The
SYNC fi eld
is 128 bits (symbols) in length and contains a string of 1s
which are scrambled prior to transmission. The receiver uses this fi eld to acquire
the incoming signal and to synchronize the receiver’s carrier tracking and timing
prior to receiving the SFD. The SFD fi eld contains information to mark the start of
the PPDU frame. The SFD specifi ed is common for all IEEE 802.11 DSSS radios.
All information bits transmitted by the DSSS PMD are scrambled using a
self-synchronizing 7-bit polynomial. An 11-bit Barker code (1,
1, 1, 1,
1, 1,
1, 1,
1,
1,
1) is used for spreading. In the transmitter, the 11-bit Barker code
is applied to a modulo-2 adder together with each of the information bits in the
PPDU. The output of the modulo-2 adder results in a signal with a data rate that
is 10 times higher than the information rate. The result in the frequency domain
is a signal that is spread over a wide bandwidth at a reduced RF power level.
Every station in the IEEE 802.11 network uses the same 11-bit sequence. At the
receiver, the DSSS signal is convolved with the same 11-bit Barker code and cor-
related. The minimum requirement for processing gain (
G
p
) in North America
and Japan is 10 dB.
PPDU
PPDU
Data Clock
11-bit Barker code
Scrambling
Despreading
Demodulation
Descrambling
Timing Clock Recovery
Spreading
Mask-Filling
Modulation
11-bit Barker code
Figure 21.7 Transmit and receive DSSS PPDU.
PLCP-Preamble
PLCP Header
SYNC
128-bits
SFD
16-bits
Signal
8-bits
Service
8-bits
Length
16-bits
CRC
16-bits
MPDU
PPDU
