satcommsa2004part4

Satellite Communications APart 4Access Schemes in Satellite Networks-Professor Barry G Evans-EEM.scmA

SatComms A - part 4 - B G Evans

Satellite Network organisation

EARTH STATION TRAFFIC MATRIX:


Satellite Networks-Fixed and Demand Assignment-


Basic multiple access techniques

FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)


Various layers of multiple accessThere are two layers of multiple access:Access to any earth station by several usersAccess 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


FDMA Techniques


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


FDMA-1 carrier per station-With N earth stationsEach earth station transmits to one carrier modulated by a multiplex of the signals to the other earth stationsThe satellite repeater handles N carriers


215.bin

One carrier per station


FDMA throughput


FDMA SummaryAccess Channel: give frequency bandAdvantagesUse of existing hardware to a greater extent than other techniquesNetwork timing not requiredDisadvantagesAs 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 capacityThe frequency allocation may be difficult to modifyUplink power coordination is required


TDMA Satellite SystemIn 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)


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 synchronisationMeasurements 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 summaryAccess Channel: given time slot within time frameAdvantagesDigital signalling provides easy interfacing with developing digital networks on groundDigital circuitry has decreasing costHigher throughput compared to FDMA when number of accesses is largeDisadvantagesStations transmit high bit rate bursts, requiring large peak powerNetwork control is requiredGeneration and distribution of burst time plans to all traffic stationsProtocols to establish how stations enter the networkProvision of redundant reference stations with automatic switchover to control the traffic stationsMeans 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 accessAdvantagesDisadvantagesFDMANetwork timing not requiredIntermodulation products cause degradation and poor power utilisationCompatible to existing hardwareUplink control power requiredTDMANo mutual interference between accessesNetwork control requiredUplink power control not neededLarge peak power transmission for earth stationMaximum use of satellite transponder power, most efficientBeing digital in nature interface with analogue system is expensiveCDMANetwork timing not requiredWide bandwidth per user requiredAnti-jamming capabilityStrict code sync.needed


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


Random Access Schemes (2)Simplest system is ALOHA transmit packets and if collide, retransmit with random time difference.Performance via throughput versus delayThroughput = N L/RN= no transmissions= packet generation rate (S-1)L= packet length (bits)R= transmission bit rate (bits/s)ALOHA doesnt need synchronisationMaximum throughput 18%


Random Access Schemes (3)SLOTTED-ALOHA confines transmission to slot boundaries and needs time synchronisationMaximum throughput is increased to 36%

As system rapidly becomes unstable as collisions build up, usual to operate below maxima


Random Access Schemes (4)For variable length messages need to employ more complex scheme e.g. slotted reject ALOHAUse multi-packet message and only re-transmit sub-packets that collideIncreases throughput (0.37) independent of message length


Random Access Schemes (5)Comparison of random access


Random Access Schemes (6)Comparison performances

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


Random Access Schemes (7)Reservation TDMA

RSF= Reservation Sub FrameISF = Information Sub FrameRSF used to book space in next ISF frame according to demandRSF can be operated in fixed TDMA, ALOHA, S-ALOHA, etc.

ith frame(i+1) frameRS F iISFiR S F i+1ISF(i+1)


Random Access Schemes (8)SummarySelect RA scheme for traffic type and delay/throughput ( number of txs)Take care to achieve stabilityALOHA: short bursty trafficS-ALOHA: short bursty traffic better throughputS-R.ALOHA: variable length messagesRA-TDMA: stream or file transfers


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satcommsa2004part4