EEM.scmA 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- Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.3 Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.14 Recovery of data messages Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.15 Frame Structure -Example: INTELSAT/EUTELSAT Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.16 Synchronisation -Problem statement- Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.17 Synchronisation -Problem statement• Space-time graph illustrating TDMA synchronisation Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.18 Synchronisation -Determination of ‘stat of local TDMA frame’ instant Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.19 TDMA synchronisation Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.23 TDMA throughput Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.28 Direct sequence systems -power spectrum of data and of spread signal- Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.29 Direct sequence systems -practical receiver implementation- Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.30 CDMA -Frequency hopping systems Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.31 Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.32 Code generation Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.35 Exercise- Capacity of a CDMA system Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.36 Multiple access -Comparison of multiple access techniques Autumn2004 © University of Surrey SatComms A - part 4 - B G Evans 4.37 Advantages/disadvantages of various multiple access techniques Type of multiple access FDMA TDMA CDMA Autumn2004 © University of Surrey 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 Autumn2004 © University of Surrey 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
