1. Adwait Dongare, Revathy Narayanan et al. Carnegie Mellon University
Charm: Exploiting Geographical Diversity Through Coherent Combining in Low-Power Wide-Area Networks
Adwait Dongare, Revathy Narayanan et al.
Carnegie Mellon University

2. Background LPWAN technologies growing rapidly
10-years battery life
Low data rate
x km communication distance
Not all devices experience the promised 10 year battery life
Severe battery drain when signals are highly attenuated
LPWANs are largely user-deployed
Device deployment is unplanned

3. Use Multiple Gateways to Decode Weak Signals
Motivation
Use Multiple Gateways to Decode Weak Signals

4. Challenges Identify significantly weak signals at the gateway
Packets are commonly undetectable
How to detect the signals in real time
Identify common signals between gateways at the cloud
Decode packers from multiple damaged signals

5. Architecture

6. Locally Detecting Weak Signals

7. Locally Detecting Weak Signals
LoRa Signals:
Sub-sample with 1/δf:
Add received signals will reinforce the signal part while alleviate the noise part

8. Mitigating Frequency Offsets
LoRa gateway experiences arbitrary shift in frequency (CFO)
continuous frequency shift ΔfCFO , hard to eliminate
continuous phase shift 2π ΔfCFO t
The straw man approach
The phase shift between two preamble symbols is 2π ΔfCFO t
The ΔfCFO can be solved by estimating the phase shift
But the preamble symbols is insufficient to overcome noise
Improved solution
Estimate ΔfCFO modulo δf

9. Signal Detection Algorithm

10. Programmable Hardware Design
Hardware Platform
Compressing Data Stream
Reduce computation overhead
Sum 64 samples into a 7-bit sample
Programmability
Use Microsemi IGLOO AGL250 FPGA
Programmable for developers

11. Time Synchronization at the Cloud
Require Phase-Level Time Synchronization

12. Time Synchronization at the Cloud
Phase-Based Time-Sync Below the Noise Floor
Conduct coarse time synchronization of the gateways via NTP
Consider a small range of possible offsets
Find the offset whose phase difference varies minimally

13. Joint Decoding at the Cloud
Select Signal Candidate
Use geographic location of gateways
The arrival time of the packet
Use OPTICS clustering algorithm
Decoding Algorithm

14. Opportunistic Fetching of Information
Transmitting all weak signals is inefficient
The cloud may receive decoded data from another gateway
Two-Phase Data Fetch
Gateway buffers I/Q streams
For potential reception, gateway first reports its signature
The cloud performs clustering to decide whether to pull data

15. Results
Sensitivity & Battery Life Improvement

16. Results
Coverage & Data Rate Improvement

17. Summary Local Ultra-Low SNR Packet Detection
Hardware Implementation of Packet Detection Algorithm
Phase Level Time Synchronization
Joint Decoding Algorithm