Keysight WLAN Measurement Guide 93
Concepts
WLAN Standards
802.11b
The 802.11b operates in the 2.4 GHz and uses direct sequence spread
spectrum (DSSS) techniques to spread the energy in a single carrier over a
wider spectrum.
See “Direct Sequence Spread Spectrum (DSSS)” on
Two coding schemes are used in the 802.11b to spread the
spectrum of a single carrier. Complementary code keying (CCK) is mandatory,
while packet binary convolutional coding (PBCC) is optional. CCK is used to
increase the 802.11b peak data rate to 11 Mbps using QPSK modulation.
“Complementary Code Keying (CCK)” on page 102.
. PBCC makes use of
forward error correction to improve the link performance when noise is the
limitation.
See “Packet Binary Convolutional Coding (PBCC)” on page 103.
.
The 802.11b allows 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps operation using
various modulation schemes. The 1 and 2 Mbps rates use DBPSK and DQPSK
modulation schemes. For 5.5 and 11 Mbps operation, CCK modulation is used.
To achieve 5.5 and 11 Mbps rates, the spreading length is first reduced from 11
to 8. This increases the symbol rate from 1 Msps to 1.375 Msps. So, for 5.5
Mbps bit rates one needs to transmit 5.5/1.375 or 4 bits/symbol and for 11
Mbps, 8 bits/symbol.
The approach taken for 802.11b, which keeps the QPSK spread spectrum
signal and still provides the required number of bits/symbol, uses all but two of
the bits to select from a set of spreading sequences. It uses the remaining two
bits to rotate the sequence.
An important difference between the sets of spreading sequences used here
and the single Barker code sequence used for the 1 and 2 Mbps rates is that
these sequences are complex. In other words, the signal is a (D)QPSK signal
with QPSK spreading.
shows a plot of 802.11b signal burst. For all
802.11b bit rates, the preamble and header are sent at the 1 Mbps rate. The
header is 192 µs long (192 bits). This translates to a total of 2112 chips. The
payload data is then appended using one of the four modulation rates.
Table 3-5
802.11b Rate-dependent parameters
Data rate (Mbps)
Modulation
Symbol rate (MSps)
1
DBPSK
2
DQPSK
5.5
CCK
1.375
PBCC
11
CCK
1.375
PBCC