1. Transmitter CCA Issues in 2.4 GHz June 2006 06-0869r0
July 2004
doc.: IEEE /0658r0
June 2006
Transmitter CCA Issues in 2.4 GHz June r0
Richard van Nee
Geert Awater
VK Jones
Rolf de Vegt
Richard van Nee, Airgo Networks
Bruce Kraemer, Conexant

2. Importance of TX & RX CCA Support
June 2006
IEEE MAC is based on CSMA/CA
CSMA/CA only works if both the following conditions are met:
Transmitter needs to ensure that each transmitted packet can be detected by other devices such that they can properly defer
Receiver needs to detect presence of any valid packet such that it can properly defer
Current draft 11n 40 MHz mode in 2.4 GHz does not fulfill 1) and highly complicates 2) because it uses 20 MHz spacing between control channel and extension channel while typical 2.4GHz channels are spaced by 25 MHz
This presentation summarizes the issues surrounding the 2.4 GHz channel spacing issue and proposes some solutions
Richard van Nee, Airgo Networks

3. June 2006
2.4GHz Installed base interoperability highly problematic with current draft .11n 40MHz mode
Both the Control and Extension channel cannot be centered on the widely-used adjacent 2.4GHz channels (e.g. channel 1 and 6 or channel 6 and 11)
The control and extension channels would be 1 and 5, or 6 and 2 for example; i.e. misaligned by 5MHz
Richard van Nee, Airgo Networks

4. Impact of Draft .11n Operation on Legacy .11g Networks
June 2006
Impact of Draft .11n Operation on Legacy .11g Networks
Two .11g networks:
Vendor A AP, Vendor A Client
Vendor B AP, Vendor B Client
Network 2 turned on 30 seconds after Network 1
Network 1 suffers when Network 2 is not aligned in frequency
6 feet
6 feet
40 feet,
2 walls
One Network on Channel 6
One Network on Channel 5
Both on Channel 6
Richard van Nee, Airgo Networks

5. Draft 802.11n 40MHz in the 2.4GHz Band; The Impact
June 2006
Draft n 40MHz in the 2.4GHz Band; The Impact
The technical impacts of an extension channel with 5 or 10 MHz offset from an Overlapping BSS
Protection mechanisms do not work
Collisions will occur between adjacent-channel legacy stations and 40MHz n transmissions
Net result is dramatic degradation of n and adjacent-channel legacy BSS network throughput
802.11n networks will not be able to assure QoS using 40MHz signals
The adjacent-channel legacy network will be confused by any n transmission in the Extension channel, crushing the throughput and reliability of these Legacy Networks
Richard van Nee, Airgo Networks

6. Channel Utilization USA
June 2006
Channel Utilization USA
Total of 1088 Access Points
Measurements taken in San Francisco and Silicon Valley using the Netstumbler tool
Richard van Nee, Airgo Networks

7. Channel Utilization Europe
June 2006
Channel Utilization Europe
Total of 1722 Access Points
Measurements taken in Netherlands, Belgium and Italy
Richard van Nee, Airgo Networks

8. Summary of Channel Utilization
June 2006
USA: More than 90% of APs is on channels {1,6,11}
Europe: 70% of APs is on channels {1,6,11}
Japan: 76% of APs is on channels {1,6,11}
Richard van Nee, Airgo Networks

9. June 2006
Likelihood of Overlapping Networks Not Deferring Correctly due to 40MHz operation
Consideration: Networks, in the two channels adjacent to the control and extension channel center frequencies are not deferring correctly (e.g. if the Ext. Ch is on Ch 5, Networks on Ch 2,3,4,6,7 are not deferring correctly)
Richard van Nee, Airgo Networks

10. June 2006
25MHz Operation, together with a 1,6,11 channel allocation policy provides the preferred solution
= Preferred Solution: only use channels 1,6,11 with 25 MHz spacing
Richard van Nee, Airgo Networks

11. Proposed changes
June 2006
In 2.4 GHz, use 25 MHz spacing between control channel and extension channel for legacy duplicate mode
In 2.4 GHz, use 25 MHz spacing between control channel and extension channel for the legacy part of a 40 MHz mixed mode preamble up to and including HT-SIG
In 2.4 GHz, add 11b legacy duplicate mode
* Straw poll in the April 6 teleconference indicated a majority was in favor of changes 1) and 2), while 3) got mostly abstains
Richard van Nee, Airgo Networks

12. 11b Legacy Duplicate Mode
June 2006
In addition to using 25 MHz spacing, it makes a lot of sense to use legacy duplicate 11b rates in 2.4 GHz instead of having only legacy duplicate OFDM rates
For proper defer behavior in the presence of 11b devices that cannot detect any legacy duplicate OFDM packets
For proper defer behavior in the presence of networks outside the usual channel grid {1,6,11}: Barker rates of 1&2 Mbps with a frequency shift of 5 or 10 MHz will still be received correctly although sensitivity is decreased by several dBs, depending on the filtering
Richard van Nee, Airgo Networks

13. 11b rates With Channel Offset
June 2006
Two 1 Mbps networks on Channel 1, second network turned on after 20 seconds
Two 1 Mbps networks on Channels 1 and 2, second network turned on after 20 seconds
Networks properly defer in the presence of a channel offset
when using 11b Barker rates – they share fairly
Richard van Nee, Airgo Networks

14. June 2006
Conclusions
20 MHz spacing is a bad choice for 2.4 GHz as many legacy devices cannot properly receive the preamble, causing severe CCA problems
Using 25 MHz channel spacing solves the 2.4 GHz TX-CCA problem for 94% of all legacy APs in the US and 85% in Europe
To solve potential TX-CCA problems for the small percentage of APs that are off the {1,6,11} grid, we propose to introduce an optional legacy duplicate 11b mode
Richard van Nee, Airgo Networks