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Presented By:
M.Vamsi Krishna
B.kiran sai phaneswar

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ABSTRACT:

A wireless digital communication system is a technique that allows people to communicate each other at a distance. In this paper we tried to explain one of the latest technologies of digital wire less communication systems - The CDMA and the application of this on cellular phones.CDMA is an acronym for code division multiple access. Each CDMA subscriber is given with a separate code for communication that reduces interferences & increases privacy. In this paper we explained about the transmission, reception of CDMA signals for cell phones, the channelising, modes of communication, features and advantages. Code Division Multiple Access Technology emerged as an alternative to the GSM cellular architecture and has shared in the past decadeâ„¢s explosive growth in the wireless market. CDMA like GSM has seen incremental improvements in capacity through out this period. Our contribution to this paper is implementation of technology behind
CDMA using FLASH TECHNOLOGY
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INTRODUCTION:

The wireless digital communication system allows people to communicate at a distance. A digital communication system mainly consists of,
1. Network components and equipment
2. The interrelationship between the components
3. Types of calls.
Wireless digital communications has dramatically increased in popularity, resulting in the need for technologies that allow multiple users share the same frequency. These are called multiple access systems. The three types of multiple access system are:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Frequency Division Multiple Access:
Each FDMA subscriber is assigned a specific frequency channel. No one else in the same cell or a neighboring cell can use the frequency channel while it is assigned to a user. This reduces interference, but severely limits the number of users.
Time Division Multiple Access:
TDMA users share a common frequency channel, but use the channel for only a very short time. They are each given a time slot and only allowed to transmit during that time slot. When all available time slots in a given frequency are used, the next user must be assigned a time slot of another frequency.

Code Division Multiple Access (CDMA):

CDMA users share a common frequency channel. All users are on the same frequency at the same. However, each pair of users is assigned with a specific code that reduces interferences while increasing privacy.
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Comparison of multiple access systems: The following table summarizes

some of the technical aspects of multiple access technologies. The technologyused determines the channelâ„¢s capacity. TDMA triples the capacity of FDMA, but CDMA capacity can be up to seven times that of TDMA.

Working of CDMA: This includes
1. CDMA signal generation
2. The types of codes used in CDMA
3. Forward and reverse page link code channels
4. CDMA call processing
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As the name indicates, CDMA uses codes to convert between analog voice signals and digital signals. CDMA also uses codes to separate voice and control data into data streams called channels.
Generating a CDMA signal for cellular phones:
The following fig. shows cell phone transmitter using CDMA technology.
There are five steps in generating a CDMA signal.
1. Analog to digital conversion
2. Vocoding
3. Encoding and interleaving
4. Channelising the signals
5. Conversion of the digital signal to radio frequency signal
Analog to Digital Conversion:
The first step of CDMA signal generation is analog to digital conversion, sometimes called A/D conversion. CDMA use s a technique called Pulse Code Modulation (PCM) to accomplish A/D conversion.

Voice compression:
The second step of CDMA signal generation is voice compression. CDMA uses a device called a vocoder to accomplish voice compression. Vocoders are located at the BSC (base control stations) and in the phone. People pause between syllables and words when they talk. CDMA takes advantage of these pauses in speech activity by using a variable rate vocoder. A CDMA vocoder varies compression of the voice signal into one of four data rates based on the rate of the user's speech activity. The four rates are: Full, 1/2, 1/4 and 1/8. The vocoder uses its full rate when a person is talking very fast. It uses the 1/8 rate when the person is silent.
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There are two types of vocoders, 8kbps and 13kbps.The 8kbps vocoders are having high capacity but the Recently the CDMA community adopted a new 8 kbps vocoder. This new vocoder is usually referred to as the EVRC (Extended Variable Rate Coding). It combines the quality of 13 kbps quality with the capacity of the 8kbps vocoder.

Encoders and Interleaves:
Encoders and interleaves are built into the BTS (base station transceiver subsystem) and the phones. The purpose of the encoding and interleaving is to build redundancy into the signal so that information lost in transmission can be recovered.
The type of encoding done at this stage is called "convolution encoding. A digital message consists of four bits (A, B, C, D) of vocoded data. Each bit is repeated three times. These encoded bits are called symbols. The decoder at the receiver uses a majority logic rule. Thus, if an error occurs, the redundancy can help recover the lost information. Interleaving is a simple but powerful method of reducing the effects of burst errors and recovering lost bits.
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In the example shown here the symbols from each group are interleaved (or scrambled) in a pattern that the receiver knows.
Burst errors:
A burst error is a type of error in received digital telephone signals. Burst errors occur in clumps of adjacent symbols. These errors are caused by fading and interference Encoding and interleaving reduce the effects of burst errors.
Channelising:
The encoded voice data is further encoded to separate it from other encoded voice data. The encoded symbols are then spread over the entire bandwidth of the CDMA channel. This process is called channelisation.

Types of codes:
CDMA uses two important types of codes to channelise users. Walsh codes channelise users on the forward page link (BTS to mobile). Pseudorandom Noise (PN) codes channelize users on the reverse page link (mobile to BTS)
Walsh codes:
Walsh codes provide a means to uniquely identify each user on the forward link. Walsh codes have a unique mathematical property--they are "orthogonal." In other words, Walsh codes are unique enough that a receiver applying the same Walsh code can only recover the voice data. All other signals are discarded as background noise.
Pseudorandom Noise (PN) code:
Pseudorandom Noise (PN) codes uniquely identify users on the reverse link. A PN code is one that appears to be random, but is not. The PN codes used in CDMA yield about 4.4 trillion combinations of code. This is a key reason why CDMA is so secure.
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Digital to Radio Frequency (RF) conversion:
The BTS combines channelised data from all calls into one signal. It then converts the digital signal to a Radio Frequency (RF) signal for transmission.

Reception of CDMA signals for cellular communication: After the CDMA signal is transmitted, the receiver must reverse the signal generation process to recover the voice, as follows:
1. Conversion of RF to digital signal
2. Dispreading the signal
3. De-interleaving and decoding
4. Voice decompression
5. .Digital to analog conversion

Channelising of CDMA signals in cellular communication:

A code channel is a stream of data designated for a specific use or person. This channel may be voice data or overhead control data. Channels are separated by codes. The forward and reverse links use different types of channels.

Forward page link channels:
The forward page link uses four types of channels to transmit voice and control data to the mobile. The types of forward page link channels are,
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1. Pilot
2. Sync
3. Paging
4. Traffic
Reverse page link channels:
The reverse page link uses two types of channels to transmit voice and control data to the BTS. The types of reverse page link channels are,
1. Access
2. Traffic

Pilot channel:
The BTS constantly transmits the pilot channel. The mobile uses the pilot signal to acquire the system. It then uses the pilot signal to monitor and adjust the power needed in order to transmit back to the BTS.

Sync channels:
The BTS constantly transmits over the sync channel so the mobile can synchronize with the BTS. It provides the mobile with the system time and the identification number of the cell site. The mobile ignores the sync channel after it is synchronized.
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Paging channel:
CDMA uses up to seven paging channels. The paging channel transmits overhead information such as commands and pages to the mobile. The paging channel also sends commands and traffic channel assignment during call set- up. The mobile ignores the paging channel after a traffic channel is established.

Forward page link channel:
CDMA uses between fifty- five and sixty-one forward traffic channels to send both voice and overhead control data during a call. Once the call is completed, the mobile tunes back in to the paging channel for commands and pages.

Access channel:
The mobile uses the access channel when not assigned to a traffic channel. The mobile uses the access channel to: Register with the network Originate calls Respond to pages and commands from the base station Transmit overhead messages to the base station.

Reverse page link channel:
The reverse traffic channel is only used when there is a call. The reverse traffic channel transmits voice data to the BTS. It also transmits the overhead control information during the call.


Call processing stages:

The reverse traffic channel is only used when there is a call. The reverse traffic channel transmits voice data to the BTS. It also transmits the overhead control information during the call.
1. Initialization mode
2. Idle mode
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3. Access mode
4. Traffic mode
Initializing mode:
During initialization, the mobile:
1. Aquires the system via pilot code channel.
2. Synchronises with the system via the sync code channel.
Idle mode:
The mobile is not involved in a call during idle mode, but it must stay in communication with the base station:
1. The mobile and the base station communicate over the access and

paging code channels

2. The mobile obtains overhead information via the paging code channel.
Access mode:
The mobile accesses the network via the Access code channel during call origination. The Access channel and Paging channel carry the required call set-up communication between the mobile phone and the BTS until a traffic channel is established.
Traffic mode:

During a Land To Mobile call (LTM):
1. The mobile receives a page on the paging channel.
2. The mobile responds on the access channel.
3. The traffic channel is established and maintained throughout the call.

During a Mobile To Land call (MTL):
1. The call is placed using the Access channel.
2. The base station responds on the paging channel.
3. The traffic channel is established and maintained throughout the call.
Features and advantages of CDMA in cellular phones:
CDMA has several unique features that make it a cost-effective, high quality wireless solution. In this module you will learn what those features are and how they provide advantages.
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Additional features of CDMA in cellular phones:

The following features are unique to CDMA technology:
1. Universal frequency reuse
2. Fast and accurate power control
3. Rake receiver
4. Different types of hand off

Frequency reuse:
Each BTS in a CDMA network can use all available frequencies. Adjacent cells can transmit at the same frequency because code channels, not frequency channels, separate users. This feature of CDMA, called "frequency reuse of one, eliminates the need for frequency planning.
Fast and accurate power control:
Power control is a CDMA feature that enables mobiles to adjust the power at which they transmit. This ensures that the base station receives all signals at the appropriate power. The CDMA network independently controls the power at which each mobile transmit.

Rake receiver:

The rake receiver is an important feature of CDMA technology. Signals sent over the air can take a direct path to the receiver, or they can bounce off objects and then travel to the receiver. These different paths, called multi-paths, can result in the receiver getting several versions of the same signal but at slightly different times.

Types of handoff:
Handoff is the process of transferring a call from one cell to another. This is necessary to continue the call as the phone travels. CDMA has three primary types of handoff:
1. Hard
2. Soft
3. Idle
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Advantages of cell phones by implementing CDMA:
CDMA technology has numerous advantages including:

1. Coverage
2. Capacity
3.Clarity
4. Compatibility
5. Customer satisfaction

Conclusion:
Finally we conclude that CDMAâ„¢s popularity has steadily risen over the past few years and is now widely spreaded because CDMA provides crystal clear voice quality. It is one of the new techniques of Wireless communications. CDMA also provides enhanced services such as short messaging, e- mail, internet access etc. The CDMAâ„¢s future may have little to do with cell phones like, Wireless local loop, Globalstar satellite communications, wireless data services etc.


References:
1.Google search.
2.Mobile and Cellular communications by Lee

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INTRODUCTION
Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdma One and CDMA2000 (which are often referred to as simply CDMA), which use CDMA as an underlying channel access method. One of the basic 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 multiplexing.
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 signalling , 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 communicate with each other.
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 not other people. 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.
Chapter 2
CDMA
2.1 MULTIPLEXING:
One of the basic 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 multiplexing.
2.2 FDMA:
FDMA: Frequency Division Multiple Access (FDMA) is the most common analog system. It is a technique whereby spectrum is divided up into frequencies and then assigned to users. With FDMA, only one subscriber at any given time is assigned to a channel. FDMA has been used for first generation analog systems.
2.2.1 Features:
• In FDMA all users share the satellite simultaneously but each user transmits at single frequency.
• FDMA can be used with both analog and digital signal.
• FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
• FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
• Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.
• Each user transmits and receives at different frequencies as each user gets a unique frequency slot .
It is important to distinguish between FDMA and frequency-division duplexing (FDD). While FDMA allows multiple users simultaneous access to a certain system, FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a base-station). Furthermore, frequency-division multiplexing (FDM) should not be confused with FDMA. The former is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data page link layer.
2.2.2 Disadvantage:
Crosstalk which causes interference on the other frequency and may disrupt the transmission. One frequency is assigned to individual users ,it will remain idle, if that user is not sending any data then there is wastage of time.
2.3 TDMA:
TDMA was the next to come, it was the opening of digital world. In this technology whole the frequency band is assigned vertically in the form of fixed number of slots to individual users for a specific time. With TDMA channel bandwidths are wider than FDMA.
Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using his own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones.
2.3.1 TDMA characteristics:
• Shares single carrier frequency with multiple users
• Non-continuous transmission makes handoff simpler
• Slots can be assigned on demand in dynamic TDMA
• Less stringent power control than CDMA due to reduced intra cell interference
• Higher synchronization overhead than CDMA
• Advanced equalization may be necessary for high data rates if the channel is "frequency selective" and creates Inter symbol interference
• Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA
• Frequency/slot allocation complexity
• Pulsating power envelope: Interference with other devices
2.3.2 DISADVANTAGE:
TDMA is the dominant technology for the second generation mobile cellular networks.
TDMA decreases spectrum capacity , hence v go for CDMA.
2.4 CDMA:
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. CDMA increases spectrum capacity.
CDMA works on the principle of Spread spectrum. Here with the help of a CODE, data signal is spread like a noise like signal which is unable to detect by others it provide security .Since the spreaded signal is below the noise level , noise has no effect on the signal .its provides noise reduction. CDMA was developed during second world war in order to transmit signals in military.
Since it is suitable for encrypted transmissions, it has long been used for military purposes. CDMA increases spectrum capacity by allowing all users to occupy all channels at the same time. Transmissions are spread over the whole radio band, and each voice or data call are assigned a unique code to differentiate from the other calls carried over the same spectrum.
CDMA allows for a “soft hand-off”, which means that terminals can communicate with several base stations at the same time. The dominant radio interface for third-generation mobile, or IMT-2000, will be a wideband version of CDMA with three modes.
2.4.1 General Principles of CDMA:
Code division multiple access (CDMA) is a multiple access technique where different users share the same physical medium, that is, the same frequency band, at the same time. The main ingredient of CDMA is the spread spectrum technique, which uses high rate signature pulses to enhance the signal bandwidth far beyond what is necessary for a given data rate.
In a CDMA system, the different users can be identified and, hopefully, separated at the receiver by means of their characteristic individual signature pulses (sometimes called the signature waveforms), that is, by their individual codes. Nowadays, the most prominent applications of CDMA are mobile communication systems like cdmaOne (IS-95), UMTS or cdma2000.
To apply CDMA in a mobile radio environment, specific additional methods are required to be implemented in all these systems. Methods such as power control and soft handover have to be applied to control the interference by other users and to be able to separate the users by their respective codes. Basics of mobile radio networks are also presented, and methods of controlling the interference are discussed.