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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-ofbound 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 to the cell operators and their subscribers
                                       st

                 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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posted:6/20/2010
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