May 2007doc.: IEEE 802.11-07/0761r3

20/40 Coexistence at 5 GHz
Date: 2007-05-16
Author(s):
Name / Company / Address / Phone / Email
Brett Douglas / Cisco Systems / 301 S. Prospect Rd, Ste. 4
Bloomington, IL61704-4908 / 1 309 661 7656 /
Douglas Chan / Cisco Systems / 170 West Tasman Drive
San Jose, CA95134 / 1 408 527 9344 /
Brian Hart / Cisco Systems / 170 West Tasman Drive
San Jose, CA95134 / 1 408 525 3346 /

Abstract

Many have expressed the concern that if we place constraints on 40 MHz operation in 5 GHz, we will not be able to use 40 MHz at all. The only constraint we propose to place in the 5 GHz band is that an AP must scan all the channels it might use. If it detects a legacy beacon or a primary channel beacon in a particular channel, that channel cannot be used as the APs secondary channel.

This submission addresses the following CIDs:

187, 2470, 2963, 3254, 3264, 3278

The analysis below compares the beacons at 2.4 GHz and at 5 GHz, and shows this to be a quite reasonable constraint.

Motivation

The CCA mechanism in the secondary channel is extremely weak:

  1. It guarantees a channel busy state only when the signal on the secondary channel is -62 dBm or higher.
  2. It does not honor NAV on the secondary channel.

As a result, if it uses the same transmit power as a transmitter on the secondary. A 40 MHz STA or AP can transmit signals that are as strong as -62 dBm when received by a secondary device that is transmitting.

Presumeably, an AP or STA would use 40 MHz mode when it has lots of traffic to transmit. This combination of high duty cycle and poor CCA can cause severe degradations to the following operations on the secondary channel.

A BSS that is carrying QOS traffic such as voice or video.

A mesh network that is using the secondary channel for backhaul for multiple mesh nodes and numerous clients.

Beacon Analysis

The table below compares the factors that determine the beacon range, the beacon coverage area for 2.4 GHz and 5 GHz. From this, we can derive a minimum density where 40 GHz operation may be prohibited.

Factor / 2.4 GHz
(1 Mbps Beacon) / 5 GHz
(6 Mbps Beacon)
Transmit Power / 20 dBm / 17 dBm
RX Sensitivity / -93 dBm / -86 dBm
Propagation Model / 22 + 35 log (R)
R in ft. / 30 + 35 log (R)
R in ft.
BeaconRange / 398 ft / 122 ft
Beacon Area / 498K sq ft. / 47K sq ft.
Number of Channels / 3 channels / 20 channels
Min Density for Beacons on all Chan. / One AP/166K sq. ft / One AP/2331 sq. ft.

Reference Points

One Acre Lots 43,560 sq ft < 166K sq. ft.

That means 40 MHz mode in the 2.4 GHz will not be able to be used in affluent neighborhoods in most of the USA.

Typical Enterprise Deployment: Cell Size 3000 to 8000 sq. ft.

That means you could take an existing dense enterprise deployment: (Cell Size 3000 sq ft), install a 40 MHz capable 5 GHz AP, and it could find beacon-free channels to use as its secondary channel.

Conclusion

The following constraint:

“An AP must scan all the 5 GHz channels it might use. If the AP detects a legacy beacon or a primary channel beacon in a particular channel, that channel cannot be used as the AP’s secondary channel.”

Does not place an unreasonable constraint on the use of 40 MHz PHY modes in the 5 GHz band.

20/40 Coexistence at 5 GHzpage 1Douglas, et al