1. 40 MHz Coexistence in 2.4 GHz Tutorial
July 2007
doc.: IEEE /2090r0
November 2008
40 MHz Coexistence in 2.4 GHz Tutorial
Date:
Authors:
Eldad Perahia (Intel)
Eldad Perahia (Intel)

2. July 2007
doc.: IEEE /2090r0
November 2008
Abstract
Provide an overview of coexistence and protection mechanisms in n D7.0
Eldad Perahia (Intel)
Eldad Perahia (Intel)

3. Outline Coexistence Conditions
November 2008
Outline
Coexistence Conditions
Overview of Coexistence and Protection Mechanisms
Details of Coexistence Conditions
Comparison between n and /2
Eldad Perahia (Intel)

4. Coexistence Conditions
November 2008
Coexistence Conditions
Legacy overlapping BSS (OBSS) on a channel that partially overlaps the 40 MHz channel
Legacy Overlapping BSS on channel that is completely overlapping with 40 MHz primary channel
Non devices
Eldad Perahia (Intel)

5. Overview of Coexistence and Protection Mechanisms
November 2008
Overview of Coexistence and Protection Mechanisms
Overlapping BSS scanning
The AP (or some of its associated HT STAs) is required to scan all of the channels of the current regulatory domain in order to ascertain the operating channels of any existing 20 MHz BSSs and 20/40 MHz BSSs
40 MHz Intolerant bit in 2.4 GHz
40 MHz Intolerant bit allows ANY device to indicate to an AP that it may not operate in 40 MHz anywhere in the 2.4 GHz band
A station may also broadcast the 40 MHz Intolerant bit to overlapping BSS to force them to stop 40 MHz operation
Mixed environments of legacy b or g require protected transmissions
Eldad Perahia (Intel)

6. November 2008
Legacy overlapping BSS (OBSS) on a channel that partially overlaps the 40 MHz channel
Requirements regarding establishing a 20/40 MHz BSS
An AP must perform an Overlapping BSS scan prior to establishing a BSS
An AP can not establish a 20/40 MHz BSS if there is an OBSS on a partially overlapping channel
E.g. 20/40 MHz BSS primary channel 1 and OBSS on channel 2-9 not permitted
Complete overlap of a 20 MHz BSS with the primary channel of the 20/40 MHz BSS is permitted
This is similar to the situation of an g BSS overlapping an b BSS
Recommends protection mechanisms, similar to 11g
Monitoring overlapping channels during 20/40 MHz BSS operation
Active 20/40 MHz HT stations are required to periodically perform periodic Overlapping BSS scans to determine that no overlapping BSSs exist
Determines when conditions which allowed the establishment of the 20/40 MHz BSS do not change to conditions that would disallow the existence of the 20/40 MHz BSS
Scan for beacons, frames with Forty MHz Intolerant field set to 1
If an overlapping BSS is detected on a partially overlapping channel, the station reports this to the AP
Upon report, the AP must immediately switch the BSS to 20 MHz operation
Eldad Perahia (Intel)

7. Protection of legacy devices in BSS or primary channel OBSS
November 2008
Protection of legacy devices in BSS or primary channel OBSS
Complete overlap of a 20 MHz BSS with the primary channel of the 20/40 MHz BSS is permitted
In this situation, it is recommended to protect HT transmissions and certain HT sequences from stations that may not recognize these formats and thus not defer correctly, just as in 11g
Non-member protection: non-HT STA in OBSS is detected on primary channel, but all STAs in BSS are HT STA
If at least one member of the BSS in non-HT
non-HT mixed mode: requires protection of HT transmission, just as in 11g
802.11b stations present requires use of RTS/CTS or CTS-to-Self with DSSS/CCK format
802.11g stations present
Use of the HT mixed format frame inherently provides protection
Some HT transmissions may not be interpreted correctly by non-HT stations and require protection
RIFS Burst
Greenfield Format
Eldad Perahia (Intel)

8. Coexistence with non-802.11 Devices
November 2008
Coexistence with non Devices
To promote sharing of the spectrum resource in mixed environments, any device is able to prohibit the operation of a 20/40 MHz BSS
A very strict constraint on establishing a 20/40 MHz BSS includes the allowance for any device to explicitly prohibit the operation of the 20/40 BSS mode FOR ANY REASON
To prohibit use of 40 MHz in 2.4 GHz, a device transmits a management frame containing a value of 1 for the Forty MHz Intolerant field
Receivers of such frames are not allowed to establish a 20/40 MHz BSS
Example usage:
Dual radio device with both n and non radios has knowledge of non communications and may transmit Forty MHz Intolerant to disable neighboring 20/40 MHz BSSs
employs carrier sense multiple access, which defers access on a packet-by-packet basis in presence of non signal
Eldad Perahia (Intel)

9. Comparison between 802.11n and 802.15.1/2
November 2008
Comparison between n and /2
802.11n /2
Standardized ways of detecting "alien" systems
None
none
Standardized ways of communicating about "alien" systems
Yes - Forty MHz Intolerant bit; various .11k reports
Yes - AFH channel blacklist report
Mandatory requirement to act on receipt of communicated information
Yes - Recipient of Forty MHz Intolerant bit required to disable 40 MHz
NO – Master may to decide whether to honor or not a channel blacklist
Other general comparison points
use of spectrum is deterministic unlike /2 and hence it is easier to design coexistence solutions around it
employs carrier sense multiple access, which defers access on a packet-by-packet basis in presence of non signal. No such mechanism in
max PPDU duration limit in 11n takes care of the “max contiguous transmission time” issue
Eldad Perahia (Intel)

10. Detection of “Alien” Systems
November 2008
Detection of “Alien” Systems
Although does not mandate a means of detection non-BT devices in the protocol, a manufacturer has to find an effective way of doing this, or they will not compete effectively in the marketplace
The same considerations will apply to n devices - i.e. the need to "play nice" is there to ensure competitive performance and maintain the manufacturer's reputation
Eldad Perahia (Intel)

11. November 2008
Summary
IEEE n has implemented several coexistence mechanisms for 40 MHz coexistence with legacy and non devices in 2.4 GHz
OBSS scanning of partially overlapping channels prior to establishing a 20/40MHz BSS
OBSS scanning of partially overlapping channels during operation of a 20/40MHz BSS
Forty MHz Intolerant bit to disable 40 MHz operation
Protection mechanisms
IEEE n goes well beyond /2 in mandating coexistence behavior in response to Forty MHz Intolerant bit
Eldad Perahia (Intel)