Satellite Communications A Part 4

1. EEM.scmA Satellite Communications APart 4 Access Schemes in Satellite Networks-Professor Barry G Evans- SatComms A - part 4 - B G Evans

2. Satellite Network organisation EARTH STATION TRAFFIC MATRIX: SatComms A - part 4 - B G Evans

3. Satellite Networks-Fixed and Demand Assignment- SatComms A - part 4 - B G Evans

4. SatComms A - part 4 - B G Evans

5. Basic multiple access techniques FREQUENCY DIVISION MULTIPLE ACCESS (FDMA) SatComms A - part 4 - B G Evans

6. 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

7. FDMA Techniques SatComms A - part 4 - B G Evans

8. 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

9. 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

10. One carrier per station SatComms A - part 4 - B G Evans

11. FDMA throughput SatComms A - part 4 - B G Evans

12. 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

13. 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

14. Burst Generation SatComms A - part 4 - B G Evans

15. Recovery of data messages SatComms A - part 4 - B G Evans

16. Frame Structure-Example: INTELSAT/EUTELSAT SatComms A - part 4 - B G Evans

17. Synchronisation -Problem statement- SatComms A - part 4 - B G Evans

18. Synchronisation -Problem statement- • Space-time graph illustrating TDMA synchronisation SatComms A - part 4 - B G Evans

19. Synchronisation-Determination of ‘stat of local TDMA frame’ instant SatComms A - part 4 - B G Evans

20. TDMA synchronisation SatComms A - part 4 - B G Evans

21. Synchronisation of multiple beam TDMA systems SatComms A - part 4 - B G Evans

22. 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

23. Frame efficiency SatComms A - part 4 - B G Evans

24. TDMA throughput SatComms A - part 4 - B G Evans

25. 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

26. CDMA-Spread spectrum communications SatComms A - part 4 - B G Evans

27. 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

28. Direct sequence systems SatComms A - part 4 - B G Evans

29. Direct sequence systems-power spectrum of data and of spread signal- SatComms A - part 4 - B G Evans

30. Direct sequence systems-practical receiver implementation- SatComms A - part 4 - B G Evans

31. CDMA-Frequency hopping systems SatComms A - part 4 - B G Evans

32. SatComms A - part 4 - B G Evans

33. Code generation SatComms A - part 4 - B G Evans

34. Code Synchronisation-direct sequence systems- SatComms A - part 4 - B G Evans

35. Exercise- Capacity of a CDMA system SatComms A - part 4 - B G Evans

36. Exercise- Capacity of a CDMA system SatComms A - part 4 - B G Evans

37. Multiple access-Comparison of multiple access techniques SatComms A - part 4 - B G Evans

38. Advantages/disadvantages of various multiple access techniques SatComms A - part 4 - B G Evans

39. 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

40. 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

41. 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

42. 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

43. Random Access Schemes (5) • Comparison of random access SatComms A - part 4 - B G Evans

44. 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

45. 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

46. 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