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

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

3 64-QAM - Perfect & Failure

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

5 COFDM DTTB Block Diagram Error Correction Error Correction

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

7 Convolutional Coder 1-BitDelay1-BitDelay1-BitDelay1-BitDelay1-BitDelay1-BitDelay DataInput Y Output X Output

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

9 Inner Coding n Convolutional coder generates the X & Y codes ConvolutionalEncoder Puncturing InterleaverData CodedData XY n Puncturing operation selects X & Y in sequence n Result then scrambled with an interleaver

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

11 Reed Solomon (RS) n RS is a Block data correcting Code n Hamming type cyclic Polynomial sequence  Code Generator Polynomial: g(x) = (x+ 0 )(x+ 1 )(x+ 2 )...(x+ 15 ), =02 Hex  Field Generator Polynomial: p(x) = x 8 + x 4 + x 3 + x n Has special ability to correct multiple bursts of errors in a code block n DVB-T uses 204 bytes for each 188 byte Packet (ATSC uses 207 bytes for each 187 byte Packet) n Can correct 8 bytes in each 204 byte packet

12 Error Protection - Order Outer Code RS(204,188)DataInput InterleaverInterleaver Inner Code FEC(2/3) ErrorProtectedData Mapper 188Bytes 204Bytes 204Bytes 306Bytes 6 bits x 1512 Carriers 6 bits x 6048 Carriers 2448Bits 64 QAM

13 Transmitted Symbol Transmitted Symbol Guard Interval Useful Symbol Useful Symbol TGTGTGTG TSTSTSTS TUTUTUTU 1/41/41/41/4 1/81/81/81/8 1 / 16 1 / 32 Guard

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

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

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

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

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

19 Mobile Digital TV Onboard a Tram in Cologne - Germany

20 Bus Route 7 Singapore

21 Doppler Echo us Coax Echo Level E/D (dB) Frequency Offset (Hz) COFDM 8-VSB

22 VHF - Band III UHF DOPPLER SHIFT (  Hz) COFDM 2K, 3dB degrade COFDM 2K SPEED (Km/Hr) AIRCRAFT Vehicles Over Cities for current implementations for current implementations ATSC seeseparatecurves see separate curves COFDM implementations will inherently handle post and pre-ghosts equally within the selected guard interval. DTTB Systems Doppler Performance Limits

23 VHF - Band III UHF VSB, “Fast Mode”, 3dB degrade SPEED (Km/Hr) Vehicles 8VSB Aircraft for current implementations 8VSB implementations of equalisers are likely to cater for post ghosts up to 30 uSec and pre-ghosts up to 3 uSec only. DOPPLER SHIFT (  Hz) ATSC 8-VSB Doppler Performance Limits

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

25 7 MHz COFDM Modulator Spectrum 2k 1/32 Guard Frequency Offset (MHz) Power Spectrum Density (dB)

26 7 MHz COFDM Modulator Spectrum 8k 1/32 Guard Frequency Offset (MHz) Power Spectrum Density (dB)

27 7 MHz COFDM Modulator Spectrum 8k 1/32 Guard 2k 1/32 Guard Frequency Offset (MHz) Power Spectrum Density (dB)

28 C/N - Signal Level Performance Receiver Signal Level (dBuV) C/N Threshold (dB)

29 8VSB vs COFDM Australia

30 8VSB vs COFDM Latest

31 DVB-T - Bit Rates [2k] Code Rate QPSK 16 - QAM 64 - QAM QPSK 16 - QAM 64 - QAM 64 - QAM 16 - QAM D/Tu = 1/4D/Tu = 1/8D/Tu = 1/32 1/2 2/3 3/4 5/6 7/ us32 us8 us 7 MHz

32 DVB-T - C/N Values Code Rate QPSK 16 - QAM 64 - QAM QPSK 16 - QAM 64 - QAM 64 - QAM 16 - QAM 1/2 2/3 3/4 5/6 7/ GAUSSIAN RICEANRAYLEIGH Simulated Theoretical Thresholds (bandwidth independent)