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Satellite Communications-II

Satellite Communications-II. Dr. Nasir D. Gohar. Satellite Communications-II. WHY MULTIPLE ACCESS? Users/Earth Stations Share the Transmission Resource i.e. Radio Spectrum

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Satellite Communications-II

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  1. Satellite Communications-II Dr. Nasir D. Gohar

  2. Satellite Communications-II WHY MULTIPLE ACCESS? Users/Earth Stations Share the Transmission Resource i.e. Radio Spectrum Aim is to develop Efficient Techniques that Maximize System Capacity thru Dynamic Resource Allocation and Spectrum Reuse Simple FDM/FM Satellite Systems become Inefficient is BW Utilization and Economically Impractical Pre-Assigned or Demand-Assigned Channel Allocation In case of Pre-Assigned System, a given number of available voice-band channels from each earth station are assigned to a dedicated destination….Some-times wastage of Precious BW Resource In case of Demand-Assigned System, Resources allocation is on need basis, versatile and efficient usages of Radio Spectrum, but a Complex Mechanism is required at all Earth Stations/Users

  3. Satellite Communications-II • Each earth station requires two dedicated pairs of Tx/Rx frequencies to communicate with any other station • As many communication partners, same number of transponders(RF-RF duplex translator/repeater) • Transponder BW 36 MHzwhich is mostly wasted A PRE-ASSIGNED/DEDICATED SYSTEM

  4. Satellite Communications-II • Domsat operated by Telsat, Canada • Group A (12 Radio Ch) use H Polarization • Group B (12 Radio Ch) use V Polarization • Radio Ch. BW=36 MHz • Inter-Channel Guard band =4MHz • 10 MHz band on each side extra to avoid Inter-System Interference • Total BW = 500 MHz ANIK-E FREQUENCY & POLARIZATION PLAN

  5. Satellite Communications-II TWO TYPES OF DUPLEXING A Duplex Link allows simultaneous transmission of information in both directions Frequency Division Duplex (FDD) – two frequency channels for each up/down link i.e. one frequency channel for Tx and other for Rx Time Division Duplex (TDD) – a single frequency channel shared by both Tx and Rx

  6. Satellite Communications-II THREE MULTIPLE ACCESS TECHNIQUES Satellite Multiple Accessing/Destination means more than one users/earth stations can access to one or more Radio Channels (Transponders) on board FDMA TDMA CDMA FH-CDMA DS-CDMA

  7. Satellite Communications-II CATEGORIZATION OF MA TECHNIQUES Narrow-band Systems – Total system BW is divided into a large number of narrow-band radio channels FDMA/FDD –Each user is assigned two narrow-band radio channels, one for up-link and other for down-link TDMA –When each narrow-band radio channel is divided into number of time slots, and each user is assigned two time slots, one for Tx and other for Rx. Hybrid TDMA/FDMA or TDMA/FDD – when two slots {same position in time) of the user are allocated in two different narrow-band radio channels TDMA/TDD – when two slots of the user are allocated in the same narrow-band radio channel Wide-band Systems – Total spectrum/BW is shared by all users all the time Wide-band TDMA, each user is allocated two time slots to use the entire spectrum. TDMA/FDD and TDMA/TDD both configurations are possible. Wide-band CDMA, entire spectrum is used by each user all the time but with use of orthogonal codes. CDMA/FDD and CDMA/TDMA both configurations are possible.

  8. Satellite Communications-II FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)-THE CONCEPT Given Radio Spectrum (RF BW) is divided into a large number of narrow-band radio channels called sub-divisions Each sub-division has its own sub-carrier called IF Carrier A control mechanism is required to ensure that each user/earth station uses only its own assigned sub-division at any time SCPC- a system where each sub-division carries only one 4-kHz voice channel MCPC-a system where several speech/voice band channels are frequency-division multiplexed to form a group, super-group or even master-group FDM/FM/FAMA- a system using a fixed MCPC format over a long period of time DAMA- a system that allows all users continuous and equal access to the entire transponder BW by assigning carrier frequencies on a temporary basis as per demand

  9. Satellite Communications-II FDMA-Examples Intelsat IV and V used FDMA/FM/FAMA system SPADE DAMA Satellite System – SPADE ES Tx

  10. Satellite Communications-II FDMA-Examples SPADE DAMA Satellite System – Carrier Frequency Assignment

  11. Satellite Communications-II FDMA-Examples SPADE DAMA Satellite System – Frame Structure of Common Signaling Channel (CSC)

  12. Satellite Communications-II TIME DIVISION MULTIPLE ACCESS (TDMA)-The Basic Concept

  13. Satellite Communications-II TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT Primary Multiplex Frame Block Diagram

  14. Satellite Communications-II TIME DIVISION MULTIPLE ACCESS (TDMA)-The CEPT Primary Multiplex Frame Timing Sequence

  15. Satellite Communications-II FDMA and TDMA – A Comparison In TDMA, only one carrier from any of several Earth Stations is present at Satellite at any time FDMA requires each Earth Station capable of transmitting and receiving on multitude of carrier frequencies (FDMA/DAMA) TDMA is more amenable to digital transmission (storage, processing, rate-conversion etc.) than FDMA TDMA requires precise synchronization

  16. Satellite Communications-II THREE MULTIPLE ACCESS TECHNIQUES Code Division Multiple Access (CDMA)-The Concept No restrictions on any user/earth station on time and frequency slots usages, rather any user can use allocated BW or all system BW at any time, however, using a special chip code to spread its low-bandwidth signal over the entire allocated spectrum… Spread Spectrum Multiple Access

  17. Satellite Communications-II Code Division Multiple Access (CDMA)-The Concept (Cont’d) Types Of CDMA Orthogonal Codes Correlation and Cross-Correlation How Spreading and De-Spreading is done? Processing Gain, G = Chip Rate/Date Rate Next

  18. Satellite Communications-II Correlation and Cross-Correlation Back

  19. NDG Notes

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  21. Back NDG Notes

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  23. Satellite Communications-II • FH-Spread Spectrum NDG Notes

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  25. Satellite Communications-II • DS-Spread Spectrum Back NDG Notes

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  28. Back NDG Notes

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  31. Back NDG Notes

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  36. Satellite Communications-II • SATELLITE RADIO NAVIGATION • Navstar GPS NDG Notes

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