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2.2 Generation of Direct Sequence Spread Spectrum
To generate a spread spectrum signal one requires:
1. A modulated signal somewhere in the RF spectrum
2. A PN sequence to spread it
2.3 Features of Direct Sequence Spread Spectrum
DSSS has some features as following:
1. DSSS phase-modulates a sine wavepseudorandomly with a continuous string of
pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an
information bit. That is, each information bit is modulated by a sequence of much faster chips.
Therefore, the chip rate is much higher than the information signal bit rate.
2. DSSS uses a signal structure in which the sequence of chips produced by the transmitter is
known a priori by the receiver. The receiver can then use the same PN sequence to counteract the
effect of the PN sequence on the received signal in order to reconstruct the information signal.
2.4 Transmission of Direct Sequence Spread Spectrum
Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a
"noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency
much higher than that of the original signal, thereby spreading the energy of the original signal
into a much wider band.
The resulting signal resembles white noise, like an audio recording of "static". However, this
noise-like signal can be used to exactly reconstruct the original data at the receiving end, by
multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This
process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN
sequence with the PN sequence that the receiver believes the transmitter is using.
For de-spreading to work correctly, the transmit and receive sequences must be synchronized.
This requires the receiver to synchronize its sequence with the transmitter's sequence via some
sort of timing search process. However, this apparent drawback can be a significant benefit: if the
sequences of multiple transmitters are synchronized with each other, the relative synchronizations
the receiver must make between them can be used to determine relative timing, which, in turn, can
be used to calculate the receiver's position if the transmitters' positions are known. This is the basis
for many satellite navigation systems.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain.
This effect can be made larger by employing a longer PN sequence and more chips per bit, but
physical devices used to generate the PN sequence impose practical limits on attainable processing
gain.
If an undesired transmitter transmits on the same channel but with a different PN sequence
(or no sequence at all), the de-spreading process results in no processing gain for that signal. This
effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows
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