Overview
Code division multiple access (CDMA) is a modulation and multiple-access
scheme based on spread-spectrum communication. In this scheme, multiple users
share the same frequency band at the same time, by spreading the spectrum of their
transmitted signals, so that each user's signal is pseudo-orthogonal to the signals
of the other users.
CDMA
Signals In a CDMA system, each signal consists of a different pseudorandom
binary sequence (called the spreading code) that modulates a carrier, spreading
the spectrum of the waveform. A large number of CDMA signals share the same frequency
spectrum. If CDMA is viewed in either the frequency or time domain, the multiple
access signals overlap with each other. However, the use of statistically orthogonal
spreading codes separates the various signals in the code space.
CDMA
Receivers A CDMA receiver separates the signals by means of a correlator
that uses the particular binary sequence to despread the signal and collect the
energy of the desired signal. Other users' signals, whose spreading codes do not
match this sequence, are not despread in bandwidth and, as a result, contribute
only to the noise. These signals represent a self-interference generated by the
system. The output of the correlator is sent to a narrow-bandwidth filter. The
filter allows all of the desired signal's energy to pass through, but reduces
the interfering signal's energy by the ratio of the bandwidth before the correlator
to the bandwidth after the correlator. This reduction greatly improves the signal-to-interference
ratio of the desired signal. This ratio is also known as the processing gain.
The signal-to-noise ratio is determined by the ratio of the desired signal power
to the sum of all of the other signal powers. It is enhanced by the processing
gain or the ratio of spread bandwidth to baseband data rate.
CDMA
Channel Assignments A CDMA digital cellular waveform design uses a pseudorandom
noise (PN) sequence to spread the spectrum. The sample rate of the spreading sequence
(called the chip rate) is chosen so that the bandwidth of the filtered signal
is several times the bandwidth of the original signal. A typical system might
use multiple PN sequences. In addition, it might use repeated spreading codes
of known lengths to ensure orthogonality between signals intended for different
users. The channel assignment is essentially determined by the set of codes that
are used for that particular link. Thus, the signal transmitted at any time in
a logical channel is determined by:
*
The frequency of operation for the base station * The current symbol *
The specific orthogonal spreading code assigned for the logical channel *
The PN spreading code
