Interference Mitigation –Where We Stand,

Where We Might Go

Steve Ellingson & Kyehun LeeVirginia Polytechnic Institute & State University

URSI 2007 Ottawa July 23, 2007

One Perspective on RFI: 25—90 MHz

NTSCCh 4Ch 3Ch 2 Ch 5 Ch 6

Can observe

here

Can observe

here

Can observe

here

Can observe

here

Can observe here –but need good linearity and

< 1 kHz channelization

About 100 sec of noise-limited sensitivity using >95% of contiguous 5 MHz band

around 38 MHz

Many of the spectral channels remain noise-limited for hours…

Rural mountainous area about 40 miles SW of Asheville, NC

Time-Frequency Perspective

Wideband junk

Wideband junk

Wideband junkSe

lf-G

ener

ated

(PC

)

6-m

Am

ateu

r Rad

io

Ionospheric enhancement

Ionospheric enhancement

Citi

zen’

s B

and,

oth

er H

F

NC

Sta

te P

olic

e

GalacticBackground

( TA ~ 9000 K )

Not visible: Impulsive (~125 kHz) RFI

Almost all of this stuff can be either avoided or edited out.

The real problems are the associated problems of time (=money) and labor, respectively!

TR ~ 300 K(i.e., sky-noisedominated)

Digital TV (ATSC)

ATSC NTSC

ATSC test transmissionspotted at VLA – Jul 07

Ch 3ATSC

Craig County VA – Oct 2005

US federal law banishes NTSC in favor of ATSC by Feb 2009

ATSC is “spectrallywhite” – blots outentire 6 MHz

ATSC is being rolledout now

Also, TV Ch 52-69 (698-746 MHz)moving to Ch 2-51

Emerging threat forVHF- and UHF-bandradio astronomy

Ch 2

Ch 4

Ch 5

FM

Ch 3

Ch 6

MJD

From Clegg briefing to CORF, 2006 Fall

RFI “Lines of Defense”

• Regulation / Frequency Coordination

• Avoidance Avoiding contaminated frequency bands, Scheduling to avoid satellites

• Analog Filtering / High Dynamic Range Receivers

• “Pre-detection” Signal ProcessingExcision: Detection & Blanking, Adaptive Filtering, Null FormingCanceling: -- “look through”; e.g., Model-and-subtract methods

• “Post-detection” Signal ProcessingPost-correlation & Cross-spectral techniques

• Data Editing

Real-TimeTechniques

Some Successes in “Operational” RFI Mitigation• Haystack Deuterium Array / 327 MHz Line Detection

• Combination of techniques (See Rogers talk, this conf.)

• VLA 74 MHz / P-Band• Editing visibilities (See Lane et al. talk, this conf. / RFI2004)

• Bruny Island Radio Spectrometer (Erickson) • Solar observing in HF all the way down to ionospheric cutoff• Spectrometer senses and avoids narrowband RFI

• Transient Searches (Transient searching is RFI mitigation)• Many examples of effective use of multi-site anti-coincidence• Example: GBT-Arecibo (Bhat et al. 2005 / RFI2004)

• Project Phoenix (SETI Inst.) – early adopters

• No doubt there are others I have overlooked…

Some “Emerging” (not yet operational) Techniques

• Radar Pulse Blanking • Actually, “old school”; e.g., Arecibo pulse-pattern-synchronous blanker • Many others have explored this; useful to some extent right now• Effectiveness limited by detection sensitivity, multipath spread,

“mangled” (semi-correlated multipath) pulses – room for improvement

• Spatial Nulling for Arrays • Especially vs. satellites (ATA, some FPA concepts)

• Adaptive Canceling• “Reference antenna approaches” of all kinds; pre-/post-correlation• “Model & subtract” methods

• LOFAR is testing/demonstrating a whole battery of techniques • See Boonstra talk, this conf.

Mitigation of the notorious 1330/1350 MHz radar @ Arecibo using a digital receiver

Ellingson & Hampson (2003), Astrophysical J. Sup. Ser., 147,167.

200 MSPSA/Ds

I/Q Conv., LPF,Pulse Blanker

1KFFT

I/F to PC

SDP,Integrate

Implemented completely inAltera Stratix FPGAs

Before: Radar pulses corrupt spectrum

After: Radar pulses excised(~4% of the data is blanked)

L-Band Radar Blanking: One Example

Work continues at OSU (Johnson) including application to L- and C-band remote sensing

Mitigation of Iridum – Blanking vs. NullingArgus

• Array of 24 spiral antenna elements• Tsys ~ 215 °K per element• 1200-1800 MHz Tuning Range• 60 kHz processed bandwidth

Time Series

Matched Filter

Output

Rank DetectorPulse Detector

Top: RFI mit offMiddle: SL CancelingBottom: Blanking

Detector:Total power pulseW = 8 msβ = 10σ at PMF output

SL Canceling:Projecting out estimatedspatial signature of burst

Cancel 56 ms windowStart 16 ms before triggerNo data loss

Blanking:Blank 56 ms windowStart 16 ms before trigger~ 20% of data is blanked

PSD calculation:∆ν = 100 Hz∆t = 10 msτ = 58.3 s

Blanking: Better Performance;Nulling: No Data Loss. More info: Proc. RFI2004

Mitigation of Broadcast FM?

• Strong source of RFI in 88-108 MHz (U.S.)

• Bandwidth ~ 200 kHz

• Baseband is analog audio + many other components, total ~75 kHz: Processing gain!

• Simpler version used to convey audio in NTSC

• Prone to multipath; especially apparent in weak signal areas

A Canceller for Broadcast FM *

* Work of Kyehun Lee, VT

Architecture

• Analyze band; determine # of signals & form coarse estimate of associated center frequencies

• Extract carriers one at a time, demodulate, estimate model parameters

• Reconstruct noise-free version using extracted model parameters

• Subtract synthesized carriers from telescope output.

Estimation Block

Looks complicated, but is only slightly more complex than a high-performance commercial FM receiver.

This version does not account for channel characteristics, such as mutlipath.

Broadcast FM Canceller: Demonstration *

* Work of Kyehun Lee, VT

Before / AfterSimulated signal

(no channel effects)

Before / AfterOff-the-air signal

(includes channel effects)

Somewhat toxic: Current algorithm suppresses uncorrelated in-band spectral content (i.e., underlying radio astronomy) by about 40%.

Note detailed model knowledge helps a lot with this.

Prospects good for further improvement, especially with site diversity: Using information from sites closer to the transmitter.

Toxicity to simulated spectral line

([0..1]; 1 is perfectly safe)

Simple “Chirp” Model

Complete parametric

model

Canceling for ATSC?

• Maybe yes! ATSC has much in common with GLONASS and GPS, which are relatively easy to cancel

• 8VSB digital modulation – “finite alphabet” property • Amenable to demod-remod; “reasonable” computational complexity• Model-based method can be used to enhance “reference signal” methods,

which have already shown some promise with wideband signals

• Maybe not… ATSC has aspects in common with Broadcast FM

• Decorrelated multipath (more complicated model)• Low processing gain (info BW ~ occupied BW) – potential for toxicity• Possibility for big gains via site diversity (“reference signal relay”)

(e.g., Ellingson, Bunton, and Bell (2001), ApJS., 135, 87.)

Concluding Remarks

Signal processing-based approaches to RFI mitigation will be more important for radio astronomy not necessarily because RFI is becoming worse, norbecause instruments are becoming more sensitive, but because users want to produce more data

Should (and might) receive greater emphasis in coming years:- Continue automating the simple, effective things that are done manually now- Multi-pronged attack on ATSC (& related OTA DTV protocols) - Nulling vs. Satellites (Been ripe for a while – ATA may address)- Site Diversity (Anti-Coincidence, “Reference Signal Relay”,…) – Transients- New instruments: understand the algorithms, understand the interplay between

instrument and algorithm, and include the appropriate“hooks” (short integration times, high spectral resolution, complex coefficients, …).