Choir:EmpoweringLow-PowerWide-AreaNetworksinUrban
Settings
RashadEletreby
DianaZhang,Swarun KumarandOsmanYağan
1http://www.witechlab.com/LoRa/ChOIR.html
ImagineaworldwhereeverysingleobjectisconnectedtotheInternet…
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Fewkbps
Severalmilesaway
10yearbatterySimpleandcheapRFinterface
3
SmartInfrastructure SmartHomes SmartVehicles
Low-PowerWide-AreaNetworking(LP-WAN)
LongRange
• Upto10KMsinruralareas
LowDatarate
• Orderofkilobitspersecond
LowCost
• <$5
LowPower
• Upto10yearsofbatterylife
Low-PowerWide-AreaNetworking(LP-WAN)
InitiativesfromIndustry(LoRa,SIGFOX)andstandardizationbodies(3GPPLTEM,NBIoT)
KeyChallenges
6
KeyChallengesInterference
Collisionsemergefromthesheer densityofnodesandthesimplicity ofthecurrentMACprotocols(e.g.,transmit
assoonaswakeup)
LPWANrangesdropby10xinurban areasduetoexcessivemultipath,shadowing,etc.
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Range
8
WiFi/CellularWirelesssensor
networksLPWANs
LoRaWAN
Sigfox
MegaMIMO
SAM
ZigZag
Glossy
ACR
Pastwork
….
…. ….
ChoirScalability
• Decodes10’sofcollidedtransmissions
Range
• Extendstherangeofteamsofcooperatingnodes
Preservingsimplicity
• Fullyimplementedatasingle-antenna basestation
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basestationoveranareaof10Km2 inPittsburgh
:Chirps
ChirpinT.D. Chirponaspectrogram
Data encoding
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:1-bitencoding
𝒏 bits->dividetheBWto𝟐𝒏 initialfrequenciesIn general,
‘0’ ‘1’
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:2-bitencoding
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:2-bitencoding
13
ChoirinactionInterference
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Range
CollisionofchirpsDifferentdata
+
15
Collisionofchirps
+
Samedata
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
16
80 100 120 140 160 180FFT Bin
0
100
200
300
400
500
Abs.
FFT
Hardwareimperfections
𝑓
𝑓 + 𝛿𝑓&
𝑓 + 𝛿𝑓'
Localoscillatormismatch
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Hardwareimperfections
Packet1
Packet2
Sub-symboltimingoffsets
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Time
Frequency
TO
Chirpsaresignalswhosefrequencyincreaseslinearlywithtime
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Time
Frequency
TO
Anoffsetintimemapstoanoffsetinfrequency!FO
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Time
Frequency
Twochirpswithanoffsetinfrequency!FO
Collisionofchirps
+
Samedata
80 100 120 140 160 180FFT Bin
0
100
200
300
400
500
Abs.
FFT
Hardwareoffsets!22
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Decodingdata
U1data: U2data:
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!U1data+U1hardwareoffsets=125U2data+U2hardwareoffsets=130
90 100 110 120 130 140 150 160 170FFT Bin
0
100
200
300
400
500
Abs.
FFT
Symbol 1
125 130
DecodingdataPreamble Sym.1 Sym.2 Sym.n…
Preamble Sym.1 Sym.2 Sym.n…
Peaklocationsareusedtoestimatehardwareoffsets
Hardwareoffsetsremainconstantacross
thepacket
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U1data+U1hardwareoffsets =125U2data+U2hardwareoffsets=130
Symbol1:
DecodingdataPreamble Sym.1 Sym.2 Sym.n…
Preamble Sym.1 Sym.2 Sym.n…
Howtomeasureaccuratehardwareoffsetsacrossthepreamble?
Peaklocationsareusedtoestimatehardwareoffsets
Hardwareoffsetsremainconstantacross
thepacket
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Decodingdata𝑓&∗, 𝑓'∗ = 𝑎𝑟𝑔𝑚𝑖𝑛{34∈ 3467,3487 ,39∈ 3967,3987 } 𝑦𝐶
6& − ℎ&𝑒?'@34A + ℎ'𝑒?'@39A'
𝑓B ->initialfrequencyoffsetestimateofuseriℎB ->channelestimateofuseriΔ ->binsizeoftheFFT𝐶6& ->conjugatenominalchirp𝑦 ->receivedsymbol𝑓B∗ ->correctfrequencyoffsetofuseri
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Whichpeakcorrespondstowhichuser?
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Whichpeakcorrespondstowhichuser?
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 2
27.2 189.6
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 1
352.2107.6
30
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 2
27.2 189.6
Whichpeakcorrespondstowhichuser?
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 1
352.2107.6
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 2
27.2 189.6
Whichpeakcorrespondstowhichuser?
0 100 200 300 400 500 600FFT Bin
0
100
200
300
400
500
600
Abs.
FFT
Symbol 1
352.2107.6
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User1 User2Integerpartdependsonbothdataandhardware
offsets
Fractionalpartdependsonlyonhardware
offsets
1 Near-fareffect
2 Inter-symbolinterference
3 Handlingageneralnumberofcollisions
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Wegeneralizethissolutiontoaccountfor…
ChoirinactionInterference
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Range
RangeExtension
62°F
67°F
65°F
Eachnodeisout-of-range!
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RangeExtension
62°F
67°F
65°F
Eachnodeisout-of-range!
35
63°F
65°F
Canweexploitdatacorrelationstoobtainacoarse-grainedviewofthesenseddata?
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0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25
Abs.
FFT
65 °F
62 °F 67 °F
Noisefloor
Objective
0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25
Abs.
FFT
64.5 °F
Noisefloor
37
0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25Ab
s. F
FT
65 °F
62 °F 67 °F
Noisefloor
Approach
0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25
Abs.
FFT
64.5 °F
Noisefloor
ChoirReceivefilter
38
0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25Ab
s. F
FT
65 °F
62 °F 67 °F
Noisefloor
0 100 200 300 400 500 600FFT Bin
0
5
10
15
20
25
Abs.
FFT
64.5 °F
Noisefloor
ChoirReceivefilter
Implementation
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Evaluation
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Hardwareoffsets
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100 120 140
CD
F
Observed CFO+TO (Hz)
ObservedIdeal
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HardwareoffsetsaretrulydiverseacrossLPWANradios
Resolvinginterference
20000
40000
60000
80000
100000
120000
140000
2 3 4 5 6 7 8 9 10
Net
wor
k T
hrpt
(bi
ts/s
ec)
# Users
Ideal
42
Resolvinginterference
0
20000
40000
60000
80000
100000
120000
140000
2 3 4 5 6 7 8 9 10
Net
wor
k T
hrpt
(bi
ts/s
ec)
# Users
Ideal ALOHA
43
Resolvinginterference
0
20000
40000
60000
80000
100000
120000
140000
2 3 4 5 6 7 8 9 10
Net
wor
k T
hrpt
(bi
ts/s
ec)
# Users
Ideal ALOHA
ChOIR
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29x
Extendingrange
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Numberofcollaboratingnodes
Range
1 1Km
10 2.5Km
30 2.65Km
2.65X
ConclusionObjective
Results Scalability
• Decodes10’sofcollidedtransmissions
Range
• Extendstherangeofteamsofcooperatingnodes
Preservingsimplicity
• Fullyimplementedatasingle-antenna basestation
Platform
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