Adjacent Band Interference to Consumer Receivers - GPS

Adjacent Band Interference to
Consumer Receivers

Tom Powell
The Aerospace Corporation

National Space Based PNT Advisory Board
7 May 2013

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© The Aerospace Corporation 2013

Introduction

• The issue of adjacent-band interference to radio receivers has
received recent attention
– In particular, adjacent band interference to GPS receivers

• Leads to the question: How well do other types of radio receivers
withstand adjacent band interference?

• The Aerospace Corporation tested a number of common consumer
radio receivers against adjacent band interference signals
– Digital Television (Samsung LN52B530)
– FM Radio (Sony STRDH100)
– 3 types of GPS receivers
• Garmin Montana 650t, uBlox LEA-6A, Novatel OEM 628

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Test Design

Other FM Radio Other Digital TV Channels 7-13 Other Non-TV
Bands Frequency Band Non-FM, Non-TV Frequency Bands Frequency Band Frequency Bands

88 108 174 216
MHz MHz MHz MHz

FM Adjacent Band TV Adjacent Band
Interference Signal Interference Signal

Other GPS L1
Non-GPS Bands Frequency Band

1559 1610
MHz MHz

GPS L1 Adjacent Band
Interference Signal

• All testing conducted in a controlled, laboratory environment

– Conductive or anechoic chamber - No external transmissions!

• Continuous Wave (CW) signal transmitted at varying power and
frequency offset from each device’s operating band

– Interference signal transmitted outside of device’s allocated band

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Start with CW tone in adjacent band

Power

Desired Adjacent
Signal Band Band

Interference to
Desired Signal

Power Ratio

Interference Signal Frequency (f)
(CW Tone)

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Increase power …

Power

Desired Adjacent
Signal Band Band

Interference to Frequency (f)
Desired Signal

Power Ratio

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… until device fails

Power

Desired Adjacent
Signal Band Band

Interference to Loss of Frequency (f)
Desired Signal Service

Power Ratio

• Failure criterion was total loss of track of all signals
• Used this criterion because signal-to-noise ratio degradation or

similar metrics were not available from all devices tested
• Total loss of service enabled “apples to apples” comparison

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Record the CW signal power (I) and
frequency offset (Df) at failure point;
Compute (I/S)

Power Desired Adjacent
Signal Band Band
Interference to
Desired Signal S I

Power Ratio Frequency (f)

(SI )

Frequency
Offset Df

Desired Signal Interference
Band Edge Signal (CW)
Frequency

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Increase CW signal offset, repeat

Power

Desired Adjacent
Signal Band Band

Frequency (f)

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Increase CW signal offset, repeat

Power

Desired Adjacent
Signal Band Band

Frequency (f)

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Result is locus of (I/S) points where device failed

Power

Desired Adjacent
Signal Band Band

Frequency (f)

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Power and Frequency Offset Considerations

• Interference to desired signal ratio (I/S) is used because desired
signal powers vary widely by service
– Simulated GPS signal power = -158 dBW (-128 dBm)
– TV signal power = -51 dBm (77 dB stronger than GPS)
– FM signal power = -67.5 dBm (60.5 dB stronger than GPS)

• Frequency offset is plotted as a percentage of desired band edge
– Referenced from band edge to consider only adjacent band
signals – avoid interference signals in desired band
– Rationale for normalizing versus frequency
• Filter “Quality Factor” (BW/f) also scales with frequency
• Commonly used to describe filter roll-off

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More resistance to interference

Adjacent Band Interference Ratio (I/S) vs. Frequency Offset (%)

120

100

I/S (dB) 80

Adjacent Band Interference Ratio (I/S) vs. Frequency Offset (%)

1620

1400

280 Greater frequency separation

60I/S (dB)0.5% 1.0% 1.5% 2.0% 2.5% 3.0% 3.5%
0.0%
Frequency Offset (%)
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Test Results – (I/S)

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Test Results – (I/S)

OmniSTAR OmniSTAR
Corrections Corrections
MSS Downlink MSS Downlink
1557.8 MHz 1540.0 MHz

(0.14%) (1.28%)

http://www.omnistar.com/Support/OmniSTARSatelliteInfo.aspx

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Test Results – Alternative Power Metric – I/(S+N)

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Summary and Conclusions

• Summary of test results
– 3 different types of consumer receivers tested
– All were susceptible to adjacent band interference

• Conclusions
– Any radio receiver can eventually be overloaded by adjacent
band signals of sufficient power
– Compatibility assessments should consider relative signal
powers of adjacent band services

All trademarks, service marks, and trade names are
the property of their respective owners.

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BACKUPS

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Notes on Novatel EOM 628 GPS Receiver

• Novatel OEM 628 is wide-band high precision GPS receiver
• Designed to receive differential correction signals from

Geosynchronous satellites operating in the Mobile Satellite Service
(MSS) band (1525-1560 MHz) below GPS L1
• As a result, some data points fell inside pass band of OEM 628 filter,
even though they were outside of the GPS L1 band
– Novatel more susceptible to interference within 1% of band edge

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Desired Signal Below Thermal Noise: N >> S
e.g. GPS

Power

Desired Adjacent
Signal Band Band

I

No S

Frequency (f)

B B = Receiver Bandwidth (MHz)
No = Noise Power Density (W/MHz)

N = Receiver Noise Power (W) = B×No

I I
S+N N
( ) ( ) ( )I = ≈ for N >> S
S+BNo

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Desired Signal Above Thermal Noise: S >> N
e.g. TV and FM

Power

Desired Adjacent
Signal Band Band

S I

No

Frequency (f)

B B = Receiver Bandwidth (MHz)
No = Noise Power Density (W/MHz)

N = Receiver Noise Power (W) = B×No

I I
S+N S
( ) ( ) ( )I = ≈ for S >> N
S+BNo

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