access techniques

Satellite Communications

Access Schemes in Satellite Networks

EARTH STATION TRAFFIC MATRIX:

Satellite Network organisation

Satellite Networks-Fixed and Demand Assignment-

Basic multiple access techniques

FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)

• 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

Various layers of multiple access

FDMA Techniques

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

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

One carrier per station

FDMA throughput

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

• 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)

TDMA Satellite System

Burst Generation

Recovery of data messages

Frame Structure-Example: INTELSAT/EUTELSAT

Synchronisation -Problem statement-

Synchronisation -Problem statement-

• Space-time graph illustrating TDMA synchronisation

Synchronisation-Determination of ‘stat of local TDMA frame’ instant

TDMA synchronisation

Synchronisation of multiple beam TDMA systems

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

Frame efficiency

TDMA throughput

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

CDMA-Spread spectrum communications

• 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).

Direct sequence systems

Direct sequence systems-power spectrum of data and of spread signal-

Direct sequence systems-practical receiver implementation-

CDMA-Frequency hopping systems

Code generation

Code Synchronisation-direct sequence systems-

Exercise- Capacity of a CDMA system

Multiple access-Comparison of multiple access techniques

Advantages/disadvantages of various multiple access techniques

Type of multiple access Advantages Disadvantages

FDMA Network timing not required Intermodulation products cause degradation and poor power utilisation

Compatible to existing hardware

Uplink control power required

TDMA 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

CDMA Network timing not required Wide bandwidth per user required

Anti-jamming capability Strict code sync.needed

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


• 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%


• 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

Cha

nnel

thro

ughp

ut (S

)

Channel load (G)

0.18

0.36S-ALOHA(S=Ge-G)

ALOHA(S=Ge-2G)


• 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


• Comparison of random access


• Comparison performances

– For stream or file traffic need to use reservation TDMA (DA-TDMA) schemes

ALOHAS-ALOHA S-R.ALOHA

DA-TDMA

Del

ay

Throughput


• 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.

ith frame (i+1) frameRS Fi

ISFiR S F i+1

ISF(i+1)


• 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

access techniques

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