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gsmwsAn Opportunity forRural Cellular Service
Shaddi Hasan, Kurtis Heimerl, Kate Harrison, Kashif Ali, Sean Roberts, Anant Sahai, Eric Brewer
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Shaddi HasanUC Berkeley TIER/Endaga
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Thanks to...Tapan Parikh
Peter Bloom, Ciaby, and RhizomaticaSteve Song and Village Telco
David Haag, Scotty and Heidi Wisely, and the OA crewThe OpenBTS community
USAID DIL, NSF, Blum Center for Developing Economies... Many others!
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RURALCELLULAR NETWORKS
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WHITE MEANS NO COVERAGE
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WHITE MEANS NO SPECTRUM IN USE
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ONE BILLION PEOPLE WITHOUTCOVERAGE
Source: GSMA
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COMMUNITYCELLULARNETWORKSMicro-scale GSM networks that rural communities build and run themselves.
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280 Subscribers$1,000/mo revenue
300,000 SMS/Voice Min.Critical Infrastructure
“Local, Sustainable, Small-Scale Cellular Networks”, Heimerl et al. ICTD 2013
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Oaxaca, Mexico
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PROBLEMLimited room for CCNs in today’s regulatory frameworks.
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So,
How should CCNs be regulated?
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GSM WHITESPACELet CCN operators
use spectrum on a secondary basis that licensed carriers aren’t using.
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GSM WHITESPACESafety Don’t interfere with existing licensed operators.
Backwards Compatibility Don’t require new or modified client devices.
Spectrum Flexibility Avoid another “garage door opener” fiasco.
Independence Don’t make CCNs and Big Telcos talk to each other.
Trustworthiness Let regulators control what spectrum is used and where.
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And why those goals in particular?
Why GSM Whitespace?
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Regulators CCN Operators Carriers
Safety X X XBackwards
Compatibility X
Spectrum Flexibility X X
Independence X X
Trustworthiness X
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Regulators CCN Operators Carriers
Safety X X XBackwards
Compatibility X
Spectrum Flexibility X X
Independence X X
Trustworthiness X
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Regulators
1) Control over emerging CCN trend➔ Put rules in place that encourage good practices
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Regulators
1) Control over emerging CCN trend➔ Put rules in place that encourage good practices➔ Incorporate a database for monitoring and control
◆ Gives long-term regulatory flexibility
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Regulators
2) Improved rural communication access➔ Current mechanism: USO
◆ Expensive◆ Ineffectual!
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Regulators CCN Operators Carriers
Safety X X XBackwards
Compatibility X
Spectrum Flexibility X X
Independence X X
Trustworthiness X
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CCN Operators
1) Stable regulatory environment➔ Small-scale businesses: getting shut down rare, but disastrous
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CCN Operators
1) Stable regulatory environment➔ Small-scale businesses: getting shut down rare, but disastrous➔ Stability encourages investment
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CCN Operators
2) Use existing client devices➔ GSM phones are EVERYWHERE
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CCN Operators
2) Use existing client devices➔ GSM phones are EVERYWHERE➔ In Papua, 1500 unique phones detected in village
◆ No power◆ No cellular coverage (yet!)◆ Primarily used for listening to music (not smartphones)
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CCN Operators
3) Little to no overhead➔ Yo ho ho! Pirate’s life isn’t bad
◆ Enforcement is unlikely◆ As easy as running an unlicensed network
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CCN Operators
3) Little to no overhead➔ Yo ho ho! Pirate’s life isn’t bad
◆ Enforcement is unlikely◆ As easy as running an unlicensed network
➔ Little power or ability to negotiate with carriers◆ Village schools aren’t going to send lawyers to Jakarta◆ Minimal formal economy
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Regulators CCN Operators Carriers
Safety X X XBackwards
Compatibility X
Spectrum Flexibility X X
Independence X X
Trustworthiness X
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Existing License Holders
1) Garage door openers, v2.0➔ Problem:
Make sure CCNs don’t become reliant on a particular channel.
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Existing License Holders
1) Garage door openers, v2.0➔ Problem:
Make sure CCNs don’t become reliant on a particular channel.➔ Solution:
Require CCNs to change channels frequently and proactively.
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Existing License Holders
2) Sharing overhead➔ Problem:
Don’t want to interact with 100’s of CCNs.
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Existing License Holders
2) Sharing overhead➔ Problem:
Don’t want to interact with 100’s of CCNs.➔ Solution:
Use sensing as primary sharing mechanism.Make database usage optional.
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Existing License Holders: Even More Incentives
➔ Share spectrum to fulfill rural service obligations◆ DB gives visibility into what spectrum CCNs use to provide rural service◆ License holders could receive credit for CCN activity in their spectrum
➔ Opens up new rural markets◆ CCNs prove rural markets, without investment from incumbents◆ CCN customers call incumbents’ customers: free money
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Nothing bad happens when a CCN uses unused spectrum...
➔ CCNs don’t have to talk to anyone
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Nothing bad happens when a CCN uses unused spectrum...
➔ CCNs don’t have to talk to anyone➔ CCNs aren’t using spectrum anyone else is using
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Nothing bad happens when a CCN uses unused spectrum...
➔ CCNs don’t have to talk to anyone➔ CCNs aren’t using spectrum anyone else is using➔ Licensed users can still use spectrum as they please
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Nothing bad happens when a CCN uses unused spectrum...
➔ CCNs don’t have to talk to anyone➔ CCNs aren’t using spectrum anyone else is using➔ Licensed users can still use spectrum as they please➔ Sufficiently low sensing threshold restricts sharing to
underserved areas only.
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...but plenty of good does.
➔ Rural areas get communications service
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...but plenty of good does.
➔ Rural areas get communications service➔ Rural entrepreneurs get a sustainable business
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...but plenty of good does.
➔ Rural areas get communications service➔ Rural entrepreneurs get a sustainable business➔ Existing carriers keep building out their networks like they
always have
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GSM WHITESPACESafety Don’t interfere with existing licensed operators.
Backwards Compatibility Don’t require new or modified client devices.
Spectrum Flexibility Avoid another “garage door opener” fiasco.
Independence Don’t make CCNs and Big Telcos talk to each other.
Trustworthiness Let regulators control what spectrum is used and where.
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Crazy Possibilities -> Good Practices
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DownlinkGSM850 (25MHz)
GSM1900 (60MHz)
Uplink200kHz per channel
GSM900 (25MHz)GSM1800 (75MHz)
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DownlinkUplink
Carrier A Carrier B Carrier C
7 concurrent voice calls100’s SMS/min100’s active subscribers
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DownlinkUplink
Carrier A Carrier B Carrier C
7 concurrent voice calls100’s SMS/min100’s active subscribers
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I use channel A. My neighbors use channels B, C, D...!D
E
A
B
C
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A: -90dBmB: -70dBmC: -93dBm
D
E
A
B
C
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Moving your call to cell B.
Bye!D
E
A
B
C
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D
E
A
B
C
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1996’s hottest gadget.
Also a cognitive radio.
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Key idea #1:Use phones to scan for in-use channels.
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Key idea #2:Constantly change channels to prevent squatting.
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Key idea #3:Use a database to monitor and control CCNs.
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Ch. 20
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Ch. 20
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Plz scan:Ch. 30, 40, 48, 70, ...
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30: Clear!40: Clear!48: Clear!
30: Clear!40: Clear!48: Clear!
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Ch. 30
After a few hours...
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30: Clear!40: Clear!48: -97dBm!
Ch. 48
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Note to self: don’t use
ch. 48!
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Ch. 9830: Clear!40: Clear!48: Clear!
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20: -95dBm!30: Clear!40: Clear!48: Clear!
Ch. 20Ch. 20
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20: -95dBm!30: Clear!40: Clear!48: Clear!
Ch. 20Ch. 20
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Solution: Simulate Handover!
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Handover happens here.
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Someone else is on a channel we’re using!
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We should switch to a new safe channel.
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Phones handover to the new channel.
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Worst case detection speed = cycle time
~90 sec
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90 sec << rate carriers add rural base stations
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90 sec = probably excessive
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Lab Experiments + Real World Deployment
Evaluation
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github.com/shaddi/gsmws(Runs on OpenBTS)
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Detecting a new primary user
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Papua: Measured spectrum usage
In-use channel
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Papua: Measured spectrum usage
In use + “safe” chans
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Papua: Measured spectrum usage
Two detection events(probably spurious reports)
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Papua: Measured spectrum usage
➔ Switched channel every night, after power failures
➔ No impact on usage of network
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Future work
➔ Field trial in Mexico◆ Full system deployment◆ Lots of real users◆ Detect real intereference events
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Shaddi [email protected]/~shaddi
We’re looking for telco and regulator partners for GSMWS trial deployments.