40 MHz Coexistence in 2.4 GHz Tutorial

1. 40 MHz Coexistence in 2.4 GHz Tutorial Authors: Date: 2008-11-11 Eldad Perahia (Intel)

2. Abstract Provide an overview of coexistence and protection mechanisms in 802.11n D7.0 Eldad Perahia (Intel)

3. Outline • Coexistence Conditions • Overview of Coexistence and Protection Mechanisms • Details of Coexistence Conditions • Comparison between 802.11n and 802.15.1/2 Eldad Perahia (Intel)

4. 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 Eldad Perahia (Intel)

5. 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 Eldad Perahia (Intel)

6. 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 • 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 802.11g BSS overlapping an 802.11b 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 • 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 • 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 non-802.11 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 Eldad Perahia (Intel)

9. Other general comparison points 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 Comparison between 802.11n and 802.15.1/2 Eldad Perahia (Intel)

10. 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 Eldad Perahia (Intel)

11. 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 Eldad Perahia (Intel)