IEEE 802.11 18.104.22.168.2 Channel numbering
"Channel center frequencies are defined at every integral multiple of 5 MHz above Channel starting frequency. The relationship between center frequency and channel number is given by Equation (32):
"Channel center frequency = Channel starting frequency + 5 ???¡ª nch (MHz)
where nch = 0,1,?¡é?€?|200."
This definition provides a unique numbering system for all channels with 5 MHz between center frequencies. Channel starting frequency is typically 5000 MHz.
So OFDM channel 40 is centered at 5200 MHz. Since the normal channel spacing is 20 MHz the next channel is channel 44 centered at 5220 MHz.
This design allows for channel spacing of both 10 and 5 MHz to use the channel numbers in between.
I hope this helps. Thanks. /criss