November 2008
doc.: IEEE 802.11-08/1360r0
40 MHz Coexistence in 2.4 GHz Tutorial
Date: 2008-11-11
Authors:
Name
Company
Eldad Perahia
Intel
Matt Fischer
Broadcom
Harish
Ramamurthy
Bruce Kraemer
Marvell
Submission
Address
Phone
email
eldad.perahia@in
tel.com
mfischer@broadc
om.com
harishr@marvell.
com
bkraemer@marve
ll.com
Marvell
Slide 1
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Abstract
Provide an overview of coexistence and protection
mechanisms in 802.11n D7.0
Submission
Slide 2
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Outline
•
•
•
•
Coexistence Conditions
Overview of Coexistence and Protection Mechanisms
Details of Coexistence Conditions
Comparison between 802.11n and 802.15.1/2
Submission
Slide 3
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Coexistence Conditions
• Legacy 802.11 overlapping BSS (OBSS) on a channel
that partially overlaps the 40 MHz channel
• Legacy 802.11 Overlapping BSS on channel that is
completely overlapping with 40 MHz primary channel
• Non-802.11 devices
Submission
Slide 4
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
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 802.11b or 802.11g
require protected transmissions
Submission
Slide 5
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Legacy 802.11 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
•
–
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 802.11g BSS overlapping an 802.11b BSS
Recommends protection mechanisms, similar to 11g
Monitoring overlapping channels during 20/40 MHz BSS operation
–
•
E.g. 20/40 MHz BSS primary channel 1 and OBSS on channel 2-9 not permitted
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
Submission
Slide 6
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
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
Submission
Slide 7
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Coexistence with non-802.11 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 802.11n and non-802.11 radios has knowledge of non802.11 communications and may transmit Forty MHz Intolerant to disable
neighboring 20/40 MHz BSSs
•
802.11 employs carrier sense multiple access, which defers access on a
packet-by-packet basis in presence of non-802.11 signal
Submission
Slide 8
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Comparison between 802.11n and 802.15.1/2
802.11n
802.15.1/2
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
–
–
–
Submission
802.11 use of spectrum is deterministic unlike 802.15.1/2 and hence it is easier to design
coexistence solutions around it
802.11 employs carrier sense multiple access, which defers access on a packet-by-packet basis
in presence of non-802.11 signal. No such mechanism in 802.15.1
max PPDU duration limit in 11n takes care of the “max contiguous transmission time” issue
Slide 9
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Detection of “Alien” Systems
• Although 802.15.1 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 802.11n devices i.e. the need to "play nice" is there to ensure
competitive performance and maintain the
manufacturer's reputation
Submission
Slide 10
Eldad Perahia (Intel)
November 2008
doc.: IEEE 802.11-08/1360r0
Summary
• IEEE 802.11n has implemented several coexistence
mechanisms for 40 MHz coexistence with legacy 802.11
and non-802.11 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 802.11n goes well beyond 802.15.1/2 in
mandating coexistence behavior in response to Forty
MHz Intolerant bit
Submission
Slide 11
Eldad Perahia (Intel)