1. Komunikasi Satelit, Sukiswo, ST, MT 1 Multiple Access For Satellite Sukiswo sukiswok@yahoo.com

2. Komunikasi Satelit, Sukiswo, ST, MT2 Outline  Traffic Parameters  Access Techniques  Performance Evaluation – efficiency  FDMA  TDMA  CDMA  Fixed and On-Demand Assignment  Random Access

3. Komunikasi Satelit, Sukiswo, ST, MT 3 Traffic Parameters

4. Traffic intensity The traffic intensity A is defined as A =R call.T call (Erlang) Where R call = mean number of calls per unit time (s -1 ) T call = mean duration of a communication (s) Komunikasi Satelit, Sukiswo, ST, MT4

5. Call Blocking Probability Komunikasi Satelit, Sukiswo, ST, MT5  It is assumed that the number of users generating calls is much greater than the number of communication channels C provided between end terminals and blocked calls are not stored.  Under these conditions, the Erlang B formula indicates the probability that n channels are occupied (n  C):

6. Call Blocking Probability Komunikasi Satelit, Sukiswo, ST, MT6

7. Burstiness  Once the connection is established, transfer of information may occur.  Burstiness relates to the intermittent transfer of data.  Information is conveyed in the form of data bursts that are generated at random intervals.  The burstiness is defined as the peak bit rate of the active information source divided by the average bit rate: BU =R/ L where R is the peak bit rate (bit/s), is the message generation rate (s -1 ) and L is the length of the message (bits). Continuous information corresponds to low burstiness (stream traffic with BU of order 1–5) whereas highly intermittent traffic is characterised by high burstiness (BU=10 3 to 10 5 ). Komunikasi Satelit, Sukiswo, ST, MT7

8. Traffic Routing Komunikasi Satelit, Sukiswo, ST, MT8  Given a demand for traffic in a network of N earth stations, it is necessary to establish an adequate information transfer capacity between each pair of stations.  This capacity is calculated as a function of demand expressed by traffic intensity and acceptable blocking probability (a typical value is 0.5 to 1%).  Let CXY be the capacity, expressed as a number of communication channels for transfer of traffic from station X to station Y

9. Traffic Routing Komunikasi Satelit, Sukiswo, ST, MT9 a. One carrier per station-to-station link b. One carrier per transmitting station

10. Komunikasi Satelit, Sukiswo, ST, MT 10 Access Techniques

11. Komunikasi Satelit, Sukiswo, ST, MT11 Multiple access  Multiple access are techniques that allow several carriers from several earth stations to access the satellite.  The problem of multiple access arises when several carriers are handled simultaneously by a satellite repeater which is a nodal point of the network.  The satellite repeater consists of several adjacent channels (called transponders), whose bandwidth is a fraction of the total repeater bandwidth.

12. Komunikasi Satelit, Sukiswo, ST, MT12 Multiple access  In terms of multiple access, there are two aspects to be considered: –multiple access to a particular repeater channel (i.e. a transponder). –multiple access to a satellite repeater.

13. Komunikasi Satelit, Sukiswo, ST, MT13 Access to Transponder  Three multiple access fundamental techniques: –frequency division multiple access (FDMA); –time division multiple access (TDMA); –code division multiple access (CDMA).

14. Access to Satellite Repeater  Multiple access to a particular repeater channel (transponder) implies prior multiple access to the satellite repeater.  Access to a satellite repeater is achieved as a function of the frequency and polarisation of the carrier.  For every carrier with a given polarisation and frequency there is obligatory FDMA access to the repeater together with FDMA, TDMA or CDMA access to each channel. Komunikasi Satelit, Sukiswo, ST, MT14

15. Komunikasi Satelit, Sukiswo, ST, MT15 Access to Satellite Repeater  Several types of multiple access as defined above can be combined.

16. Performance Evaluation  The efficiency (  ) of a MA scheme is ratio of multiple access capacity to single access capacity at transponder saturation.  = multiple access capacity / single access capacity at transponder saturation  The capacity of a carrier is equal to the information bit rate R b conveyed by the carrier, sometimes called the carrier throughput.  The efficiency of a multiple access scheme then is the ratio of the sum of the throughputs of all accessing carriers to the maximum throughput of a single carrier in the transponder.  Efficiency then appears as a normalised throughput Komunikasi Satelit, Sukiswo, ST, MT16

17. Komunikasi Satelit, Sukiswo, ST, MT 17 FDMA

18.  The bandwidth of a repeater channel is divided into sub- bands; each sub-band is assigned to one of the carriers transmitted by the earth stations.  With this type of access, the earth stations transmit continuously and the channel conveys several carriers simultaneously at different frequencies.  It is necessary to provide guard intervals between each band occupied by a carrier to avoid interference as a result of imperfections of oscillators and filters.  The receiver selects the required carrier in accordance with the appropriate frequency. Komunikasi Satelit, Sukiswo, ST, MT18

19. TDM/PSK/FDMA  The baseband signals at the earth station are digital.  They are combined to form a time division multiplex (TDM) signal.  The TDM multiplex signal contains the total traffic from the transmitting earth station to all other stations.  The binary stream representing this multiplex signal modulates a carrier by phase shift keying (PSK) which accesses the satellite repeater channel at a particular frequency at the same time as other carriers on other frequencies from other stations.  To minimise intermodulation products, and consequently the number of carriers, traffic routing is preferably performed according to the ‘one carrier per transmitting station’ principle. Komunikasi Satelit, Sukiswo, ST, MT19

20. TDM/PSK/FDMA Komunikasi Satelit, Sukiswo, ST, MT20

21. SCPC/FDMA  The baseband signals at the earth station each modulate a carrier individually.  This is called ‘single connection per carrier’ (SCPC).  Each carrier accesses the satellite repeater channel on its particular frequency at the same time as other carriers on other frequencies from the same or other stations.  Information routing is thus performed according to the ‘one carrier per link’ principle Komunikasi Satelit, Sukiswo, ST, MT21

22. FDMA Efficiency Komunikasi Satelit, Sukiswo, ST, MT22

23. FDMA Efficiency Komunikasi Satelit, Sukiswo, ST, MT23

24. FDMA Efficiency Komunikasi Satelit, Sukiswo, ST, MT24

25. Komunikasi Satelit, Sukiswo, ST, MT 25 TDMA

26.  The earth stations transmit one after another bursts of carrier with duration T B.  All bursts of carrier have the same frequency and occupy the full repeater channel bandwidth.  Hence the satellite repeater channel carries one carrier at a time.  Bursts are inserted within a periodic time structure of duration T F, called a frame. Komunikasi Satelit, Sukiswo, ST, MT26

27. TDMA Komunikasi Satelit, Sukiswo, ST, MT27 Operation

28. TDMA Komunikasi Satelit, Sukiswo, ST, MT28 Burst generation

29. TDMA : Burst generation Komunikasi Satelit, Sukiswo, ST, MT29  The burst corresponds to the transfer of traffic from the station considered.  The bit rate R which modulates the carrier is thus given by: R =R b (T F /T B ) bit/s  With the technique of one carrier per station-to-station link, the station transmits N-1 bursts per frame, where N is the number of stations on the network and the number of bursts P in the frame is given by P = N(N -1)  Hence, for example, if Rb=2 Mbit/s and (T F /T B )= 10, modulation occurs at 20 Mbit/s

30. TDMA : Frame structure Komunikasi Satelit, Sukiswo, ST, MT30

31. TDMA Komunikasi Satelit, Sukiswo, ST, MT31 Burst reception

32. TDMA efficiency  In single carrier operation, the maximum throughput is R R=B , where B is the channel bandwidth (Hz)  is the spectral efficiency of the modulation (bit/s Hz).  With multiple access, the actual total throughput is,  Where  t i represents the sum of the times not devoted to transmission of traffic (guard times plus burst headers). The TDMA efficiency (  ) is thus: Komunikasi Satelit, Sukiswo, ST, MT32

33. TDMA efficiency  The efficiency depends on the number P of bursts in the frame. Let p be the number of bits in the header and g the equivalent duration in bits of the guard time.  Assuming that the frame contains two reference bursts, this gives: where R is the bit rate of the frame (bit/s).  The efficiency as a function of the number of accesses, that is the number of stations N on the network, depends on the traffic routing technique adopted.  in the case of a ‘one carrier per link’ technique, P =N(N-1)  in the ‘one carrier per transmitting station’ routing technique, P =N. Komunikasi Satelit, Sukiswo, ST, MT33

34. Frame duration considerations  A long frame duration requires a higher storage capacity in the transmitting and receiving earth station buffer memories. Interface-to-interface delivery delay =Round-trip propagation time + 2TF (s)  In practice, frame durations typically range from 750 u s to 20 ms. Komunikasi Satelit, Sukiswo, ST, MT34

35. Example  The variation of throughput as a function of the number of bursts of traffic P, equal to the number N of traffic stations or the number of accesses, can be examined by inserting the values of the INTELSAT / EUTELSAT standard.  Taking p=560, g =128, R=120,832 Mbit/s and TF=2 ms gives:  Notice the relatively slow decrease of efficiency as a function of the number of accesses.  For instance, for 50 accesses the efficiency is still 85%. Komunikasi Satelit, Sukiswo, ST, MT35

36. Example Komunikasi Satelit, Sukiswo, ST, MT36

37. Komunikasi Satelit, Sukiswo, ST, MT 37 CDMA

38. Komunikasi Satelit, Sukiswo, ST, MT38  Code division multiple access operates on the principle of spread spectrum transmission.  The code sequence which serves to spread the spectrum constitutes the ‘signature’ of the transmitter.

39. CDMA Komunikasi Satelit, Sukiswo, ST, MT39

40. CDMA Komunikasi Satelit, Sukiswo, ST, MT40  The receiver recovers the useful information by reducing the spectrum of the carrier transmitted in its original bandwidth.  This operation simultaneously spreads the spectrum of other users in such a way that these appear as noise of low spectral density.

41. CDMA Komunikasi Satelit, Sukiswo, ST, MT41

42. CDMA  With code division multiple access (CDMA), network stations transmit continuously and together on the same frequency band of the satellite repeater channel.  The signature consists of a binary sequence, called a code, which is combined with the useful information at each transmitter.  Two techniques are used in CDMA: –direct sequence (DS); –frequency hopping (FH). Komunikasi Satelit, Sukiswo, ST, MT42

43. Direct sequence CDMA Komunikasi Satelit, Sukiswo, ST, MT43

44. Direct sequence CDMA Komunikasi Satelit, Sukiswo, ST, MT44

45. Spectrum Direct sequence CDMA Komunikasi Satelit, Sukiswo, ST, MT45

46. Frequency hopping CDMA Komunikasi Satelit, Sukiswo, ST, MT46

47. Frequency hopping CDMA Komunikasi Satelit, Sukiswo, ST, MT47 Spectral distribution in FH-CDMA for RH « Rb.

48. CDMA efficiency  The efficiency of CDMA can be considered as the ratio of the total capacity provided by a repeater channel in the case of single access, that is a single carrier modulated without spectrum spreading, and that of a repeater channel transmitting several CDMA carriers simultaneously.  The total capacity of the repeater channel is then the product of the capacity of one carrier and the number of carriers, that is the number of accesses. Komunikasi Satelit, Sukiswo, ST, MT48

49. CDMA efficiency  The maximum total throughput is equal to N max R b.  The throughput of a single carrier modulated without spectrum spreading and occupying a bandwidth B N would be the chip rate R c.  The throughput  of CDMA is thus given by the ratio: Komunikasi Satelit, Sukiswo, ST, MT49

50. Example  Consider a CDMA network occupying the whole of a 36MHz satellite repeater channel. The receiving bandwidth is B N = 36 MHz. It is assumed that each carrier has a capacity equal to 64 kbit/s. With BPSK modulation of theoretical spectral efficiency  =1bit/s Hz, the chip rate is R c =B N /  =36 Mbit/s and the spreading ratio Rc/Rb is 36x10 6 /64x10 3 =563. Table shows the maximum number of accesses, the maximum total throughput of the network and the resulting efficiency for a required bit error probability. The CDMA efficiency, of the order of 10%, is low compared, for example with that of TDMA. The values in the table are optimistic: thermal noise is neglected, user codes are assumed to be orthogonal and no account is taken of degradation due to the demodulator. Komunikasi Satelit, Sukiswo, ST, MT50

51. Example Komunikasi Satelit, Sukiswo, ST, MT51

52. Komunikasi Satelit, Sukiswo, ST, MT 52 Fixed and On-Demand Assignment

53. Fixed & On-Demand Assignment  Traffic routing implies access by each carrier transmitted by the earth stations to a radio-frequency channel.  For each of the three fundamental modes (FDMA, TDMA and CDMA) described in the previous sections, each carrier is assigned a portion of the resource offered by the satellite, i.e. a satellite channel (a frequency band, a time slot or a fraction of the total power keyed to a code) or a part of it.  This assignment can be defined once and for all (a fixed assignment) or in accordance with requirements (on- demand assignment). Komunikasi Satelit, Sukiswo, ST, MT53

54. Fixed Assignment  With fixed assignment, the capacity allocated each earth station is fixed independently of the traffic demand from the terrestrial network to which it is connected.  An earth station can receive a traffic request from the network to which it is connected greater than the capacity which is allocated to it.  It must then refuse some calls; this is a blocking situation, in spite of the fact that other stations may have excess capacity available.  Because of this, the resource constituted by the satellite network is poorly exploited. Komunikasi Satelit, Sukiswo, ST, MT54

55. Fixed Assignment  The total capacity of the repeater channel is shared among 20 stations.  Each station thus has 1520/20=76 communication channels available.  These channels are shared among the 19 destinations.  There are, therefore, 76/19= 4 channels per destination.  The maximum traffic intensity A must be determined such that the blocking probability B(C=4, A)=E C=4 (A) remains less than 0,01.  It is found, by using equation (6.2) or Figure 6.1, that A= 0,87 erlang, that is 0,217 erlang per communication channel. Komunikasi Satelit, Sukiswo, ST, MT55

56. On-Demand Assignment  With on-demand assignment, the satellite network resource can be assigned in a variable manner to the various stations in accordance with demand.  There will, therefore, be the possibility of transferring capacity from stations with low demand to stations with excess demand. Komunikasi Satelit, Sukiswo, ST, MT56

57. On-Demand Assignment  The total capacity S of the repeater channel can be assigned to any station whatever the destination.  One must realise the condition B(S=1520, A)=E S=1520 (A) < 0,01 which leads to A=491 for the 1520 communication channels and hence an intensity A/1520=0,98 erlang per communication channel. Komunikasi Satelit, Sukiswo, ST, MT57

58. Komunikasi Satelit, Sukiswo, ST, MT 58 Random Access

59.  This type of access is well suited to networks containing a large number of earth stations where each earth station is required to transmit short, randomly generated messages with long dead times between messages.  The principle of random access is to permit transmission of messages almost without restriction in the form of limited duration packets, towhich correspond bursts of modulated carriers, which occupy all or part of the bandwidth of the repeater channel.  It is, therefore, multiple access with time division and random transmission. Komunikasi Satelit, Sukiswo, ST, MT59

60. Random Access  The performance of random access is measured in terms of the normalised throughput and the mean transmission delay.  Normalised throughput has been defined previously as efficiency and is restated here as the ratio of the volume of traffic delivered at the destination to the maximum volume which could be transmitted in the available bandwidth.  Random access has been the object of numerous studies since 1970.  Its practical application has become important in the context of private networks using small stations (very small aperture terminals, VSAT) which have been widely developed to provide satellite communication between computers and distant terminals. Komunikasi Satelit, Sukiswo, ST, MT60

61. ALOHA Protocol Komunikasi Satelit, Sukiswo, ST, MT61 Principle of the ALOHA random multiple access protocol without collision

62. ALOHA Protocol Komunikasi Satelit, Sukiswo, ST, MT62 Principle of the ALOHA random multiple access protocol with collision

63. Pure ALOHA Komunikasi Satelit, Sukiswo, ST, MT63

64. Slotted ALOHA Komunikasi Satelit, Sukiswo, ST, MT64

65. ALOHA and Slotted ALOHA Komunikasi Satelit, Sukiswo, ST, MT65

66. Transmission efficiency. Komunikasi Satelit, Sukiswo, ST, MT66

67. Mean transmission time Komunikasi Satelit, Sukiswo, ST, MT67

68. Comparison of efficiency Komunikasi Satelit, Sukiswo, ST, MT68