Electronics Project:CDMA Signal on AWGN
Implementation
of CDMA signals on AWGN:
Code division multiple access (CDMA) is a channel access method used
by various radio communication technologies. One of the concepts in data
communication is the idea of allowing several transmitters to send information
simultaneously over a single communication channel. This allows several users
to share a band of frequencies. This concept is called multiple access. CDMA
employs spread-spectrum technology and a special coding scheme (where each
transmitter is assigned a code) to allow multiple users to be multiplexed over
the same physical channel. By contrast, time division multiple access (TDMA)
divides access by time, while frequency-division multiple access (FDMA)
divides it by frequency. CDMA is a form of spread-spectrum signaling, since the
modulated coded signal has a much higher data bandwidth than the data being
communicated.
An analogy to
the problem of multiple access is a room (channel) in which people wish to talk
to each other simultaneously. To avoid confusion, people could take turns
speaking (time division), speak at different pitches (frequency division), or
speak in different languages (code division). CDMA is analogous to the last
example where people speaking the same language can understand each other, but
other languages are perceived as noise and rejected. Similarly, in radio CDMA,
each group of users is given a shared code. Many codes occupy the same channel,
but only users associated with a particular code can communicate.
Additive white Gaussian noise (AWGN) is a channel model in which the
only impairment to communication is a linear addition of wideband or white
noise with a constant spectral density (expressed as watts per hertz of bandwidth)
and a Gaussian distribution of amplitude. The model does not account for fading,
frequency selectivity, interference, nonlinearity or dispersion. However, it
produces simple and tractable mathematical models which are useful for gaining
insight into the underlying behavior of a system before these other phenomena
are considered. Wideband Gaussian noise comes from many natural sources, such
as the thermal vibrations of atoms in conductors (referred to as thermal noise
or Johnson-Nyquist noise), shot noise, black body radiation from the earth and
other warm objects, and from celestial sources such as the Sun.
The AWGN
channel is a good model for many satellite and deep space communication links.
It is not a good model for most terrestrial links because of multipath, terrain
blocking, interference, etc. However, for terrestrial path modeling, AWGN is
commonly used to simulate background noise of the channel under study, in
addition to multipath, terrain blocking, interference, ground clutter and self
interference that modern radio systems encounter in terrestrial operation.
This
project builds the CDMA transmitter and receiver model with AWGN channel to study
the real time effect of channel on transmitter and receiver.
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