Abstract :
Mobile
communications are rapidly becoming more and more necessary for everyday
activities. With so many more users to accommodate, more efficient use of
bandwidth is a priority among cellular phone system operators. Equally
important is the security and reliability of these calls. One solution that has
been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM.
Code Division
Multiple Access is a new technology used in wireless communication devices.
This technology made its commercial debut in the early nineties. Significant
advantage of the CDMA is the fact that unlike other modulation schemes it does
not have to allocate part of the frequency for each user. It allocates whole
frequency spectrum to each user, distinguishing each signal with the unique
pseudo-random sequence.
CDMA stands
for "Code Division Multiple Access." It is a form of spread-spectrum,
an advanced digital wireless transmission technique. Instead of using
frequencies or time slots, as do traditional technologies, it uses mathematical
codes to transmit and distinguish between multiple wireless conversations. Its
bandwidth is much wider than that required for simple point-to-point
communications at the same data rate because it uses noise-like carrier waves
to spread the information contained in a signal of interest over a much greater
bandwidth. However, because the conversations taking place are distinguished by
digital codes, many users can share the same bandwidth simultaneously.
What is
CDMA?
Code
Division Multiple Access (CDMA) is a digital cellular spread-spectrum
modulation technique that implements distributed voice and data networks.
CDMA works
by converting speech into digital information, which is then transmitted as a
radio signal over a wireless network. CDMA does not assign a specific frequency
to each user, but the full available spectrum. Each signal is encoded
differently using a unique code. This way CDMA enables a large number of users
to share the same frequency band at the same time,
without interference. The receiving device is instructed to use the code to extract the
data out of the received signal.
Generating
the CDMA signal
In the CDMA technique the signal transmissions among the multiple
users completely overlap in both time and frequency. The separation between the
users is made by assigning each user a unique
code. Generally,
CDMA converts analog voice signal to a digital signal, encodes the digital
signals, and separates voice and control data into data streams called
channels.
Generating
a CDMA signal is a five steps process:
The first
step is analog to digital conversion or A/D. The incoming voice signal is an
analog signal meaning that it is changing constantly, taking on all possible
values of amplitude range. The CDMA uses a digital signal for its further
manipulations. That digital signal is characterized by discrete states.
In that
step the analog voice signal is quantized to form a digital signal
consists of a few levels. The second step is voice coding or Vocoding. Voice encoding is the
process of compressing the audio into as small a stream of bits as possible.
The vocoder takes advantage of the pauses in speech to accomplish maximum
compression. The Vocoder’s rate must be variable to fit the rate of the users
speech activity.
The third
step is encoding and interleaving. This step purpose is to reduce the errors
when receiving the signal.
Interleaving
is a method of reducing the effects of burst errors and recovering lost bits.
The symbols are interleaved such that originally neighboring symbols will be
transmitted far away from each other. In addition to that, the various encoding methods
add redundancy to the signals to help the recovery of information at the
receiver in case of errors.
The forth step is channelizing.
The signal
of each user if further encoded to create a separation between different users.
A unique identification code is given to each user and the signals of all users
are transmitted together, sharing the same frequency and time. The CDMA
receiver decodes the signal by multiplying it by a decoding sequence of the
desired user.
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