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Doc.: IEEE 802.11-06/543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 1 Transmitter CCA Issues in 2.4 GHz April 2006 06/543r0 Richard van.

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Presentation on theme: "Doc.: IEEE 802.11-06/543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 1 Transmitter CCA Issues in 2.4 GHz April 2006 06/543r0 Richard van."— Presentation transcript:

1 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 1 Transmitter CCA Issues in 2.4 GHz April /543r0 Richard van Nee

2 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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: 1.Transmitter needs to ensure that each transmitted packet can be detected by other devices such that they can properly defer 2.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

3 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 3 Draft n TX CCA Issues Draft n 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

4 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 4 11b TX CCA Issue Draft n 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

5 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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)

6 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 6 Protection Mechanisms do not work for 40 MHz operation in the 2.4 GHz band Draft n 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

7 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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 40 feet, 2 walls 6 feet Both on Channel 6 One Network on Channel 6 One Network on Channel 5

8 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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

9 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 9 Required Transmitter Change DAC Generate baseband legacy signal Shift by +10 MHz Shift by -15 MHz To RF j 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

10 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 10 Spectrum of Extension Channel Blue: No extra filtering Green: 5 th order Butterworth with cutoff frequency of 20 MHz

11 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 11 Impact of Filtering on Extension Channel Defer Legacy Signal Field Error Ratio versus SNR for channel D-NLOS Blue: No extra filtering Green: 5 th order Butterworth with cutoff frequency of 20 MHz

12 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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 5 th order filter giving 11 dB attenuation at one band edge of the extension channel, the PER performance is degraded by less than a dB

13 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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

14 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 14 Proposed change 1 In 2.4 GHz, use 25 MHz spacing between control channel and extension channel for legacy duplicate mode

15 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 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

16 doc.: IEEE /543r0 Submission April 2006 Richard van Nee, Airgo NetworksSlide 16 Proposed change 3 In 2.4 GHz, add 11b legacy duplicate mode


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