Silicon Flatirons Center, UC Boulder

Risk-informed Interference AnalysisPutting spectrum allocation decisions on a more quantitative footing

Pierre de VriesSenior Adjunct Fellow, Silicon Flatirons Center, UC Boulder

Presentation at TPRC 4326 September 2015v. 0.4

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ContextInsatiable demand for spectrum rights leads to– Squeezing services together ever more tightly– Ever-tougher harm/benefit trade-offs

This entails regulatory judgments about harmful interference – Informed by engineering, typically (unfortunately) worst case analysis

Most agencies nowadays complement worst case with quantitative risk analysis (NRC, EPA, FAA, FDA, NASA, DHS, etc.) -- but not the FCC

Project goal – Put harmful interference analysis on a more quantitative, statistical footing– … in order to yield better decisions about allocations and rules

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Worst case (deterministic extreme value) analysisDoes not represent reality accurately – Most parameters that influence harm take a range of values

Over-conservative– Provides too much protection; doesn’t serve public interest, nor

economically efficientCan lead to false confidence that the resulting rules will avert harm– There are many kinds of radio interference– e.g. LightSquared/GPS: fixated on OOBE, but ABI was the real problem

BUT: Worst case is simple, and will continue to be used– Quickly gives a black & white answer

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Risk examples: AccidentsAccident on Mount Everest– High likelihood, high

consequence– Overall risk: very high

Skydiving accident– Low likelihood, high

consequence– Overall risk: moderate

Falling off a bicycle– Low likelihood, low

consequence– Overall risk: low

    Likelihood

Low Medium High

Consequence

High

Medium

Low

EverestSkydiving

BicyclingRisk

Unicycling

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Some terminologyRisk– The combination of likelihood and consequence for multiple hazards– Kaplan and Garrick’s risk triplet: What can go wrong? How likely is it? What

are the consequences? – There are other, complementary approaches: economics, psychology, socio-

cultural analysisQuantitative risk assessment (QRA)– Apply risk triplet using numerical estimates of likelihoods and consequences

Risk-informed interference assessment– A systematic analysis of the likelihood and consequence of interference

hazards caused by the interaction between radio systems

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Risk-informed interference assessmentCurrent scope – Planning, e.g. allocation, rulemaking, waivers– Not post-deployment operation (e.g. adjudication & enforcement, service provider ops)– Leave aside cost/benefit analysis

Related work in spectrum (quantitative risk analysis generally: 30+ years of literature)– Michael Marcus, IEEE-USA (2012): Noted MCL vs. stochastic modeling, flagged lack of FCC

policy– Grunwald, Alderfer & Baker (TPRC 2014): Holistic analysis of 5 GHz Wi-Fi/Globalstar

interference – Littman & De Vries (TPRC 2014): Lessons for FCC from use of QRA in nuclear regulation– FCC TAC (2015): Noted value of risk assessment in allocation decisions, proposed a

method– Cui & Weiss (TPRC 2015): QoS and monetary risk for different kinds of spectrum sharing

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Four element method

1. Make an inventory of all significant harmful interference hazard modes

2. Define a consequence metric(s) to characterize the severity of hazards

3. Assess the likelihood and consequence of each hazard mode

4. Aggregate the results to inform decision making

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Applying the method: MetSat case study

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Meteorological Satellite (MetSat) & LTE

1675–1710 MHz– weather satellite receiving earth

stations– NOAA and DoD

Focus on polar orbiting satellites – leave aside geostationary ones

1695–1710 MHz– AWS-3 cellular mobile uplink

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NTIA Fast Track, CSMAC WG-1 studiesDetermine an interference

protection criterion (IPC): aggregate i/f power not to be exceeded in MetSat receiver

Assume a “sea” of LTE mobilesCalculate the smallest radius

without mobiles that satisfies IPC

Exclude/limit mobile ops within this zone

Essentially worst case

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1. Make an inventory of hazards

Interference types– Co-channel– Out-of-band emission (OOBE)– Adjacent band interference (ABI)

Types of interferer– Point sources and aggregate

interference– Unintentional and intentional

radiators– Operators: well-meaning,

ignorant or malicious

Working taxonomy

– Interfering system (“transmitters”)

– Affected system (“receivers”)

– Coupling between transmitters & receivers

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Transmitter characteristics (LTE)

Cellular mobiles– Transmit power per mobile, co-

channel and out-of-channel – Frequency channel width– Percentage loading of base

station – Location and density of mobiles– Location and density of base

stations

CDF of total EIRP per scheduled mobile

Source: CSMAC (2013), Appendix 3-3

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Receiver characteristics (MetSat)

Source: NTIA (2010) “Fast Track Report”

Transmitter-Receiver Coupling

– Propagation loss from transmitter to receiver

– Additional losses– Antenna heights: satellite

receiver and mobile transmitters

– Earth station antenna elevation and azimuth

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Propagation loss:field measurements and fit

Source: Phillips, Sicker & Grunwald (2012)

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2. Define consequence metric(s)Corporate, aka operational: Cost, ability to complete the mission – Receiver metrics not available (indications of high baseline

outage)Service– Availability: the percentage of time that the link margin is not

met– Quality: bit-error ratio

RF– Fraction of interference-free margin consumed by interference– Interfering signal power levels (IPC) to be exceeded no more

than 20% or 0.0125% of the time

– Interference-to-noise ratio in the receiver

3. Assess likelihood & consequenceConsider co-channel interference, a la Fast Track and

CSMAC WG-1Calculate received interfering power for each exclusion

radiusDetermine the probability a given power is exceeded1. Choose -121

dBm interference protection criterion (IPC)

2. Select 80th percentile, so that IPC is exceeded no more than 20% of the time

3. Results in a 34 km protection radius

4. Vertical “slice” through distribution yields distribution of interfering power at the 34 km radius; see next slide

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Probability of exceeding IPC versus radiusVertical slice through

range/interference chart

Choose -121 dBm interference protection criterion (IPC)

Meets ITU-R SA.1026 long-term IPC:– Below IPC more than

80% of time – i.e. exceeded less

than 20% of the time

Satisfies -121 dBm IPC at 34 km exclusion distance

Interference limit met > 80% of the time

Likelihood

Consequence

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4. Aggregate risks Non-RF: equipment failure,

operator error, …Co-channel interference– sunspot activity– long-term low-level i/f– short-term high-level i/f

Interference from adjacent band– OOBE– ABI

OOBE (hypothetical)

ITU-R long-term criterion: with 34 km exclusion, -121 dBm exceeded less than 20% of time

ITU-R short-term criterion: with 34 km exclusion, -118 dBm exceeded less than 0.0125% of time (hypothetical)

Likelihood

Consequence

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Conclusions

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Benefits of risk-informed interference assessmentA common currency for comparing – different interference mechanisms– competing assessments

More comprehensive analysis – increases the chances of identifying unexpected harmful

interference mechanismsObjective information for decision makers – balancing the benefits of a new service and its adverse

technical impact on existing services

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Action items for regulatorsEducate– Get the community thinking & talking via papers, workshops and consultations– Develop know-how through lectures and in-house training

Set a good example– Quantify likelihoods and consequences in own findings – Request (ideally, require) disclosure and analysis of likelihood & consequence

in filingsStart small, but start soon– Changing the culture is going to take a long time– Pilot approach on low risk/impact proceedings, e.g. waivers for services at

fixed locations

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Action items for the Executive & Legislature

Oversight– Make risk-informed assessment an oversight requirement– Don’t fall for nightmare scenarios– Support and encourage regulators that use risk-informed

interference assessmentsRequire spectrum regulators to do risk analysis– US: Extend existing requirements for risk and cost/benefit

analysis (cf. Executive Orders, OMB directives) to independent agencies

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Summary

Risk analysis considers the likelihood-consequence combinations for multiple hazards, and complements a “worst case” analysis

This will yield better spectrum allocation decisionsFour element method: (1) inventory hazards; (2) define

metrics; (3) assess likelihood & consequence; (4) aggregateAdding this to the toolkit requires culture change, so start

small – but start soon

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We are not able in life to avoid risk but only to choose between risks

Stanley Kaplan & John Garrick (1981)

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Backup

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Risk Chart

      Likelihood

  Qualitative descriptors   Rare Unlikely Possible Likely Certain

    Quantscales Determined case by case

Consequence

Very High Severity

Determined case by case

High Severity

Medium Severity

Low Severity

Very Low Severity

Risk

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Deterministic methods and worst case analysisDeterministic methods: evaluate risk in terms of a predetermined set of

causes characterized by single-valued parameters– potentially interfering transmitter operating at a fixed distance – at a fixed transmit power– to a specific receiver– single-valued path loss

“Worst” case: parameters take extreme values– transmitter at closest distance – maximum allowed transmit power– the least interference-resistant receiver on the market– direct path without any intervening obstructions

Deterministic method doesn’t necessarily entail using extreme values (but usually does)

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Utility of extreme value analysis

Extreme value analysis can be useful– if worst case assumptions show that a particular hazard

doesn’t pose a risk, it can be omitted from subsequent analysis

– if best case assumptions indicate that a hazard poses risks even in favorable circumstances, further analysis is needed

Typically, though, worst case is used “illogically”– worst case assumptions showing harm used to justify

further analysis, and perhaps even determine rules

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Interference analysis: a schematic

– Interfering (transmitting) system

– Affected (receiving) system

– Coupling between transmitters & receivers

Affected System Characteristics

Interfering System Characteristics

Affected System Locations

Interfering System Locations

CouplingCharacteristics

Likelihood and Consequence Metrics

Risk Assessment

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Probability of exceeding IPC versus radiusHorizontal slice through

range/interference chartAs radius increases,

probability that IPC will met increases

Choose -121 dBm interference

Meeting IPC 80% of time means exceeded 20% of the time: ITU-R SA.1026 criterion

34 km exclusion distance

Interference limit met > 80% of the time