1. University of Canberra Advanced Communications Topics
Television Broadcasting into the Digital Era
Lecture 3
Digital
Modulation Systems
by: Neil Pickford 1

2. Enabling Technologies
Source digitisation (Rec 601 digital studio)
Compression technology (MPEG, AC-3)
Data multiplexing (MPEG)
Transmission technology (modulation)
Digital TV has Key Technologies that make it possible. Most production within the current TV stations already happens in the digital domain using standards such as Rec 601 digital video. It only becomes analog when it is transmitted over the air to the viewer.
Display technology has not reached the level needed for HDTV to be fully implementable at present.

3. Digital TV Transmission Technology
The transmission system is used to transport the information to the consumer.
The system protects the information being carried from the transmission environment
The transmission system is a “data pipe”
Transports data rates of around 20 Mb/s
Transports data in individual containers called packets
Being Digital, it is just like any computer data. You put a byte of data into the pipe, you must receive the same byte out the end of the pipe. If it is different at the other end it simply does not work. The testing of these systems had primarily been done (95%) looking at Bit Error Rates (BER), not the displayed pictures. The data is transported in 188 byte groups called packets.

4. Digital TV Transmission Systems
DTV & HDTV systems fall into three groupings
Europeans - Digital SDTV MHz on UHF - DVB-T (COFDM)
Americans - Digital HDTV MHz VHF/UHF - ATSC (8-VSB)
Japanese - Integrated Broadcasting - ISDB (BST-OFDM)

5. Developed by the advance television systems committee ATSC
8-VSB - USA
Developed by the advance television systems committee ATSC
Developed for use in a 6 MHz channel
A 7 MHz variant is possible but has not been produced.
Uses a single carrier with pilot tone
8 level amplitude modulation system
Single Payload data rate of Mb/s
Relies on adaptive equalisation
Existing AM technology highly developed
ATSC developed out of the Grand Alliance which resulted at the end of the initial digital television race in America.

6. COFDM - Europe
Developed by the digital video broadcasting project group - DVB
Uses similar technology to DRB
Uses 1705 or 6817 carriers
Variable carrier modulation types are defined allowing Payload data rates of 5-27 Mb/s in 7 MHz
Developed for 8 MHz channels
A 7 & 6 MHz variants have been produced and tested.
Can use single frequency networks - SFNs
New technology with scope for continued improvement & development
Multicarrier System
Many different combinations of system parameters
To change the bandwidth of the system you change the master system clock. As the clock rate increases the system carrier spacing & thus bandwidth decreases as an inverse proportion.

7. ISDB - Japan
Japanese are developing integrated services digital broadcasting (ISDB)
System integrates all forms of broadcasting services into one common data channel which can be passed by satellite, cable or terrestrial delivery systems
Video services
Sound services
Bulk data services
Interactive data services

8. ISDB - Concept
Main TV menu
Newspaper - Categories - Headlines Downloaded overnight
Television Schedule
Weather
Preview of other stations
Time, ,Interactive services
16:9 Display.
Uses Band Segmented Transmission - Orthogonal Frequency Division Multiplex (BST-OFDM)

9. Terrestrial Transmission Problems
Multipath interference - ghosts
Noise interference - snow
Variable path attenuation - fading
Interference to existing services
Interference from other services
Channel frequency assignment - where to place the signal
The current Analog TV system has lots of problems which can arise in the transmission path and are reflected in a poor quality picture.
We also have problems with planning where to place these services, because we have to avoid known interference mechanisms.

10. Digital Modulation - Functions
Spreads the data evenly across the channel
Distributes the data in time
Maintains synchronisation well below data threshold
Employs sophisticated error correction.
Equalises the channel for best performance
Digital modulation distributes the information across the channel bandwidth so the rectangular spectrum shown earlier is produced. This makes more efficient use of the spectrum.
Highly rugged transmission techniques are used for the system synchronisation data such as QPSK and 2 level data. This is because if the system clock is lost, the whole system immediately stops working and may take some time to reaquire lock.
Error correction techniques such as Reed Solomon and Viterbi Coding are used to correct errors in the data.

11. Conventional Single Carrier Multicarrier/Spread Spectrum
Digital Modulation
Two techniques:
Conventional Single Carrier
8VSB, QPSK or QAM
Multicarrier/Spread Spectrum
OFDM
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace 38

12. 8-VSB & COFDM - Spectrum 8-VSB COFDM
This is a spectrum analyser plot of the two digital systems being considered in Australia. The little hump on the left side of the 8-VSB spectrum (yellow) is the pilot carrier.
These are averaged spectrums and have shoulder levels of around dB
The spectrums are basically rectangular in shape. The COFDM signal is wider since it is a 7 MHz system in a 7 MHz channel while 8-VSB is 6 MHz wide.

13. Sin(x)/x Amplitude dB A Carrier phase or frequency
As power is an absolute
quantity the negative
amplitudes also
resolve as real power
A Carrier phase or frequency
modulated at a low symbol rate
will exhibit a Sin(X)/X type
spectral occupancy
Frequency

14. Digital Modulation Amplitude, dB Frequency Fc - Fs Fc + Fs Fc
Low Symbol Rate
Medium Symbol Rate
High Symbol Rate
Amplitude, dB
Spectrum of a Conventional Multi-Phase Keyed Carrier Fc at a Symbol Rate Fs
Sin(X)/X shaping
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency
Fc - Fs
Fc + Fs Fc 38

15. PSK

16. BPSK Modulation
I AXIS 1
180 Deg
Phase
Change

17. QPSK Modulation
Q AXIS
1 0
1 1
QPSK
Distance
I AXIS
0 0
0 1

18. 16QAM Modulation Q AXIS 1 0 1 1 I AXIS 0 0 0 1 16-QAM Distance 1 1 1 0
1 0
1 1
16-QAM
Distance
I AXIS
0 0
0 1

19. 64QAM Modulation Q AXIS 1 0 1 1 I AXIS 0 0 0 1 64-QAM Distance 111011
111111
1 0
1 1
111001
64-QAM
Distance
110011
110111
110001
I AXIS
001110
001000
001100
0 0
000110
0 1
000000
000100

20. Hierarchical Modulation
Distance
Q AXIS
1 0
1 1
QPSK
Distance
I AXIS
0 0
0 1

21. Digital Modulation Amplitude Frequency Fc - Fs Fc + Fs Fc
Typical Filtered Spectrum to give about half original bandwidth
Occupied Channel Bandwidth
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency
Fc - Fs
Fc + Fs Fc 38

22. Normal FDM Guard Band Amplitude, dB Carrier 1 Carrier 2 Frequency
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency 38

23. Orthogonal Modulation
Amplitude, dB
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency 38

24. Orthogonal Modulation
Amplitude, dB
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency 38

25. COFDM - Orthogonal Carriers
COFDM is Coded Orthogonal FDM
If you observe the carrier spacing they are much closer together. This thing called “Orthogonality” is the key. It means that the peak of the yellow carrier (and all others) coincides with a null on every other carrier. Each peak sits directly above a null. You cannot generate these carriers by having individual oscillators mixed together. They are all generated at the same time using an Inverse Fast Fourier Transform (IFFT) using the same clock.
Frequency

26. Almost Rectangular Shape
Spectrum of COFDM DTTB
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 referes 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

27. OFDM Amplitude, dB Frequency Frequency Fcentre
Occupied bandwidth is: No. of Carriers x Spectral Width. Create with FFT
Amplitude, dB
As we have discussed, each IRD contains components which are responsible for the decoding, demultiplexing and demodulation of signals and components which are responsible for decrypting of signals.
NDS licenses its technologies to a number of hardware manufacturers. These include Thomson, Sony, Hughes, Sanyo, Samsung and Panasonic for conditional access, and Pace and Mitsubishi for digital IRD technology
NDS receives upfront integration fees and ongoing royalties under these license arrangements
Core operating software and electronic program guide software are new offerings first being shipped this quarter to StarTV, NetSat and Innova initial licensee - Pace
Frequency
Frequency
Spectral Width
2k is 4x wider than 8k
Fcentre 38

28. DIGITAL TERRESTRIAL BROADCASTING
Among the four Digital Broadcasting standards available, three are based on the Coded Orthogonal Frequency Division Multiplex modulation.... Why ?
Distant transmitter
Nearest transmitter
The Terrestrial Broadcasting has to cope with multipath propagation and Doppler effects: COFDM is the response for these impairments !

29. COFDM : HOW ?
1 - Organize time & frequency partitions in the RF channel
time
frequency RF
Channel
bandwidth
frequency
sub-band
time
segment

30. COFDM : HOW ? 2 - Spread sub-carriers over “time vs frequency” cells
symbol
Make sub-carriers orthogonal to avoid “inter-carriers” interference

31. COFDM : HOW ?
3 - Insert Guard Interval to avoid “inter-symbol” interference
time
frequency
Useful symbol
duration
Guard Interval
OFDM
symbol
Guard interval introduces a first loss in transport capacity

32. COFDM : HOW ?
4 - Insert “Synchronization Pilots” Helps Receivers to lock onto the signal
time
frequency
FFT
time windows
for receivers
OFDM Frame
(68 OFDM symbols)
Synchronization markers introduce the second loss in transport capacity

33. Protected DATA (convolutionnal error protection codes)
COFDM : HOW ?
5 - Prepare data to be carried on OFDM symbols
time
frequency
DATA to broadcast
Protected DATA (convolutionnal error protection codes)
Protection codes introduce the third loss in transport capacity

34. COFDM : HOW ?
6 - Map bits onto OFDM: Spread contiguous data bits over distant sub-carriers
time
frequency
DATA to broadcast
Protected DATA 1
Create frequency diversity to improve robustness against fading

35. DTTB - Channel Estimation
The Terrestrial transmission channel is continuously varying (position & time)
Variations occur in Amplitude, Phase & Frequency
To correct for this variation Information needs to be added to the transmission to quantify the channels response at any instant - Pilots
Equalisers in the Digital receiver use this information to remove these transmission impairments

36. DVB-T - Carriers + Pilots
0.977/3.906 kHz
SYMBOLS
Kmin
1704 for 2K or
6816 for 8K
Kmax =
Kmax
SYMBOLS IN SEQUENCE - 68 PER BLOCK.
Modulated Carriers
Scattered Pilots

37. Co- Incident with Scattered pilots
DVB-T Super Frame
45/177
TPS Carriers
Fixed Pilots
Frame 1
Frame 2
Frame 3
Frame 4 54 48 34 50
17/68
1512:
1705
in 2k
6048:
6817
in 8k
Co- Incident with Scattered pilots
Super
Frame
4 x 68
Symbols =
N MPEG
Packets

38. TPS Pilots
Transmission Parameter Signalling is added on selected carriers within the OFDM spectrum (17 pilots for 2k & 68 pilots for 8k)
TPS pilots Carry:
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 (1/2, 2/3, 3/4, 5/6, 7/8)
Guard Interval (1/4, 1/8, 1/16, 1/32)
System Bandwidth (6, 7, 8 MHz)

39. DVB-T Transmission Frame
2k mode Hz - Kmax=1704
8k mode Hz - Kmax=6816
Kmin=0
Carrier Spacing & Position
Kmax
Data
TPS - Pilot
Scattered Pilot
Continuous Pilot
Symbol Duration 256 us (2k) or 1024 us (8k)

40. DVB-T - Estimating the Channel
B=3/4A+1/4E C
C=1/2(A+E) D
D=1/4A+3/4E E E
A to E ms (2k) ms (8k)
For a varying transmission channel
DVB-T estimation is 23.5 times faster than ATSC

41. DVB-T - Estimating the Channel
A B C D E F G
A B C D E F G
B=2/3A+1/3D
C=1/3A+2/3D
E=2/3D+1/3G
F=1/3D+2/3G
A to D kHz (2k) kHz (8k)
For a varying transmission channel
DVB-T estimation is 23.5 times faster than ATSC

42. Estimation Rate DVB-T Guard Interval
Equaliser is updated every symbol period (256 us). 1/12 of data carriers are pilots
Full Channel estimate is available every 4 symbols (1.024 ms)
Guard Interval
OFDM is better than Single carrier systems under Multipath Echo conditions due to the addition of a Guard interval in the modulation system.
The Guard interval is added onto the symbol time wasting some potential data capacity.

43. 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

44. 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

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

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

47. 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

48. 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?

49. Bus Route 7 Singapore

50. 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]