On a normal 20MHz channel there are 64 OFDM subcarriers of which 48 are available for data. Of the remander 4 are used for synchronisation and the remainder are not transmitted to act as guard bands to avoid adjacent channel interference. There are spectral masks which show the signal rolloff outside the channel. An example of this can be seen at http://www.rfcafe.com/references/electrical/wlan-masks.htm
My problem is trying to find out what happens with 10MHz and 5MHz channels. If I follow the 20MHz design approach I get the following:
- 10MHz channel, 32OFDM subcarriers, 4 used for synchronisation, 6+6 for guard band leaving 16 for user data
- 5MHz channel, 16OFDM subcarriers, 4 used for synchronisation, 6+6 for guard band leaving 0 for user data - this doesn’t look right
Does anyone know what the subcarrier usage is for 5MHz and 10MHz channels?
I need to explain to a regulator what the out of band emissions are likely to be for a 10MHz channel so working out the OFDM carrier assignment is important if I am to adapt the 802.11 spectrum mask to the narrow channels.
I’ve done a couple of tests which can be seen at the end of this message where the 10MHz channel appears to roll off at the same rate as the 20MHz channel. However I’d like to be sure, this information will also allow me to work out the expected data throughputs for different configurations.
anyone got any comments?
andy
