IEEE 802.11j OFDM goes off half-clocked
Last Post: December 1, 2004:
My employer has provided me a copy of the IEEE 802.11j amendment. Most of its 40 pages amends the clause 17 OFDM PHY introduced by the IEEE 802.11a amendment.
The new OFDM now has a half-clocked 10MHz channel spacing. Quoting:
"Insert the following paragraph at the end of 17.1: The OFDM system also provides a Ã¢Â€Âœhalf-clockedÃ¢Â€Â operation using 10 MHz channel spacings with data communications capabilities of 3, 4.5, 6, 9, 12, 18, 24, and 27 Mbit/s. The support of transmitting and receiving at data rates of 3, 6, and 12 Mbit/s is mandatory when using 10 MHz channel spacing. The half-clocked operation doubles symbol times and clear channel assessment (CCA) times when using 10 MHz channel spacing. The regulatory requirements and information regarding use of this OFDM system in 4.9 GHz and 5 GHz bands is in Annex I and Annex J."
So IEEE 802.11 clause 17 compliant radios are henceforth a/j ?? Just kidding.
This new operating mode wreaks havoc with all the nice tables you love to create and test. For example, these two consecutive paragraphs:
"Change the text of 184.108.40.206 as follows: The slot time for the OFDM PHY shall be 9 Ã‚Âµs for 20 MHz channel spacing, which is the sum of the RXto-TX turnaround time, MAC processing delay, and CCA detect time (< 4 Ã‚Âµs) and shall be 13 Ã‚Âµs for 10 MHz channel spacing. The propagation delay shall be regarded as being included in the CCA detect time. Where dot11RegulatoryClassesRequired is true, the value of the slot time shall be increased by the value of 3 Ã‚Âµs ÃƒÂ— coverage class. The default value of coverage class shall be zero.
"NOTEÃ¢Â€Â”Distributed coordination function (DCF) operation over larger BSS diameters is facilitated by relaxing some PHY timing parameters, while maintaining compatibility with existing implementations in small BSS diameters.1"
But after thoroughly amending the clause 17 general description of OFDM, we find in Annex J that the new 10 MHz channel spacing only applies to Japan! Cheers.