Scout: An Asymmetric Vehicular
Network Design over TV Whitespaces
Tan Zhang, Sayandeep Sen, Suman Banerjee
University of Wisconsin Madison
Motivation
• Growing trend of vehicular Internet access
Infotainment
Vehicle Intelligence
Road Safety
Motivation
• Internet connectivity available in global
vehicles by 2020 [Cisco report]
300 million
25%
900 million
75%
Motivation
• Present-day approaches
WiFi
Cellular
Small (0.2km)
Congested
Large (10km)
Motivation
• New opportunity in TV whitespaces
– FCC recently released unoccupied TV channels for
unlicensed usage
“Thispropagation
new unlicensed spectrum
will be
Good
characteristics
a powerful platform for innovation…”
in lower frequency
- FCC Chairman
TV
Vacant
Large (2km)
Motivation
• New opportunity in TV whitespaces
Up to 180MHz additional spectrum
-50
-60
Power (dBm)
Design
-70 robust communication protocols to use
-80
whitespaces
for vehicular Internet access
-90
-100
-110
-120
-130
-140
Collected at Madison, WI
-150
500
520
540
560
580
600
620
Frequency (MHz)
640
660
680
700
Outline
• Our asymmetric network design – Scout
– Asymmetric architecture
– Scouting radio based channel estimation
• System implementation
• Evaluation
• Summary
A
Metro Hotspot
Internet
Whitespaces
Gateway
Whitespaces
Base Station
Problem of Power Asymmetry
Asymmetric
Power Limits (FCC 10-174)
Internet
Whitespaces
Gateway
Whitespaces
Base Station
Primary
Incumbents
A Naïve Symmetric Network
16x base stations
Scout – An Asymmetric Network
Internet
Cellular BS
Additional advantages:
• Offloading the majority of traffic
Whitespaces BS
• Zero interference from mobile clients
Challenges in Asymmetric Networks
• Cellular path has high latency
Technology
One-way Latency
3G
100 – 150ms
4G
25 – 40ms
Whitespaces
< 5ms
Delayed Feedback
Fast Changing Environment
Poor Protocol
Decisions
Challenges in Asymmetric Network
Whitespaces BS
Internet
Cellular BS
Can the client foresee channel at a future location?
ACK
6Mbps
Intuition behind Scouting Radio
Whitespaces BS
Internet
Receiver Scout
Rear
Front
Cellular BS
Scouting Based Rate Adaptation
Candidate
Whitespaces
BS
Rates
Internet
9Mbps
6Mbps
1Mbps
NACK
Rear(receiving)
Front
Front(scouting)
6Mbps
Current
ReceptionLocation
Location
Future Reception
Cellular BS
System Implementation
• Downlink: translate WiFi to TV whitespaces
– Wide Band Digital Radio
• Uplink: use a 3G link
Evaluation
• Experiment setup
– Mount a base station on top of a 8-floor building
– Drive along a 1.5km road stretch
Advantage of Scouting Radio
• Approach: measure how accurate a previous
observation to the current channel condition
• Metric: packet loss rates
• Traffic: 200 byte UDP packets at fixed 12Mbps
Single Static Single 10mph and 25mph
Scout 25mph
Advantage of Scouting Radio
Singlehas
hashigh
27%variation
- 34% estimation
error
Single
under mobility
Typical 3G Delay
Scout has 5x lower estimation error
Lag (ms)
Static scenarios have low variation
Application for Rate Adaptation
• Integrate with SampleRate and RRAA
• UDP throughput in trace-driven emulation
38% and 39% median improvement
1
CDF
0.8
0.6
0.4
0.2
Scout-RRAA
Scout-Sample
0
0
20 40 60 80 100 120 140
Throughput Gain(%)
Trace from 10 drives
Post HotMobile Work
• Improving other protocol decisions
– Inter-packet FEC
– Intelligent traffic duplication
• Dual-radio reception diversity
– Combine packets received at both radios
• Improve TCP throughput by 3 – 10x
Post HotMobile Work
Front
BS
Rear
Front
Rear
Conclusion
• TV whitespaces is ideal for providing vehicular
network connectivity
• We presented an asymmetric architecture to
extend network coverage
• We presented a scouting radio to improve
channel estimation under feedback delay