electronics by lakshmidhar

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									             CDMA – A REVOLUTIONARY TECHNOLOGY

CH. POORNIMA                                                                     S. SHRAVANI
                           Email: srikondashravani@yahoo.com
            Second year, B. Tech, Electronics and Instrumentation Engineering
                       Avanthi College of Engineering, Narsipatnam


       The world is demanding more from wireless communication technologies than ever
before. More people around the world are subscribing top wireless services. it is necessary to
study various characteristics of existing cellular system and how they support seamless mobile
communication to know how a wireless could behave in the real world.


       CDMA – Code Division Multiple Access is the multiple access system that is now
attracting the attention as a core technology for the next generation mobile communication
system. CDMA consistently provides better capacity for voice and data communications than
other commercial mobile technologies, allowing more subscribers to contact at any given time. It
is a “Spread Spectrum” technology, allowing many users to occupy the same time and
frequency allocations in a given band / space. As its name implies CDMA assigns unique codes
to each communication to differentiate it from others in the same spectrum. The provisions and
description of CDMA is made in the present paper.


       In a CDMA system, different spread-spectrum codes are selected and assigned to each
user, and multiple users share the same frequency. CDMA is a system based on spread spectrum
technology, which makes it less susceptible to the noise and interference by substantially
spreading over the bandwidth range of modulated signal. However, high efficiency of frequency
usage has been demonstrated in CDMA since the introduction of power control to adjust the
antenna emitting power so that the near-far problem could be solved.


Spread Spectrum:
       Spread spectrum is a transmission technique wherein data occupy a larger bandwidth than
necessary. Bandwidth spreading is accomplished before the transmission through the use of a
code that is independent of the transmitted data. The same code is used to demodulate the data at
the receiving end.


       Originally designed for military use to avoid jamming (interface created on purpose to
make a communication channel unusable), spread spectrum modulation is now used in personal
communication systems also due to its superior performance in an interference – dominated
environment.


Direct Sequence Spread Spectrum (DSSS):
       In the DS method, the radio signal is multiplied by a pseudorandom sequence bandwidth
is much greater than that of the signal itself, thereby spreading its bandwidth. This is a
modulation technique wherein as pseudorandom sequence directly phase modulates a (data
modulated) carrier, thereby increasing the bandwidth if the transmission and lowering the
spectral power density.


Frequency Hopping Spread spectrum (FHSS):
       In a FH method, a pseudorandom sequence is used to change the radio signal frequency,
across a broad frequency band in a random fashion. A spread-spectrum modulation technique
implies that the radio transmitter frequency hops from channel to channel in a predetermined but
pseudorandom manner. The RF signal is dehopped at the receiver end using a frequency
synthesizer controlled by a pseudorandom sequence generator synchronised to the transmitters
pseudorandom sequence generator. A frequency hopper may be fast hopped, where there are
multiple hope per data bit, or slow hopped, where there are multiple data bits per hop.


The CDMA Revolution:
       The great attraction of CDMA technology from the beginning has been the promise of
extraordinary capacity increase over narrowband multiple access wireless technologies. Simple
models suggest that the capacity improvement may be more that 20 times that of the existing
narrowband cellular standards, such as AMPS.
Background:
           An idealized multiple access mobile radio system consists of a family of base stations, or
‘cells’ geographically distributed over the service area, and mobile stations. We use the term
‘mobile’ generically to mean any subscriber station, whether it moves or not.


           Spectrum for mobile wireless is normally allocated in frequency division duplex (FDD)
paired bands. For the use of time division duplex (TDD), such operation inherently limits the
coverage area, and have not achieved widespread acceptance.


           Communication between base stations and mobile stations is established by a negotiation
upon call origination. Once communication is established between base and mobile movement of
the mobile is detected and the service is handed over from one base station to another. One cell
at a time services each mobile in the narrowband services. The concept of handoff is extended to
a multi-way simultaneous ‘soft’ handoff in the CDMA standards.


Cell Capacity:
           The load of a cell is typically characterized by the following two important random
parameters:
    (i)       The average number of MSs requesting the service (average call arrival rate).
    (ii)      Average length of time the MS requires the service (average holding time).


What is the point of using CDMA:
           It is unaffected by cell sectorisation.


Cell Sectoring :
           We have been primarily concentrating on what is known as omni directional antennas,
which allow transmission of radio signals with equal power strength in all directions. It is
difficult to design such antennas, and most of the time, an antenna covers an area of 60 degrees
or 120 degrees; these are called directional antennas, and cells served by them are called sectored
cells.
       The advantages of sectoring are that it requires coverage of a smaller area by each
antenna and hence lower power is required in transmitting radio signals. It also helps in
decreasing interference between cochannels. It is also observed that the spectrum efficiency of
the overall system is enhanced.


Frequency Reuse:
       Earlier cellular systems employed FDMA, and the range was limited to a radius of 2 to
20 KM. The same frequency band or channel used in a cell can be ‘reused’ in another cell as
long as the cells are for apart and the signal strength do not interface with each other. This, in
turn, enhances the available bandwidth of each cell. The distance between the two cells using the
same channel is known as the ‘reuse distance’.


Magic of CDMA:
       CDMA offers an answer to the capacity problem. The key to its high capacity is the use o
noise like carrier waves as was first suggested decades ago by Claude Shannon. Instead of
partitioning either spectrum or time into disjoint ‘slots’ each user is assigned a different instance
of the noise carrier. While those waveforms are not rigorously orthogonal, they are nearly so.
Practical application of this principle has always used digitally generated pseudo noise, rather
than true thermal noise. The basic benefits are preserved, and the transmitters and receivers are
simplified because large portions can be implemented using high density digital devices.


       The major benefit of noise like carriers is that the system sensitivity to interference is
fundamentally altered. Use of noise like carriers, with all users occupying the same spectrum,
makes the effective noise the sum of all other user signals. Te receiver correlates its input with
the desired noise carrier, enhancing the signal to noise ratio at the detector. The enhancement
overcomes the summed noise enough to provide an adequate SNR at the detector. Because the
interference is summed, the system is no longer sensitive to worst case interference, but rather to
average interference. Frequency reuse is universal, that is multiple users utilize each CDMA
carrier frequency.
Near – Far Problem:
       The near-far problem stems from a wide range of the signal levels received in wireless
mobile communication systems. The effect called adjacent channel interference, becomes
serious when the difference in the received signal strength is high. For this reason, the out-of-
bound radiation must be kept small. If power control technique is used, the system can tolerate
higher relative adjacent channel interference levels. The near-far problem, becomes more
important for CDMA systems where spread spectrum signals are multiplexed on the same
frequency using low cross correlation codes. The question is how to address the near-far
problem. One simple solution is power control.


Power control:
       Power control is simply the technique of controlling the transmit power so as to affect the
received power, and hence the CIR. For example, in free space, the propagation path loss
depends on the frequency of transmission and the distance between transmitter and receiver.


Advanced Mobile Phone Systems (AMPS):
       From a historical point of view we consider AMPS (Advanced Mobile Phone Systems)
as the first representative of wireless system.


       AMPS is the first generation cellular systems used in the United States. AMPS is the
first cellular phone technology created by AT&T bell labs with the idea of dividing the entire
service area into logical divisions called cells. Each cell is allocated one specific band in the
frequency spectrum.


Characteristics of AMPS:
       AMPS uses the frequency band from 824 MHz to 849 MHz for transmission from MSs to
the BS (reverse link or uplink) and frequency band between 869 MHz to 894 MHz form the BS
to MS (forward link or downlink).
General Working of AMPS:
           When a BS powers up, it has to know its surroundings before providing any service to the
MSs. Thus, it scans all the control channels and tunes itself to the strongest channel. Then it
sends its system parameters to all the MSs present in its service area. Each MS updates its SID
(system identification number) and establishes its paging channels only if its SID matches the
one transmitted by the BS. Then the NMS goes into the idle state, responding only to the beacon
and page signals.


           If a call is placed to an MS, the BS locates the MS through the IS-41 message exchanges.
Then the BS pages the MS with an order. If the MS is active, it responds to the page with its
MIN, ESN, and so on. The BS then sends the control information necessary for the call, for
which the MS has to confirm with a supervisory audio tone (SAT), indicating completion of a
call. If a call is to be placed fro man MS, the MS first sends the origination message to the BS on
the control channel. The BS passes this to the IS-41 and sends the necessary control signals and
orders to the MS.


IS – 41:
           IS – 41 is an interim standard that allows handoffs between BSs under control of different
MSCs and allows roaming of a MS outside its home system.
           ·   Registering of the NMS with a visiting MSC (Mobile Switching Centre)
           ·   Allowing for cell origination in a foreign MSc.
           ·   Allowing the MS to roam from one foreign system to another.


Advantages:
   (i)         To explore a reuse pattern, the frequency spectrum is divided among seven cells,
               improving the voice quality as each user is given a higher bandwidth.
   (ii)        One important aspect of AMPS is that it allows both cell sectoring and splitting.
   (iii)       AMPS is capable of supporting about 100,000 customers per city, and the system is
               aimed to reduce blocking probability to about 2% during busy hours.
2G – cdmaOne


cdmaOne:
       The family of IS-95 CDMA Technologies represents the end-to-end wireless system and
all the necessary specifications that govern its operation. cdmaOne provides a family of related
services including cellular, PCS and wireless.


IS-95A: The first CDMA cellular standard:
       IS 95A described the structure of the wideband 1.25 MHz CDMA channels, power
control, call processing, hand-offs, and registration for system operation. In addition to voice
services, many IS-95A operators provide circuit switched connection at 14.4 kbps.


2G – cdmaOne Advantages:
       When implemented in a cellular network, cdmaOne technology offers numerous benefits
                                                               st
to the cell operators and their subscribers when compared to 1 generation.
       ·   Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times
           that of system.
       ·   Improved call quality with better and more consistent sound as compared to AMPS
           system.
       ·   Simplified system planning though the use of same frequency in every sector.
       ·   Enhanced privacy
       ·   Improved coverage characteristics, allows for the possibility of fewer cell sites
       ·   Increased talk time for portables.
       ·   Bandwidth on demand


2G – cdmaOne Deployments:
       cdmaOne is the fastest growing 2G wireless technology reaching 100 million subscribers
after only of commercial deployment.
3G – CDMA2000


CDMA 2000 is 3G:
       3G is the term used to describe next generation mobile services which provide better
quality voice, high-speed internet and multimedia services. While there are many interpretations
of what 3G represents, the only definition accepted universally is the one published by the
International Telecommunication Union (ITU). ITU, working with industry bodies from around
the world, defines that approved technical requirements and standards as well as the use of
spectrum for 3G systems under the IMT-2000 (International Telecommunication Union 2000
program).


       The ITU requires that IMT-2000 (3G) networks, among other capabilities, deliver
improved system capacity and spectrum efficiency over the 2G systems and support data
services at minimum transmission rates of 144 kbps in mobile (outdoor) and 2 Mbps in fixed
(indoor) environments.


Advantages of CDMA2000:
       CDMA2000 is a very efficient and robust technology. Supporting both voice and data the
standard was devised and tested in various spectrum bands.


       The unique features, benefits and performance of CDMA2000 make it an excellent
technology for high-voice capacity and high-speed packet data.


Increased Voice Capacity:
       Voice is the major source of traffic and revenue for wireless operators, but packet data
will emerge in coming years as an important source of incremental revenue. CDMA2000
delivers the highest voice capacity and packet data throughput using the lease amount of
spectrum for the lowest cost.


       Voice capacity improvement in the forward link is attributed to faster power control,
lower code rates (¼ rate) and transmit diversity.
Increased Battery Life:
       CDMA2000 significantly enhances battery performance. Benefits include:
       ·   Quick paging channel operation
       ·   Improved reverse link performance
       ·   New common channel structure and operation
       ·   Reverse link gated transmission
       ·   New MAC(message authentication code) states for efficient and ubiquitous idle time
           operation


Applications Overview:
CDMA – offering a world of opportunities for multimedia services and applications.
       CDMA2000 was designed with the internet in mind, making it the ultimate platform on
which to build innovative applications. CDMA2000 users are experiencing a host of advanced
services including web browsing, e-commerce, MMS (multimedia messaging services).
Streaming video, games, enterprise solutions and email. To meet future demand for data
services, CDMA2000 operators are building their portfolios at a rapid pace, creating valuable
opportunities for application developers and content providers. CDMA2000 is an ideal platform
for building advanced applications.


Enterprise Solutions:
       The inherence thigh-speed data capabilities and security aspects of CDMA2000 make it
an ideal technology for workers to stay connected while mobile. Gaining access to email, the
internet and corporate intranets, via handsets or laptops equipped with modern cards are key
drivers for companies looking to take advantage of mobile high speed data services.


       Many CDMA2000 operators offer a wide range of services for the enterprise sector.
Some specific industries already making use of wireless data are public safety, insurance,
healthcare and pharmaceutical.
Multimedia Messaging Service (MMS):
           MMS enable graphics, pictures, video or music to be attached to the messages and sent to
mobile devices or computers. These services are widely deployed by CDMA2000 operators in
Korea and Japan and are getting momentum elsewhere around the world, including the US.
Carriers are seeing good uptake of MMS and enjoying the resulting revenue.


CDMA Operator Services:
           Operators have introduced a variety of applications which enable their consumers and
enterprises customers to access information, surf the web, download music and video, send
pictures and play games.


Conclusion:
   (i)        CDMA is the fastest growing wireless technology and it will continue to grow at a
              faster pace than any other technology
   (ii)       CDMA was designed with internet in mind, making it the ultimate platform on which
              to build innovative applications.
   (iii)      Developing countries like India are facing a demand for new business and residential
              telephone service – because of unique features and benefits of CDMA technology,
              more and more operators are choosing CDMA based wireless local loop networks to
              deliver these services.
   (iv)       Thus, the main use of CDMA is to reduce the signal to noise ratio and more number
              of users can login at same time in small frequency band.

								
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