40 MHz ChannelizationBy CWNP On 09/07/2007 - 31 Comments
When 40 MHz channels are used in 802.11n networks, two 20 MHz channels are bonded together. The two 20 Mhz channels are designated as primary and secondary and are designated by two fields: (Primary, Secondary) where the Primary is the number of the primary channel and the Secondary is a positive or negative integer indicating whether the secondary channel is one channel above or one channel below the primary channel). 40 MHz channels MUST consist of immediately adjacent 20 MHz channels allowed within the regulatory domain.
When transmitting using either 20/40 MHz mode or PCO mode, both 20 and 40 MHz transmissions are present in the BSS. When duplicate frame formatting is used, the 20 MHz spectral mask (as with clause 17 OFDM transmissions) is used on both adjacent channels simultaneously. When 20 MHz transmissions are used, a 20 MHz spectral mask is used on the primary channel only. When a 40 MHz spectral mask is used, it spans both adjacent channels, but is shaped just like a 20 MHz transmission. So, there are cases where you have one transmission spanning both channels, cases where both channels are used at the same time (with identical spectral masks), and cases where only the primary channel is used.
While I haven't taken a look at this on a spectrum analyzer due to a lack of PCO-capable and 20/40 MHz capable gear, I'm guessing that it's going to be so fast switching back and forth between possibly all three scenarios (depending on operating mode) that a 40 MHz wide blur will likely be observed on a spectrum analyzer.