1. 40MHz in 2.4 GHz band Date: 2006-27-06 Authors: Name Company Address
Month Year
doc.: IEEE yy/0XXXr0
June 2006
40MHz in 2.4 GHz band
Date:
Authors:
Name
Company
Address
Phone
Assaf Kasher
Intel
Eldad Perahia
Adrian Stephens
Gal Basson
Solomon Trainin
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Assaf Kasher (Intel)
Assaf Kasher (Intel)

2. Addresses Comments with CIDs
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Addresses Comments with CIDs
104, 258, 286, 288, 430, 431, 689, 705, 706, , 1493, 1558, 1560, 1561, 1728, 2848, 3471, 3501, 3502, 3602, 4570, 4571, 7010, 7193, 7195, 7312, 7313, 7314, 7315, 7871, 7925, 8138, 8186, 8194, 8282, 10272
Assaf Kasher (Intel)
Assaf Kasher (Intel)

3. There are many problems with 40MHz channel spacing in 2.4GHz:
Month Year
doc.: IEEE yy/0XXXr0
June 2006
40MHz operation in 2.4GHz
There are many problems with 40MHz channel spacing in 2.4GHz:
Channel availability
Coexistence mechanisms between legacy 11g/b on the extension channel and the 40MHz BSS are problematic.
40MHz devices are bad neighbors to devices operating in adjacent channels.
Assaf Kasher (Intel)
Assaf Kasher (Intel)

4. Channel Availability The 2.4GHz band is only 70MHz wide
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Channel Availability
The 2.4GHz band is only 70MHz wide
2402MHz-2472MHz (Practical OFDM use in US)
A single 40MHz channel leaves space for only one more 20MHz channel.
Best case – only when channel planning is available. Out of the box configurations may not allow even for this
There are no other frequencies a BSS can move to if it hits interference.
5MHz channel spacing – Any channel can be the center of a 20MHz or 40MHz channel -
Default AP settings are 1,11,6,3,5 in most 11g AP vendors.
2.4GHz spectrum is congested with Over 100 Million WLAN plus countless Bluetooth devices, cordless phones, baby monitors, microwave ovens, etc.
Assaf Kasher (Intel)
Assaf Kasher (Intel)

5. Coexistence with Overlapping BSS in the extension channel.
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Coexistence with Overlapping BSS in the extension channel.
No practical way to respect the NAV on the extension channel.
Ideally 40MHz devices would maintain two NAVs, one per 20MHz channel, based upon CCA and RTS/CTS etc packets on the control and extension channels
It is not possible to receive packets on the extension channel when transmitting on the control channel, so this ideal can never be achieved.
We can set the extension channel to be 20MHz/25MHz apart, but the overlapping BSS can be either 20MHz or 25MHz apart.
20/25 MHz does not cover all the deployed cases (channels 3, 5)
What will be the algorithm for deciding 20 or 25 or 10
Need for supporting both OFDM and CCK protection mechanism
CCK protection may make the 40MHz channel use very inefficient.
RTS/CTS exchange in rate 1Mbps is 618usec long. The 40MHz use must be 507usec long to be worth it.
Assaf Kasher (Intel)
Assaf Kasher (Intel)

6. 40Mhz devices are very bad neighbors
Month Year
doc.: IEEE yy/0XXXr0
June 2006
40Mhz devices are very bad neighbors
News magazines articles on 40MHz devices show that a 40MHz devices (from various vendors) cause significant degradation in the throughput of Adjacent 11g devices
The throughput of the legacy devices may fall in some cases to zero.
Similar results with various vendors.
The wider spectral mask of 40MHz devices cause higher interference for BSS in an adjacent channel.
Assaf Kasher (Intel)
Assaf Kasher (Intel)

7. Possible Solutions Disallow 40MHz operation in 2.4GHz
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Possible Solutions
Disallow 40MHz operation in 2.4GHz
There isn’t really enough bandwidth in 2.4GHz to allow this type of operations (Only 70 MHz).
Crowded spectrum
5MHz channel spacing
The wider spectral mask of 40MHz devices cause higher interference for BSS in an adjacent channel
5GHz has plenty of BW to incorporate 40 MHz (in some regulatory domains: ~0.5GHz).
Assaf Kasher (Intel)
Assaf Kasher (Intel)

8. Possible Solutions (Cont.)
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Possible Solutions (Cont.)
Allow 40MHz operations in 2.4GHz only when there are no devices in or near the extension channel. Otherwise: mandate BSS transition to 20 MHz
“In” the extension channel means devices 20MHz off the control channel
“Near” the extension channel means devices 20/25/30MHz off the control channel
A 40MHz BSS shall switch to 20MHz operation when a device is transmitting in the extension channel –
When the device in the extension channel passes the detection threshold for 20% duty cycle (time) for 10 seconds.
Switch to 20MHz in 10 seconds periods
Assaf Kasher (Intel)
Assaf Kasher (Intel)

9. Possible Solutions (cont.)
Month Year
doc.: IEEE yy/0XXXr0
June 2006
Possible Solutions (cont.)
Central Approach
Extension channel measurement is done only at the AP. If the AP senses transmission in the extension channel as above, it switches the BSS to 20MHz.
Distributed Approach
A STA (not necessarily an AP) looks for extension channel interference. If it passes the thresholds above, it shall stops transmitting in 40MHz and requests the AP to stop transmitting to it using 40MHz.
If the AP senses transmission in the extension channel as above, it switches the whole BSS to 20MHz.
Assaf Kasher (Intel)
Assaf Kasher (Intel)

10. Month Year
doc.: IEEE yy/0XXXr0
June 2006
Switching Back to 40MHz
An AP can switch back to 40MHz 300 seconds after it has switched to 20MHz.
Assaf Kasher (Intel)
Assaf Kasher (Intel)