1. השתלמות בתקשורת לוויינים רמי נוידרפר כל הזכויות שמורות למחבר. השימוש בחומר המופיע במצגת זו לצרכים מסחריים אסור בהחלט. מותר השימוש לצורכי הוראה ולימוד עצמי בלבד

2.  חלק חמישי - אפנונים, קידוד ושיטות גישה 3 June 2008 מבוא לתקשורת לוויינים

3. מטרת הלימוד הכרת שיטות גישה מרובת משתמשים ללוויין – FDMA, TDMA, CDMA, וצירופים שלהם הבנת היתרונות והמגבלות של כל אחת מהשיטות הכרת תקנים מרכזיים של קידוד לווייני MPEG2,DVB-S,DVB-S2 הבנת השפעת דחיסת אותות על קיבול הלוויין הבנת פעולת לוויין חדיש עם עיבוד אותות על גבי הלוויין

4. ריבוב מול גישה מרובת משתמשים ריבוב Multiplexing מספר ערוצי תקשורת על גבי אותו עןרק, כשהריבוב נעשה בנקודת מוצא אחת ריבוי משתמשים Multiple Access - מספר רב של מתשמשים בנקודות קצה שונות משתמשים באותו מקלט / משדר

5. שיטות גישה מרובות משתמשים Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

6. Multiple Access Schemes

7. Frequency Division Multiple Access (FDMA) assigns individual channels (i.e frequencies) to individual users

8. Frequency-Division Multiple Access Factors which limit the number of subchannels provided within a satellite channel via FDMA ◦ Thermal noise ◦ Intermodulation noise ◦ Crosstalk

9. צורות FDMA Fixed-assignment multiple access (FAMA) ◦ The assignment of capacity is distributed in a fixed manner among multiple stations ◦ Demand may fluctuate ◦ Results in the significant underuse of capacity Demand-assignment multiple access (DAMA) ◦ Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations

10. FAMA-FDMA FAMA – logical links between stations are preassigned FAMA – multiple stations access the satellite by using different frequency bands Uses considerable bandwidth

11. דוגמה

12. DAMA-FDMA Single channel per carrier (SCPC) – bandwidth divided into individual VF channels ◦ Attractive for remote areas with few user stations near each site ◦ Suffers from inefficiency of fixed assignment DAMA – set of subchannels in a channel is treated as a pool of available links ◦ For full-duplex between two earth stations, a pair of subchannels is dynamically assigned on demand ◦ Demand assignment performed in a distributed fashion by earth station using CSC

13. Time Division Multiple Access (TDMA):  The spectrum is divided into time slots, in each slot only one user can transmit or receive  TDMA shares a single carrier with several users

14. מדוע עוברים לTDMA Cost of digital components continues to drop Advantages of digital components ◦ Use of error correction Increased efficiency of TDM ◦ Lack of intermodulation noise

15. FAMA-TDMA Operation Transmission in the form of repetitive sequence of frames ◦ Each frame is divided into a number of time slots ◦ Each slot is dedicated to a particular transmitter Earth stations take turns using uplink channel ◦ Sends data in assigned time slot Satellite repeats incoming transmissions ◦ Broadcast to all stations Stations must know which slot to use for transmission and which to use for reception

17. FAMA-TDMA Uplink

18. FAMA-TDMA Downlink

19. Code Division Multiple Access (CDMA):  All the users in this system, use the same carrier frequency and may transmit simultaneously  Lets take a closer look at it!

20. CDMA Spread Spectrum Systems  Wireless Communication technique that uses transmission bandwidth substantially larger than the signal bandwidth  Many users can simultaneously use the same bandwidth without significantly interfering with one another  There are many applications including Global Positioning System, Wireless LAN e.g. Bluetooth CDMA  In CDMA user data is multiplied with a pseudo-noise (PN) sequence called the spreading signal  This causes the spectrum to be spread to several orders of magnitude greater than the minimum signal bandwidth

21. PN Sequence  PN sequence is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receiver  The rate of the PN code called the chip rate, must be much higher than the rate of the information signal

22. איך זה פועל ?  The information signal is demodulated at the receiver by cross-correlation with a locally generated replica of the user’s PN sequence.  The signal is de-spread and is restored to the original signal.  Cross-correlation with PN sequence of other users results in a very small noise at the receiver.

23. Why CDMA (I) ?  Exploitation of multipath – RAKE Receiver  Satellite path diversity exploitation for: Improved quality of service Fading effects mitigation  Immunity from jamming signals, external interference and multipath distortion  Full frequency reuse easing resource allocation

24. Why CDMA (II) ? f1f1 f1f1  Soft handoff – Improved quality of Service BS

25. אתגרים  Nonlinearity in satellite communications causes degradation in performance  Power Control - Near / Far Effect  Open loop power control  Closed loop power control  Synchronization

26. שיטות משולבות - דוגמה

27. שיטות שידור – אנלוגיות ודיגיטליות שיטות אפנון וקידוד לתקשורת לוויינית

28. שיטות שידור אנלוגיות בד " כ FM משתמשים ב sub-carrier לנתונים אני לא אלמד את הנושא מאחר וכל הלוויינים היום, בוודאי לוויני השידור, מעבירים אותות דיגיטליים בלבד לעניות דעתי חבל לבזבז על זה זמן בתוכנית הלימודים. ממילא דומה מאד לשידורי רדיו וטלוויזיה ארציים

29. Broadcast Satellites: the WARC Plan Features Frequency Band 11.7 to 12.5GHz (Europe & Africa) 40 channels spaced at 19.18MHz Orbital positions –generally a 6 0 spacing Frequency modulation –deviation 13.5MHz/Volt, i.e. a bandwidth of about 27MHz 5 channels for each country Circular polarisation Sound –a single channel on a sub-carrier Video –PAL or SECAM composite

30. מערכת שידור לוויינית – מודל עקרוני

31. שידור דיגיטלי - מבוא MPEG Compression Techniques MPEG Packets DVB-S Transmission

32. מה נלמד ? Why compression? MPEG-2 compression toolbox, including: ◦ Temporal and spatial redundancy ◦ Discrete Cosine Transform, DCT DVB channel adaptation, including: ◦ Forward error correction (FEC) encoding ◦ Modulation and the effects of nonlinearity Quality of service and picture impairments Contribution and distribution

33. למה דחיסה ? ITU-R BT.601-5 specifies 27Msamples/s at 8bits/sample = 216Mbits/s. MPEG-2 can deliver consumer quality video at ~1Mbits/s to 6Mbits/s. Typical broadcast satellite transponders have 27- 36MHz bandwidth, cost roughly £2-3m/year, and can carry 30-40Mbit/s OR one FM TV channel. Transponder cost/channel is much lower for MPEG-2 compression than FM-TV. Digital format allows many more applications.

34. אלמנטים של מערכת שידור STUDIO Camera Tape Film File server Contribution Electronic Programme Guide (EPG) MPEG-2 Encoder Subscriber Management System Conditional Access System Multiplexer Modulator MPEG-2 Encoder

35. MPEG-2 Video Compression Toolbox for bit-rate reduction includes: ◦ Removal of temporal redundancy: inter-frame compression ◦ Removal of spatial redundancy (DCT): intra-frame compression ◦ Quantisation of DCT coefficients ◦ Variable length coding (VLC)

36. Temporal redundancy Three classes of video frame: I-frames, make no reference to other frames P-frames, predicted from earlier I- or P-frames B-frames, predicted from both past and future frames Only P- and B-frames use temporal redundancy.

37. Temporal redundancy Use motion estimation to predict the next frame. Use DCT to encode the difference between predicted and actual. Intraframes Predicted frames

38. Spatial redundancy Operates on blocks of 8x8 pixels. Discrete Cosine Transform (DCT) converts spatial elements to frequency domain (lossless). Scaling related to human vision’s perceptual sensitivity. Quantisation controlled by feedback from rate buffer.

39. Spatial redundancy Pixel values for a block taken from a typical picture Increasing horizontal frequency Values after DCT processing Increasing vertical frequency 176 171 185 203 206 203 193 178 110612-22124620 145-15-16103710 98-4-204511 52-15-812-20 18-10 1-20 9-4-3-2100 -42 1-3210 -13100112

40. Spatial redundancy Increasing horizontal frequency Increasing vertical frequency DCT values after quantisation and scaling: 138100000 8 00000 50000000 2 000000 10000000 00000000 00000000 00000000

41. Spatial redundancy Conversion to serial data by zig-zag scanning: Run length coding removes long strings of zeros. Variable length coding replaces common values with shorter symbols (c.f. Morse code). 138100000 8 00000 50000000 2 000000 10000000 00000000 00000000 00000000

42. MPEG audio Uses a psychoacoustic algorithm based on the characteristics of the human hearing system. Divides the audio spectrum into sub-bands. The model determines the just-noticeable level of noise for each sub-band, and adjusts quantisation. Loud sounds reduce the ability to hear quiet sounds at other frequencies, so the quiet sounds may not need to be transmitted. MPEG 2 AAC is better than MP3 by a factor of ~50%

43. MPEG system layer Video encoder Audio encoder Data encoder Other programmes Other data Elementary streams Programme streams Transport stream

44. Broadcast transmission - enter the DVB! MPEG defines the Transport Stream but not how to carry it. DVB defines framing structure, channel coding and modulation for satellite (DVB-S) in EN 300 421. DVB is a European project, but DVB-S has been adopted around the world. Other types of DVB DVB-C, DVB-T, DVB-H DVB-S 2 is coming in

45. Channel adaptation Channel Adaptation: the processes involved in taking a Transport Stream and converting it to a form suitable for transmission on the satellite. Energy dispersal Outer FEC encoder Interleaver Inner FEC encoder Baseband shaping QPSK Modulation To RF channel Transport stream

46. Outer FEC encoding Reed-Solomon (204,188) encoding adds 16 bytes to each MPEG packet. 188 bytes 16 bytes RS 204 bytes 188 bytes 204 bytes

47. Interleaver Interleaver: breaks up bursts of errors, so that the performance of the Reed-Solomon error corrector in the receiver is enhanced. Achieved by changing the sequence of transmission of bytes, then performing the inverse function in the receiver.

48. Inner FEC encoder Provides a second layer of forward error correction. Target BER in receiver after error correction is 10 -11, corresponding to roughly one uncorrected error per hour. Target BER can be achieved with channel BER<10 -2. Choice of code rates of 1/2, 2/3, 3/4, 5/6, 7/8 allows trading of bandwidth and error performance.

49. שינוי קצב המודם DVB specifies modem performance in IF loop to achieve quasi error-free performance: Note: E b /N 0 = 10log(C/N 0 ) - 10log(bit rate). The bit rate referred to in this table is the useful bit rate before RS encoding. Inner code rateE b /N o (dB) 1/24.5 2/35.0 3/45.5 5/66.0 7/86.4

50. אפנון Modulation cannot be AM because the satellite TWTA must operate at saturation to deliver maximum power. Modulation must therefore be some form of phase shift keying (PSK). Requirement for the smallest possible receiving antennas means that the modulation must be rugged, i.e. able to be demodulated at low C/N. Must be spectrally efficient (bits/Hz) to maximise transponder payload.

51. BPSK Modulation BPSK has largest inter-symbol distance. QPSK has half BPSK’s symbol rate, so half the bandwidth. Inter-symbol distance is down 3dB relative to BPSK, but so is received noise power! I Q 0 1 I Q 0,0 1,10,1 1,0 BPSK constellation QPSK constellation

52. Baseband shaping AmplitudeNyquist bandwidth Slow roll-offMedium roll-offFast roll-off

53. Modulation performance Typical receiver performance in a linear channel: Measured Theoretical Note: in this case the bit rate used to calculate E b /N 0 from C/N 0 is the channel rate.

54. Effect of TWTA on spectrum Spectrum of 11Mbits/s (gross rate) QPSK signal after passing through a wideband TWTA at saturation.

55. Example payload calculation Q. 30MHz of bandwidth is available. If the inner code rate is 3/4, what is the bit-rate available to the MPEG stream? A. The relationship between bandwidth at -20dB relative to mid-band and the symbol rate is BW = 1.28 x symbol rate. Therefore, symbol rate = 30 / 1.28 = 23.4Msym/s QPSK has two bits per symbol, so the gross bit rate is 23.4 x 2 = 46.8Mbits/s.

56. Example payload calculation The rate after the inner layer of error correction is 46.8 x 3/4 = 35.1Mbits/s. The rate after the outer (RS) layer of error correction is 35.1 x 188/204 = 32.3Mbit/s. (Inner code)(Outer code) MPEG stream to decoder From demodulator 46.8Mbits/s35.1Mbits/s32.3Mbits/s Convolutional decoding (3/4) RS (204,188) decoding

57. Quality of service The two concatenated error correcting codes give an abrupt failure as C/N degrades. Above the failure point, picture quality is the same as that leaving the studio. Picture Quality C/N FM Digital FM threshold Digital threshold

58. שידור למשדרים Broadcaster to broadcaster connections: ◦ Programme exchange ◦ Feeds to cable head-ends (primary distribution) ◦ Digital Satellite News Gathering (DSNG) DVB-DSNG (EN 301 210): ◦ Specifies QPSK, same as DVB-S ◦ Adds 8PSK and 16QAM

59. DVB-S2 PRINCIPLES The DVB-S2 system has been designed for several satellite broadband applications: broadcast services for standard definition TV and HDTV; interactive services, including Internet access, for consumer applications; professional applications, such as Digital TV contribution and News Gathering, TV distribution to terrestrial VHF/UHF transmitters; data content distribution and Internet trunking.

60. New Standard DVB-S2 – 2003 Achieves 35-40% increase in throughput for same bandwidth Greater than 20 combinations of modulation and coding schemes offer ◦ Spectrum efficiency 0.5  4.5 bits/unit bandwidth ◦ C/N from –2  16dB Backward compatibility with DVB-S Opens up range of new services and reduced costs

61. New standard DVB-S2 – 2003 Standard optimised for range of satellite transponder characteristics and satellite channels Variable coding and modulation allows change on frame to frame basis Allows MPEG2, MPEG4, IP and ATM input streams Adaptive M&C can be operated between forward/return (RCS) to secure 4-8dB added advantages

62. Modulation schemes DVB-S2 The four possible DVB-S2 constellations before physical layer scrambling

63. Framing Structure: the system train Pictorial representation of the physical layer framing structure

64. DVB-S2 Performance Required C/N versus spectrum efficiency, obtained by computer simulations on the AWGN channel (idea demodulation) (C/N refers to average power) Operates C/N’s –2.4dB with QPSK/1/4 to 16dB with 32APSK/9/10 (for PER of 10-7) Note: 20-35% capacity increase over DVB-S

65. Spring2005 © University of SurreySatComms B - General - B G Evans65 DVB-S2 Range of C and M Examples of useful bit rates Ru versus LDPC code rate per unit symbol rate Rs

66. Comparison DVB-S and S2 (CCM)

67. Adaptive Coding and Modulation for one-to-one services When DVB-S2 is used for interactive point-to-point applications like IP unicasting, its gain over DVB-S is even greater, if Adaptive Coding and Modulation (ACM) schemes are used. In fact ACM allows us to recover the so called “clear sky margin” (4 to 8 dB of power), typically wasted in conventional “constant coding and modulation” satellite links, thus doubling or even tripling the average satellite throughput and reducing dramatically the service cost