rpm 2004esf workshop on active protection of passive radio services cagliari, 28 october 2004 1 rfi...

ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004

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RFI Mitigation for Radio AstronomyRFI Mitigation for Radio Astronomy

An Overview

Rob MillenaarASTRON/CRAF


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Overview

• Introduction• The Problem• Mitigation

– methods– implementation– results– prospects

• Summary & Conclusions


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Introduction

• Sensitivity– progressing science demands more sensitivity and more

frequencies– and technology in principle makes this possible

• Frequency Range– radio astronomy has been blessed with reserved frequencies,

but...• Nature/Physics

– goes beyond the reserved ranges• Mankind

– wants the passive service ranges and transmits in pretty much all of the rest

‘It is the astronomer’s duty to study the Universe. The radio astronomer does this by performing measurements at the highest sensitivity and at frequencies dictated by the natural phenomena being investigated, in spite of obstacles created by mankind.’


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The Problem

• The Sensitivity Issue– Sensitivities needed to do astronomical observations -

next generation instruments ~100x as sensitive– Levels of RFI sources around us are going up, up, ...


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The Problem

• Available spectrum– allocated only ~2% at cm wavelengths– suffering from o.o.b. radiations from other spectrum

usersWSRT frequency coverage

10

100

1000

100 1000 10000f (MHz)

Ts

ys

(K

)


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The Problem

• Available spectrum, SKA frequency range:


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The Problem

• Future spectrum developments (not all is bad news)– migration to higher frequencies– digital tv, radio

• lower eirp’s vs. more transmitter antennas• wide bands vs. spikes

– satellites are becoming smarter - not higher power but smarter illumination (but there will be more of them)

– licence free transmissions• wireless devices like Bluetooth, WLAN• UWB • SRR• PLC (!)


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Characteristics of RFI

• RFI to us, useful (legitimate) signal to others...• Signal characteristics:

– basic properties• signal power (R/N ratio)• signal frequency• signal modulation

– origin properties• internal• external• stationary• moving: airborne, space• coherency: direct, scattered (multipath)

– time properties• stationary• nonstationary• cyclostationary


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Mitigation

• RFI Mitigation is about– prevention of the occurrence of unwanted signals– recognition of the unwanted signals once prevention

has failed– and dealing with them by applying techniques and

strategies that either • remove them• or minimize their influence on the wanted signal

– while the wanted, astronomical signal • stays free from artefacts • and is observed as close to the theoretical sensitivity as

possible.


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Mitigation MethodsPrevention

• Prevention of RFI– Quiet zones, siting

• but no escape from LEO’s (in ever increasing numbers)

– And all the other Regulatory measures• to Regulate or to Mitigate?

ultimate sensitivity only way for unprotected freq.

we need both– Prevent Self-Generated RFI

• increasingly difficult


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Self-generated RFI: fouling up one’s nest


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• Preventing detrimental effects of RFI– Super linear RF circuitry– Filtering out strong signals while preserving as much of

the spectrum as possible and at same sensitivity (High Temp. Superconducting Filters)

– Use Multi-bit sampling (2)


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Mitigation Methods - How

Repairing the damage that is done.• How: In general we can apply a method in some

domain (time/frequency/spatial):– excision

• flagging/blanking• filtering• thresholding• beamforming/nulling in (sparse) arrays and fpa’s• higher order statistics of gaussian noise + (non gaussian)

RFI input signal (probability distribution analysis)

t t t


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Mitigation Methods - How

Repairing the damage that is done.• How: In general we can apply a method in some

domain (time/frequency/spatial):– canceling

• adaptive interference cancellation (with ref. channels)

– other• anticoincedence (interferometry)• targeted methods: Iridium, radar, bursts, DME


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Mitigation Methods - Howadaptive cancellation

Using reference antennas

F. Briggs


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Mitigation Methods - Howadaptive cancellation

Without dedicated reference antennas


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Mitigation Methods - Where

• The various methods for RFI mitigation can be applied at many stages in the chain of signal processing parts:

antenna

receiverIF chain

ADCcorrelator/tpd

data processingcalibration

analog

digital

software

real-time

off-line

• Some methods can be applied both early and late in the chain.


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• Where: – Pre-correlation

• excision by higher order statistics• excision by thresholding• spatial filtering: beamforming and nulling• adaptive cancellation

R R

IF ADC COR DP

…..


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• Where: – Correlation

• excision by blanking, spatial nulling• fast correlators - need high dump rate• the natural suppression of non-sidereally moving sources

of RFI by fringe and delay steering

R R

IF ADC COR DP

…..


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Mitigation Methods - Wherecorrelator

• At the correlator RFI suppression naturally takes place because of the required fringe and delay steering to compensate for the Earth’s rotation (example: WSRT)

=10°

=80°


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• Where: – Post-correlation

• adaptive cancellation• off-line editing (excision)• a calibration nightmare: RFI mitigation affects the

observation by dynamically changing the: – frequency coverage– observation times– uv coverage– beamshapes

R R

IF ADC COR DP

…..


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Mitigation Methods

• Combining methods– time, frequency and spatial domain– some methods exclude others (esp. blanking, excision)– no silver bullets– need for a Mitigation Strategy Machine

• database of RFI sources (know your enemy)– fixed and mobile terrestrial– satellites– perform monitoring operations

• multidimensional parameter space– time– spectrum– modulation– track or position

• artificial intelligence, neural networks


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Mitigation Methods

• Making the most of RFI Mitigation techniques:– sense the signals at strategic points in system– use data from RFI database and monitoring equipment– apply mitigation measures/algorithms at appropriate

points– let some smart Mitigation Strategy Machine decide:

• what technique(s) to apply• with what parameters• at which spot(s) in the system

R R

IF ADC COR DP

…..

MSM

RFImonitoring

RFIdatabase


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Implementation

• Technology– asic’s (application specific ic)

– dsp chips (digital signal processing)

– fpga (field programmable gate array)

• using IP blocks (fft, fir filters, etc)

– mmic’s (monolithic microwave integrated circuit)

– digital radio– beamformers, analog and digital

• Rapid prototype development/demonstrators through:– c.o.t.s. technology test boards– c.o.t.s. fpga boards


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Implementation

• Example RFI Mitigation System for the WSRT– how to implement RFI

mitigation in an existing backend

– pre-correlation processing system for one of 8 20 MHz wide bands has been built

– future post-correlation mitigation sub-systems


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ImplementationFlexible algorithm implementation in fpga’s

excision in freq. domain


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ImplementationFlexible algorithm implementation in fpga’s

adaptive noise canceller


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Resultsexcision in time and frequency

• Effelsberg 100m single dish RT with ‘portable’ RFI mitigation system– Continuum observations at

1625MHz, 20MHz wide– Median filtering, excision in

time and frequency domain using thresholding.


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Resultsexcision in time and frequency

• image built from 16 scans

before after


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Resultsadaptive cancellation

• WSRT example, using neighboring telescopes as reference antennas– observation at 355MHz– autocorrelation RT5


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• WSRT example, using neighboring telescopes as reference antennas– observation at 355MHz– crosscorrelation RT5&6


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• WSRT example, using neighboring telescopes as reference antennas– map


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Results

• Despite the positive results, at this time that must be put into perspective:– There is a selection effect: we tend to attack problems

that we think are solvable– In some case much handtuning goes into the

algorithms– We can (often) deal with moderate strength RFI; now

how about really strong RFI...– ... and really weak, broadband RFI (UWB)?– We need much more field experience with ‘real’ routine

astronomy


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Prospects

• Moore’s law will allow more and faster signal processing

• Much work on the theoretical foundation takes place now next generation benefits

• There is an increase in the exchange of ideas, implementations and results among groups worldwide:– through international fora and publications– URSI World assembly– (SKA) RFI workshops– (IUCAF/CRAF) Summer school– RadioNet initiatives – less obvious: the telecom industry contributes as well,

by vigorous research in signal processing and cost effective implementation


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Words of Warning

• RFI mitigation is not without risks: – danger of throwing away the baby... (lose the signal of

interest while suppressing the rfi)– generation of artefacts– reduction of sensitivity to the astronomical signal -

dynamic range– reduces the chances of ‘discovery’– complicates calibration

• Need to Regulate AND Mitigate


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Summary & Conclusions

• Apply the appropriate mitigation method for the:– type of instrument– type of observation

• continuum• spectral • pulsar• transitional

– type of RFI

• And do all you can to protect:– RA769 – sites

“RFI Mitigation results are always better than expected” Ron Ekers

but seriously...

rpm 2004esf workshop on active protection of passive radio services cagliari, 28 october 2004 1 rfi...

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