Luận văn nghiên cứu và phát triển kỹ thuật chỗng nhiễu giao thoa trong bộ thu gnss research and development of advanced interference mitigation techniques in gnss receivers

Chapter 2 Notch Filter for GNSS Narrowband Interference Mitigation.. New Design of Narrowband Interference Mitigation Module for GNSS Receiver.... LIST OF ABRREVIATIONS ACARS Airorall

MINISTRY OF EDUCATION AND TRAINING

NGUYEN THI THANH TU

RESEARCH AND DEVELOPMENT OF ADVANCED INTERFERENCE MITIGATION TECHNIQUES

BQ GIAO DUC ¥A DAO TAO

TRUONG DAI HOC BACH KHOA HA NOI

NGUYEN THE THANH TU

NGHIÊN CỨU VÀ PHÁT TRIEN

KỸ THUẬT CHONG NHIEU GIAO THOA

TRONG BO THU GNSS

_ LUẬN VĂN THẠC SĨ KHOA HỌC _

K¥ THUAT MAY TINH VA TRUYEN THONG

NGƯỜI HƯỚNG DẪN KHOA HOC

1 T8 Tạ Hải Tùng

2 I'S Beatrice Motella

Hà Nội — 2015

ACKNOWLEDGMENTS

In the first words of this thesis, | am extremely grateful to those who in various ways

contributed to all the results presented in this thesis

Voremost, | would like to express my sincere gratitude to my supervisors Dr ‘la Llai Tung and Dr Beatrice Motella for their patience, encouragement, and broad research vision, Their guidance helped me in all the Limne of this study and writing of this thesis

Tn addition, T deeply acknowledge Prof Gustavo Relforte for his enthusiastic supports

I would like to thank the supports from all the NAVIS members I am thankful to Tran Trung Hieu, Nguyen Dinh Thuan, Truong Minh Duc, who gave me a lot of helpful advice | also appreciate Ms Nguyen ‘Thi lla Phuong, who has always encouraged me since the very beginning of my study

The list could not be complete without the Growing NAVIS project, funded by the Furopean Commission under the FP7 Call Galileo.2011.43-1 —Tnternational Activities (Grant Agreement No 287203), for supporting my intomship at ISMB from May to July, 2013

Last, and most importantly, | would like to thank my parents, my sister and her family for their love that definitely is my limitless support

COMMITMENT

1 commit myself to be the person who was responsible for conducting this study All reference figures were extracted with clear derivation The presented results are truthful and have not published in any other person's work

NGUYEN Thi Thanh Tu

TOM TAT LUAN VAN

Ngày nay, các hệ thống dinh vi toan cau st: dung vé tinh (GNSS Global Navigation

Satelite System) đóng vai trỏ rất quan trong trong rất nhiều lĩnh vực khác nhau Một số img dung GNSS dan sur quan trong có thế kế đên như: giám sát các phương tiện giao thông, đồng bộ trong truyền thông và bán đề Do đó, yêu cầu về tính chính xác và liên tục đổi với các bộ thu GNSS ngày cảng trở nên cấp thiết

Việc sử dụng kỹ thuật trải phê trực tiếp, mang lại nhiều wu diém trong chống nhiễu cho

cac hé théng GNSS nhu GPS, Galileo ‘fuy nhién, các tin hiệu GNSS nhận được tại

antenna thường có năng lượng thắp, dẫn đến việc chủng để bi tác động bởi cáo nguồn nhiều khác nhau Tắt eã cóc tin hiệu được truyền trong phạmn vị tân sở gần với băng tan tín hiệu GNSS đều có thể trở thành nguồn gây nhiều cho bộ thu GNSS

Có nhiễu loại nhiều khác nhau tác động lên bộ thu GNSS, trong dỏ, nhiễu bằng hẹp (narrow-band interference _ NIHI) là nhiều có ảnh hưởng lớn nhất tới hiệu năng bộ thu

GNSS Các nghiên cứu gần đây sử dụng Notch filler trong lọc nhiễt ig hep cho bộ

thu GN88 cho thấy đây là phương pháp khả thú và hiệu quả Các bộ lọc notch filter dáp

từng xưng vô hạn có độ phúc tạp không caa vá có thể 48 dang triển khai một cách và

hiệu quả

Trong khuôn khổ luận văn, tác giả đề xuất một thiết kế mới cho khối chồng nhiều bằng,

hep trong bé thu GNSS Khéi này có khả năng phân loại nhiều băng hẹp: và tự động

diễu chỉnh cầu hình notch filter, sao cho chất lượng tỉn hiệu dầu ra dạt tốt nhất

Luận văn được xây dựng gồm 5 chương như sau:

—_ Chương 1 Fundamentals Mõ tả sơ lược vẻ một hệ thông GNSS nói chung, va nguyên tắc hoạt động cúa hệ thông, Irình bảy các khái niệm liên quan đến nhiều (interference), ảnh hưởng của nhiễu lên hiệu năng bộ fhu và tổng quan vẻ cáo phương pháp phòng chống,

—_ Chương 2 Nofch [iller for GNSS narrowband interference mitigation Gidi thiệu Notch fiter, bộ lọc cỏ khả năng loại bó nhiễu băng hẹp trong bộ thu

GNSS, cáo tham sổ quan trọng của Noich ñilter, và một cải tiên của Notch filter

giúp bộ lọc có khả năng tự phát hiện lân số của nhiều bằng hợp:

— Chương 3 New design of notch filters on GNSS narrowband interference

miligation Để xuất một thiết kế mới của khối chống nhiều băng hợp trong bộ

tha GNSS

[rinh bảy các kết quá đánh giá để xuất đã

Chuong 4 Performance analy:

néu trong chuong 3

— Chung 5 Conelusinn Tỏm lắt nội dung luận vẫn, các két quả đã đạt được

Chapter 2 Notch Filter for GNSS Narrowband Interference Mitigation

2.3 Using Notch Filter on GNSS Narowband Interference Mitieatien

3.2 New Design of Narrowband Interference Mitigation Module for GNSS

Receiver

3.2.1 Interference Characterization Block

3.22 Notch Filter Configuration

Chapter 4 Performance Analysis

4.1 Bandwidth Estimation Algorithm

4.2 Qualiy of Fitered Signal, cọc neo

LIST OF FIGURES

Figure 1.1, Fundamental of a satellite navigation system cesses

Kigure 2.1 l'requency response of notch filter for two difference pole factors

Figure 2.2 Spectrum at the front-end output at different time instants (from [4]) 28 Figure 2.3 Power of filter output when notch frequency vazies

Figure 2.4 Spectrum of an interfered signal before and after filtering - 30

Figure 2.6, High-level block diagram of the GNSS receiver ehai

Figure 3.1 Proposed Interference Detection/ Estimation Unit 3

Figure 3.4 The signal power at the output of a NF and its derivative Bandwidth of NF

is fixed at 30 KHz and whose frequency swcops over the spectrum range

Ligure 3.5 Bandwidth estimation algorithm

Figure 3.6 Interference bandwidth estimation for different NF's bandwidth 45

Figure 4.1 Simulation results: estimation of the interference bandwidth (A; =

Figure 4.2 Simulation results: estimation of the interference bandwidth (B; = 25 kHz)

56

Figure 4.3 Estimated interference bandwidth over time, in case of variable band 5 4

Figure 4.4, Theoretical and practical C/NO when bandwidth of interference (By =

Figure 4.5, Theoretical and simulated C/NO in case of CWI (C/N0 = 45 đB,P, =

LIST OF TABLES

‘Table 1.1 Interference threshold versus interference bandwidth for GPS receivers in

Table 1.2 Potential interference sourecs [6]

‘Table 1.3 Possible interference mitigation techniques to a standard GPS receiver [17]

LIST OF ABRREVIATIONS

ACARS Airorall Communication Addressmg and Reporting System

CANO Carrier to Noise density ratio

cWL Continuous Wave Interference

Direct Sequence-Spread Spectru

EGNOS Buropean Geostationary Navigation Overlay Service

FFT Fast Fourier Transform

FIR Finite Impulse Response

GNSS Global Navigation Satellite System

IcB Interference Classification Block

TF Trlennediate Frequertuy

IR Infinite Impulse Response

IRNSS Indian Regional Navigation Satellite System

LMS Least Mean Square

MSAS Multi-functional Satellite Augmentation System

NBL Narrow-band interference

NCB Notch Filter Configuration Block

Q785 Quagi-Zenith Satellite System

RET Radia Frequency Interference

SNR Signal Lo Noise Ratio

WAAS Wide Area Augmentation System

WBI Wide-band Interference

INTRODUCTION

Nowadays the Global Navigation Satellite Systems (GNSSes) play a fundamental role

in several fields, belonging to very different areas Tracking of vehicles fleets (trains, trucks, or vessels), synchronization of communications and energy distribution networks, logistics, and mappings are examples of important civil applications, Within this scenario, the requirements that GNSS receivers have to fulfill, both in terms of

accuracy and continuity, are becoming more and more stringent

GNSS systems like GPS and Galileo are based on the Direct Sequence-Spread Spectrum (DS-S8) technique, which intrinsically gives thom a high level of robustness

At the same time, it has to be considered that the satellite signals arrive at the receiver

anterma with an extremely low level of power This makes them vulnerable to different

disturbances All the system transmitting at carrier frequencies close to the band of interest are potential sources of interference for a GNSS receiver Even small leakages

out of their allocated bandwidth can become threats for the receiver,

Among many types of disturbances, narrow band interference (NRT) might cause serious receiver porformance degradation Recently, notch filters, which pass all frequencies except those in a narrow stop or rejection band centered on a central lrequeney, have boon considered as an ollective teclengue (or mitigating NBT There are several ways to implement a notch filter, among which the infinite impulse

response uoich filter (TR NF) bas a low complexity and efficient implementation Tn

this thesis, through the assessment of the carer to noise density ratio of the filtered

signal, a new design of the interference mitigation module based on the IIR NI is proposed This module has an ability to classify the incoming interference into CWI or band-limited interference ‘Then, it configures the notch: filter appropriately so that the

interference suppression is optimized

‘The thesis includes 5 chapters as follows:

Chapter 1, Fundamentals This clapler gives an overview of Global Navigation Satellite Systems, interferences and their effects of interference on performance of

GNSS receivers And a summary of interference mitigation techniques is presented

Chapter 2 Notch filter for GNSS narrowband interference mitigation: 1n this

chapter, Notch filler, one of the interference miligalion techniques, is introduced Tt is

modified into Adaptive notch filter in order to detect automatically the interference

frequency The application of notch filter in GNSS interference mitigation is also introduced through block diagram of receiver

Chapter 3 New design of notch filters on GNSS narrowband interference

mitigation This chaptcr presents the impact of notch Glters on the quality of GNSS signal via carrier to noise density ratio ‘Then, a new design of the narrowband

interference mitigation module using notch filler, which oplimives the carrier to noise

density ratio of the filtered signal, is proposed

Chapter 4 Performance analysis In this chapter, the performance analysis of the proposed design in chapter 3 is proved

of the International Conference on Space, Aeronautical and Navigation

Flectronics 2013, Hanoi, December 2-3, 2013

2, Tu Thitthanh Nguyen, Tung Hai Ya, Beatrice Motella, "An Adaptive Bandwidth Notch Filter for GNSS Narrow Band Interference Mitigation", The First NAFOSTED Conference on Tuformalion and Computer Science (NICS 2014), 13-14 March 2014, Llanoi

3, Tu Thi-Thanh Nguyen, Beatrice Motella, Tung Hai Ta, “A New Design of GNSS Narrowband Interlerence Mitigation Using Notch Filter”, TON’s Pacific PNT Conference 2015 (accepted)

11

GNSS Overview

A satellite navigation system is a system of satellites allowing electronic receivers to

determine their location (longitude, latitude, and altitude) by moans of the signal

transmission time from satellites [15] Depending on the scale and purpose of use, it

can be classified as:

Global Navigation Satellite System: A satellite navigalion system with globat

coverage such as: GPS (United States), Galileo (EU), GLONASS (Russian), and

COMPASS (China) Among them, GPS and GLONASS were the full orbital

vonstellations, Galileo are in the process of being developed, and Compass, the satellite navigation system is covering China, has been expanded into a global

system

Regional Navigation Satellite System: Some countries such as Indian, Japan lave developed thar own salcllite navigation system Indian Regioual

Navigation Satellite System (LRNSS) was approved in 2006 with the intention

of the system to be completed by middle of 2015 The Quasi-Z.enith Satellite

System (QZSS) was authorized by the Japanese government in 2002 and expected to be fully operational status by the end of 2017

Augmentation System: in order to improve the accuracy, integrity and

availability of GNSSes, especially GPS, augmentation systems have been developed There are two types: local area augmentation systems with the limited coverage; and wide area augmentation systems ‘Ihe latter uses satellites

lo broadcast augmentation information to users Examples are WAAS (USA), EGNOS (EU), MSAS (Japan) |18].

1.11 Fundamentals of satellite navigation

The position of a GNSS receiver in space can be found from the intersection of 3

spheres as described in Figure 1.1 The center and the radius of each sphere are the

position of a satellites and the distance measured from the receiver to the satellite,

Figure 1.1 Fundamental of a satellite navigation system

Let us denote (X„y„,Z„) 1s the receiver position, which is also the unknown

variables: and s;(x;,Y;,2;) is the position of i" satellite, which can be calculated using

the message transmitted by the corresponding satellite In order to

determine u(%,; Vi Z,) it is necessary to know positions and distances to the receiver

of at least 3 satellites

The distance from the receiver to a satellite is measured based on the transmission time

of the signal between them:

p=clr+ (ðt, — ðt°)] (ll) where € is the speed of light, r is the transmission time, and 6t,, — dt* = 6¢ is the bias between the receiver clock and the satellite clock, as shown on Figure 1.2 This bias exists because the atomic clocks on all the satellites are designed with a very high

accuracy, while the low cost clock on the receiver does not meet this requirement

Pseudorange

Figure 1.2 Clock misalignments in a GNSS system

The bias d¢ is a new variable For this reason, it is necessary to know the positions of at

least 4 satellites The receiver's position is found by solving the system of equations

pr = VG =u)? + Oi = ad? + i — Au? + ECT + St)

po = VG — Xu)? + 2 — Yu)? + Ge — Au}? + C(t + 60) tu

ps = \[O% — Xu)Ê + (yy — Yu)Ê + 2a — Z2) + cứ + ôĐ) ~

(x4 — Xu)? + a — Yu)? + Za — Bu)? + cứ + ðt)

1.12 GNSS receiver structure

In general, a conventional GNSS receiver includes 3 main functioning blocks: RF

front-end, Digital signal processing, and Navigation processing as seen in Figure 1.3

Figure 1.3 High-level architecture of a GNSS receiver

These blocks are described as follows [9, 15]

e Radio frequency front-end: After arriving at the receiver's antenna, the GNSS

signal (with the noise and the interference components) is filtered, down-

converted to the Intermediate Frequency (IF), and sampled by the front-end

© The digital signal processing is divided into 2 stages: acquisition, tracking and

data demodulation

© The function of signal acquisition is to provide GNSS satellites in view,

their code delay and Doppler shift frequency To do these tasks, the

acquisition stage performs a search in three dimensions, namely PRN

identification, code delay and Doppler shift Each set of three parameters

forms a local replica signal The correlation between this local signal and

the incoming one is used to decide the best-match set The estimated set

is then transferred to the tracking stage for finer estimation

° The estimates of Doppler shift and code delay in the acquisition process

are initial values with a low accuracy Thus, they are fed for tracking

provess to be estimated contimously and more accurately ‘This process tries to generate local replica signal with the least bias with respect to the code and carrier in received signal Once the signal of a satellite is tracked, the navigation data bits can be readily recovered and then the navigation message is obtained

* Navigation processing: ‘The obtained navigation message contains the satellite’s

position, When af least 4 salcllites are Imcked, the posilioning process can be performed by solving a system of equations (1.2) as introduced in section 1.1.1

12 Interference Threat

it has to be considered that the satellite signals arrive at the receiver antenna with an

extremely low level of power (approximately 20 dB below the noise floor) This makes

them vuluerable (o different disturbances, among which the Radio Frequency

Interference (RU'1) from external sources is one of the main threats As defined in [16]:

Interference noise is noise that comes in at the same frequency (ies) as the wanted

signal and can mask (or overwhelm) parts of ihe desired signal All the sysloms transmitting at carrier frequencies close to the band of interest are potential sources of

interference for GNSS receivers Ever small leakages out of their allocated bandwidihy

can become threats for the receivers, Moreover, the growing number of wireless communication infrastructures increases the probability that some out of band energy

affects the perlormance of GNSS receivers

Normally, the RF interference is classified as wide-band (WB) or narrow-band (NB), depending on the ratio between the interference and the desired GNSS signal bandwidths [5] In this sense, an interference might be classified differently, depending,

on the specific GNSS signal that is considered (e.g the same interference can be

classified as wide for the GPS L1 CYA code and narrow for the GPS P(Y) code) The

limit for a narrow-band interference is a single tone [5], usually refer to as continuous wave (CW)

‘Table 4.1 Interference threstiold versus Interference bandwidth

for GPS receivers in track moile [16]

Interferences affect the performance of GNSS receivers, especially in the

synchronizalion processes For asquisition process, they increase the noise Moor,

therefore, reduce the ability of signal detection [1] This might make the receiver

unable to find its position, because it can not see enough satellites For tracking

‘process, interferences increase (he code tracking error, and then might lead to the loss

of track in the receiver [6] In [16], the author showed the susceptibility of GPS

reueivers versus the interference bandwidth (Table 1.1) Tt cant be

mi that signals with

bandwidth up to 700 Hz is the most severe threat with the lowest thresholds of -126.5 dBm.

‘The potential sources of non-intentional interference might come from the harmonics

of some telecommunication signals such as: VIIFCOM, SATCOM Communications,

Aircrall Communication Addressing and Reportirys Sysiem (ACARS), TY charmels, 4M channels, as illustrated in ‘Table 1.2 [6]

‘Table 1.2 Potential interference sources [6]

In Vietnam, the Circular No 29, dated 27 December 2013, issued by The Ministry of

Information And Communications stated that three organizations of broadcasting the

digital terrestrial lelevision services in Vielam have the nght to use the charmel 27

{518 526 MIIz], at the center frequency 522 Milz It can be seen in the ‘Table 1.2, the

3" harmonic of this channel is inside L1 band: thus, it is one of the potential

interference sources [or 1.1 band of GNSS signals

1.3 Interference Mitigation ‘Techniques

As mentioned above, the presence of interfer

¢ might cause @ serious decree `

Teceiver performanoe Therefore, it is necessary to research and develop interference

monitoring techniques including: detection, characterization, localization and

mitigation [9] Among them, the mitigation techniques play an important role to ensure

that the receiver still works in the interfered environment Interference mitigation

lechrriques cari be claasilied into 3 categories, dependiry on ils position on Lhe recewer diagram [17]: A/D converter, Post-correlation techniques, and Pre-correlation techniques They are shown in ascending order of complexity in Table 1.3 Obviously,

the more complex these Leclniques are, the more effective they achieve

« A/D Converter: GNSS receivers often use a single bit A/D converter to reduce

the cost and the power consumption ITowever, in the presence of CWI, the loss

of lock threshold of these receivers might decrease up to 5-7 dB Adaptive A/Ds use more than one bit for quantization and adaptively adjust their signal thresholds according to the received power

« Post-correlation techniques: are implemented by modifying the tracking loops of

Te

cr in order to improve the tmeking thresholds

« Pre-correlation techniques: are applied prior to the tracking loop Considering the frequorey domain, a tarrowband inlterlerenee can be removed by a filler,

without significant altering the characteristic of GNSS signal ‘Ihe filter used for

23

removing interference is the notch filter For spatial domain, an antenna array can be used to reduce (or remove) the interference according to its direction The combination belween fillers and beamfonwing techniques give # high performance in interference suppression

‘Table 1.3 Possible interference mitigation techniques to a standard GPS receiver [17]

receiver

Veclar Loops Integration with INS

Amplitude Domain Processing | 70~ 40 dB agaist

narrow band Can be

TemporalFFT domain Filters | interferences implemented

20 40 dBagainst | cither intemally narrow bard + or external to

Space-Time Filters interferences

Among many types of interference NBI might cause a serious receiver performance

degradation, For this kind of disturbing signal, the NF has proved to be an efficient

mitigation technique, since it can be considered as a good compromise between

efficiency and complexity ‘hus, this thesis focuses on the class of narrow-band interferers, where the term ‘narrow’ has to be considered with respect to the GPS Ll CYA code and the Galileo EI signals (ie, the interference bandwidth is within tens of kdlohertz).

CHAPTER 2 NOTCH FILTER FOR GNSS NARROW BAND

A notch filter (NF) is a filter that passes all frequencies except those in a stop or

rejection band centered on a central frequency Therefore, a notch filter can be defined

by two parameters: the notch bandwidth By, and notch frequency fy Notch frequency

is the center frequency to be removed, and notch bandwidth is the width of frequency

region in which the signal power is reduced by 3 dB

Normalized Frequency (x7 rad/sample)

Figure 2.1 Frequency response of notch filter for two difference pole factors

‘There are several ways to implement a notch filter, normally categorized into three main classes: the Fast Fourier Transform (FT'T)-based, the Finite Impulse Response (FIR), and the Infinite Trpulse Response (TR) notch filter [4] FFT — based NF used

JL algorithm to evaluate the signal spectrum 1f any frequency component passes a predefined threshold, it will be considered as a CWI and removed by setting at zero This algorithin brings the capability of working with multiple CWT lo FFT-based NF The second type is FIR filter, which removes the interference by combining several weighted input samples This lype of filler is oflen designed with linear phase response

in order to ensure that thore is no distortion in the output of filter Both FFT-based and 1L NIV have their own advantages [lowever, these types of notch filter require high computation cost Tn recen years, TR nolch filters are considered for GXSS interference mitigation because of their low computational requirements, efficient implementation and low number of parameters to be adapted [2]

The IIR two-pole NF has the following transfer function [2]

1—21Re{zg}z 1+ |Zo|z ?

NO Weight aa, + Vaal Ee? GD)

where % = reJ2*f% is the zero, which orresponds †o the notch fequency /y; and k„

(with 0 < ky <1) is the pole contraction factor, which decides the notch bandwidth

To exploit effectively the capability of NF for removing interference signals, the zeros should be constrained to be on the unit circle [7], and this type of NF will be

considered in this thesis

Figure 2.1 reports the frequency responses of a notch filter with two different pole factors It is clear that the pole factor ke decides the width of the notch: the closer ke is

to unity, the narrower the noteh is Tn other words, the notch bandwidth is characterized

by the pole factor ka, as it decides which frequency components will be removed

tạ

22 Adaptive Frequency Notch Filter

When using NI‘ to remove interference, it is necessary to know the notch frequency (or interference frequeney) However, in reality, this parameter is unknown and might vary over time as shown on Figure 2.2, Thus, an adaptive notch filter (ANE) is designed to

estimate and track it automatically To do this, it is important to note that if the notch

frequency matches the interference carrier (fx = fingh the power of signal after filtering will be a minimum It is because the interference power is often much higher than the usual received signal (includes useful signal and noise) This statement is proved on Figure 2.3, where the power of output filler is mvestigated with varying

Figure 2.2 Spectrum at the front-end output at different time instants (from [4]}

Thus, the ANF can be implemented by usin Teast Mean Square (IMS) algorithin ta anininize the power of filler ontpul, as follows [2] The vero a9 of transfer fimetion

(2.1) is updated iteratively If the interference is present, the angle of % will converge

to the interference frequency

The ileralive rule be update Z9 3s:

Zn[Lm + 1] = Zu[n] — wV„„(|y[n]|?) (2.2)

where V denotes the stochastic gradient, j: is the algorithm step and y[m] is the output

of the notch filter [2]

Figure 2.3 Power of filter output when noteh frequency varies

‘The power spectrums of signal before and after filtering are shown on Figure 2.4, An interference source produces a very high peak in the spectrum in Figure 2.4.(a), and it

is removed by ANF and disappears in Figure 2.4.(b)

Figure 2.5 shows the evolution of adapted frequency during the adaptation process It

converges lo the interference Lequency after aboul one thousand samples (equivalent

to about Ims for this data set).

b Spectrum after filtering

Figure 2.4 Spectrum of an interfered signal before and after filtering

Time index - sample number

Figure 2.5 Adapted notch frequency of an ANF

23 Using Notch Filter on GNSS Narrowband Interference Mitigation Notch filter has been proved to be an efficient interference mitigation technique Thus,

several interference mitigation modules using the notch filter were proposed Figure 2.6 shows a high-level block diagram of the GNSS receiver chain, where the

Interference mitigation module is present After arriving at the receiver's antenna, the

GNSS signal (with the noise and the interference components) is filtered, down- converted to the Intermediate Frequency, and sampled by the front-end Then, the

‘Interference Mitigation module’ is in charge of

e Sampled signal goes through an ANF

¢ A detection control system detects the presence of interference depending on the

convergence of notch filter: