DSSS

DSSS
DHSS systems mix high-speed bit patterns with the information being sent to spread the RF carrier. Each
bit of information has a redundant bit pattern associated with it, effectively spreading the signal over a
wider bandwidth. These bit patterns vary in length and in the rate at which they are mixed into the RF
carrier. They are called chips or chipping codes and range from 11 bits to extremely long sequences. The
speed at which they are transmitted is called the chipping rate. To an observer, these sequences appear
to be noise and are also called pseudorandom noise codes (Pncodes). Pncodes are usually introduced
into the signal through the use of hardware-based shift registers, and the techniques used to introduce
them are divided into several groups including Barker codes, Gold codes, M-sequences, and Kasami
codes.
These spreading codes also allow the use of statistical recovery methods to repair damaged
transmissions. Another side effect of spreading the signal is lower spectral density-that is, the same
amount of signal power is distributed over more bandwidth. The effect of a less spectrally dense signal is
that it is less likely to interfere with spectrally dense narrowband signals. Narrowband signals are also
less likely to interfere with a DSSS signal because the narrowband signal is spread as part of the
correlation function at the receiver.
The frequency channel in IEEE 802.11 DSSS is 22 MHz wide. This means that it supports three
nonoverlapping channels for operation. This is why only three IEEE 802.11b DSSS systems can be colocated.
In addition to spreading the signal, modulation techniques are used to encode the data signal through
predictable variations of the radio signal. IEEE 802.11 specifies two types of DPSK modulation for DSSS
systems. The first is BPSK and the second is QPSK. Phase-shift keying (PSK), as the name implies,
detects the phase of the radio signal. BPSK detects a 180-degree inversion of the signal, representing a
binary 0 or 1. This method has an effective data rate of 1 Mbps. QPSK detects 90-degree phase shifts.
This doubles the data rate to 2 Mbps. IEEE 802.11b adds CCK and packet binary convolutional coding
(PBCC), which provide data rates up to 11 Mbps.[5]