Harnessing Mobile Multiple
Access Efficiency with Location
Input
Wan Du* and Mo Li
School of Computer Engineering
Nanyang Technological University, Singapore
Main access to WLAN
“Smart phones overtake client pcs in 2011,” 2012.
www.canalys.com/newsroom/smart-phones-overtake-client-pcs-2011
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Pervasive Location Information
• Outdoors
– GPS (meters)
• Indoor Localization
– Sound (centimeter)
– WiFi (meter)
– Camera (meter)
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Location Based Applications
• Navigation
• Augmented reality
• Fine-grained location in supermarkets
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Key Observation
Location error of <<
localization
Indoor: <1m
Outdoor: <13.7m
Communication range
of WiFi
Indoor: >50m
Outdoor: >200m
• Improving the communication efficiency
using location input
– Hidden terminal and exposed terminal problems
in mobile WLAN
• In two campus WLAN of CENTAUR, 40% links of
exposed terminals and 10% links with 70%
throughput reduction due to hidden terminals.
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outline
•
•
•
•
Problem review and State-of-the-Art
Design of CO-MAP
Implementation and Evaluation
Conclusions
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Hidden Terminal
Collision!
• Detect this relation
• Prevent concurrent transmissions
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State-of-the-Art
• Extra coordination channel
– DC-MAC (TPDS 2012)
• New hardware or USRP implementation
• Conflict map based scheduling
– RXIP (INFOCOM’ 12)
• Overhead of map learning
• Centralized control for downlinks
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Exposed Terminal
Collision!
• Detect this relation
• Enable concurrent transmissions
• Multiple exposed terminal problem
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State-of-the-Art
• Extra coordination channel
– Attached-RTS (TPDS 2012)
• New hardware or USRP implementation
• Conflict map based scheduling
– CMAP (NSDI’ 08) and CENTAUR (MobiCom’ 09)
• Overhead of map learning
• Multiple exposed terminal problems
• Centralized control for downlinks
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Co-Occurrence MAP - Overview
log normal shadowing
propagation model
Fast
Uniform
Co-Occurrence MAP
Exposed Terminals
Enchanced
CSMA
Maximize spatial reuse
Hidden Terminals
Distributed
Dynamic
packet size
Minimize collision
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Exposed Terminal
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Exposed Terminal
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Concurrent Transmissions
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Concurrent Transmissions
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Concurrent Transmissions
Multiple Exposed Terminals 
Enhanced CSMA 
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Concurrent Transmissions
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Concurrent Transmissions
ACK Lost Problem 
Windowed ACK

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Hidden Terminal
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Hidden Terminal
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Hidden Terminal
Important Parameters:
Number of HTs
Packet Size
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Dynamic Packet Length for
Hidden Terminals
Packet size
Probability of node i
transmiting in slot s
Number of
Number of
hidden terminal
contending nodes
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Implementation
• Testbed of six laptops
– Intel Wireless 4965AGN network adapter
– MAC80211 and iwlegacy wireless drivers.
• Three Components
– CO-MAP
– Header and concurrent ET transmission
– Packet length adaptation
• Data rate adaptation – Minstrel (Default)
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Implementation
• Header in data packets
– Thirteen bytes (address and CRC) in PHY header
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Evaluation – Exposed Terminal
78%
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Evaluation – Hidden Terminal
39%
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Large Scale Network on NS-2
• Network layout
– Three APs separated about 60m
– Nine clients.
– Thirty topological configurations
• 48% exposed links and 19% hidden terminals
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Large Scale Network on NS-2
39%
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Large Scale Network on NS-2
39%
19%
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Tolerance to Position Inaccuracy
Location
Error Range
Misclassification percentage
Wrong ET
Missing ET
Wrong HT
Missing HT
1m
0.2%
0.3%
0.2%
0.2%
5m
1.2%
1.4%
1.1%
0.8%
10m
2.1%
2.3%
2.4%
1.4%
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Conclusion
• A practical work leveraging pervasive
location information to improve spatial
reuse and reduce hidden collisions in
mobile WLAN
• Distributed design with rapid construction
of conflict map
• Successful practice using sensor hints in
protocol design
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Thanks. Questions?
Wan DU, duwan@ntu.edu.sg
Research Fellow @ NTU, Singapore