Flexible Channelization for 802.11 Wireless LANs

Zafar Ayyub Qazi*, Zhibin Dou and Prof. Samir Das**Department of Computer Science (WINGS lab), Stony

Brook UniversityDepartment of Computer Science, Tianjin University

802.11 b/g Wireless LANs becoming increasingly crowded Only certain predefined channels and fixed channel widths are used Improve spectrum efficiency by choosing an appropriate channel width and center frequency for each transmission Goal: To understand how to employ flexible channelization when using multiple potentially interfering links

Problem

Challenges Increasing channel width potentially increases throughput However for a constant transmit power, power per unit frequency reduces

for larger channel widths [1][Chandra et. al.] leading to reduced SNR and possibly poor connectivity in longer links

In choosing center frequency we need to take into account that partially overlapped channels might not be harmful[2][Mishra et. al.]

• [1] R. Chandra, R. Mahajan, T. Moscibroda, R. Ragvendra and P. Bahl. A case for adapting channel width in wireless networks. SIGCOMM ‘08 • [2] A. Mishra, V. Shrivastava, S. Banerjee, W. Arbaugh. Partially Overlapped Channels Not Considered Harmful. SIGMetrics’06

Experimental Study

• Experiments with commodity 802.11 a/b/g wireless cards with different channel widths and center frequencies in different scenarios.

Sender A Receiver A

Sender B Receiver B

Link A

Link B

Insights

10MHz (Link A)

5MHz (Link B)0

50

100

150

200

250

5MHz of Center Frequency Separation

Links togtherLinks alone

Channel Width

Send

ing

Rate

(pps

)

Asymmetrical Carrier Sensing at the Transmitter

Insights No one configuration provides best performance in all

the cases Impact on Carrier Sensing Smaller channel widths have higher energy per hertz

hence more links can carrier sense each other at lower channel widths

Impact on Hidden and Exposed terminals Number of exposed and hidden terminals may vary with

channel widths. Lower channel widths may possibly allow more links to carrier sense but they may also interfere over longer distances

Possible Ways Forward

• Modeling Link capacity using SINR at the Receiver and the Interference level at the the Sender:

Challenge: Computing a conflict graph for all channel widths, center frequencies—huge overhead!

• TDMA Based Approach: It might a good option for centralized Wireless LANs, what about the distributed Wireless LAN settings?