Harnessing Mobile Multiple Harnessing Mobile Multiple Access Efficiency with Location Access Efficiency with Location InputInput

Wan Du* and Mo Li

School of Computer Engineering

Nanyang Technological University, Singapore

Main access to WLAN

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“Smart phones overtake client pcs in 2011,” 2012. www.canalys.com/newsroom/smart-phones-overtake-client-pcs-2011

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

• 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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Location error of

localization

Indoor: <1m

Outdoor: <13.7m

<< Communication range of

WiFi

Indoor: >50m

Outdoor: >200m

outline• Problem review and State-of-the-Art• Design of CO-MAP• Implementation and Evaluation• Conclusions

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Hidden Terminal

• Detect this relation• Prevent concurrent transmissions

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Collision!

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

• Detect this relation• Enable concurrent transmissions• Multiple exposed terminal problem

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Collision!

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

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Co-Occurrence MAP

log normal shadowing

propagation model

Exposed Terminals Hidden Terminals

Minimize collisionMaximize spatial reuse

Dynamic

packet size

Enchanced

CSMA

Fast

Uniform

Distributed

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

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Multiple Exposed Terminals

Enhanced CSMA

Concurrent Transmissions

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Concurrent Transmissions

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ACK Lost Problem

Windowed ACK

Hidden Terminal

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Hidden Terminal

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Hidden Terminal

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Important Parameters:

Number of HTs

Packet Size

Dynamic Packet Length for Hidden Terminals

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Probability of node i transmiting in slot s

Packet size

Number of hidden terminalNumber of

contending nodes

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

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78%

Evaluation – Hidden Terminal

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39%

Large Scale Network on NS-2

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• Network layout– Three APs separated about 60m– Nine clients.– Thirty topological configurations

• 48% exposed links and 19% hidden terminals

Large Scale Network on NS-2

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39%

Large Scale Network on NS-2

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39%

19%

Tolerance to Position Inaccuracy

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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%

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