University of Canberra Advanced Communications Topics

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University of Canberra Advanced Communications
Topics

• Television Broadcasting into the Digital Era

Lecture 4 DTTB Types Digital Modulation Systems
by Neil Pickford
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Digital Terrestrial TV - Layers
3
Digital Television Encode Layers
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Digital Television Decode Layers
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Set top Box (STB) - Interfacing

• Domestic and Professional interfaces still to be
  defined
• Most probably Transport Stream via IEEE 1394
  (Firewire)
• Baseband Audio RGB/YUV Video signals.
• STB can convert between line standardsso you do
  not have to have a HD display.
• Display and transmitted information must be at
  same Frame/Field rate. (25/50)

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DTTB - Content Services

• DTTB was designed to carry video, audio and
  program data for television
• DTTB can carry much more than just TV
• Electronic program guide, teletext
• Broadband multimedia data, news, weather
• Best of internet service
• Interactive services
• Software updates, games
• Services can be dynamically reconfigured

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DVB Data Containers

• MPEG Transport Stream is used to provide DVB
  data containers which may contain a flexible
  mixture of
• Video
• Audio
• Data services
• Streams with variable data rate requirements can
  be Statistically Multiplexed together.
• Allows Six 2 Mb/s programs to be placed in a 8
  Mb/s channel

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Examples of DVB Data Containers
Channel bandwidth can be used in different ways
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Video Program Capacity
For a payload of around 19 Mb/s

• 1 HDTV service - sport high action
• 2 HDTV services - both film material
• 1 HDTV 1 or 2 SDTV non action/sport
• 3 SDTV for high action sport video
• 6 SDTV for film, news soap operas
• However you do not get more for nothing.
• More services means less quality

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Spare Data Capacity

• Spare data capacity is available even on a fully
  loaded channel.
• Opportunistic use of spare data capacity when
  available can provide other non real time data
  services.
• Example 51 secondBMW commercial

The Commercial wasshown using 1080
Lines Interlaced. 60 Mb of data was transferred
during it. In the Final 3 seconds the BMW Logo
was displayed allowing 3 Phone Books of data to
be transmitted.
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Enabling Technologies

• Source digitisation (Rec 601 digital studio)
• Compression technology (MPEG, AC-3)
• Data multiplexing (MPEG)
• Transmission technology (modulation)
• Display technology (large wide screens)
• Production

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Digital Television - Types

• Terrestrial (DTTB)
• DVB-T / 8-VSB
• Free to air TV (broadcasting)
• Narrowcasting/value added services
• Untethered - portable reception

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Digital Terrestrial Television Broadcasting - DTTB

• Regional free to air television
• Replacement of current analog PAL broadcast
  television services
• Operating in adjacent unused taboo channels
  to analog PAL service
• Carries a range of services HDTV, SDTV, audio,
  teletext, data
• Providing an un-tethered portable service

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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
• Current Australian analog television uses the
  PAL-B AM modulation system

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Digital TV Transmission Technology

• The transmission system is a data pipe
• Transports data rates of around 20 Mb/s
• Transports data in individual containers called
  packets

4
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Digital TV Transmission Systems

• Australia has been following Digital TV HDTV
• Europeans - Digital SDTV - 8 MHz on UHF -
  DVB-T (COFDM)
• Americans - Digital HDTV - 6 MHz VHF/UHF -
  ATSC (8-VSB)
• Japanese - Integrated Broadcasting - ISDB
  (BST-OFDM)

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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 19.39 Mb/s
• Relies on adaptive equalisation
• Existing AM technology highly developed

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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

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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

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ISDB - Concept

• Proposed to use band segmented transmission -
  orthogonal frequency division multiplex
  (BST-OFDM)

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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

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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

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Digital Modulation

• Two techniques
• Conventional Single Carrier
• 8VSB
• Multicarrier/Spread Spectrum
• OFDM

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8-VSB COFDM - Spectrum
8-VSB COFDM
25
Sin(x)/x
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Digital Modulation
Amplitude, dB
Spectrum of Conventional Multi-Phase Keyed
Carrier Fc at Symbol Rate Fs
Sin X/X shaping
Frequency
Fc - Fs
Fc Fs
Fc
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Digital Modulation
Amplitude, dB
Low Symbol Rate Medium Symbol Rate High Symbol
Rate
Frequency
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PSK
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BPSK Modulation
I AXIS
0
1
180 Deg Phase Change
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QPSK Modulation
Q AXIS
1 0
1 1
QPSK Distance
I AXIS
0 1
0 0
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16QAM Modulation
Q AXIS
1 1 1 0
1 1 1 1
1 0
1 1
16-QAM Distance
1 1 0 0
1 1 0 1
I AXIS
0 0 1 0
0 0 1 1
0 1
0 0
0 0 0 0
0 0 0 1
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8VSB Modulation
Q AXIS
1 1 1 0
1 1 1 1
1 0
1 1
16-QAM Distance
1 1 0 0
1 1 0 1
I AXIS
0 0 1 0
0 0 1 1
0 1
0 0
0 0 0 0
0 0 0 1
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Hierarchical Modulation
Hierarchical Distance
Q AXIS
1 1 1 0
1 1 1 1
1 0
1 1
1 1 0 0
1 1 0 1
QPSK Distance
I AXIS
0 0 1 1
0 0 1 0
0 1
0 0 0 1
0 0 0 0
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Digital Modulation
Amplitude
Typical Filtered Spectrum to give about half
original bandwidth
Occupied Channel Bandwidth
Frequency
Fc - Fs
Fc Fs
Fc
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8-VSB Digital Modulation
Amplitude
Application of Vestigial Sideband Filter to give
reduced spectral occupancy BUT with destruction
of pure Amplitude modulation causing incidental
Phase modulation but some power in a small carrier
Occupied Channel Bandwidth eg 6 MHz in US
Frequency
Fc - Fs
Fc Fs
Fc
8VSB uses symbol Rate with period 93 nanoseconds
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Normal FDM
Guard Band
Amplitude, dB
Carrier 1
Carrier 2
Frequency
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Traditional SCPC Modulation
MinimumCarrier Spacing
Frequency
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Orthogonal Modulation
Amplitude, dB
Frequency
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Orthogonal Modulation
Amplitude, dB
Frequency
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COFDM - Orthogonal Carriers
Frequency
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Spectrum of COFDM DTTB
Carrier Spacing 2k Mode 3.91 kHz 8k Mode 0.98 kHz
AlmostRectangularShape
1705 or 6817 Carriers
6.67 MHz in 7 MHz Channel
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OFDM
Occupied bandwidth is No. of Carriers x Spectral
Width. Create with FFT
Amplitude, dB
Frequency
Frequency
Spectral Width 2k is 4x wider than 8k
Fcentre
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DIGITAL TERRESTRIAL BROADCASTING
Among the four Digital Broadcasting standards
available, three are based on the Coded
Orthogonal Frequency Division Multiplex
modulation.... Why ?
The Terrestrial Broadcasting has to cope with
multipath propagation and Doppler effects COFDM
is the response for these impairments !
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COFDM HOW ?

• 1 - Organize time frequency partitions in the
  RF channel

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COFDM HOW ?

• 2 - Spread sub-carriers over time vs frequency
  cells

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COFDM HOW ?

• 3 - Insert Guard Interval to avoid inter-symbol
  interference

Guard interval introduces a first loss in
transport capacity
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COFDM HOW ?

• 4 - Insert Synchronization Pilots Helps
  Receivers to lock onto the signal

Synchronization markers introduce the second
loss in transport capacity
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COFDM HOW ?

• 5 - Prepare data to be carried on OFDM symbols

DATA to broadcast
Protection codes introduce the third loss in
transport capacity
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COFDM HOW ?

• 6 - Map bits onto OFDM Spread contiguous data
  bits over distant sub-carriers

Create frequency diversity to improve robustness
against fading
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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
• Equalisers in the Digital receiver use this
  information to remove these transmission
  impairments

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Data Multiplex - 8-VSB
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8-VSB Segment Sync Data
Symbol Duration 93 ns
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Digital Modulation - 8-AM
7
6
5
4
3
2
1
0
Before Equaliser
After Equaliser
8-VSB - Coaxial Direct Feed through Tuner on
Channel 8 VHF
3 Bits/Symbol
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8-VSB - Field Sync
Field Sync is Repeated Every 24 ms
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8-VSB - Field Sync
7
6
5
4
3
2
1
0
Before Equaliser
After Equaliser
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8-VSB Frame
832 Symbols per Data Segment of 77.3 µs
Training Sequence
313 Data Segments of 24.2 ms
Data
Sync
Training Sequence
Data
The Training Sequence is only 0.3 of signal
time Total Sync is only 0.8 of time
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DVB-T - Carriers Pilots
0.977/3.906 kHz
Kmin
Kmax
SYMBOLS IN SEQUENCE - 68 PER BLOCK.
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DVB-T Super Frame
Super Frame 4 x 68 Symbols N MPEG Packets
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DVB-T Transmission Frame
2k mode - 3906 Hz - Kmax1704 8k mode - 977 Hz
- Kmax6816
Kmin0
Carrier Spacing Position
Kmax
Data
TPS - Pilot
Scattered Pilot
Continuous Pilot
Symbol Duration 256 us (2k) or 1024 us (8k)
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DVB-T - Estimating the Channel
A
A
B
B3/4A1/4E
C
C1/2(AE)
D
D1/4A3/4E
E
E
A to E - 1.024 ms (2k) - 4.096 ms
(8k)
For a varying transmission channel DVB-T
estimation is 23.5 times faster than ATSC
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DVB-T - Estimating the Channel
A B C D E F G
A B C D E F G
B2/3A1/3D
C1/3A2/3D
E2/3D1/3G
F1/3D2/3G
A to D - 11.724 kHz (2k) - 2.931
kHz (8k)
For a varying transmission channel DVB-T
estimation is 23.5 times faster than ATSC
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Channel Estimation Equalisation
ATSC
Time
DVB-T
Time
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Estimation Rate Comparison

• ATSC Equaliser is updated every 24 ms (260 000
  symbols)
• DVB-T Equaliser is updated every symbol period
  (256 us). 1/12 of data carriers are pilots
• DVB-T Full Channel estimate is available every 4
  symbols (1.024 ms)
• For a varying transmission channel DVB-T
  estimation is at least 23.5 times faster than ATSC

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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