1. Enabling Reliable, Asynchronous, and Bidirectional Communication in SNOW
Abusayeed Saifullah*, Mahbubur Rahman*, Dali Ismail,
Chenyang Lu, Jie Liu, Ranveer Chandra
*co-primary author
Wayne State University
Washington University in St Louis
Microsoft Research

2. Low-Power Wide-Area Network (LPWAN)
Overcomes scalability and range limits of traditional wireless sensor networks.
A key technology driving the Internet of Things (IoT).
LoRa
SigFox
nWave
RPMA
NB-IoT
EC-GSM-IoT
LTE Cat M1 5G

3. SNOW (Sensor Net. over White Space)
LPWAN by exploiting the TV white spaces.
White Space: unused TV channels between MHz.
SNOW 1.0 [SenSys ’16, Best Paper Nominee]
Long range  nodes directly Tx to the base station (BS).
The BS accesses cloud for white space database.
Spectrum split into narrow orthogonal subcarriers.
Each node transmits on a subcarrier.
Multi-Rx at the BS using a single antenna-radio.

4. Decoding for Multi-Rx at BS
Aggregate OFDM signal
in time domain
Orthogonal signals from sensor nodes on orthogonal subcarriers.

5. SNOW 2.0 Uplink: Multi-Rx with single-antenna radio (SNOW 1.0).
Downlink: Multi-Tx with single-antenna radio
Send different data to different nodes using a single Tx
Bidirectional: concurrent uplink and downlink comm.
Fully Asynchronous
Reliability
ACK for
each Tx

6. SNOW 2.0 Dual-Radio Design
Simultaneous Tx/Rx at BS is enabled by using two radios.
One radio is dedicated for Tx, the other for Rx.
BS uses wide white space spectrum
Each node uses one half-duplex narrowband radio

7. PHY Design Using D-OFDM
D-OFDM: Distributed OFDM
Tx on narrow OFDM subcarriers  energy and spectrum efficiency
Individual subcarrier modulation: BPSK
Uplink
Each node independently encodes and transmits.
BS runs decoder based on global FFT to decode asynchronous Tx.
Downlink: needs an inverse implementation.
Different from MIMO radio that rely on multiple antennas
Single antenna is important for low frequency band (54-698MHz).

8. Downlink Communication
BS encodes different data on different subcarriers.
Different from traditional broadcast.
Performs IFFT.
Makes a single transmission.
From the received OFDM signal, a node independently decodes data from signal component on its subcarrier.
Can encode anytime on a subcarrier
Distributed decoding

9. MAC Protocol White space spectrum split into orthogonal subcarriers.
Each node is assigned a subcarrier.
When number of nodes> subcarriers, a subcarrier is shared.
Location-aware subcarrier allocation
Attempt to assign different
subcarriers to hidden
terminals.
Also ensure a subcarrier
is not overly congested.
Tx using a lightweight CSMA/CA protocol (like TinyOS)
Static interval random backoff
Try to assign different subcarriers to u and w. u w

10. MAC Protocol: Handling ACK
D-OFDM allows us to encode data anytime on any subcarrier  allows ACK of asynchronous Tx.
Rx radio keeps receiving while Tx radio keeps sending.
When there is nothing to transmit, Tx radio can sleep.
CSMA/CA helps avoid interference on ACK.
ACK for u will be sensed by v. u v

11. Other Features Peer to peer communication Handling dynamics
Spectrum dynamics through backup subcarrier
Subcarrier swap
Load balancing
Node join/leave
SNOW can exploit
fragmented white
space spectrum.
Fragmented white space spectrum
Signal strength (dBm)
Frequency (MHz)

12. Implementation Implemented in GNU Radio Experiment with USRP device
Connected to laptop or
Raspberry pi
All the packets (up to # of subcarriers) are decoded within 0.1ms  comparable to a single packet decoding time scalability.

13. Design Parameters Key design parameters Default settings
Bit rate: target bit rate at least 50kbps.
Packet size
Subcarrier bandwidth
Bit spreading factor
Default settings
Tx power: 0dBm
Subcarrier bandwidth: 400kHz
BS bandwidth: 6MHz
Packet size: 40bytes
Bit spreading factor: 8
Determined based on target bit rate, Shannon-Hartley Theorem, Nyquist Theorem, and experiment.

14. Urban Deployment
Deployment in Detroit, Michigan

15. Distance vs. Tx Power
400kHz bandwidth
Approx. 8km
at 0dBm

16. Reliability Uplink Downlink 400kHz 0 dBm bandwidth
SNOW is reliable in both directions

17. Max Achievable Throughput
Throughput increases linearly with the number of subcarriers due to parallel receptions at BS.

18. Indoor Deployment Lower frequency propagates
well through obstacles.
Deployment in CS building in Wayne State University

19. Rural Deployment (Rolla, Missouri)
Throughput increases linearly with the number of subcarriers due to parallel receptions at BS.

20. Benefits of SNOW over Other LPWANs
Limited infrastructure
NB-IoT, 5G, LTE Cat M1, EC-GSM-IoT need infrastructure.
Bidirectional
Most non-cellular LPWANs are primarily uplink only.
Scalability increases
with the spectrum
availability due to
high parallelism.
LoRa, SigFox achieve
scalability assuming
very low traffic.
Abundant spectrum
Spectrum availability in the US counties

21. Comparison with LoRa (QualNet Simulation)
Setup
Used a LoRa gateway with 8 parallel demodulation paths.
An equal spectrum given to LoRa and SNOW.
Energy consumption and latency are quite steady in SNOW
As parallelism in SNOW is much higher.

22. Conclusion SNOW is an LPWAN over the TV white spaces.
Can be exploited by the future IoT and CPS.
Can help shape and evolve IEEE m standard.
Potential advantages over existing LPWANs.
SNOW achieves scalability, robustness, energy efficiency
Asynchronous, parallel Tx and parallel Rx
Reliable, bidirectional
Simple design using single antenna radio
Future work
Coexistence
Mobility
Security.