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CCA- and slot-time relations

CCA- and slot-time relations. Jan Boer Agere Systems. IEEE Std 802.11g/D2.8, May 2002 clause 19.4.5 CCA.

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CCA- and slot-time relations

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  1. CCA- and slot-time relations Jan Boer Agere Systems Jan Boer, Agere Systems

  2. IEEE Std 802.11g/D2.8, May 2002clause 19.4.5 CCA b) With a valid signal (according to the CCA mode of operation) present at the receiver antenna within 5 µs of the start of a MAC slot boundary, the CCA indicator shall report channel busy before the end of the slot time. Refer to Figure 47 (should be 58)of IEEE Std 802.11, 1999 Edition for a slot time boundary definition. d) The start of a valid OFDM transmission at a receive level equal to or greater than the minimum 6 Mbit/s sensitivity (-82 dBm) shall cause CCA to indicate busy with a probability >90% within 4 µs. • b) is copied out of 11b, d) is copied out of 11a • Looks inconsistent, d) is overspecified for the 11b (or 11g) slottime (20µs) Jan Boer, Agere Systems

  3. Jan Boer, Agere Systems

  4. Slottime relations n n+1 n+2 Slottime 20µs CCA del RX/TX CCA del RX/TX CCA del RX/TX MAC slot boundery Station A (wants to transmit at slot n+1, performs CCA in slot n) CCA idle Transmit Station B (wants to transmit at slot n+2, performs CCA in slot n+1) CCA active Defer At a slottime of 20µs the CCA time can be 15µs independent on the modulation received Jan Boer, Agere Systems

  5. Proposed change • In 11b the CCA time is specified to be 15µs (minus the MAC proc. delay), with a slottime of 20µs • To copy the 11a CCA time to 11g is overspecifying. Also for OFDM the CCA time can be 15µs at a 20µs slottime. This longer time can be used to make the the probability of the CCA indicator more reliable. • Proposal: b) With a valid signal (according to the CCA mode of operation) present at the receiver antenna within 5 µs of the start of a MAC slot boundary, the CCA indicator shall report channel busy before the end of the slot time. In case of a valid OFDM signal at a receive level equal or greater than –82dBm the probability of the CCA indicator to report channel busy shall be > 90%. Refer to Figure 58of IEEE Std 802.11, 1999 Edition for a slot time boundary definition. Delete d) Jan Boer, Agere Systems

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