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

Almost Rectangular Shape Spectrum of COFDM DTTB 7 MHz Carrier Spacing 2k Mode 3.91 kHz 8k Mode 0.98 kHz Almost Rectangular Shape These 1000s of carriers form the observed rectangular spectrum. There are two modes for the system 2K & 8K. This reefers to the size of the FFT used to generate and demodulate them. The 2k system has 1705 carriers and the 8k system uses 6817 carriers. The extra locations in the FFT are used to ensure that the signals have a sharp roll-off and good out of channel performance. 1705 or 6817 Carriers 6.67 MHz in 7 MHz Channel 7.61 MHz in 8 MHz Channel

64-QAM - Perfect & Failure These figures are real 64-QAM system constellations for both a no noise “perfect” condition and at the failure threshold due to white noise. In the system on the right the data eye is no longer visible but the system is still just working. Again notice in the perfect case that there are some errant points in-between the main constellation points.

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]

COFDM DTTB Block Diagram Error Correction

Forward Error Correction (FEC) 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 Encode N bits Tx/Rx N+Code Decode N+Code+Error N bits

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

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

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

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

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

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

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

COFDM - Multipath TRANSMITTER A REFLECTIONS DIRECT PATH 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

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

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

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

Mobile Services Antenna Performance Doppler Needs to be Rugged 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?

Mobile Digital TV Onboard a Tram in Cologne - Germany A trial in Germany has DTTB on a Tram showing portable operation.

Bus Route 7 Singapore - 1999

Doppler Echo - 7.5 us Coax COFDM 8-VSB -5 -10 Echo Level E/D (dB) -15 COFDM 8-VSB -5 -10 Echo Level E/D (dB) -15 -20 For ATSC at all doppler offsets more than 1 Hz the echo levels less than 20 dB are required for the system to work. DVB-T has a broad range where echo levels around 0 dB can be tolerated. -25 -500 -200 200 500 Frequency Offset (Hz)

DTTB Systems Doppler Performance Limits for current implementations 300 250 UHF 200 VHF - Band III DOPPLER SHIFT (Hz) COFDM 2K, 3dB degrade 140 COFDM 2K 100 50 Some interesting doppler effects due to large vehicles were experienced during the field test. ATSC failed when Busses & Trucks passed the field vehicle. The data on this plot comes from the Lab tests but demonstrates the doppler speed sensitivity of the DVB-T system. Note the small red circle in the bottom left corner. 100 200 300 400 500 600 700 800 900 1000 ATSC see separate curves SPEED (Km/Hr) Vehicles AIRCRAFT Over Cities COFDM implementations will inherently handle post and pre-ghosts equally within the selected guard interval.

ATSC 8-VSB Doppler Performance Limits for current implementations 10 UHF VHF - Band III DOPPLER SHIFT (Hz) 8VSB, “Fast Mode”, 3dB degrade 5 8VSB 1 This is a magnification of the lower left corner of the previous plot. It shows that ATSC is much more sensitive to flutter due to moving vehicles than DVB-T. The ATSC failures due to flutter occurred at normal signal strengths and were not in what would have been classed as low signal areas. 2 6 10 23 30 100 SPEED (Km/Hr) Vehicles Aircraft 8VSB implementations of equalisers are likely to cater for post ghosts up to 30 uSec and pre-ghosts up to 3 uSec only.

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

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

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

7 MHz COFDM Modulator Spectrum -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 1 2 3 4 5 6 7 8 Frequency Offset (MHz)

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

8VSB vs COFDM Australia

8VSB vs COFDM Latest

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

Simulated Theoretical Thresholds (bandwidth independent) DVB-T - C/N Values GAUSSIAN RICEAN RAYLEIGH Code 16 - 64 - QAM 16 - 64 - 16 - 64 - 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)