Factors to Consider for Intersystem EMC

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• Factors to Consider for Intersystem EMC
• ITU-R Radar Seminar
• Geneva
• 24 September 2005
• Frank Sanders
• Chief, ITS Telecommunications Theory Division,
• U.S. Department of Commerce, NTIA
• 325 Broadway, Boulder, CO
• fsanders_at_its.bldrdoc.gov
• 303.497.7600

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Introduction

-Proposals have been made to operate
communication systems in radar bands.
-These proposals represent a change from the
historical approach of separate band allocations
due to perceived electromagnetic compatibility
issues between radar and communication services.
-This talk addresses the technical challenges
analyzing intersystem EMC.
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Some justifications offeredfor sharing

1) Radar systems make little use of existing
spectrum allocations.
2) Radar receivers are inherently robust against
interference.
3) Perhaps interference to radars might be
accommodated on a statistical basis.
4) Spectrum might be shared by communication
devices that sense radar signals and avoid local
radar frequencies.
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Evaluation of these ideas
may create opportunities for new patterns of
spectrum use. But careful technical consideration
of EMC is needed to test these concepts. We will
consider the four ideas from the previous slide
in this presentation
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Measured spectrum utilization of radars
Radars usually consist of high power transmitters
operating in concert with very sensitive
receivers. EIRP transmit levels are often more
than 1 GW, while receiver noise figures are often
only a few decibels above the theoretical thermal
noise limit.

Representative radar emission parameters as a
function of mission.
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Measured spectrum utilization of radars, continued
-Some spectrum surveys have not observed radar
emissions. Is this evidence that radars dont
utilize spectrum allocations very much?
-But published NTIA spectrum surveys show
significant spectrum utilization by radars on an
ongoing basis at every location in the US where
such measurements have been done (for example, in
Denver, San Diego, Los Angeles, and San
Francisco).
Why might this be?
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Measured spectrum utilization of radars, continued
San Diego, California measured radar utilization
of L-band (2 weeks)
Source NTIA spectrum survey report

Maximum
Mean
Minimum
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Measured spectrum utilization of radars, continued
San Diego, California measured radar utilization
of C band (2 weeks)
Source NTIA spectrum survey report
Maximum
Mean
Minimum
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Measured spectrum utilization of radars, continued
Why dont some spectrum surveys show radar
signals whereas the NTIA results do show
substantial usage?

Radar emissions dont show up with conventional
survey techniques. Instead, radars need to be
observed with specialized, computer-controlled
techniques. NTIA publications (e.g., a recent
NTIA Report on RSEC measurement procedures)
explain how. See also ITU-R New Recommendation
M.1177.
Specialized RF front ends are also needed
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M.1177 radar meas. front-end

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Radar receiver performance in the presence of
interference
In some EMC studies that have been performed by
NTIA, radars have been intentionally subjected to
low levels of interference from various waveforms.

Protocols Radar receiver is disconnected from
its antenna. In place of live targets, test
targets are injected into the radar receiver.
Target power level is adjusted so that the
probability of detection of these targets is as
close as possible to 90 (Pd0.9).
Interference is injected with a selected
modulation. Target losses are counted as a
function of interference power level relative to
radar inherent receiver noise level (I/N).
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Radar receiver performance in the presence of
interference

Example radar plan position indicator (ppi)
display during interference testing. The bright
wedge is a time-lapse effect of the radar ppi
scan line.
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Radar receiver performance in the presence of
interference

Equipment room of a radar station
Examination of a radar receiver circuit card
during interference testing
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Radar receiver EMC performance in the presence of
interference

Example data showing target losses as a function
of I/N ratio for a search radar.
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Radar receiver EMC performance in the presence of
interference

Example data showing target losses as a function
of I/N ratio for a maritime search radar.
Pulsed signals representative of other Radar
emissions.
Communication-type signals
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Radar receiver EMC performance in the presence of
interference
Typical I/N ratios at which measurable target
losses begin to occur. Based on results of
measurements to date and grouped by radar mission.

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Radar receiver EMC performance in the presence of
interference
Do target losses only occur at the edge of radar
coverage?
Interference causes loss of targets within some
number of decibels (call it X dB) of radar
inherent noise...
therefore target losses dont depend upon range,
per se. To the extent that distant targets are
weaker than closer targets, losses would
correspond somewhat to range. But target cross
sections vary
so some targets are weak and close. Since X dB
is X dB, target losses dont strictly translate
into range reduction. Weak targets are not
necessarily unimportant targets.
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Radar receiver EMC performance in the presence of
interference
Target losses at low I/N levels of interference
are insidious.
Low-level interference doesnt make any weird
features appear. Targets simply fade away, so
there is no way to tell that targets have been
lost.
Low-level interference has caused all targets to
disappear. But there is no overt indication of
Interference.
10 targets with no interference present
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Radar receiver performance in the presence of
interference
Example of ppi screen display effects of
high-level interference (such as dramatized in
movie portrayals).
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Technical consideration of a statistical basis
for radar interference
The statistical approach raises several questions
1) Which targets is it acceptable to lose?
2) How can communication systems coordinate
activities with radars to assess target losses?
3) How can target losses be correlated with
communication system interference?
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Mitigation of interference by avoiding radar
frequencies
-In dynamic frequency selection (DFS),
communication units sense radar signals on the
communication systems frequency, and then move
operations to another frequency.
-DFS depends upon communication systems to sense
radar signals and then avoid the conflicted
frequencies.
-No theoretical reason has been shown why this
approach cannot work it is possible in
principle. But it needs to be tested.
-In the USA, NTIA, other Government agencies, and
industry are currently working together to test
this technology in the 5 GHz part of the spectrum.
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Summary
-Radar activity fills allocated spectrum bands at
surveyed locations in the US. But proper
monitoring techniques are necessary for
successful surveys.
-In tests performed to date, radars typically
begin losing targets when interference is at
about -10 dB I/N. (That is, at 10 dB average
power below radar receiver inherent noise.)
-Effects of low-level interference are insidious
targets fade away without overt indications of
interference.
-Proposals to allow interference on a statistical
basis are in early stages the technical
feasibility of this approach is being studied.
-An alternative approach, dynamic frequency
selection, may be feasible. It typically requires
substantial amounts of listening time. The
concept is undergoing tests at this time.
Institute for Telecommunication Sciences
Boulder, Colorado