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University of Canberra Advanced Communications Topics

University of Canberra Advanced Communications Topics. Television Broadcasting into the Digital Era. Lecture 5 DTTB Transmission Error Correction. by: Neil Pickford. Spectrum of COFDM DTTB. 7 MHz Carrier Spacing 2k Mode 3.91 kHz 8k Mode 0.98 kHz. Almost Rectangular Shape.

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University of Canberra Advanced Communications Topics

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  1. University of Canberra Advanced Communications Topics Television Broadcasting into the Digital Era Lecture 5 DTTB Transmission Error Correction by: Neil Pickford

  2. Spectrum of COFDM DTTB 7 MHz Carrier Spacing 2k Mode 3.91 kHz 8k Mode 0.98 kHz AlmostRectangularShape 1705 or 6817 Carriers 6.67 MHz in 7 MHz Channel 7.61 MHz in 8 MHz Channel

  3. 64-QAM - Perfect & Failure

  4. OFDM - Features • Multicarrier - many carriers sharing • Reduced C/N compared to Analogue • Resistant to echoes, Interference etc • Low symbol rate per carrier • ~ 1 kBaud: Long Symbol Period, can Extend with Guard Interval • With FEC becomes COFDM • Uses Fast Fourier Transform [FFT] • ”2k” and “8k” versions • Single Frequency Networks [SFN]

  5. COFDM DTTB Block Diagram Error Correction

  6. Forward Error Correction (FEC) N bits Encode Tx/Rx Decode N bits N+Code N+Code+Error • Broadcast transmission • One way process - Tx to Rx • Not possible to repeat any errored data • Forward Error Correction is a technique used to improve the accuracy of data transmission • Extra redundant bits are added to the data stream • Error correction algorithms in the demodulator use the extra FEC bits to correct data errors • C OFDM uses a Convolutional FEC code

  7. Convolutional Coder 1111001 X Output 1-Bit Delay 1-Bit Delay 1-Bit Delay 1-Bit Delay 1-Bit Delay Data Input 1-Bit Delay Y Output 1011011 6 5 4 3 2 1 0

  8. Puncturing Codes (FEC) • The X and Y outputs of the Convolutional coder are selected in a Puncturing pattern

  9. Inner Coding X Coded Data Data Y • Convolutional coder generates the X & Y codes • Puncturing operation selects X & Y in sequence • Result then scrambled with an interleaver Convolutional Encoder Puncturing Interleaver

  10. Viterbi Decoder • A special type of data decoder designed to work with convolutional FEC codes • Uses the past history of the data to identify valid future data values • Element in the Receiver Only

  11. Reed Solomon (RS) • RS is a Block data correcting Code • Hamming type cyclic Polynomial sequence • Code Generator Polynomial: g(x) = (x+l0)(x+l1)(x+l2)...(x+l15), l=02 Hex • Field Generator Polynomial: p(x) = x8 + x4 + x3 + x2 + 1 • Has special ability to correct multiple bursts of errors in a code block • DVB-T uses 204 bytes for each 188 byte Packet(ATSC uses 207 bytes for each 187 byte Packet) • Can correct 8 bytes in each 204 byte packet

  12. Error Protection - Order Inner Code FEC (2/3) Data Input Interleaver Interleaver Error Protected Data Mapper 188 Bytes 204 Bytes Outer Code RS (204,188) 306 Bytes 204 Bytes 2448 Bits 6 bits x 1512 Carriers6 bits x 6048 Carriers 64 QAM

  13. Guard Interval TG TU 1/8 TS 1/16 1/32 Transmitted Symbol Guard Useful Symbol 1/4

  14. COFDM - Multipath TRANSMITTER A REFLECTIONS 1 Microsecond = 300 Metres DIRECT PATH SYMBOL PERIOD [1 ms] RECEPTION POINT SIGNAL Several µseconds disturbance from echoes. OFDM inherently resistant. 8VSB needs Time Domain Equaliser, symbol period short at 93ns

  15. COFDM - Multipath TRANSMITTER A REFLECTIONS 1 Microsecond = 300 Metres DIRECT PATH GUARD INTERVAL SYMBOL PERIOD RECEPTION POINT SAFE AREA SIGNAL

  16. COFDM - Pre-Echo GUARD INTERVAL SAFE AREA TRANSMITTER A REFLECTIONS 1 Microsecond = 300 Metres SYMBOL PERIOD RECEPTION POINT SIGNAL

  17. COFDM - SFN TRANSMITTER B TRANSMITTER A REFLECTIONS 1 Microsecond = 300 Metres DIRECT PATH GUARD INTERVAL [Variable] SYMBOL PERIOD RECEPTION POINT SAFE AREA SIGNAL

  18. Mobile Services • Antenna Performance • Poor Directivity, Low Gain • Multipath Dominated environment • Doppler • High Speeds for Main Roads and Railways • Low Speeds for Public Transport in Cities • Needs to be Rugged • Choose version of DVB-T that is suitable • Low Bit Rate, Low C/N, Long Guard Interval?

  19. Mobile Digital TV Onboard a Tram in Cologne - Germany

  20. Bus Route 7 Singapore - 1999

  21. Doppler Echo - 7.5 us Coax 0 COFDM 8-VSB -5 -10 Echo Level E/D (dB) -15 -20 -25 -500 -200 0 200 500 Frequency Offset (Hz)

  22. DTTB Systems Doppler Performance Limits for currentimplementations 300 250 UHF 200 VHF - Band III DOPPLERSHIFT(Hz) COFDM 2K, 3dB degrade 140 COFDM 2K 100 50 0 0 100 200 300 400 500 600 700 800 900 1000 ATSC seeseparatecurves SPEED (Km/Hr) Vehicles AIRCRAFT Over Cities COFDM implementations will inherently handlepost andpre-ghostsequally within the selected guard interval.

  23. ATSC 8-VSB Doppler Performance Limits for current implementations 10 VHF - Band III UHF DOPPLERSHIFT(Hz) 8VSB, “Fast Mode”, 3dB degrade 5 8VSB 1 0 0 2 6 10 23 30 100 SPEED (Km/Hr) Vehicles Aircraft 8VSB implementations of equalisers are likely to cater forpost ghostsup to30 uSecandpre-ghostsup to3 uSec only.

  24. TPS Pilots • Transmission Parameter Signalling is added on selected carriers within the OFDM spectrum(17 for 2k & 68 for 8k) • TPS Carries: • Frame Number in Super Frame: 00 / 01 / 10 / 11 • Constellation Type QPSK / 16-QAM / 64-QAM • OFDM Mode 2k or 8k • Constellation Mode Normal/Hierarchical + a value • Inner FEC Code rate • Guard Interval • System Bandwidth

  25. 7 MHz COFDM Modulator Spectrum 0 -10 -20 Power Spectrum Density (dB) -30 -40 2k 1/32 Guard -50 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 Frequency Offset (MHz)

  26. 7 MHz COFDM Modulator Spectrum 0 -10 -20 Power Spectrum Density (dB) -30 -40 8k 1/32 Guard -50 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 Frequency Offset (MHz)

  27. 7 MHz COFDM Modulator Spectrum 0 -10 -20 Power Spectrum Density (dB) -30 -40 8k 1/32 Guard 2k 1/32 Guard -50 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 Frequency Offset (MHz)

  28. C/N - Signal Level Performance 28 24 20 16 C/N Threshold (dB) 12 8 4 0 10 15 20 25 30 35 40 45 50 55 60 Receiver Signal Level (dBuV)

  29. 8VSB vs COFDM Australia

  30. 8VSB vs COFDM Latest

  31. DVB-T - Bit Rates [2k] 16 - 64 - 16 - 64 - 16 - 64 - QPSK QPSK QPSK QAM QAM QAM QAM QAM QAM 1/2 4.35 8.71 13.06 4.84 9.68 14.51 5.28 10.56 15.83 2/3 5.81 11.61 17.42 6.45 12.90 19.35 7.04 14.07 21.11 3/4 6.53 13.06 19.59 7.26 14.51 21.77 7.92 15.83 23.75 5/6 7.26 14.51 21.77 8.06 16.13 24.19 8.80 17.59 26.39 7/8 7.62 15.24 22.86 8.47 16.93 25.40 9.24 18.47 27.71 D/Tu = 1/4 D/Tu = 1/8 D/Tu = 1/32 7 MHz 64 us 32 us 8 us Code Rate

  32. DVB-T - C/N Values 64 - QAM GAUSSIAN RICEAN RAYLEIGH 16 - 16 - 64 - 16 - 64 - Code QPSK QPSK QPSK Rate QAM QAM QAM QAM QAM 1/2 3.10 8.80 14.4 3.60 9.60 14.70 5.40 11.20 16.00 2/3 4.90 11.1 16.5 5.70 11.60 17.10 8.40 14.20 19.30 3/4 5.90 12.5 18.00 6.80 13.00 18.60 10.70 16.70 21.70 5/6 6.90 13.5 19.30 8.00 14.40 20.00 13.10 19.30 25.30 7/8 7.70 13.9 20.10 8.70 15.00 21.00 16.30 22.80 27.90 Simulated Theoretical Thresholds (bandwidth independent)

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