Introduction
IEEE 802.11 (a/b/g)
11
Operating Manual 1171.5283.12 ─ 18
Parameters
Value
Occupied bandwidth
16.6 MHz
Channel spacing
20 MHz
The table shows the main parameters of OFDM. 64-point IFFT is used to generate the
52 subcarriers. 12 of the 64 possible carriers are not used. One is the carrier in the
middle of the band, which would otherwise be impaired by the carrier leakage of the
I/Q modulator, the others are the remaining carriers at the upper and lower end of the
spectrum. The required subcarrier offset of 312.5 kHz is implicitly observed when the
time signal generated by the IFFT with a sampling rate of 20 MHz is output. These 20
MHz are also called 'kernel sample rate'. An OFDM symbol generated in this way
would have a period of 3.2 µs. To compensate for multipath propagation, a so-called
guard interval with a duration of 0.8 µs is attached to each symbol so that a total sym-
bol interval of 4 µs is obtained.
Either BPSK, QPSK, 16QAM or 64QAM modulation can be used on the subcarriers.
Prior to the modulation, the raw data are convolutionally coded with code rates of ½ to
¾ being possible.
The frame structure can be seen in the figure below (also indicated in the "PPDU Con-
figuration" dialog):
2.1.1
PLCP Format
The physical layer convergence protocol (PLCP) is a protocol layer between medium
access control and the actual physical transmission layer (PHY). It is mainly used to
adapt the different transmission formats of the 802.11 standards to the MAC layer
which is identical for all transmission methods. Moreover, this protocol informs the
receiver on the type of signal sent to allow for a correct demodulation.
The PLCP generates the PLCP protocol data unit (PPDU) frames which are physically
transmitted.
2.1.2
PLCP Preamble
Each frame starts with the PLCP preamble made up of 10 short and 2 long symbols.
The receiver uses the short symbols mainly for signal detection, AGC, coarse fre-
quency adjustment and time synchronization. The long symbols are used to determine
the transmission function of the channel and to set the equalizer of the receiver accord-
Physical Layer OFDM
