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EEM.scmA. Satellite Communications A Part 4. Access Schemes in Satellite Networks -Professor Barry G Evans-. Satellite Network organisation. EARTH STATION TRAFFIC MATRIX:. Satellite Networks -Fixed and Demand Assignment-. Basic multiple access techniques.

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satellite communications a part 4

EEM.scmA

Satellite Communications APart 4

Access Schemes in Satellite Networks-Professor Barry G Evans-

SatComms A - part 4 - B G Evans

satellite network organisation
Satellite Network organisation

EARTH STATION TRAFFIC MATRIX:

SatComms A - part 4 - B G Evans

basic multiple access techniques
Basic multiple access techniques

FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)

SatComms A - part 4 - B G Evans

various layers of multiple access
Various layers of multiple access
  • There are two layers of multiple access:
    • Access to any earth station by several users
    • Access to the satellite by all earth stations
      • At each layer, the access problem is solved using one or a combination of the basic multiple access techniques

SatComms A - part 4 - B G Evans

fdma techniques
FDMA Techniques

SatComms A - part 4 - B G Evans

fdma 1 carrier per link
FDMA-1 carrier per link-
  • With N earth stations:
    • Each earth station transmits (N-1) carriers to the other stations
    • The satellite repeater handles N(N-1) carriers

SatComms A - part 4 - B G Evans

fdma 1 carrier per station
FDMA-1 carrier per station-
  • With N earth stations
    • Each earth station transmits to one carrier modulated by a multiplex of the signals to the other earth stations
    • The satellite repeater handles N carriers

SatComms A - part 4 - B G Evans

one carrier per station
One carrier per station

SatComms A - part 4 - B G Evans

fdma throughput
FDMA throughput

SatComms A - part 4 - B G Evans

fdma summary
FDMA Summary
  • Access Channel: give frequency band
  • Advantages
    • Use of existing hardware to a greater extent than other techniques
    • Network timing not required
  • Disadvantages
    • As the number of accesses increases, intermodulation noise reduces the usable repeater output power (TWT back-off). Hence there is a loss of capacity relative to single carrier/transponder capacity
    • The frequency allocation may be difficult to modify
    • Uplink power coordination is required

SatComms A - part 4 - B G Evans

tdma satellite system
TDMA Satellite System
  • In a TDMA system, each earth station transmits traffic bursts, synchronized so that they occupy ASSIGNED NON-OVERLAPPING time slots. Time slots are organised within a periodic structure called TIME FRAME.
  • A burst is received by all stations in the downlink beam and any station can extract its traffic from any of the bursts
  •  a BURST = link from one station to several stations (TDMA=one-link-per-station scheme)

SatComms A - part 4 - B G Evans

burst generation
Burst Generation

SatComms A - part 4 - B G Evans

recovery of data messages
Recovery of data messages

SatComms A - part 4 - B G Evans

frame structure example intelsat eutelsat
Frame Structure-Example: INTELSAT/EUTELSAT

SatComms A - part 4 - B G Evans

synchronisation problem statement
Synchronisation -Problem statement-

SatComms A - part 4 - B G Evans

synchronisation problem statement1
Synchronisation -Problem statement-
  • Space-time graph illustrating TDMA synchronisation

SatComms A - part 4 - B G Evans

synchronisation determination of stat of local tdma frame instant
Synchronisation-Determination of ‘stat of local TDMA frame’ instant

SatComms A - part 4 - B G Evans

tdma synchronisation
TDMA synchronisation

SatComms A - part 4 - B G Evans

synchronisation of multiple beam tdma systems
Synchronisation of multiple beam TDMA systems

SatComms A - part 4 - B G Evans

open loop synchronisation
Open loop synchronisation
  • Measurements of round trip delay are performed by three ranging stations using closed loop synchronization.
  • Satellite position is derived by triangulation and range from each ordinary station to satellite is calculated at reference station.
  • Satellite-to-station range information and frame timing is distributed to all ordinary stations by reference station

SatComms A - part 4 - B G Evans

frame efficiency
Frame efficiency

SatComms A - part 4 - B G Evans

tdma throughput
TDMA throughput

SatComms A - part 4 - B G Evans

tdma summary
TDMA summary
  • Access Channel: given time slot within time frame
  • Advantages
    • Digital signalling provides easy interfacing with developing digital networks on ground
    • Digital circuitry has decreasing cost
    • Higher throughput compared to FDMA when number of accesses is large
  • Disadvantages
    • Stations transmit high bit rate bursts, requiring large peak power
    • Network control is required
      • Generation and distribution of burst time plans to all traffic stations
      • Protocols to establish how stations enter the network
      • Provision of redundant reference stations with automatic switchover to control the traffic stations
      • Means for monitoring the network

SatComms A - part 4 - B G Evans

cdma spread spectrum communications
CDMA-Spread spectrum communications

SatComms A - part 4 - B G Evans

slide27
Transmitter spreads baseband signal from bandwidth W to B.
  • B/W = spreading factor (100 to 1 000 000).
  • Receiver despreads only signal with proper address.
  • Received signals with other addresses and jammer are spread by receiver and act as noise.
  • Addresses are periodic binary sequences that either modulate the carrier directly (DIRECT SEQUENCE SYSTEMS) or change the frequency state of the carrier (FREQUENCY HOPPING SYSTEMS).

SatComms A - part 4 - B G Evans

direct sequence systems
Direct sequence systems

SatComms A - part 4 - B G Evans

cdma frequency hopping systems
CDMA-Frequency hopping systems

SatComms A - part 4 - B G Evans

code generation
Code generation

SatComms A - part 4 - B G Evans

code synchronisation direct sequence systems
Code Synchronisation-direct sequence systems-

SatComms A - part 4 - B G Evans

exercise capacity of a cdma system
Exercise- Capacity of a CDMA system

SatComms A - part 4 - B G Evans

exercise capacity of a cdma system1
Exercise- Capacity of a CDMA system

SatComms A - part 4 - B G Evans

advantages disadvantages of various multiple access techniques
Advantages/disadvantages of various multiple access techniques

SatComms A - part 4 - B G Evans

random access schemes 1
Random Access Schemes (1)
  • FDMA/TDMA/CDMA fixed access have been designed for circuit/stream traffic
  • Bursty data traffic –e.g. packets- more efficiently dealt with via random access schemes
  • In random access there is no permanent assignments –resource is allocated when needed on a random basis

SatComms A - part 4 - B G Evans

random access schemes 2
Random Access Schemes (2)
  • Simplest system is ALOHA –transmit packets and if collide, retransmit with random time difference.
  • Performance via ‘throughput versus delay’
  • Throughput = N  L/R
    • N= no transmissions
    • = packet generation rate (S-1)
    • L= packet length (bits)
    • R= transmission bit rate (bits/s)
  • ALOHA doesn’t need synchronisation
  • Maximum throughput 18%

SatComms A - part 4 - B G Evans

random access schemes 3

0.36

S-ALOHA(S=Ge-G)

Channel throughput (S)

0.18

ALOHA(S=Ge-2G)

Channel load (G)

Random Access Schemes (3)
  • SLOTTED-ALOHA confines transmission to slot boundaries and needs time synchronisation
  • Maximum throughput is increased to 36%
  • As system rapidly becomes unstable as collisions build up, usual to operate below maxima

SatComms A - part 4 - B G Evans

random access schemes 4
Random Access Schemes (4)
  • For variable length messages need to employ more complex scheme e.g. slotted reject ALOHA
  • Use multi-packet message and only re-transmit sub-packets that collide
  • Increases throughput (0.37) independent of message length

SatComms A - part 4 - B G Evans

random access schemes 5
Random Access Schemes (5)
  • Comparison of random access

SatComms A - part 4 - B G Evans

random access schemes 6

S-ALOHA

S-R.ALOHA

ALOHA

DA-TDMA

Delay

Throughput

Random Access Schemes (6)
  • Comparison performances
    • For stream or file traffic need to use reservation TDMA (DA-TDMA) schemes

SatComms A - part 4 - B G Evans

random access schemes 7
Random Access Schemes (7)
  • Reservation – TDMA
  • RSF= Reservation Sub Frame
  • ISF = Information Sub Frame
  • RSF used to book space in next ISF frame according to demand
  • RSF can be operated in fixed TDMA, ALOHA, S-ALOHA, etc.

SatComms A - part 4 - B G Evans

random access schemes 8
Random Access Schemes (8)
  • Summary
    • Select RA scheme for traffic type and delay/throughput ( number of tx’s)
    • Take care to achieve stability
    • ALOHA: short bursty traffic
    • S-ALOHA: short bursty traffic –better throughput
    • S-R.ALOHA: variable length messages
    • RA-TDMA: stream or file transfers

SatComms A - part 4 - B G Evans