Charm: Exploiting Geographical Diversity Through Coherent Combining in LPWANs OR When They Go Low, We Go Lower (Power) IPSN 2018, Porto Adwait Dongare , Revathy Narayanan, Akshay Gadre, Anh Luong, Artur Balanuta, Swarun Kumar, Bob Iannucci, Anthony Rowe Electrical And Computer Engineering Carnegie Mellon University, Pittsburgh PA and Silicon Valley CA 1
Low-Power Wide-Area Networking (LPWAN) 2
LPWAN’s potential Sub-GHz ISM band chirp spread-spectrum (CSS) 10km range in line-of-sight Low data rate (0.25 kbps – 27 kbps) 5+ year battery life Thousands of devices per gateway 3
OpenChirp In Pittsburgh • openchirp.io CMU’s LoRaWAN network in Pittsburgh • 4 outdoor gateways + multiple indoor gateways 4
Coverage 12km 8km 4km 0,0 8km 12km 16km 4km 5
Penetration Inside Buildings Bi-directional packet success rate 100 % gateway 50 % 0 % 0 m 5 m 10 m 72 % 59 % 67 % 72 % 58 % 72 % 61 % 36 % 40 % 52 % 18 % 67 % 52 % 20 % 66 % 44 % 6
Client Device Battery Life 200 Current (mA) MCU + TX 120.7 mA 100 MCU + MCU sleep + MCU + RX radio sleep radio sleep 16.6 mA • Wireless transmissions 3.7 mA 287 nA 0 0 0.5 1 1.5 2 2.5 dominate energy usage Time (s) Packets per hour • Increasing data rate and 1 5 10 60 100 10 DR0 (980 bits/s) 9 reducing retransmissions DR1 (1760 bit/s) 8 Battery lifetime (years) DR2 (3125 bit/s) significantly improves battery 7 DR3 (5470 bit/s) halve retransmissions DR4 (12500 bit/s) 6 life 5 2.2 → 4.5 years 4 increase data rate 3 2.2 → 4 years 2 1 0 10 1 10 2 10 3 10 4 Packets per day 7
LPWANs have lots of gateways….. Can we use them to improve network performance? 8
Coherent Combining gateway 1 • Multiple gateways hear the client same weak transmission gateway 2 cloud • Coherent combining in the cloud gateway 3 • e.g. Cloud Radio Access gateway 1 Networks (C-RAN) in cellular communication gateway 2 gateway 3 time 9
Challenges • High bandwidth connectivity to the cloud • Nanosecond-scale synchronization • Expensive computing resources for large number of streams • Latency 10
Charm • Practical coherent combining Leverage diversity from multiple gateway receivers • Software architecture • Scaleable two-phase protocol • Local packet detection • Hardware platform Auxiliary low-cost SDR-like platform for gateways 11
LoRaWAN y 1 RX 1 h 1 Packet h 2 y 2 x TX RX 2 Cloud Packet MAC h N … y N RX n 12
Charm: Two-Phase Protocol y 1 RX 1 h 1 Packet? h 2 y 2 x TX RX 2 Cloud Joint MAC Packet? Decoder h N … y N Request RX n 13
Charm: Local Packet Detection chirp spread-spectrum packet frequency preamble sync data time known preamble frequency matched filtering time amplitude threshold time Uses only the preamble and sync header 14
Charm: Enhanced Packet Detection chirp spread-spectrum packet frequency time subsample (folding) f time known signal pattern matched filtering f amplitude time threshold time Uses the entire packet 15
Charm: Gateway Hardware • Semtech SX1257 frontend with MicroSemi IGLOO FPGA RX SPI antenna • Outputs radio I/Q stream like LNA Ethernet SPI FPGA RF frontend RPi 3 TX (IGLOO) (SX1257) antenna ΔΣ -I/Q an SDR LPRAN board • Auxiliary hardware for existing gateways and interfaces with raspberry Pi 16
Practical Coherent Combining With Charm • High bandwidth connectivity to the cloud upload samples on request • Nanosecond-scale local packet detection synchronization simplifies synchronization • Expensive computing resources for large number of selective combination of sample streams streams • Latency LoRaWAN ~1 sec to ACK 17
Benchmark: Packet Detection 18
Benchmark: Improved Network And Device Performance Combined signal SNR Results into improved increases logarithmically battery life on client devices 19
Simulation: Dense Deployment coverage improvement data rate improvement LoRaWAN Charm Improvements Increase in coverage area: 46.60% Improved region Data Rate Battery Life (by area) 2x 2x 35.33% 4x 4x 22.30% 8x 8x 2.26% 20
Simulation: Random Deployment coverage improvement data rate improvement LoRaWAN Charm Improvements Increase in coverage area: 74.59% Improved region Data Rate Battery Life (by area) 2x 2x 33.70% 4x 4x 25.82% 8x 8x 3.48% 21
Future Work • Collisions: can we decode collisions? time • Scalability: can we avoid continuously streaming to the time cloud? • Hardware Architecture: how can we leverage new radio Analog Devices front-ends ADALM-Pluto 22
Conclusions • Decode weak transmissions through coherent Gateway 1 combining • Charm’s two-phase protocol Client Gateway 2 • On-demand upload - save bandwidth Cloud • Local packet detection - simplify synchronization requirements and computation User Deployed • Selective combination - better scalability Gateway 3 • SDR-like auxiliary hardware to capture I/Q streams • Performance • Improve coverage up to 98% • Improve battery life up to 8x • Effectively reduce coverage holes • No changes on low-power devices 23
Thank you! Q&A Adwait Dongare (adongare@cmu.edu) 24
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