Introduction to interference limits policy presented at Silicon Flatirons roundtable in DC on November 13, 2012

1. Interference Limits Policy
An Introduction
Silicon Flatirons Roundtable on Receivers, Interference & Regulatory
Options
13 November 2012
Pierre de Vries
Silicon Flatirons Center, UC Boulder
pierredv@hotmail.com
1

2. The Problem
Improve receiver interference tolerance
… without mandating receiver performance
2

3. The Problem (2)
• When can a receiver stop a neighbor transmitting at
authorized levels inside that neighbor’s band?
Receiver Transmitter
Received □ Always
signal
strength
□ Sometimes
□ Never
Frequency
✓
□ Don’t Know
Harmful Interference?
3

4. Interference Limits: a Definition
• Explicit, up-front statement of the interference that a system needs to tolerate
before it can bring a harmful interference claim
– A harm claim threshold NOT an attempt to characterize actual the interference environment
– It doesn’t attach to a receiver as such: a system = transmitter + receiver
• Every allocation’s interference limit will be a customized combination of signal
strength, probability levels, etc.
Percentage of locations and times
field strength where field strength cannot be
px , pt exceeded
Field strength (all polarizations, any No protection beyond this
modulation, at specified height) above point, i.e. receiving system
which receiving system can claim has to tolerate any interfering
harmful interference signals
Band to be allocated frequency 4

5. Choosing Receiver Interference Limits
field strength Receiver interference limit with
probability of not being exceeded;
chosen to accommodate current
and planned conditions Interference level chosen
to reflect planned, not
px , pt current, conditions
px , pt
px , pt
Current
Probability of
interference
resulting signal
level
strength above
limit
frequency
Band to be allocated
5

6. Connecting interference limits to receiver performance
Interference Limit
rules
Design input: Estimate of Design requirement:
expected RF interference Business case
environment
Design requirement: Design requirement: Design requirement: Design requirement:
Desired signal RF interference to be Quality of service Cost constraints
characteristics tolerated
Regulation: System Design Industry standards,
Transmitters, receivers Process best practices
Specification: Specification: Specification: Specification:
Transmitter deployment: Transmitter performance Receiver performance Interference Protection
power, height, spacing, … Ratios
6

7. Implementation
• Start with band boundaries where problems can be
foreseen
• Use multi-stakeholder process to seek
consensus, then move to NOI, rule-making as
necessary
• FCC can ratchet up harm claim thresholds to drive or
reflect improvement in system interference tolerance
where market forces are insufficient
• Interference limits may not be sufficient in some
cases, e.g. decoupled receivers, unlicensed
devices, safety of life, sharing with government 7

8. Examples
A. Protect incumbent neighbor: cellular downlink
B. Protect incumbent receiver: TV
1. Single value (at contour)
2. Thresholds vary across service area
8

9. A. Protect incumbent neighbor: Cellular downlink
• For new allocation, to allow incumbent or anticipated
cellular neighbor next door
• Choose harm claim threshold to allow operation of
cellular downlink
– i.e. new allocation won’t be able to claim harmful
interference given envisaged cellular operation
• Use modeled resulting field strengths for generic base
station deployment as basis for thresholds
– In-block cellular field strength ≡ floor for adjacent-block
harm claim threshold, and vice versa
9

10. Ofcom/Transfinite downlink simulation
10

11. Simulation parameters
• Ofcom commissioned Transfinite (2008) to compute the signal
strengths of a variety of services that use the UHF band
• For an IMT-2000 downlink
– Frequency: 826 MHz
– Bandwidth: 3.84 MHz within 5 MHz (standard WCDMA channel bandwidth)
– Transmitter height 30 m; transmitter separation distance 1.86 km
– Total EIRP all users: 22.7 dBW
– Power attenuation at frequency offset of 5 MHz: 46.2 dB
– Baseline propagation model: ITU-R Rec. P.1546 version 1546-2, configured
for 50% of time and a location variability standard deviation of 5.5 dB
– Simulation file: seven base stations in hexagonal cellular structure, 479 test
point in central cell, aggregate pfd determined at height of 10 m
11

12. Example: Determining px for base stations
PFD at 10 m doesn’t exceed
-42 dB(W/m2) per MHz
(+104 dB(μV/m) per MHz)
at more than 5% of
locations
PFD at 10 m doesn’t exceed
-60 dB(W/m2) per MHz
at more than 50% of
locations
5%
-42
Source: Ofcom/Transfinite 2008. Modeled PFD of IMT-2000 base station population, 10 m altitude

13. Results
• Downlink in-band field strength at 10 m altitude is 104
dBμV/m per MHz or less at 95% of locations; use this
as the out-of band interference limit
• Downlink adjacent band field strength at 10 m altitude
is 57 dBμV/m per MHz or less at 95% of locations; add
3 dB assuming interferers on both sides to give 60
dBμV/m per MHz as the in-band interference limit
13

14. field strength
Cellular-inspired Interference Limit
dBμV/m per MHz
NTE at >5% of locations, >50% of time; observed at 10 meter altitude
104
Interference limit for
Cellular DL in this block operator in this block
60
§15.209 limit: 54 dBμV/m
per MHz at 3 meters
Frequency
MHz
14

15. B. Protect incumbent: TV receivers
• Approach
– Assume desired signal strength D and adjacent channel protection
ratio D/U
– Calculate max undesired signal strength U from requirement
U ≤ D – D/U
• Options
1. Single harm claim threshold using Part 73 D/U ratios and
desired signal at service contour
2. Location-dependent threshold: Use interference thresholds and
measurement protocol defined in 47 CFR 73.616 (e)(1) and OET
69 to define interference limits that vary with grid cell across
service area
15

16. Option 1. Part 73, at contour only
• Single harm claim threshold using Part 73 D/U ratios and
desired signal at service contour
– Part 73: field strength at the NLSC is 41 - 20 log[615/(channel mid-
freq)] dBu/6MHz.
• To simplify, assume we’re looking at 615 MHz, so D = 41 dBu/6MHz.
– Part 37.616: the {-1, 0, +1} channel D/Us are {-28, +23, -26} dB
• So harm claim thresholds U = D – D/U = {69, 18, 67}
dBu/6MHz:
– “An end user cannot claim harmful interference unless their
receiver can operate satisfactorily for U = {69, 18, 67} dBu/6MHz
given D = 41 dBu/6MHz”
16

17. Option 1: Notes
• Doesn’t meet interference limit paradigm of harm
claim threshold defined probabilistically across license
area
– Provides no guidance to the regulator or adjacent licensees
about the maximum allowed signal that could be delivered
by a neighbor when D > 41, i.e. inside the contour, closer to
the transmitter.
• FCC could use branding program to label receivers that
meet this criterion
• Who’s entitled to protection: service (broadcaster) or
receiver (consumer)?
17

18. 2. Part 73 applied across service area
• A TV licensee may not may not claim harmful
interference unless the interfering signal exceeds the
thresholds set in Part 73.616
– Calculate desired field strength D using Longley-Rice in each
2x2 km grid cell, following OET Bulletin 69
– Use D/U ratios for co- and first-adjacent channels in Part
73.616 (e)(1) to calculate D for each cell
– Apply F(50,10) undesired signal test
18

19. Part 73.616 (e)(1)
“. . . The threshold levels at which interference is considered to occur
are:
(i) For co-channel stations, the D/U ratio is 15 dB.
This value is only valid at locations where the signal-to-noise ratio is 28 dB or
greater.
At the edge of the noise-limited service area, where the signal-to-noise (S/N)
ratio is 16 dB, this value is 23 dB.
At locations where the S/N ratio is greater than 16 dB but less than 28 dB, D/U
values are computed from the following formula:
D/U = 15 10log10*1.0/(1.0−10−x/10)+ , where x = S/N-15.19 (minimum signal to noise
ratio)
(ii) For interference from a lower first-adjacent channel, the D/U ratio is
−28 dB.
(iii) For interference from an upper first-adjacent channel, the D/U ratio
is −26 dB.”
19

20. Part 73-inspired harm claim thresholds
• Within each reference cell with calculated median desired signal
strength D, a receiver may not claim harmful interference unless the
interfering signal exceeds {D+28, D+26} dBu/(6 MHz) on the {-1, +1}
channels at more than 50% of locations, more than 10% of the time.
• Within each reference cell with a calculated median desired signal
strength D, a receiver may not claim harmful interference unless the
interfering signal exceeds the following values in dBu/(6 MHz)
D = 41.6 - Ka: 18.6 - Ka
41.6 - Ka >= D < 53.6 - Ka: D - Ka - 15+10log10[1.0/(1.0 -10^{-x/10})],
where x = D + Ka - 40.8, Ka = dipole factor
D >= 53.6 - Ka: 38.6 - Ka
… on licensed channel at more than 50% of locations, more than 10% of the
time.
20

21. Option 2: Notes
• Using just Part 73 and OET 69 results in a limited
(incomplete?) interference limit policy
• Extending scope will require balancing transmitter and
receiver interests: starting point for negotiation.
– Part 73.116 only protects the first-adjacent channels
• adding more protected channels would be desirable for receivers
– D/U in rules don’t reflect current state of the art
• reflecting more negative D/U for larger values of D, as ATSC A/74
does, would be desirable for adjacent transmitters operating near
the TV tower
21

22. For discussion
• Cellular
– modeling approaches
– Near/far: femto vs. macrocell in adjacent blocks
• TV
– Compare/contrast options
– Negotiating between cellular and TV
• Harder cases
– Different duty cycles: radar vs. comms
– Different wave forms
22

23. Resources
Executive summary: http://sdrv.ms/ReceiverLimits
TPRC Paper: http://ssrn.com/abstract=2018080, slide deck
at http://bit.ly/TPRC2012InterferenceLimits
23

24. Backup
24

25. Why Bother?
• FCC TAC (2011) listed 9+ cases where receiver
performance was a significant issue limiting the regulator’s
ability to allocate spectrum for new services
– 800 MHz Public safety vs. Nextel, SiriusXM vs. WCS, AWS-1 vs.
AWS-3, TV whitespace, LightSquared vs. GPS, etc.
• Impact
– Gains foregone when new services cannot be deployed
• Innovation, investment, consumer welfare, competition, …
– Unused 20 MHz AWS-3 band @ 50 cents/MHz.POP = $3 bn at
auction, x10 (?) for consumer surplus
– LightSquared claimed surplus of $120 bn, FAA claimed impact of
$70 billion on aviation
25

26. Solutions: The transmit/receive trade-off
(1) Reduce Tx signal (2) Improve Rx filters
Receiver Transmitter
Received
signal
strength
Frequency
After poor receiver After Tx power reduction After better receiver
filtering & poor receiver filtering filtering
Received
signal
strength
Frequency

27. Enforcement: What’s interfering signal strength?
For each measurement point
in a verification area, observe
over time; fraction that
exceed E must be < px
Verification area, ←5 km→
observe on 100m grid
Verification time window, observe every 100 ms 60 sec
For every verification window,
fraction of observations that exceed E should be < pt
27

28. Additional Measures
• Receiver interference limits: necessary but not always sufficient
• What if users aren’t trusted to deploy receivers that function
“satisfactorily” given the limits?
– Decoupled receivers: sold and operated independently of licensee
– Unlicensed devices: no license required
– In bands shared with Feds, perhaps even licensees...
• So: add device performance requirements
– Self-certification
• warranty-of-fitness, self-certification to individual or industry standard
– Mandated performance
• front-end selectivity, mandated industry standard
28

29. Alternative approaches
Decoupled receivers in licensed
Service on other side of block boundary Licensed
service; unlicensed devices
Similar service type, no change Nothing beyond customary
Harm claim thresholds
envisaged transmitter rules
Different service across boundary, but Harm claim thresholds, perhaps
Harm claim thresholds
occupancy matches interference limits self-certification
Currently low intensity use across
Self-certification, perhaps
boundary, but change to more intensive Harm claim thresholds
receiver mandates
service planned
Self-certification, perhaps Perhaps self-certification,
Performance-critical services
receiver mandates probably receiver mandates
29

30. Comparison with Interference Temperature
• Interference temperature is like saying anybody can come into your yard when
you’re having a party, as long as they don’t increase the noise above a given level.
• Interference limits are rules that say how loud the noise in the neighbor’s yard can
be before you can call the cops.
Interference Temperature Receiver Interference Limits
Focus on solving out-of-band, cross-allocation
Focus on in-band, co-channel operation
interference
Designed to facilitate and encourage second party, Does not grant second party rights in a primary
co-channel operation licensee’s frequency block
Aims to create additional operating rights Adjunct to existing definition of operating rights
Only needs to be measured when concern that limit
Needs to be measured at all locations at all times
is being exceeded
Deterministic values Probabilistic
30

31. Benefits of Interference Limits
• User value
– Regulator delegates system design decisions, e.g. Tx vs. Rx performance
– Reduces business risk
• Receivers: guarantee of no interference from future allocations
• Transmitters: no harmful interference claims from poor receivers
• Both: better estimate of deployment costs from knowing interference risks
– Increases economic efficiency: adjust Tx and Rx rights by negotiation to
reach social welfare optimum
• Regulatory value
– Allows technology-neutral rules
– Allows future repurposing of quiet bands
– Facilitates dynamic sharing by automatic calculation of permissions
• Increase usage by clarifying responsibility for mitigating H.I.
31

32. Pros and Cons of Interference Limits
Pro Con
Parameters are technology- and service-neutral Compliance validation requires modeling
(though parameters choice encodes assumptions) and/or measurement, with assumptions about
propagation models and/or sample statistics
Delegates system implementation choices to Probabilistic metric makes it hard to apportion
operators and manufacturers, thus provides flexibility blame if multiple transmitters combine to
exceed reception interference limit
Can ratchet up receiver performance indirectly by Short list of parameters may omit a key
increasing limit. Ratchet is technology-independent parameter that is vital to the effective
since it specifies operating environment, not receiver management of a particular case
performance
Easier to adjust interference limits across assignment Doesn’t capture nuances of harmful
boundaries to reach social welfare optimum than interference mechanisms, e.g. different
renegotiating device performance standards impact of different modulations
Uniformity of approach in all cases makes rulemaking Interference limits attached to a transmitter
easier and more predictable license are insufficient when receivers are not
controlled by a licensee, e.g. decoupled
receivers and unlicensed