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Transmitter CCA Issues in 2.4 GHz April 2006 06/543r0

Transmitter CCA Issues in 2.4 GHz April 2006 06/543r0. Richard van Nee vannee@airgonetworks.com. Importance of TX & RX CCA Support. IEEE MAC is based on CSMA/CA CSMA/CA only works if both the following conditions are met:

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Transmitter CCA Issues in 2.4 GHz April 2006 06/543r0

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  1. Transmitter CCA Issues in 2.4 GHzApril 200606/543r0 Richard van Nee vannee@airgonetworks.com Richard van Nee, Airgo Networks

  2. Importance of TX & RX CCA Support • 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. Draft 802.11n TX CCA Issues • Draft 802.11n 40MHz signal consists of Control & Extension channels with 20MHz separation • Most widely used channels in 2.4GHz band have 25 MHz separation • CH1 (2412MHz), CH6(2437MHz) and CH11 (2462MHz) • Therefore, 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) • This means that legacy devices cannot properly detect packets on both control and extension channel, which was the intent of the design of both the mixed mode 40 MHz preamble, and of the duplicate legacy mode Richard van Nee, Airgo Networks

  4. 11b TX CCA Issue • Draft 802.11n 40MHz legacy duplicate mode is limited to OFDM rates • This means it cannot be used to make legacy 11b devices defer • It would be highly useful to include 11b rates in the legacy duplicate mode to ensure proper defer of 11b devices Richard van Nee, Airgo Networks

  5. 2.4GHz Installed base interoperability highly problematic with current 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) Richard van Nee, Airgo Networks

  6. Protection Mechanisms do not work for 40 MHz operation in the 2.4 GHz band • Draft 802.11n protection mechanisms do NOT work • Duplicated protection frames (using MAC protection frames and the legacy-duplicate-mode) cannot be decoded by a legacy network that is shifted by 5 MHz relative to the extension channel • The legacy portion of the Mixed-Mode Preamble cannot be received by a legacy network • The Phased Coexistence Operation (PCO) cannot work since the adjacent-channel legacy network cannot decode MAC protection frames sent on the Extension channel Richard van Nee, Airgo Networks

  7. Impact of Draft .11n Operation on Legacy .11g Networks • Two networks: • Vendor A AP, Vendor A Client • Vendor B AP, Vendor B Client • Network 2 turned on 30 seconds after • Network 1 • Throughput of Network 1 suffers dramatically when networks operate with a frequency shift of one channel 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

  8. Solution to the 2.4 TX CCA Issue • Use 25 MHz spacing instead of 20 MHz between control channel and extension channel for legacy duplicate mode and preferably also for legacy part of mixed mode 40 MHz preamble Richard van Nee, Airgo Networks

  9. j Shift by +10 MHz Generate baseband legacy signal To RF DAC Shift by -15 MHz Required Transmitter Change • Only one change required: Extension channel needs to be shifted by 15 instead of 10 MHz when producing legacy duplicate packets or legacy part of a mixed-mode 40 MHz preamble • Extension channel will get more band-edge attenuation when analog filtering is not changed; next slides show this is not a problem for getting proper defer behavior Richard van Nee, Airgo Networks

  10. Spectrum of Extension Channel • Blue: No extra filtering • Green: 5th order Butterworth with cutoff frequency of 20 MHz Richard van Nee, Airgo Networks

  11. Impact of Filtering on Extension Channel Defer • Legacy Signal Field Error Ratio versus SNR for channel D-NLOS • Blue: No extra filtering • Green: 5th order Butterworth with cutoff frequency of 20 MHz Richard van Nee, Airgo Networks

  12. Conclusions on filtering • There is no need to change filtering to accommodate the 5 MHz extra frequency shift of the extension channel • Even with a sharp 5th order filter giving 11 dB attenuation at one band edge of the extension channel, the PER performance is degraded by less than a dB Richard van Nee, Airgo Networks

  13. 11b Legacy Duplicate Mode • In addition to using 25 MHz spacing, it would make 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 MHz will still be received correctly although sensitivity is decreased by several dBs, depending on the filtering Richard van Nee, Airgo Networks

  14. Proposed change 1 • In 2.4 GHz, use 25 MHz spacing between control channel and extension channel for legacy duplicate mode Richard van Nee, Airgo Networks

  15. Proposed change 2 • 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 Richard van Nee, Airgo Networks

  16. Proposed change 3 • In 2.4 GHz, add 11b legacy duplicate mode Richard van Nee, Airgo Networks

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