Satellite Communications: medium access techniques

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Satellite Communications: medium access techniques

  1. 1. Satellite Access Techniques Francisco J. Escribano December 11, 2013 Francisco J. Escribano Satellite Access Techniques December 11, 2013 1 / 45
  2. 2. Table of contents 1 Motivation 2 Types of Access Techniques 3 Fixed Assignment FDMA TDMA CDMA SDMA and PDMA 4 On Demand Assignment 5 Random Access 6 References Francisco J. Escribano Satellite Access Techniques December 11, 2013 2 / 45
  3. 3. Motivation Motivation Francisco J. Escribano Satellite Access Techniques December 11, 2013 3 / 45
  4. 4. Motivation Motivation Sharing resources The satellite capabilities are limited. ◮ Bandwidth and power are scarce resources which need to be assigned somehow. Several strategies to share resources. ◮ Fixed, dynamic; spectrum, time, and so on. Main references: [1], [2], [3]. Francisco J. Escribano Satellite Access Techniques December 11, 2013 4 / 45
  5. 5. Types of Access Techniques Types of Access Techniques Francisco J. Escribano Satellite Access Techniques December 11, 2013 5 / 45
  6. 6. Types of Access Techniques Classification I 1 Fixed assignment, or, more comprehensively, scheduled assignment. ◮ Examples: FDMA, TDMA, CDMA. Frequency Division Multiple Access Time Division Multiple Access Code Division Multiple Access ◮ A fraction of the incumbent resource is assigned to each corresponding user. ◮ Works well in situations with intense and continuous traffic. Figure 1 : Fixed assignment space. Francisco J. Escribano Satellite Access Techniques December 11, 2013 6 / 45
  7. 7. Types of Access Techniques Classificacion II 2 On demand assignment. ◮ Example: DAMA, as opposed to PAMA (permanent assignment). Demand Assigned Multiple Access ◮ Resources are allocated only when a communication is to be established. ◮ After communication ends, the resources are de-allocated. ◮ Variable carrier frequency and frame timing, depending on the available channels. ◮ Well adapted to situations with intense, burst traffic. Figure 2 : DAMA situation. Francisco J. Escribano Satellite Access Techniques December 11, 2013 7 / 45
  8. 8. Types of Access Techniques Classification III 3 Random access. ◮ Example: ALOHA, slotted ALOHA. ◮ When an ES has data to transfer, it initiates transmission. ◮ Well adapted to situations with a large number of stations generating short-duration burst traffic. Figure 3 : Random access and collisions. Francisco J. Escribano Satellite Access Techniques December 11, 2013 8 / 45
  9. 9. Fixed Assignment Fixed Assignment Francisco J. Escribano Satellite Access Techniques December 11, 2013 9 / 45
  10. 10. Fixed Assignment FDMA FDMA Transponder complete BW is divided into channels. Each channel could have different allocated BW. Each ES has an associated channel. All ES’s transmit simultaneously. Figure 4 : Model for FDMA communication. Francisco J. Escribano Satellite Access Techniques December 11, 2013 10 / 45
  11. 11. Fixed Assignment FDMA FDMA From the point of view of quality and performance analysis, ◮ Each channel can be dealt with separately in most cases. ◮ Each channel can transmit different types of information or signals (digital, analog). ◮ It is important to know which fraction of the overall BW power is allocated to each channel. ◮ One important issue at the satellite transponder side is the possibility of intermodulation between different channels. It is a simple scheme: no synchronisation is necessary at the satellite. Figure 5 : FDMA channelization example. Francisco J. Escribano Satellite Access Techniques December 11, 2013 11 / 45
  12. 12. Fixed Assignment FDMA FDMA & FDM 1 SCPC, Single Carrier per Channel ◮ Each ES sends different communications through a different carrier/channel pair. Figure 6 : SCPC. 2 FDM/FM, Frequency Division Multiplexing ◮ The signals are first multiplexed, then sent through a unique signal that occupies all the available bandwidth. Francisco J. Escribano Satellite Access Techniques Figure 7 : FDM/FM. December 11, 2013 12 / 45
  13. 13. Fixed Assignment FDMA FDMA: Eutelsat scheme Eutelsat defines a QPSK/FDMA scheme for digital transmission, with FEC capabilities (CC) [4]. ◮ Specifications are given for an IF of 70MHz/140MHz. ◮ At the RF level, separation between same rate carriers is n × 90KHz (1/2 rate FEC), or n × 60KHz (3/4 rate FEC), where n × 64Kbps is the customer’s data rate. ◮ Each carrier transmission has to comply with severe transmission masks at the IF and RF levels. Figure 8 : 1/2 rate FEC encoder. Francisco J. Escribano Figure 9 : 3/4 rate FEC encoder. Satellite Access Techniques December 11, 2013 13 / 45
  14. 14. Fixed Assignment FDMA FDMA: Eutelsat scheme Transmission / reception masks. Figure 10 : Modulator filter amplitude response. Francisco J. Escribano Figure 11 : PSD mask at the modulator output. Satellite Access Techniques Figure 12 : Demodulator filter amplitude response. December 11, 2013 14 / 45
  15. 15. Fixed Assignment TDMA TDMA Each ES transmits a data burst during a pre-allocated period of time. Each ES burst occupies the whole carrier bandwidth. ◮ Intermodulation problem is mitigated. Periodic frame-based comm: each ES data burst accesses same time slot. Each ES’s data is buffered to access periodically the correct time slot. The system requires complex synchronisation techniques and protocols. Figure 13 : TDMA setup. Francisco J. Escribano Satellite Access Techniques December 11, 2013 15 / 45
  16. 16. Fixed Assignment TDMA TDMA A reference ES transmits a reference burst. ◮ This enables time synchronisation for the other ES’s. A frame comprises the time between two successive reference bursts. M = R b · TF Frame period, TF . Input data rate for a ES, Rb . RT = Rb · TF TB Burst duration, TB . Transmission data rate, RT Required buffer size at a ES, M Figure 14 : GNSS example. Francisco J. Escribano Satellite Access Techniques December 11, 2013 16 / 45
  17. 17. Fixed Assignment TDMA TDMA There is a essential asymmetry between uplink and downlink. On receiving the bursts with correct synchronisation, the satellite builds a frame. ◮ This requires guard intervals between consecutive burst slots, depending on the estimated delay for the different ES’s. A burst consists on a header or preamble, and a payload field. ◮ The header or preamble, apart from signalling the beginning of the burst itself, enables the carrier and bit time recovery at the receiver. ◮ The payload field carries the useful data. In a standardized system (such as INTELSAT), there could be two kinds of bursts: ◮ A traffic ES data burst (e.g. 280 symbol header, and 64-multiple payload field). ◮ A reference ES burst (e.g. 288 symbol header and no payload field). Francisco J. Escribano Satellite Access Techniques December 11, 2013 17 / 45
  18. 18. Fixed Assignment TDMA TDMA: INTELSAT frame Example of the INTELSAT TDMA frame [5]. TB = = Rb ·TF RT = 2048·106 ·2·10−3 128832·106 = = 39µs Figure 15 : INTELSAT frame fields. Francisco J. Escribano Satellite Access Techniques December 11, 2013 18 / 45
  19. 19. Fixed Assignment TDMA TDMA: synchronisation Two sides: 2 1 Transmitter synchronisation. Receiver acquisition. Figure 17 : TX synchro. Open loop timing control (10% losses). Closed loop timing control. Feedback timing control. Figure 16 : SORF marker. Francisco J. Escribano Satellite Access Techniques December 11, 2013 19 / 45
  20. 20. Fixed Assignment TDMA FDMA vs. TDMA 1 Information transmission capabilities. Bits per user, b. ◮ FDMA with M orghogonal frequency bands. ◮ TDMA with M time slots. RFDMA = M · 2 b TF RTDMA = b TF /M =M· b TF = RFDMA Average message delay. wTDMA = D =w +τ DFDMA = 0 + TF 1 M · M n=1 (n − 1) TF = M τTDMA = DTDMA = DFDMA − TF 2 1− 1 M TF M TF 2 1− 1 M ◮ TDMA is better than FDMA from the point of view of message delay. ◮ In TDMA, the average delay is lower because user could transmit in other slots. Francisco J. Escribano Satellite Access Techniques December 11, 2013 20 / 45
  21. 21. Fixed Assignment CDMA CDMA Technique based on spread spectrum. ◮ Low correlation spread sequences are required. Each ES is assigned a unique code. ◮ This ensures some degree of privacy. Figure 18 : CDMA setup. Francisco J. Escribano Satellite Access Techniques December 11, 2013 21 / 45
  22. 22. Fixed Assignment CDMA CDMA Remember the different ways of using Spread Spectrum techniques: ◮ Direct Sequence (DS). s (t) = c (t) · d (t) where c (t) is the spreading sequence at a Tc chip period, and d (t) is the data sequence, at a Ts symbol period. ◮ Frequency Hopping (FH). s (t) = ℜ d (t) · e j2πfi t where d (t) is the narrowband baseband data sequence, and fi is a carrier frequency which changes value at each i instant according to a predefined spreading sequence. ◮ Time Hopping (TH). s (t) = d (t) ∗ p (t − ni · Ts /N) where d (t) is the data sequence in the form of a Dirac delta train of symbol period Ts , p (t) is a unit amplitude pulse shape of duration Tc , and ni is an integer between 0 and N − 1, N = Ts /Tc , chosen according to a predefined spreading sequence of index i. Francisco J. Escribano Satellite Access Techniques December 11, 2013 22 / 45
  23. 23. Fixed Assignment CDMA CDMA Be it DS, FH or TH based CDMA, it is important to characterize: ◮ Spreading gain given by the SS scheme. ◮ How resources (bandwidth, data rate, SNR...) are shared as a function of Tc , Ts , Tb , according to the specific SS scheme. ◮ The effect of the interference of concurrent users, and the evaluation of the corresponding degradation of the Eb /N0 per user. For example, in a simple DS-CDMA scheme where M users interfere with the same received signal power S each, we would have an Eb /N0 : Eb N0 = B/Rb (M−1)+η/S where η is the amount of noise power, B is the spreading bandwidth and Rb is the data bit rate. Usage of CDMA may require sofisticated power control techniques. Francisco J. Escribano Satellite Access Techniques December 11, 2013 23 / 45
  24. 24. Fixed Assignment CDMA CDMA: Globalstar example [6] GLOBALSTAR is a satellite-based cellular telephone system that allows users to talk from anyplace between 70o N and S latitudes. Avoids outages caused by blockage of signals by using diversity signals from two satellites. Consists of a Walker 48 − 8 − 1 constellation: ◮ 48 low-orbiting (1400Km altitude) satellites in 8 orbits, inclined 52o with respect to the Equator. 6 satellites in each orbital plane. Users transmit and receive on the 1.6GHz band and 2.5GHz band. Satellites communicate with the gateway ground antennas on the 5GHz and 7GHz bands. CDMA provides for extensive frequency reuse through the use of orthogonal codes in 1.23MHz channels. Capacity to serve up to 30 million subscribers (not simultaneously). Francisco J. Escribano Satellite Access Techniques December 11, 2013 24 / 45
  25. 25. Fixed Assignment CDMA CDMA: Globalstar example Figure 19 : Globalstar system overview. Francisco J. Escribano Satellite Access Techniques December 11, 2013 25 / 45
  26. 26. Fixed Assignment CDMA CDMA: Globalstar example Figure 20 : Nominal link parameters. Francisco J. Escribano Satellite Access Techniques December 11, 2013 26 / 45
  27. 27. Fixed Assignment CDMA CDMA: Globalstar example Figure 21 : 16 beam user TX antenna. Figure 22 : Antenna amplifier performance. Francisco J. Escribano Satellite Access Techniques December 11, 2013 27 / 45
  28. 28. Fixed Assignment CDMA CDMA: Globalstar example Figure 23 : Example of forward traffic channel structure. Francisco J. Escribano Satellite Access Techniques December 11, 2013 28 / 45
  29. 29. Fixed Assignment CDMA CDMA: Globalstar example Figure 24 : Forward link spreading and modulation. Francisco J. Escribano Satellite Access Techniques December 11, 2013 29 / 45
  30. 30. Fixed Assignment SDMA and PDMA SDMA, PDMA Spatial Division Multiple Access (SDMA). ◮ Satellite reuses frequencies, but serving non-overlapping Earth areas. ◮ Also called Multiple Beam Fequency Reuse. Polarization Division Multiple Access (PDMA). ◮ Communications overlap in frequency and space, but are separable: orthogonally polarized beams. ◮ PDMA is highly sensitive to cross-polarization issues in atmospheric propagation. Figure 25 : Examples of SDMA and PDMA. Francisco J. Escribano Satellite Access Techniques December 11, 2013 30 / 45
  31. 31. On Demand Assignment On Demand Assignment Francisco J. Escribano Satellite Access Techniques December 11, 2013 31 / 45
  32. 32. On Demand Assignment DAMA DAMA, Demand Assignment Multiple Access, as opposed to PAMA, Permanent Assignment Multiple Access. ◮ As its name implies, resources are provided following on-demand principles. ◮ As a contrast to fixed/scheduled assignment techniques, performance and results are highly dependent on traffic profiles. ◮ Main difference with respect to xDMA fixed methods: Pure xDMA systems divide a single physical channel into a number of virtual channels, and rely on near-real-time protocols to adjust demand/throughput. DAMA philosophy is adequate when resources are sparsely utilized and there is no need for heavily planified protocols to share them. DAMA can be used along with any xDMA scheme, depending on the sparsity and traffic usage. Francisco J. Escribano Satellite Access Techniques December 11, 2013 32 / 45
  33. 33. On Demand Assignment DAMA DAMA can be applied according to a number of flavours: 1 Variable Capacity DA. 2 3 Per-Call Variable Capacity DA. Per-Call DA. 4 Fully Variable DA. Traffic and demand analysis are required to ground the correct flavour choice and the appropriate system dimensioning. DAMA is used in military sat networks due to the simplicity and robustness of implementation. A DAMA-based system is usually affected by collisions. Francisco J. Escribano Satellite Access Techniques December 11, 2013 33 / 45
  34. 34. On Demand Assignment DAMA Variable Capacity DA. ◮ Employed in TDMA sat networks. ◮ Traffic profile: slow variations and a few destinations covering a large area. ◮ System enjoys a medium DAMA gain. ◮ Useful when the traffic peak bursts are known or well characterized. ◮ Reference TS manages the whole burst planning. Francisco J. Escribano Satellite Access Techniques December 11, 2013 34 / 45
  35. 35. On Demand Assignment DAMA Per-Call Variable Capacity DA. ◮ Employed in TDMA sat networkds. ◮ Each station serves several destinations. ◮ Low traffic intensity, but fast varying. ◮ Maximum DAMA gain. ◮ Reference TS changes the burst length and position. Francisco J. Escribano Satellite Access Techniques December 11, 2013 35 / 45
  36. 36. On Demand Assignment DAMA Per-Call DA. ◮ Employed in TDMA and FDMA sat networks. ◮ Each station serves many destinations. ◮ Low traffic intensity. ◮ Medium DAMA gain. Francisco J. Escribano Satellite Access Techniques December 11, 2013 36 / 45
  37. 37. On Demand Assignment DAMA Fully Variable DA. ◮ Employed in TDMA and FDMA sat networks. ◮ Each station serves large number of destinations. ◮ Fast changing traffic intensity. ◮ The total traffic intensity at a station is low. Francisco J. Escribano Satellite Access Techniques December 11, 2013 37 / 45
  38. 38. On Demand Assignment SPADE example SPADE, SCPC PCM Multiple Access DA Equipment. ◮ As the name implies, it is the integration into one network architecture of known principles. Figure 26 : SPADE channeling scheme. Figure 27 : SPADE net diagram. Francisco J. Escribano Satellite Access Techniques December 11, 2013 38 / 45
  39. 39. On Demand Assignment SPADE example Figure 28 : Possible SPADE QPSK SCPC configuration. Francisco J. Escribano Satellite Access Techniques December 11, 2013 39 / 45
  40. 40. Random Access Random Access Francisco J. Escribano Satellite Access Techniques December 11, 2013 40 / 45
  41. 41. Random Access ALOHA Purely random access makes sense for packet-oriented networks. ALOHA is a protocol to share a common channel with available rate RT among M possible users: ◮ As soon as a terminal has data to send, it starts transmission. ◮ If a collision is detected at the satellite, the packet is retransmitted (NACK). ◮ System useful when the access to the channel is sporadic and packets are short. Figure 29 : ALOHA channel throughput. Francisco J. Escribano Satellite Access Techniques December 11, 2013 41 / 45
  42. 42. Random Access ALOHA Figure 31 : Illustration of the ALOHA protocol. In an ALOHA system, there is a great deal idle time in the channel. Figure 30 : Delay versus throughput comparison. Francisco J. Escribano ALOHA naturally accomodates traffic profiles of low intensity, short bursts. The main advantage is flexibility and simplicity of implementation. Satellite Access Techniques December 11, 2013 42 / 45
  43. 43. Random Access S-ALOHA Slotted-ALOHA. ◮ Similar to pure ALOHA, but transmissions can only happend at given predefined time slots. ◮ Throughput is higher. ◮ Delay is reduced. Figure 32 : Throughput versus load comparison. Francisco J. Escribano Satellite Access Techniques December 11, 2013 43 / 45
  44. 44. References References Francisco J. Escribano Satellite Access Techniques December 11, 2013 44 / 45
  45. 45. References Bibliography I G. Maral, Satellite Communications Systems: Systems, Techniques and Technology. Inc., 1998. T. Pratt, Satellite Communications. Chichester: John Wiley & Sons, New York: John Wiley & Sons, Inc., 2003. G. E. Corazza, Digital Satellite Communications. New York: Springer, 2007. Eutelsat Company Web Site, “SMS QPSK/FDMA System Specification,” EESS 501 G Issue 3. [Online]. Available: http://www.eutelsat.com/files/contributed/satellites/pdf/eess501.pdf A. S. Oei, R. J. Colby, R. Parthasarathy, and A. L. Stimson, “Alignment, testing and maintenance principles in the intelsat tdma/dsi network,” International Journal of Satellite Communications, vol. 3, no. 1-2, pp. 161–166, 1985. [Online]. Available: http://dx.doi.org/10.1002/sat.4600030118 F. Dietrich, P. Metzen, and P. Monte, “The Globalstar cellular satellite system,” Antennas and Propagation, IEEE Transactions on, vol. 46, no. 6, pp. 935–942, 1998. Francisco J. Escribano Satellite Access Techniques December 11, 2013 45 / 45

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