CDMA in Satellite System

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CDMA in Satellite System

• Adeel Asif

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Outline

• Basic Concepts
• Cellular System
• Frequency reuse
• Hand Off
• Satellite Communications System
• Why Satellite Communications System
• Multiple Access Schemes
• CDMA Core Concepts
• Advantages of CDMA
• Challenges of CDMA
• Conclusion

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Basic Concepts Cellular System

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Basic Concepts - Frequency Reuse
7 cell re-use pattern
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Basic Concepts Hand Off
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Satellite System

• Traditionally
• weather satellites
• satellites for navigation
• Telecommunication
• Broadcast service over a wide area
• global telephone connections
• connections for communication in remote places or
  underdeveloped areas
• ? Satellite mobile communication for 3rd
  generation systems

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Why Satellite Communications System ?

• Main Advantages
• Extend capability of existing terrestrial
  cellular system
• Coverage in remote locations
• Large coverage area
• Seamless service to subscriber in any part of the
  world
• Challenges
• Propagation delay
• Attenuation
• Low Spectral efficiency

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Typical satellite systems
Inter Satellite Link (ISL)
Mobile User Link (MUL)
MUL
Gateway Link (GWL)
GWL
small cells (spotbeams)
base station or gateway
footprint
GSM
PSTN
ISDN
User data
PSTN Public Switched Telephone Network
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Multiple Access Schemes for Satellite
Communications

• A multiple access scheme is expected to provide
  users to simultaneously access a satellite
  terminal efficiently
• In a satellite communications system, designing
  a multiple access scheme is one of the most
  challenging issues

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Frequency Division Multiple Access (FDMA) assigns
individual channels (i.e frequencies) to
individual users
Multiple Access Schemes
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Multiple Access Schemes

• Time Division Multiple Access (TDMA)
• The spectrum is divided into time slots, in each
  slot only one user can transmit or receive
• TDMA shares a single carrier with several users

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Multiple Access Schemes

• Code Division Multiple Access (CDMA)
• All the users in this system, use the same
  carrier frequency and may transmit simultaneously
• Lets take a closer look at it!

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Multiple Access Schemes
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CDMA

• Spread Spectrum Systems
• Wireless Communication technique that uses
  transmission bandwidth substantially larger than
  the signal bandwidth
• Many users can simultaneously use the same
  bandwidth without significantly interfering with
  one another
• There are many applications including Global
  Positioning System, Wireless LAN e.g. Bluetooth
• CDMA
• In CDMA user data is multiplied with a
  pseudo-noise (PN) sequence called the spreading
  signal
• This causes the spectrum to be spread to several
  orders of magnitude greater than the minimum
  signal bandwidth

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PN Sequence

• PN sequence is a binary sequence that appears
  random but can be reproduced in a deterministic
  manner by intended receiver
• The rate of the PN code called the chip rate,
  must be much higher than the rate of the
  information signal

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How it works !

• The information signal is demodulated at the
  receiver by cross-correlation with a locally
  generated replica of the users PN sequence.
• The signal is de-spread and is restored to the
  original signal.
• Cross-correlation with PN sequence of other users
  results in a very small noise at the receiver.

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Why CDMA (I) ?

• Exploitation of multipath RAKE Receiver
• Satellite path diversity exploitation for
• Improved quality of service
• Fading effects mitigation

• Immunity from jamming signals, external
  interference and multipath distortion
• Full frequency reuse easing resource allocation

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Why CDMA (II) ?

• Soft handoff Improved quality of Service

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Challenges

• Nonlinearity in satellite communications causes
  degradation in performance
• Power Control - Near / Far Effect
• Open loop power control
• Closed loop power control
• Synchronization

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Conclusion

• Satellite Communications is required to extend
  the capabilities of cellular systems in 3rd
  Generation Systems
• CDMA is preferred multiple access scheme because
  of its advantages like diversity and multipath
  exploitation, soft handoff characteristics
• CDMA however, faces challenges like performance
  degradation due to nonlinearity in satellite
  system and handling of power control and
  synchronization

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