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doc.: IEEE 802.11-11/0562r0
Submission
May 2011
Eldad Perahia, Intel CorporationSlide 1
CCA – CID 537 discussionDate: 2011-05-03
Name Company Address Phone email Eldad Perahia Intel
Corporation
2111 NE 25th Ave Hillsboro, OR 97124
503-712-8081 [email protected]
Robert Stacey Intel 503-724-
0893 [email protected]
Authors:
doc.: IEEE 802.11-11/0562r0
Submission
Overview
• Receive Procedure (22.3.22) describes how to set PHY-CCA.indication when various conditions occur, such as
– Signal loss before validating L-SIG
– check of the L-SIG parity bit is not valid
• Language in these two conditions borrows the phase “missed preamble” from 11n, however it is not defined in 11ac
• CID 537 states:– "missed preamble" language is not used anymore
• Further discussion questioned whether CCA indication applies to primary or secondary channels
May 2011
Eldad Perahia, Intel CorporationSlide 2
doc.: IEEE 802.11-11/0562r0
Submission
Initial Steps in Receive Procedure
• “… A PHY-CCA.indication(BUSY, channel-list) shall also be issued as an initial indication of reception of a signal.”
• “The channel-list parameter of the PHY-CCA.indication is absent when the operating channel width is 20 MHz and includes the element “primary” when the operating channel width is 40 MHz, 80 MHz, 160 MHz or 80+80 MHz.”– The channel-list DOES NOT include secondary channels, only
primary channel (or absent with 20 MHz)
May 2011
Eldad Perahia, Intel CorporationSlide 3
doc.: IEEE 802.11-11/0562r0
Submission
Error Conditions in Receive Procedure
• “If signal loss occurs before validating L-SIG, the VHT PHY shall maintain PHY-CCA.indication(BUSY, channel-list) until the received level drops below the CCA sensitivity level (for a missed preamble) specified in 22.3.20.5 (CCA sensitivity).”
• “If the check of the L-SIG parity bit is not valid… The VHT PHY shall maintain PHY-CCA.indication(BUSY, channel-list) until the received level drops below the CCA sensitivity level (for a missed preamble) specified in 22.3.20.5 (CCA sensitivity).”
• No definition in 22.3.20.5 for “missed preamble”
May 2011
Eldad Perahia, Intel CorporationSlide 4
doc.: IEEE 802.11-11/0562r0
Submission
Example
• Assume we define “missed preamble” to -62 dBm as in 11n
• Then consider the case where we have an invalid check of L-SIG parity bit and the level of the signal is -65 dBm on both primary and secondary
• In this case, PHY-CCA.indication on the primary switches to IDLE
• However according to the CCA sensitivity section (22.3.20.5), the PHY shall issue a PHY-CCA.indication(BUSY, {secondary}) at -72 dBm
• So we have a condition in which the PHY-CCA.indication is IDLE on the primary and BUSY on the secondary for the same signal
May 2011
Eldad Perahia, Intel CorporationSlide 5
doc.: IEEE 802.11-11/0562r0
Submission
Possible Solution
• Delete error conditions from Receiver Procedure and simply refer to 22.3.20.5 for CCA sensitivity rules, as follows
• “… A PHY-CCA.indication(BUSY, channel-list) shall is also be issued as an initial indication of reception of a signal, as specified in 22.3.20.5 (CCA sensitivity) .”
• “After the PHY-CCA.indication(BUSY, channel-list) is issued, the PHY entity shall begin receiving the training symbols and searching for L-SIG and VHT-SIG-A in order to set the maximum duration of the data stream. If signal loss occurs before validating L-SIG, the VHT PHY shall maintain PHY-CCA.indication(BUSY, channel-list) until the received level drops below the CCA sensitivity level (for a missed preamble) specified in 22.3.20.5 (CCA sensitivity). If the check of the L-SIG parity bit is not valid, a PHYRXSTART.indication is not issued. The PHY shall issue the error condition PHY-RXEND.indication(FormatViolation). The VHT PHY shall maintain PHY-CCA.indication(BUSY, channel-list) until the received level drops below the CCA sensitivity level (for a missed preamble) specified in 22.3.20.5 (CCA sensitivity).
May 2011
Eldad Perahia, Intel CorporationSlide 6
doc.: IEEE 802.11-11/0562r0
Submission
CCA Sensitivity Modification (1/2)• Replace “The start of a PPDU that occupies at least the
primary 20 MHz channel under the conditions listed in Table 22-22 shall be detected with >90% probability within a period of aCCATime (< 4μs).” with
• “The PHY shall detect the start of a PPDU that occupies at least the primary 20 MHz channel under the conditions listed in Table 22-22 within a period of aCCATime (< 4μs) and hold the CCA signal busy (PHY_CCA.indicate(BUSY,channel-list)) for the duration of the PPDU with >90% probability.”
• This permits dropping CCA due to acquisition or L-SIG failure for up to 10% of the packets
May 2011
Eldad Perahia, Intel CorporationSlide 7
doc.: IEEE 802.11-11/0562r0
Submission
CCA Sensitivity Modification (1/2)
• Remove “Any signal within the primary 20 MHz channel at or above -62 dBm” from Table 22-22
• Add “The receiver shall hold the CCA signal busy for any signal 20 dB or more above the minimum modulation and coding rate sensitivity (–82 + 20 = –62 dBm) in the primary 20 MHz channel.” after Table 22-22
May 2011
Eldad Perahia, Intel CorporationSlide 8
