ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
1rpm 2004
RFI Mitigation for Radio AstronomyRFI Mitigation for Radio Astronomy
An Overview
Rob MillenaarASTRON/CRAF
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
2rpm 2004
Overview
• Introduction• The Problem• Mitigation
– methods– implementation– results– prospects
• Summary & Conclusions
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
3rpm 2004
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.’
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
4rpm 2004
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, ...
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
5rpm 2004
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
)
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
6rpm 2004
The Problem
• Available spectrum, SKA frequency range:
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
7rpm 2004
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 (!)
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
8rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
9rpm 2004
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.
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
10rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
11rpm 2004
Mitigation MethodsPrevention
Self-generated RFI: fouling up one’s nest
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
12rpm 2004
Mitigation MethodsPrevention
• 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)
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
13rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
14rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
15rpm 2004
Mitigation Methods - Howadaptive cancellation
Using reference antennas
F. Briggs
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
16rpm 2004
Mitigation Methods - Howadaptive cancellation
Without dedicated reference antennas
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
17rpm 2004
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.
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
18rpm 2004
Mitigation Methods - Where
• Where: – Pre-correlation
• excision by higher order statistics• excision by thresholding• spatial filtering: beamforming and nulling• adaptive cancellation
R R
IF ADC COR DP
…..
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
19rpm 2004
Mitigation Methods - Where
• 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
…..
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
20rpm 2004
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°
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
21rpm 2004
Mitigation Methods - Where
• 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
…..
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
22rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
23rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
24rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
25rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
26rpm 2004
ImplementationFlexible algorithm implementation in fpga’s
excision in freq. domain
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
27rpm 2004
ImplementationFlexible algorithm implementation in fpga’s
adaptive noise canceller
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
28rpm 2004
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.
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
29rpm 2004
Resultsexcision in time and frequency
• image built from 16 scans
before after
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
30rpm 2004
Resultsadaptive cancellation
• WSRT example, using neighboring telescopes as reference antennas– observation at 355MHz– autocorrelation RT5
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
31rpm 2004
Resultsadaptive cancellation
• WSRT example, using neighboring telescopes as reference antennas– observation at 355MHz– crosscorrelation RT5&6
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
32rpm 2004
Resultsadaptive cancellation
• WSRT example, using neighboring telescopes as reference antennas– map
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
33rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
34rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
35rpm 2004
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
ESF Workshop on Active Protection of Passive Radio ServicesCagliari, 28 October 2004
36rpm 2004
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...