scale contingency to major theater of war operations.

The next test on the immediate horizon is GYPSY BRAVO. This test will be conducted in two parts. The first part will be conducted on the Fallon, Nevada range complex. The second part is bing planned for the Utah Test and Training Range. Detailed planning for GYPSY BRAVO is currently underway and results from this test should provide valuable information to warfighters, weapon system managers, and testers.

This presentation will focus on the JGPSCE test concept, test methodology, and future plans.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #2

ANTI-JAM FOR SHIPBOARD RELATIVE GPS:
K. Falcone, N.B. Jarmale, Mayflower Communications Company, Inc.; F. Allen, Naval Air Warfare Center; J. Clark, Averstar

The Navy JPALS (Joint Precision Approach and Landing System) Architecture Requirements and Definition (ARD) Program objective is to develop a Local Area Differential GPS (LDGPS) based ship landing system architecture that supports CAT II landing applications in a GPS jamming environment. The focus of the research and development in this paper is to develop GPS Anti-Jam Technology ensuring the quality of GPS carrier phase measurements necessary to meet shipboard relative GPS (SRGPS) requirements in a jamming environment. The technical issues involved are the ship's severe RF environment (EMI), the higher jamming vulnerability of carrier phase tracking, and the impact of jamming mitigation on the availability, integrity, and accuracy of GPS carrier phase measurements. The goal is to provide up to 40 dB of additional anti-jam margin on carrier phase tracking against multiple jammers. An additional task is to develop, test, and evaluate Kinematic Carrier Phase Processing Software (KCPP) to meet the SRGPS requirements.

The results presented include software simulations using the Mayflower Simulation Toolkit, along with hardware measurements using COTS products: the Mayflower GPS Receiver (RGR-6000), the Mayflower Small Affordable Anti-jam GPS Antenna Electronics (SAAGA-2A), and the CAST GPS Signal Generator (CCSG). Simulation results show that the Adaptive Temporal Filter (ATF) and Integrated Adaptive Spatial and Temporal Filter (ASTF) GPS anti-jam techniques provide the required anti-jam margin on carrier phase tracking. Laboratory results show that Mayflower ATF technology, embedded in the SAAGA-2A, does not significantly affect GPS carrier phase measurement accuracy. Post processing of the carrier phase measurements with Mayflower relative navigation filter software indicated submeter (cm-level) relative navigation accuracy which meets the shipboard autolanding accuracy requirements with a significant margin. Preliminary laboratory testing of the KCPP software in a realistic simulation environment (real-time, vehicle dynamics, error models) has demonstrated the capability to meet SRGPS requirements.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #3

TEST RESULTS OF A DIGITAL BEAMFORMING GPS RECEIVER IN A JAMMING ENVIRONMENT:
A. Brown, N. Gerein, NAVSYS Corporation

NAVSYS High Gain Advanced GPS Receiver (HAGR) uses a digital beam-steering antenna array to enable up to eight GPS satellites to be tracked, each with up to 10 dBi of additional antenna gain over a conventional receiver solution. This digital, PC-based architecture provides a cost-effective solution for military and commercial applications where more precise GPS measurements are needed in interference environments. The additional gain provided on the satellite signals by the HAGR enables sub-meter Pseudo-ranges to be observed directly on the C/A code and also improves the accuracy of the GPS carrier phase and estimates of the satellite signal strength. The directivity of the digital beams created from the antenna array also reduces mitigates the effects of jamming and interference.

The HAGR digital beam forming receiver maintains the digital beams directed at each satellite using the receiver's navigation solution and the satellite position derived from the ephemeris data.

This paper describes the operation of the HAGR digital beam steering array in a jamming environment and includes test data collected from the HAGR at the Electronic Proving Grounds, Fort Huachuca, that demonstrate the performance of a beam-steering receiver against interference sources.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #4

USE OF ANTI-JAM EQUIPMENT IN LOCATION OF INTERFERENCE SOURCES:
K.A. Falcone, Mayflower Communications Company, Inc.

The use of GPS in aircraft applications provides a necessary update for landing systems, but the low power levels used in GPS leave aircraft susceptible to unintentional interference in the GPS frequency bands. Determining the location of unintentional interference sources and stopping them is becoming important for both civilian and military aircraft. This creates a need for low cost direction finding equipment at GPS frequencies.

The focus of the research and development work described in this paper is to use COTS anti-jam hardware instead of expensive custom equipment to perform the direction finding task. In addition to providing the information necessary to determine the direction of arrival of the interference source, the anti-jam equipment will also protect a GPS receiver from the interference. Position information from this GPS receiver can then be used in combination with the direction of arrival information to locate the interference source.

Our objective is to show that direction finding can be performed using a four element digital adaptive spatial filter. Hardware simulations are used to estimate the performance of different direction finding systems, and laboratory tests are used to verify the simulation models. Simulation implementations used the Mayflower Simulation Toolkit, which incorporates a model of our COTS SAAGA-2A (Small Affordable Anti-jam GPS Antenna) anti-jam equipment enhanced with direction finding algorithms. Laboratory testing was performed on a SAAGA-2A Antenna Electronics Unit.

Direction finding performance was measured using simulations to compare square and triangular antenna array geometries, and direction finding algorithms. Direction finding algorithms include: (1) searching for nulls based on the ASF weights, (2) a monopulse technique, and (3) a simplified MUSIC (MUltiple SIgnal Classification) technique. Results
show that the ease of implementation as well as performance is determined by the combination of array geometry and algorithm. A triangular antenna geometry works better with weight based techniques, while a simplified version of the MUSIC algorithm is easier to implement on a square array.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #5

NORTH SLOPE 2000: FLIGHT TESTING MILITARY/CIVIL AVIATION GPS AGAINST NORTH WARNING SYSTEM RADARS:
V.M. Andreone, SAIC; C. Broughton, 746th Test Squadron; R. Mediavilla, Hill Air Force Base

This paper describes the work performed for the Atmospheric And Early Warning System (AEWS) program office, Hill AFB, UT from January to August 2000. This research evaluated various military and commercial aviation GPS navigation systems during exposure to unintentional jamming by ground surveillance radars. The test goal was to determine if the GPS test units could coexist with the radars and if not, determine the operational limitations to their co-existence. A derivative goal was to compare observed operational limitations to ARNS, WAAS MOPS and other published interference measures.

SAIC and the 746th Test Squadron at Holloman AFB selected for test nine GPS systems with different interference susceptibilities and representing the state of the art in receiver design at the time. These units consisted of commercial 12-channel receivers, military handheld PLGR's, and airborne MAGR's operating in GPS-only mode or using inertial aiding. A mixture of FRPA and CRPA antennas was used. The test aircraft was the 46 Test Group C-12J twin-turbo prop test bed used extensively during measurement of jamming susceptibilities. Racks containing the test receivers, a spectrum analyzer, and recording equipment were installed in the aircraft. After the equipment was checked at Holloman AFB, the aircraft was flown to Eielson AFB, AK.

The test radar sites were two AN/FPS-117 long-range radars and one AN/FPS-124 short-range radar located on Alaska's North Slope. These radars are part of the North Warning Radar (NWS) system extending from Alaska to Greenland and is operated by the AEWS program office. The 84th Radar Evaluation Squadron (84 RADES), Hill AFB UT provided support for coordination of the radars' operation and radar data recording in time synchronization with the aircraft-recorded GPS data. Additionally, the 84 RADES provided Radar Coverage Prediction (RCP) plots for the test radar sites. Using this information, a series of test flights were designed at altitudes between 1,000 to 25,000 feet MSL comprising low altitude radar site over flights to high altitude, multiple radar exposures.

The GPS data recorded in the aircraft was first processed by the 746th Test Squadron to remove format and scaling anomalies, and to limit the data to the test tracks only. The data logs include time figures of merit, position data in various coordinate systems, and carrier-to-noise and jamming-to-signal ratios. SAIC analyzed the data to identify incidences of interference. Each incidence was assessed referring to all test specimen readings, the radar-indicated data, and the individual receiver channel data.

The results demonstrate that both radar and GPS systems can coexist, when the navigation function is considered as the critical parameter. Single channel analysis shows the existence of inadvertent jamming even though the composite solution developed by averaging multiple channels held solid. Predictably, the GPS systems with more sophisticated implementations showed less channel blocking than the lower-end commercial units. Additionally, the results showed that interference standards based on single channel post-correlation carrier-to-noise measures may be unnecessarily stringent. This is attributed to a reduced interference susceptibility resulting from channel and frequency redundancies and ancillary navigation aids.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #6

TERRAIN AIDING FOR PRECISION NAVIGATION IN HEAVY GPS JAMMING:
M.K. Perrett, J.J. Krempasky, Raytheon

OVERVIEW: The Global Positioning System has been used for over a decade by civilian and military user alike. During this time all GPS users have experienced signal degradation, both from intentional and unintentional signal interference.

Navigation systems have successfully integrated various sensors to achieve the required position accuracy. Today the most common military, earth-centered-earth-fixed (ECEF), precise, navigation system is comprised of an inertial measurement system and a GPS receiver. As the dependence on GPS has increased, inexpensive jammers have become more and more available, GPS receivers (and antennas) have become more complex to defeat the threat, and the cycle repeats.

This paper lays out the design architecture of a system that utilizes a coherent radar altimeter and the digital terrain elevation database, to provide precision terrain aided navigation (PTAN). This system is integrated into an existing navigation system, which provides navigation in the presence of GPS jamming.

BACKGROUND: Last year the Space Shuttle mapped terrain height for the earth's landmass from approximately North 60 Degrees to South 60 Degrees of Latitude; this area comprises over eighty percent of the earths land mass. This data, in digital format, will be available in the first quarter of 2002, at up to the Level II accuracy (30.6 meters square per cell). The total digital terrain elevation data (DTED) base is available at several levels of precision, from Level I at 90 meters square per cell down to Level V at 0.9 meters square.

In 1999 the Raytheon Company, under contract with the United States Air Force and in cooperation with Sandia National Labs and the Honeywell Company, accomplished an initial PTAN design feasibility study for cruise missile applications. In 2000, Raytheon, Sandia National Labs and Honeywell submitted a proposal to the Office of Naval Research to design and test a GPS/IMU/PTAN based navigation system. Although the proposal was declined, significant progress has been achieved in the design architecture of a system. The United States Navy is currently looking at the feasibility of PTAN aiding on the Tactical Tomahawk cruise missile.

EXPECTED SYSTEM ACCURACY: The precision and accuracy of a PTAN position solution is dependent on several things:
-The ability of the radar to resolve a "patch" on the ground. If the radar altimeter system has a larger footprint than the DTED cell it is trying to resolve, the resultant measurement will not be precise. In order to provide GPS accuracies, DTED Level I or II is sufficient, but to provide precision strike capabilities the use of DTED Levels 3 or 4 is r
recommended.
-The flatness, roughness and uniqueness of the terrain. Each of these terrain characteristics limits the usability of the terrain for navigation.
-The navigation (Kalman) filter. The blending of the independent sensors is done in a Kalman Filter. The robustness of the navigation product is a dependent upon proper modeling of each of the navigation system sensors.

SIMULATION RESULTS: Covariance and six degree of freedom tests have shown the feasibility of a GPS/PTAN system providing required accuracies.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #7

AN IMPROVED ADAPTIVE SPATIAL TEMPORAL SELECTIVE ATTENUATOR:
l.F. Progri, W.R. Michalson, Worcester Polytechnic Institute

While the adaptive spatial and temporal selective attenuator (ASTSA) mitigates successfully the sensitivity of a finite number of spatially and temporally separated undesired signals/sources it distorts the desired signal phase. Despite the improvements in the acquisition and tracking of the C/A code and in the navigation with C/A code, there are a number of applications, which depend on the successful tracking and navigation with accumulated carrier in benign environment.

An improved ASTSA will restore its output phase adaptively; thus, enabling the successful carrier tracking and navigation with accumulated carrier phase. This paper explores the issues associated with ASTSA phase restoration and provides an improved design of the ASTSA with restored phase.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Paper #8

STUDY AND COMPARISON OF THREE INTERFERENCE MITIGATION TECHNIQUES FOR GPS RECEIVERS:
V. Calmettes, M. Bousquet, SUPAERO; F. Pradeilles, CTA

GPS system suffers from many sources of interference. A large number of mitigation techniques have been investigated to improve the performance of the GPS receivers. The aim of this paper is to analyze different pre-correlation methods used to eliminate CW, FM and pulsed interference.

The first technique consists in a pre-whitening linear filter in front of the conventional receiver. When the filtering is performed in frequency domain the computing load is important. Moreover it can result in an significant correlation loss in the presence of wide band interference such as FM or pulsed jammers. Hence we suggest a structure made out of adaptive notch filters connected in series, and controlled by the LMS algorithm [1]. This structure is modified to take into account the complex signal. An analysis of the convergence based on the statistic of the estimation error permits to define the filter parameters in the presence of CW, FM interference and for multiple jammers. Additionally the expression of the signal at the output of the DLL discriminator is determined when the interference excision is performed. It is shown that the same filter must be applied to the local code to compensate the distortion due to the filter group delay.

The second technique is derived from the detection theory. When the signal is embedded in a non gaussian noise a locally optimum test is defined. By expanding the probability density function (PDF) around the H0 hypothesis the likelihood ratio of the optimum receiver is obtained [2]. In that case the detector uses a non linearity deduced from the noise PDF. This theory leads to a non linear operator applied to the amplitude of the complex GPS signal defined by its polar components. This technique is known as Amplitude Domain Processing [3]. The expression of this operator is given in the presence of a CW, FM and pulsed jammer. The processing gain is computed by taking into account the phase distribution at the output of this non linear filter.

This technique gives impressive results when the interference consists in a single jammer. To eliminate multiple jammers the processing can be carried out in the frequency domain. The expression of the non linear operator is determined. This is a function of the frequency depending on the amplitude of the interference. Finally we present a method that can be applied when the Fourier transform allows to separate the components of the interference.

This paper permits to define a strategy for different types of interference. In the presence of narrow band, slowly time-varying interference notch filter can be applied to the input signal without introducing significant distortion. The amplitude domain processing performed in time domain is well suitable to pulsed jammer and rapidly time-varying jammer, but its performances decreases with the number of jammers. The same method can be carried out in frequency domain to eliminate fixed multiple jammers without damaging the GPS signal.

[1] Ren Jr. Landry, Vincent Calmettes, Alain Ducasse, Impact of Interference on the New COSSAP GPS Receiver and Mitigation Techniques Evaluation, Navigation 2000, ION National Technical Meeting, Long Beach, CA, 21-23 Janvier 1998.

[2] S. A. Kassam, Signal Detection in Non-Gaussian Noise, Springer-Verlag, 1988.

[3] J. Przyjemski, E. Balboni, J. Dowle, GPS Anti-Jam Enhancement Techniques, Proceedings of the 49th annual Meeting on Future Global Navigation and Guidance, Cambridge, MA, pp. 41-50, June 1993.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Alternate #1

GIJET^TM: AN AUTOMATED TOOL FOR MISSION PLANNING IN GPS-DENIED AREAS:
M. Grace, Toyon Research Corporation; S. Minarik, B. Tanju, SPAWAR PMW/PMA-156

GPS is a mission-critical system supporting U.S. military forces through all levels of conflict. However, due to the availability of accurate, low-cost, commercial GPS receivers, even adversaries may use GPS-guided weapons against U.S. forces. This creates a complex problem for electronic warfare planners who would like to selectively deny GPS to an adversary while preserving GPS for friendly forces. Under SBIR sponsorship from the Navy Space and Naval Warfare Systems Command, Toyon Research Corporation is developing the GPS Intelligent Jammer Evaluation Tool (GIJET) to address this problem. The software tool uses terrain and environmental data, advanced propagation models, complex antenna patterns, and adaptive signal processing models to optimally locate and configure jammers to deny GPS in a controlled manner. A graphical user interface simplifies data-entry, visualization of GPS jamming effectiveness, and selection of parameters to be included in the optimization while a highly flexible scripting capability allows control over the application of different multi-dimensional optimization algorithms. Since the jammer placement problem can be computationally intensive, these scripts may also be used to control the application of simplifying assumptions during the optimization. During Phase II, the software is being enhanced to solve the problems of optimal routing and mission assessment in GPS-jammed environments, as well as optimal configuration of GPS augmentation systems such as military pseudolites. GIJET will not only improve the Navy's warfighting capability and increase fleet survivability, but also will improve fleet training and simplify test planning of anti-jam GPS equipment during peacetime. The basic software architecture is also well-suited to other optimization problems involving sensors and communications systems. This paper summarizes the design, use, and capabilities of GIJET and presents an example of optimal jammer location.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Alternate #2

GPS INTERFERENCE DETECTION AND IDENTIFICATION USING MULTICORRELATOR RECEIVERS:
F. Bastide, E. Chatre, STNA, France; C. Macabiau, ENAC, France

Several types of perturbations can affect the signal processed by a GPS receiver. These perturbations are thermal noise, atmospheric disturbances, multipath and interference. Interference remains the most feared perturbation for civil aviation users because it can affect several tracking channels at a time during a long period. A large number of techniques were designed to alleviate the sensitivity of modern receivers to this perturbation. Most of these techniques are either based on spatial discrimination like adaptive antennas, on spectral selectivity such as notch filters or on amplitude detection.

A previous study [Macabiau, Chatre, Julien, 2001] showed that multicorrelator receivers enable the characterization of interference effects on the tracking loops through the analysis of the shape of the correlation peak.

The aim of this paper is to present techniques based on multicorrelator receivers that can be used to detect interferers and estimate their characteristics (power, type, central frequency, bandwidth).

The paper starts by recalling theoretical expressions of the code and phase tracking loops I and Q samples in presence of CW and FM interference. Then we present some techniques that can be used to detect interference based on reported C/N0 variation as well as observation of AR models prediction errors and variance changes in the subtraction of the ideal correlation peak from the correlator outputs.

Next interference estimation techniques are presented based on classical spectral analysis as well as Prony and AR models.

Performance of these techniques is then assessed on data collected using a multicorrelator receiver connected to a GPS signal RF generator and affected by several CW and FM interferers with different power levels, bandwidth and central frequency.

Finally, a real time tool allowing interference detection and estimation is presented.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Alternate #3

GPS SIGNAL DEGRADATION MODELING AND VALIDATION FOR GPS WEAK SIGNAL TRACKING PERFORMANCE TEST:
G. MacGougan, G. Jee, G. Lachapelle, M.E. Cannon, University of Calgary, Canada; L. Garin, J. Shewfelt, SiRF

The use of GPS receivers in cellular telephones to locate mobile users has become a viable option to meet the FCC requirement for E-911 callers. In most cases, the mobile phone will however be shaded from direct line-of-sight L1 GPS signals. The received signals will therefore be weaker than line-of-sight signals and will be affected by multipath. GPS signal fading may result in the loss of satellites tracking and increased noise, while multipath will degrade location accuracy. The loss of GPS satellites will degrade location geometry and, consequently, location accuracy.

GPS receivers intended for use with cellular phones are required to have the capability to track weaker GPS signals than direct line-of-sight signals under various scenarios like urban canyons, foliage, indoors, etc. In order to assess the receiver performance in these scenarios, acceptable testing methods are required. Since testing under a wide range of real environments is not viable, standard simulation models to conduct such tests are needed. In this paper, a GPS signal degradation model is developed for testing GPS weak signal tracking performance.

The GPS signal propagation scenarios are classified into several cases that characterize typical mobile phone user environments. Theoretical 1.5 GHz L1 GPS signal fading models of different environments will be developed based on an existing RF propagation models. These models will be verified to a reasonable extent using real data collection. A GPS simulator will then be used for characterizing receiver performance under various conditions and representative environments corresponding to the developed models.
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Session C2: JAMMING & UNINTENTIONAL INTERFERENCE
Alternate #4

ANTI-JAMMING SOLUTION OF GPS RECEIVER USING NONLINEAR ADAPTIVE PREDICTOR:
W.L. Mao, H.W. Tsao, F.R. Chang, National Taiwan University, Taiwan

The global positioning system (GPS) can provide accurate positioning and timing information that are useful in many applications. Commercial aircrafts are susceptible to having their GPS receivers jammed due to the presence of interference signal. The civil GPS receivers also have similar problems due to those intentional and non-intentional obstacles. However, it has been demonstrated that the performance of direct sequence spread spectrum system in the presence of jamming signal can be enhanced through the use of adaptive filters.

In this paper, the received signal has been passed through a filter matched to the chip interval. The equivalent baseband waveform have 3 components: GPS spread spectrum components, the additive white noise and jamming signal. They are assumed to be mutually independent. Two kinds of jamming are investigated. One is single tone continuous wave interference (CWI). The other is a periodically swept FM signal.

A nonlinear adaptive predictor (NAP), which consists of two subsections: a pipelined recurrent neural network (PRNN) and a conventional tapped delay line filter, is used to suppress the jamming signal. The PRNN is composed of M identical modules. Each module is organized by a fully connected recurrent neural network with N neurons. A gradient descent learning algorithm that minimizes the overall cost function is adopted to update the synaptic weight. The output variables of PRNN are then fed into the tapped delay line filter in order to accomplish the one step prediction. The conventional normalized least mean square (LMS) algorithm of the linear subsection will update the adjustable weights. Simulation results show that our method can indeed achieve a superior signal-to-noise ratio (SNR) performance relative to the conventional linear adaptive filter.
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