102 Keysight WLAN Measurement Guide
Concepts
Spread Spectrum Concepts
Spread Spectrum Concepts
Direct Sequence Spread Spectrum (DSSS)
DSSS (Direct Sequence Spread Spectrum) - The energy in a single carrier is
spread over a wider spectrum by multiplying data bits with a special 11-bit
pattern, called a Barker key. This is done at a chip rate of 11 MHz. This
technique can help reduce interference from narrow-band sources. The IEEE
802.11b uses an 8-bit key. As shown in figure below, to create a DSSS signal, a
lower rate signal is multiplied by a higher rate signal. For 1 Mbps, the 1 MHz
(D)BPSK signal is multiplied by an 11 MHz BPSK signal. Although not generally
true of DSSS signals, for this particular signal it can be said that the input bits
determine the phase rotation of the spreading code or that it is a (D)BPSK data
signal spread by a BPSK spreading sequence, producing a BPSK constellation.
Figure 3-6
DSSS signal spreading for 2 Mbps data rate
Complementary Code Keying (CCK)
CCK (Complementary Code Keying) - Changing both the spreading factor
and/or the modulation format varies the bit rate. CCK is used to increase IEEE
802.11b’s peak data rate from 2 to 11 Mbps, while still using QPSK (Quadrature
Phase Shift Keying) modulation. To achieve 5.5 and 11 Mbps rates, the
spreading length is first reduced from 11 to 8. This increase the symbol rate
from 1 Msps to 1.375 Msps, then taking data in 4-bit blocks (4*1.375 = 5.5) or
8-bit blocks (8*1.375 = 11), i.e. 4 bits/symbol or 8 bits/symbol. Six of the 8 bits
are used to choose 1 of 64 complementary codes, which are 8 chips long and
clocked out at 11 MHz. Thus all 8 chips are “used up” in (1/1.375) us - the time
before another byte is ready. The other 2 bits are combined with the code in
the QPSK modulator.