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

2. 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

3. 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.

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)
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

7. Convolutional Coder X Output 1111001 Data Input 1-Bit Delay 1-Bit
Y Output

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

9. 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

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
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

13. 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

14. 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

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

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

17. 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

18. 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?

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

20. Bus Route 7 Singapore

21. 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)

22. 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.

23. 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.

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
-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)

26. 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)

27. 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)

28. C/N - Signal Level Performance28 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)

29. 8VSB vs COFDM Australia

30. 8VSB vs COFDM Latest

31. 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

32. 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)