Keysight WLAN Measurement Guide 91
Concepts
WLAN Standards
OFDM systems use fixed modulation formats for the preamble. Varying data
rates are achieved by changing the modulation for the data transmission
portion of a packet. In some cases, the modulation format changes during the
data transmission. Simpler modulation formats (such as BPSK) are often used
in the early part of the burst, which contains important information such as
frequency and burst length, because these formats are less prone to bit errors.
Figure 3-1, “802.11a Spectrum View,”
shows an 802.11 a signal burst. The
spectrum is not very uniform. This is caused in part by the preambles, but
mostly by the data being transmitted.
Figure 3-1
802.11a Spectrum View
18
QPSK
3/4
2
96
72
24
16-QAM
1/2
4
192
96
36
16-QAM
3/4
4
192
144
48
64-QAM
2/3
6
288
192
54
64-QAM
3/4
6
288
216
Table 3-2
802.11a Rate-dependent parameters
Data rate
(Mbps)
Modulation
Coding
rate (R)
Coded bits per
subcarrier
(NBPSC)
Coded bits
per OFDM
symbol
(NCBPS)
Data bits
per OFDM
symbol
(NDBPS)