SPREAD SPECTRUM COMMUNICATION SYSTEM
The two most important design factors in any digital
communication system are:
1) effective channel
bandwidth utilization and
2) transmission power conservation.
Some of the most serious
issues that arise with specialized communication systems are:
1) Combating or reducing the harmful effects of jamming,
interference from other channel users, and self-interference owing to multipath
propagation are some of the key issues encountered in certain communication
systems.
2) Hiding a signal by broadcasting it with low power and
making it difficult to detect by an unwanted listener.
3) Maintaining message privacy when speaking in front of
others.
Spread-spectrum modulation is a technology that may be used
to successfully overcome these challenges.
SPREAD SPECTRUM COMMUNICATION SYSTEM
If a system
meets the following criteria, it is classified as a spread spectrum
communication system.
1) The signal
uses a lot more bandwidth than the minimum bandwidth required to convey the
data.
2) Spreading
is performed via a data-independent spreading signal, generally known as 'a
code signal'.
3) At the
receiver, despreading (recovering the original data) is accomplished by
comparing the received spread signal to a synchronized replica of the spreading
signal that was used to spread the data.
The Bandpass
modulator circuit is used in the transmitter of a digital communication system
to produce such frequency spreading of the signal.
MODEL OF SPREAD SPECTRUM DIGITAL COMMUNICATION
SYSTEM
The basic elements of a spread spectrum digital communication system are shown in the figure.
Figure: Model of spread spectrum
digital communication system
The modulator/demodulator and the channel encoder/decoder are
the fundamental elements of a digital communication system. This also has two
identical pseudorandom pattern generators. At the transmitting end, one
interface with the modulator. The second interacts with the receiver's
demodulator. The pseudo-noise (PN) binary-valued sequence is impressed on the
transmitted signal at the modulator and removed from the received signal at the
demodulator by these pseudorandom pattern generators.
BENEFICIAL
ATTRIBUTES OF SPREAD SPECTRUM SYSTEMS
Spread-spectrum
modulation was developed for military applications, where jamming
(interference) resistance is a key challenge. However, the unique
characteristics of spread spectrum modulation have civilian applications as
well. The major advantages of spread spectrum systems are listed below:
1)
Interference suppression benefits:
(i) To avoid
intentional interference (jamming), the transmitter introduces an element
of unpredictability or randomness (pseudorandomness) to each of the transmitted
coded signal waveforms. Only the intended
receiver is informed of this (not the jammer). As a result,
jamming-related interference is prevented.
(ii)
Self-interference can be seen as resolvable multipath components caused by
temporal dispersive propagation over a channel. Incorporating a pseudorandom
pattern into the broadcast signal can also help to remove this form of
interference.
2) Multiple Access
Spread spectrum techniques can be used as a multiple access
approach to distributing a communication resource among several users in a
coordinated fashion. In multiple access communication systems, when several
users share a shared channel bandwidth, interference from other users occurs.
By superimposing a separate pseudorandom pattern, also known as a code, in each
transmitted signal, the transmitted signals in this shared channel spectrum may
be identified from one another. By understanding the code or key used by the
associated transmitter, a specific receiver can recover the sent information
intended for it. Code Division Several Access (CDMA) is a communication
technology that allows multiple users to share a common channel for data
transmission at the same time (CDMA).
3)
Energy Density Reduction
By spreading
a message's bandwidth using coding and delivering the resulting signal at low
average power, a message can be hidden in the background noise. The transmitted
signal is referred to be "covert" due to its low power level. It has
a low chance of being intercepted (detected) by an untrained ear. As a result,
it's also known as a signal with a Low Probability of Intercept (LPI).
A radiometer
is a basic power meter that may be used to detect the presence of spread-spectrum
signals within a certain bandwidth (B).
4)
Fine Time Resolution
In radar and
navigation, spread spectrum signals are used to accurately determine the range
(time delay) and range rate (velocity). The time delay of a pulse as it travels
through a channel can be used to calculate distance.
5)
Message Privacy
By superimposing
a pseudorandom pattern on a transmitted message, communication privacy can be
achieved. The message can be demodulated by the intended receivers who know the
pseudorandom pattern or key used at the transmitter but not by any other
receivers who don't know the exact key.
SPREAD
SPECTRUM APPROACHES (HISTORICAL BACKGROUND)
Transmitted
Reference (TR) and Stored Reference (SR) are two spread-spectrum methods.
(i) In a TR system, the transmitter transmits
two copies of a random spreading signal (wideband carrier), one modulated with
data and the other unmodulated. For despreading (correlating) the data
modulated carrier, the receiver used the unmodulated carrier as the reference
signal.
(ii) The
spreading code signal is generated independently at both the transmitter and
the receiver in an SR system. The code sequence must be predictable, even if it
should look random to unauthorized listeners because the identical code must be
created independently at two sites. Pseudonoise (PN) or pseudorandom signals
are deterministic signals that look random.
The Stored
Reference (SR) technique, which employs a Pseudo Noise (PN) or pseudorandom
code signal, is used in modern spread spectrum systems.