Code Division Multiple Access (CDMA)
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#1
21-09-2008, 11:30 AM


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
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#2
07-03-2011, 12:40 PM

presented by:
MALLAPI SHYAM REDDY


.doc   Code Division Multiple Access.doc (Size: 26 KB / Downloads: 99)
Code Division Multiple Access (CDMA)
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
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#3
07-02-2012, 04:05 PM

Code Division Multiple Access (CDMA)


.ppt   pradeep.ppt (Size: 372 KB / Downloads: 77)

What is CDMA


CDMA used extensively in radio frequency communication systems, especially in 2G and 3G cellular telephone networks.

Basic Advantage is the way it handles a finite BW among a large number of users (more users can transmit the same data over the same Bandwidth)


Architecture


.Two frequency channels
One control channel
One data channel
.Spreading code
A common spreading code is used by all nodes over the control channel
Several terminal-specific codes can be used over the data channel
.Signal over the control channel is completely orthogonal to any signal over the data channel

How does CDMA work

The principle is the same as in wireless application. Each user is assigned a unique code (spreading length -L-) which is multiplied by each bit. This code is only known to the receiver in order to demodulate the data.

The most important part for correct detection is the code. This code must be uncorrelated from other user’s codes and be orthogonal.

Advantage Of CDMA

Many users of CDMA use the same frequency, TDD or FDD may be used

Multipath fading may be substantially reduced because of large signal bandwidth

No absolute limit on the number of users

Easy addition of more users

Impossible for hackers to decipher the code sent

Better signal quality



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#4
30-03-2012, 02:51 PM

Code Division Multiple Access (CDMA)


.pdf   CDMA.pdf (Size: 354.61 KB / Downloads: 24)


WHAT IS CDMA?
(Code Division Multiple Access) A method for transmitting realtime
signals over a shared portion of the spectrum. The foremost
application of CDMA is the digital cellular phone technology that
operates in the 800MHz band and 1.9GHz . Unlike GSM and TDMA,
which divides the spectrum into different time slots CDMA uses a
spread spectrum technique to assign a code to each conversation.
After the speech codec converts voice to digital, CDMA spreads the
voice stream over the full 1.25MHz bandwidth of the CDMA channel
coding each stream separately so it can be decoded at the receiving
end. The rate of the spreading signal is known as the “chip rate,” as
each bit in the spreading signal is called a “chip” voice conversations
use the full bandwidth at the same time.



MULTIPLE ACCESS COMPARISON
It is easier to understand CDMA if it is compared with other
multiple access technologies. The following sections describe the
fundamental differences between a Frequency Division Multiple
Access Analog technology (FDMA), a Time Division Multiple Access
Digital technology (TDMA) and a Code Division Multiple Access
Digital technology (CDMA).

FDMA - Frequency Division Multiple Access

FDMA is used for standard analog cellular. Each user is
assigned a discrete slice of the RF spectrum. FDMA permits only one
user per channel since it allows the user to use the channel 100% of
the time. Therefore, only the frequency “dimension” is used to define
channels.



CDMA - Code Division Multiple Access

IS-95 uses a multiple access spectrum spreading technique
called Direct Sequence

(DS) CDMA.

Each user is assigned a binary, Direct Sequence code during a
call. The DS code is a signal generated by linear modulation with
wideband Pseudorandorn Noise (PN) sequences. As a result, DS

CDMA uses much wider signals than those used in other
technologies. Wideband signals reduce interference and allow onecell
frequency reuse.



The International Cocktail Party
To illustrate the conceptual differences among the multiple
access technologies, the “International Cocktail Party” analogy will be
applied. Picture a large room and a number of people, in pairs, who
would like to hold conversations.


Spatial Diversity
Spatial Diversity refers to the use of two receive antennas
separated by some physical distance. The principle of spatial diversity
recognizes that when a mobile is moving about, it creates a pattern of
signal peaks and nulls. When one of these nulls falls on one antenna
it will cause the received signal strength to drop.


CDMA Modulation
Both the Forward and Reverse Traffic Channels use a similar
control structure consisting of 20 millisecond frames. For the system,
frames can be sent at either 14400, 9600, 7200, 4800, 3600, 2400,
1800, or 1200 bps. For example, with a Traffic Channel operating at
9600 bps, the rate can vary from frame to frame, and can be 9600,
4800, 2400, or 1200 bps.



CONCLUSION
The world is demanding more from wireless communication
technologies than ever before. More people around the world are
subscribing to wireless services and consumers are using their
phones more frequently. Add in exciting Third-Generation (3G)
wireless data services and applications - such as wireless email, web,
digital picture taking/sending and assisted-GPS position location
applications - and wireless networks are asked to do much more than
just a few years ago. And these networks will be asked to do more
tomorrow.
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#5
04-04-2012, 03:47 PM

Code Division Multiple Access (CDMA)



.pdf   Sohan.pdf (Size: 1.08 MB / Downloads: 56)

Introduction
1.1 Motivation
Code division multiple access (CDMA) systems using the technique of spread
spectrum communication have been used in radio communication systems in an effort to
improve radio spectral usage efficiency. The Qualcomm company in San Diego has
commercialized the technology in the last few years. Recently, there has been a lot of
interest in applying the concepts of CDMA to optical fiber communication systems.


Time-frequency space usage in (a) TDMA, (b) WDMA, © CDMA.
Both TDMA and WDMA systems have been widely studied in both industry and
academia. Although CDMA has been heavily studied in radio communications, and has
proven itself as a viable technology, it is a relatively new technology in the fiber optic
domain.



Dissertation Organization
In this chapter, we have stated the objective of this project and implimentation and given a general
introduction to the spread spectrum communication technique and CDMA systems. A
general review of the previous research on fiber optic CDMA systems, their merits and
problems will be given in Chapter 2. The key issues in designing optical CDMA systems
will also be stated.
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Arnab Mondal
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#6
11-04-2012, 07:53 PM

plz post the matlab code of CDMA system.....
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#7
12-04-2012, 10:40 AM

to get information about the topic "Code Division Multiple Access " full report ppt and related topic refer the link bellow

topicideashow-to-code-division-multiple-access-cdma?pid=80836

topicideashow-to-w-cdma-wideband-code-division-multiple-access

topicideashow-to-code-division-multiple-access-cdma

topicideashow-to-frequency-division-multiple-access-fdma

topicideashow-to-time-division-multiple-access-tdma
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#8
31-07-2012, 08:33 AM

i want future scope and ppt for implementation of cdma system using gold code spread spectrum.
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