September 2005

Slide 1

doc.: IEEE 802.11-05/0916r0

Submission

Adjacent channel interference and its impact on the Mesh MAC

Date: 2005-09-19Authors:

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Name Company Address Phone email Jack Winters Motia 103 Old Wagon Road

Middletown, NJ 07748

+1 732 208-5568- jwinters@motia.com

Syed Aon Mujtaba Agere Systems

Mathilde Benveniste Avaya Labs 233 Mt Airy Road

Basking Ridge, NJ 07920 +1-908-696-5296 benveniste@ieee.org

1110 American Parkway,Allentown, PA 18109

+1 908 342 5251 Mujtaba@agere.com

September 2005

Slide 2

doc.: IEEE 802.11-05/0916r0

Submission

Adjacent channel interferenceWhat is it?Adjacent channel interference (ACI) is caused by energy on channels that are adjacent in the RF

spectrum – ACI is serious when the receiver is close to a transmitter on an adjacent channel

2.4 – 2.483 GHz

//5 GHz band

AdjacentChannels

Frequency

September 2005

Slide 3

doc.: IEEE 802.11-05/0916r0

Submission

ACI in a WLAN

Why ACI is not a problem in a WLAN BSS– BSS stations use a single radio; all radios in a BSS use the same

channel, but at different times to transmit and receive

ACI in neighboring BSSs– Stations in different BSSs may use adjacent channels– ACI becomes a problem if the interfering signal exceeds a certain

threshold, which is data rate dependent, and is shown in the table on the next slide.

– Typically, this means that the interferer is closer to the receiver than the desired transmitter

September 2005

Slide 4

doc.: IEEE 802.11-05/0916r0

Submission

Receiver Performance Requirements(from 802.11a spec)

ACI Threshold

September 2005

Slide 5

doc.: IEEE 802.11-05/0916r0

Submission

Transmit Spectrum Mask(from 802.11a spec)

Desired Adjacent Alternate

September 2005

Slide 6

doc.: IEEE 802.11-05/0916r0

Submission

Multi-radio mesh points

• For large meshes, multi-radio MPs are needed closer to the portal of the mesh

– Traffic increases geometrically with the number of hops

Load =x

Load =3x

Portal

Load =7xLoad =15x

Load =31x

September 2005

Slide 7

doc.: IEEE 802.11-05/0916r0

Submission

Adjacent channel interference in WiFi bands

• A multi-radio MP cannot use two adjacent channels in the same unlicensed WiFi band to receive and transmit on two radios at the same time [1, 2, 3]:– TX power can be 20 dBm (or higher)– RX sensitivity can be as low as -82 dBm (or lower)– Transmit spectral mask only drops the adjacent channel by 28 dB (alternate

by 40 dB)– The system – as is – will not work!!!

– {Unless there is at least 46 (30) dB isolation between RX and TX for adjacent (alternate) channels.}

• ([2] shows 35 dB isolation) – directional antennas, or the radios are time synchronized)

• 802.11s needs to consider this issue

September 2005

Slide 8

doc.: IEEE 802.11-05/0916r0

Submission

References

[1] R. Draves, J. Padhye, and B. Zill, “Routing in multi-radio, multi-hop wireless mesh networks,” MobiCom’04.

[2] J. Robinson, K. Papagiannaki, C. Diot, N. Gao, and L. Krishnamurthy, “Experimenting with a multi-radio mesh networking testbed,” WiNMee 2005.

[3] J. H. Winters, “Smart antennas and their application to wireless ad hoc networks,” PIMRC’05.