Transmitter CCA Issues in 2.4 GHz June 2006 06-0869r0 July 2004 doc.: IEEE 802.11-04/0658r0 June 2006 Transmitter CCA Issues in 2.4 GHz June 2006 06-0869r0 Richard van Nee Geert Awater VK Jones Rolf de Vegt Richard van Nee, Airgo Networks Bruce Kraemer, Conexant

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

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

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

Draft 802.11n 40MHz in the 2.4GHz Band; The Impact June 2006 Draft 802.11n 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 802.11n transmissions Net result is dramatic degradation of 802.11n 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 802.11n transmission in the Extension channel, crushing the throughput and reliability of these Legacy Networks Richard van Nee, Airgo Networks

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

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

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

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

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

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

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

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

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