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Satellite Communications A
Part 4
Access Schemes in Satellite Networks
-Professor Barry G Evans-
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.1
Satellite Network organisation
EARTH STATION TRAFFIC MATRIX:
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.2
Satellite Networks
-Fixed and Demand Assignment-
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SatComms A - part 4 - B G Evans
4.3
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SatComms A - part 4 - B G Evans
4.4
Basic multiple access techniques
FREQUENCY DIVISION MULTIPLE
ACCESS (FDMA)
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.5
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.6
FDMA Techniques
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SatComms A - part 4 - B G Evans
4.7
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.8
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.9
One carrier per station
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SatComms A - part 4 - B G Evans
4.10
FDMA throughput
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.11
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.12
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)
4.13
•
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
Burst Generation
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SatComms A - part 4 - B G Evans
4.14
Recovery of data messages
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SatComms A - part 4 - B G Evans
4.15
Frame Structure
-Example: INTELSAT/EUTELSAT
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SatComms A - part 4 - B G Evans
4.16
Synchronisation -Problem statement-
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SatComms A - part 4 - B G Evans
4.17
Synchronisation -Problem statement• Space-time graph
illustrating TDMA
synchronisation
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SatComms A - part 4 - B G Evans
4.18
Synchronisation
-Determination of ‘stat of local TDMA frame’ instant
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SatComms A - part 4 - B G Evans
4.19
TDMA synchronisation
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SatComms A - part 4 - B G Evans
4.20
Synchronisation of multiple beam TDMA systems
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.21
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.22
Frame efficiency
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SatComms A - part 4 - B G Evans
4.23
TDMA throughput
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SatComms A - part 4 - B G Evans
4.24
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.25
CDMA
-Spread spectrum communications
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SatComms A - part 4 - B G Evans
4.26
• 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).
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.27
Direct sequence systems
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SatComms A - part 4 - B G Evans
4.28
Direct sequence systems
-power spectrum of data and of spread signal-
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SatComms A - part 4 - B G Evans
4.29
Direct sequence systems
-practical receiver implementation-
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SatComms A - part 4 - B G Evans
4.30
CDMA
-Frequency hopping systems
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SatComms A - part 4 - B G Evans
4.31
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SatComms A - part 4 - B G Evans
4.32
Code generation
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SatComms A - part 4 - B G Evans
4.33
Code Synchronisation
-direct sequence systems-
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.34
Exercise- Capacity of a CDMA system
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SatComms A - part 4 - B G Evans
4.35
Exercise- Capacity of a CDMA system
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SatComms A - part 4 - B G Evans
4.36
Multiple access
-Comparison of multiple access techniques
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SatComms A - part 4 - B G Evans
4.37
Advantages/disadvantages of various
multiple access techniques
Type of multiple access
FDMA
TDMA
CDMA
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Advantages
Disadvantages
Network timing not required
Intermodulation products
cause degradation and poor
power utilisation
Compatible to existing
hardware
Uplink control power required
No mutual interference
between accesses
Network control required
Uplink power control not
needed
Large peak power
transmission for earth station
Maximum use of satellite
transponder power, most
efficient
Being digital in nature
interface with analogue
system is expensive
Network timing not required
Wide bandwidth per user
required
Anti-jamming capability
Strict code sync.needed
SatComms A - part 4 - B G Evans
4.38
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.39
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%
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.40
Random Access Schemes (3)
• SLOTTED-ALOHA confines transmission to slot boundaries
and needs time synchronisation
• Maximum throughput is increased to 36%
Channel throughput (S)
0.36
S-ALOHA
(S=Ge-G)
0.18
ALOHA
(S=Ge-2G)
Channel load (G)
• As system rapidly becomes unstable as collisions build up,
usual to operate below maxima
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.41
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 retransmit sub-packets that collide
• Increases throughput (0.37) independent of
message length
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.42
Random Access Schemes (5)
• Comparison of random access
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SatComms A - part 4 - B G Evans
4.43
Random Access Schemes (6)
• Comparison performances
S-ALOHA
S-R.ALOHA
ALOHA
Delay
DA-TDMA
Throughput
– For stream or file traffic need to use reservation
TDMA (DA-TDMA) schemes
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.44
Random Access Schemes (7)
• Reservation – TDMA
ith frame
R
S
F
i
ISFi
(i+1) frame
R
S
F
i+1
ISF(i+1)
• 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.
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.45
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
Autumn2004 © University of Surrey
SatComms A - part 4 - B G Evans
4.46