IEEE 802.11 Direct-Sequence Spread Spectrum

IEEE 802.11 Direct-Sequence Spread Spectrum
The DSSS3 modulation spreads a signal across a wideband at very low power. The original 802.11
standards supported the DSSS data rates of 1 and 2 Mbps in the 2.4-GHz band. The widely spread
signal can be recovered by a compliant receiver despite narrowband interference (and/or jamming)
within the spectrum, and eavesdroppers may interpret any of the weak narrowband signals generated
by DSSS that they discover as background noise [453]. Under the 802.11 standard, DHSS transmitters
spread each data bit into 11 smaller pulses, called chips, and these chips are transmitted, spread over
an extended (11 times wider) spectrum, for recovery and “despreading” by the DHSS receiver [454].
This chipping process increases the likelihood that the receiver can recover the original data on the
first try. If some portion of chip is lost, the receiver can use statistical techniques to determine what
the original was, without retransmitting the chip. The signal is effectively “louder” than if the data
were transmitted raw [453]. In other words, this transmission method is robust.
The 802.11 DHSS modulation splits the 2.4-GHz band into 14 five-MHz channels, 11 of which
are available for use in the United States and Canada (not all are available elsewhere) [453]. Because
of the signal spreading, the DHSS channels that are within 30 MHz of each other may interfere with
one another. This means that only three WLANs should operate concurrently in the same area to
assure unthreatened reliability [453].
The original 802.11 standards called for differential binary phase-shift keying (DBPSK) modulation
for transmissions at 1 Mbps and differential quadrature phase-shift keying (DQPSK) for 2 Mbps
[453].