duty cycle of a WiFi Packet I am doing a simple over-the-air(WiFi 802.11a) test in a shielded chamber involving one Tx and one Rx (around 5 feet).. I need to find the duty cycle given the length of the payload (in bytes), MCS etc. 
As far as I understand , duty cycle is time a device spent on channel / total time . 
I was going through this paper https://www.academia.edu/15142561/Determination_of_the_duty_cycle_of_WLAN_for_realistic_radio_frequency_electromagnetic_field_exposure_assessment
Where the paper calculates the theoretical upper limit of duty cycle for a given MCS



The payload is 1500 bytes 
Max data rate is 54 Mbps

How is the data 248 micro secs in calculations? I am thinking,it should be 222.22 micro seconds (1500*8/54 micro seconds)

can any one please let me know  about it 

And If anyone has done such analysis on duty cycles , please do guide me thanks
Wifi uses OFDM, which uses symbols that can transport many bytes at once. That means you can't have arbitrary long packets, but always need to use the next multiple of a symbol duration.