WIDEBAND CODE DIVISION MULTIPLE ACCESS

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WIDEBAND CODE DIVISION MULTIPLE ACCESS THE
CAPACITY IN CODE DIVISION MULTIPLE ACCESS

• Presented by
• Maheshwarnath Behary
• Assisted by
• Vishwanee Raghoonundun
• Y. Koteswara Rao
• MSc Computer Networks
• Middlesex University

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Introduction

• Definition of WCDMA
• Basics of the WCDMA
• Features of the WCDMA
• Advantages of using CDMA system
• CDMA capacity
• Single cell system
• Multiple cell system
• Techniques for improving channel capacity
• Enhancements in WCDMA
• Reference

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Definition of WCDMA

• WCDMA A high speed 3G mobile wireless technology
  with capacity to offer higher data rate than
  CDMA.
• WCDMA has the capacity to easily handle
  bandwidth-intensive applications such as video,
  data, and image transmission necessary for mobile
  internet services.
• WCDMA was adopted as a standard by the
  International Mobile Telecommunications-2000 (IMT
  2000) with the intention to create a global
  standard for real time multimedia services and
  international roaming.
• WCDMA is direct spread technology, which means
  that it will spread its transmissions over a
  wide, 5MHz carrier.
• WCDMA is the leading 3G wireless standard in the
  world today.

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BASICS OF THE WCDMA

• WCDMA users share same radio resources whereas
  TDMA or FDMA users have their own radio resources
  allocated.
• User signals are differentiated based on codes.

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FEATURES OF THE WCDMA
(1) The actual data rate achieved is inversely
proportional to the coverage and mobility of the
user (2) Larger bandwidth of 10 and 20 MHz is
proposed for higher data rates. (3) Spreading
factor tb / tc. (4) Scrambling codes permit the
handling of multiple users in the network(using
the same frequency). (5) Frequency Division
Duplex used as there are two different frequency
bands, one for the uplink and the second one for
the downlink.


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ADVANTAGES OF USING CDMA SYSTEM

• A number of advantages are
• Low power spectral density. As the signal is
  spread over a large frequency-band, the Power
  Spectral Density is getting very small, so other
  communications systems do not suffer from this
  kind of communications.
• Interference limited operation. In all situations
  the whole frequency-spectrum is used.
• Privacy due to unknown random codes. The applied
  codes are unknown to a hostile user. This means
  that it is hardly possible to detect the message
  of another user.
• Random access possibilities. Users can start
  their transmission at any arbitrary time.
• Good anti-jam performance.

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CDMA CAPACITY

• Blocking probability is used to measure the
  capacity of any multiple access system.
• The capacity of CDMA systems mainly depends on
  interference. Any reduction in interference
  directly results in the increase in the capacity
  of the system.

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SINGLE CELL SYSTEM

• In a single cell system each user occupies entire
  frequency spectrum employing direct sequence
  spread spectrum waveform.
• Power control is the basic requirement and
  affects the capacity of the system greatly.
• Forward link A pilot signal is included which
  is for initial power control by the mobile to
  adjust the power inversely to the total power its
  receives.
• Reverse link Here non coherent reception
  without any pilot signal is used.
• In reverse link no pilot signal is required
  because of
• power efficiency considerations.
• an independent pilot would be needed for each
  reverse signal.
• Since the forward link combines the signals
  synchronously, its performance is much superior
  to reverse link.

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MULTIPLE CELL SYSTEM

• In multiple cell system power of each subscriber
  unit is controlled by the base station of their
  own cell.
• The maximum pilot power among cell sites decides
  which cell site the subscriber will join.
• The interference to a subscriber in another cell
  site is proportional to the attenuation in the
  path to the subscribers cell and inversely to
  the attenuation from the interfering user to his
  own cell site.
• Forward link Power control is essential since
  signals in adjacent cells fade independently and
  can cause interference in the near cell
  boundaries.
• Power control in the forward link is the
  allocation of power to each subscriber according
  to each subscribers needs.

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MULTIPLE CELL SYSTEM(CTND)

• REVERSE LINK
• The power control to a given mobile is exercised
  by the cell whose pilot signal power is maximum
  to that mobile.
• It follows that if the path loss is due to
  distance from cell site, then the mobile would be
  power controlled by the nearest cell site.
• The path loss between the subscriber and the
  cell site is proportional to r, the distance
  from the subscriber to cell site.

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TECHNIQUES FOR IMPROVING CHANNEL CAPACITY

• SECTORIZATION
• Use of directional antennas for transmission and
  reception.
• The interference seen by directional antenna is
  one third of those seen by omni directional
  antennas.
• This interference suppression improves the
  capacity of the system.
• VOICE ACTIVITY MONITORING
• The digital vocoders monitor the voice activity
  of each user.
• Each user is active only 35-40 of the time and
  the transmission can be suppressed during the
  quiet periods.
• Voice activity monitoring reduces interference by
  a value proportional to voice activity factor
  which is usually 3/8.

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ENHANCEMENTS IN WCDMA

• Superior voice and data service quality.
• Smooth evolution from GSM to WCDMA
• Open standardisation process, global standard
• Global markets and economies of scale
• Service portability and roaming

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References

• 1 fhttp//cas.et.tudelft.nl/glas/ssc/techn/tech
  niques.html
• 2 www.ittc.ukans.edu
• 3 http//www.umts-forum.org/servlet/dycon/ztumts
  /umts
• 4http//www.telecommagazine.com/default.asp?jour
  nalid2funcarticlespage0208i10year2002month
  8
• 5http//people.deas.harvard.edu/jones/cscie129/
  nu_lectures/lecture7/cdma/cdma.html
• 6 http//www.ittc.ukans.edu/krishk/Documents/cd
  ma.pdf

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END OF PRESENTATION

• THANK YOU FOR YOUR KIND ATTENTION