Satellite Communications-II Dr. Nasir D. Gohar

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

Satellite Communications-II A PRE-ASSIGNED/DEDICATED SYSTEM 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 MHz which is mostly wasted

Satellite Communications-II ANIK-E FREQUENCY & POLARIZATION PLAN 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

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

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

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.

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

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

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

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

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

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

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

Satellite Communications-II FDMA and TDMA – A Comparison 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

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

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

Satellite Communications-II  Correlation and Cross-Correlation Back

Wayne Tomasi-Ch 15NDG Notes19

Wayne Tomasi-Ch 15NDG Notes20

Wayne Tomasi-Ch 15NDG Notes21 Back

Wayne Tomasi-Ch 15NDG Notes22

Wayne Tomasi-Ch 15NDG Notes23 Satellite Communications-II  FH-Spread Spectrum

Wayne Tomasi-Ch 15NDG Notes24

Wayne Tomasi-Ch 15NDG Notes25 Back Satellite Communications-II  DS-Spread Spectrum

Wayne Tomasi-Ch 15NDG Notes26

Wayne Tomasi-Ch 15NDG Notes27

Wayne Tomasi-Ch 15NDG Notes28 Back

Wayne Tomasi-Ch 15NDG Notes29

Wayne Tomasi-Ch 15NDG Notes30

Wayne Tomasi-Ch 15NDG Notes31 Back

Wayne Tomasi-Ch 15NDG Notes32

Wayne Tomasi-Ch 15NDG Notes33

Wayne Tomasi-Ch 15NDG Notes34

Wayne Tomasi-Ch 15NDG Notes35

Wayne Tomasi-Ch 15NDG Notes36 Satellite Communications-II  SATELLITE RADIO NAVIGATION  Navstar GPS