University of Canberra Advanced Communications Topics

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University of Canberra Advanced Communications
Topics

• Television Broadcasting into the Digital Era

Lecture 4 Error Correction, DTTB Planning
System Information
by Neil Pickford
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64-QAM - Perfect Failure
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COFDM DTTB Block Diagram
Error Correction
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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

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Convolutional Coder
6 5 4
3 2 1
0
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Puncturing Codes (FEC)

• The X and Y outputs of the Convolutional coder
  are selected in a Puncturing pattern

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Inner Coding

• Convolutional coder generates the X Y codes

• Puncturing operation selects X Y in sequence

• Result then scrambled with an interleaver

Convolutional Encoder
Puncturing
Interleaver
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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

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Reed Solomon (RS)

• RS is a Block data correcting Code
• Hamming type cyclic Polynomial sequence
• Code Generator Polynomial g(x)
  (xl0)(xl1)(xl2)...(xl15), l02 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

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Error Protection - Order
188 Bytes
204 Bytes
Outer Code RS (204,188)
306 Bytes
204 Bytes
2448 Bits
6 bits x 1512 Carriers6 bits x 6048 Carriers
64 QAM
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DVB-T - Bit Rates 2k
D/Tu 1/4
D/Tu 1/8
D/Tu 1/32
7 MHz
64 us
32 us
8 us
Code Rate
Page 21 Table A1 - AS4599-1999
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DVB-T - C/N Values
GAUSSIAN
RICEAN
RAYLEIGH
16 -
16 -
64 -
64 -
16 -
Code
QPSK
QPSK
QPSK
Rate
QAM
QAM
QAM
QAM
QAM
1/2
3.10
8.80
3.60
9.60
14.70
5.40
11.20
16.00
14.4
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)
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C/N - Signal Level Performance
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24
20
16
C/N Threshold (dB)
12
8
4
0
10
15
20
25
30
35
40
45
50
55
60
Receiver Signal Level (dBuV)
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General Parameters - Aust Tests

• Parameter DVB-T ATSC
• Data Payload 19.35 Mb/s 19.39 Mb/s
• Carriers 1705 1
• Symbol Time 256 us 93 ns
• Time Interleaving 1 Symbol 4 ms
• Reed Solomon code rate 188/204 187/207
• IF Bandwidth (3 dB) 6.67 MHz 5.38 MHz

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15
8VSB vs COFDM Latest
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7 MHz COFDM Modulator Spectrum
0
-10
-20
Power Spectrum Density (dB)
-30
-40
8k 1/32 Guard 2k 1/32 Guard
-50
0
-1
-2
-3
-4
-5
-6
-7
-8
1
2
3
4
5
6
7
8
Frequency Offset (MHz)
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Frequency Planning

• Fundamental Matter - Scarce Resource
• Analogue Rules set limit to more Services
• No NEW TV Spectrum is Available
• Digital Transmission changes Rules
• Signals have different behaviour
• Digital Signals can occupy unused space -
  Taboos
• Digital Needs to fit in with Existing PAL
• Eventually Digital Only - but long wait??

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Digital Has to Fit In With PAL

• World TV channel bandwidths vary
• USA / Japan 6 MHz
• Australian 7 MHz
• Europeans 8 MHz
• Affects- tuning, filtering, interference
  system performance

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30
29
31
35
34
33
32
28
30
29
31
32
33
34
35
28
35
34
33
32
31
30
29
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Channel Spacing

• Existing analog TV channels are spaced so they do
  not interfere with each other.
• Gap between PAL TV services
• VHF 1 channel
• UHF 2 channels
• Digital TV can make use of these gaps

Ch 8
Ch 7
Ch 9
Ch 6
Ch 9A
Taboo
Taboo
Taboo
VHF Television Spectrum
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Digital Challenges

• Digital TV must co-exist with existing PAL
  services
• DTV operates at lower power
• DTV copes higher interference levels
• Share transmission infra-structure
• DTV needs different planning methods

Ch 8
Ch 7
Ch 9
Ch 6
Ch 9A
8-VSB
COFDM
VHF Television Spectrum
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DTTB PAL
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UHF Channels London
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Planning Issues

• Channel Disturbances
• Noise, at edge of area with NO interference
• Interference, Co Channel Interference and
  Adjacent Channel Interference
• Multipath, EchoesHow Many, How Large, Moving?
• Antenna Pattern?
• Static Roof Top? Directional? Wideband?
• CCIR Antenna Rec BT-419-3
• Portable Receivers? No Antenna?
• Frequency Re-Use Distances
• Terrain Data
• Propagation Models
• Protection Ratios

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Signal Strength
SIGNAL STRENGTH, MicroVolts
REGION OF SERVICE FAILURES FOR PERCENTAGE OF
TIME
MEAN
RECEIVER C/N LIMIT
TIME
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Digital Service Area Planning

• Analog TV has a slow gradual failure
• Existing PAL service was planned for50
  availability at 50 of locations
• Digital TV has a cliff edge failure
• Digital TV needs planning for90-99
  availability at 90-99 of locations

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TV System Failure Characteristic
Good
Quality
Edge of Service Area
Rotten
Far
Close
Distance
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TV System Failure Characteristic
Good
Quality
Edge of Service Area
Rotten
Far
Close
Distance
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TV System Failure Characteristic
Good
HDTV
PAL
Quality
SDTV
Edge of Service Area
Rotten
Far
Close
Distance
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Service Area Planning
PICTURE
QUALITY
DIGITAL
5
4
ANALOGUE
3
THRESHOLD OF ACCEPTABILITY
2
ANALOGUE FAILURE
C/N
1
NO SERVICE
40
45
35
15
20
25
30
10
30
Service Areas - Current
50 - 100 KILOMETRES
TRANSMITTER B
TRANSMITTER A
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Service Areas - SFN
50 - 100 KILOMETRES
TRANSMITTER B
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Digital Provides New Concepts

• Single frequency networks (SFNs) can help solve
  difficult coverage situations
• SFNs allow the reuse of a transmission frequency
  many times in the same area so long as exactly
  the same program is carried
• Allows lower power operation
• Better shaping of coverage
• Improved service availability
• Better spectrum efficiency

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MPEG Packet
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System Level Multiplexing Approaches
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Packetisation Approaches
Fixed Length
Variable Length
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Transport Stream
Link Header Format
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System Information (SI)

• Required for
• Automatic Tuning of receiver upon selection
• Program location
• EPG (Electronic Program Guide)
• API (Application Programming Interface)
• CA (Conditional Access)

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DVB SI Model
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System Information

• The DVB SI structure has its derivation in MPEG
  ISO/IEC 13818-1 and is defined in a set of
  tables.
• The primary link between DVB SI and MPEG is
  thePSI (Program Specific Information) in MPEG
  and is contained primarily in the PAT, PMT
  and CAT set of tables

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What is SI?

• SI data provides information for
• Automatic tuning to transport stream
• User Information for
• Service selection
• Event selection
• Component selection
• PSI data provides information for
• Configuration of decoder for selected Service
• DVB extensions for non-MPEG components

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PSI and DVB SI Tables
DVB OPTIONAL
MPEG DVBMANDATORY
NIT OTHER Delivery Sys.
PID0x0010
PAT
PIDOx0011
SDT OTHER TS
PIDOx0011
ST
RST
STUFFING TABLE.
RUNNING STATUS.
PROGRAMME CLOCK REF.
TIME AND DATE.
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MPEG Program PIDs

• What is a program ?
• MPEG has a definition which is different to that
  normally understood.
• A program in broadcasting is a collection of
  elements with a common time base and the same
  start and stop times.
• A program in MPEG is a collection of elements
  with a common time base only. That is a
  collection of elementary streams with same
  PCR_PID and referenced to the same program_number

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Virtual Channels PCR Timing

• A conventional Broadcaster of a TV channel or
  service having one program would be composed of a
  series of broadcaster programs or events with
  the same program_number and a common PCR_PID.
• In other words the PCR time base effectively
  creates a virtual channel which may be associated
  with a single or multiple program_numbers.
• A TV channel having multiple programs would have
  multiple program_numbers with either single or
  multiple PCR_PID between program streams.
• NOTE Services with different program_numbers
  may draw upon the same video as with the case of
  multilingual services.

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Decoding the Program

• Decoding the correct program (ie channel) ?
• Where there are several Transport Streams
  available to a decoder, in order to successfully
  demultiplex a program, the decoder must be
  notified of both the transport_stream_id (to find
  the correct multiplex) and the program_number of
  the service (to find the correct program within
  the multiplex).
• Note again the program here refers to the channel
  not the event or actual broadcast program.
• Now to the various main table purposes

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PAT, PMT CAT Tables

• PAT (Program Association Table)
• provides the link between the transport_stream_id,
  the program_number and the program_map_id (PMT).
• PMT (Program Map Table)
• when pointed to from the PAT, the PMT provides
  the associated group of elements (video, audio
  etc) with the program_number.
• CAT (Conditional Access Table)
• provides the association between CA system(s) and
  their EMM (Entitlement Management Messages)
  streams and any special parameters associated
  with them.

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DVB SI Features

• Data structured as several Tables
• Structures use fixed format for essential data,
  and descriptors for optional or variable-length
  data (similar to PSI)
• Efficient data transmission
• Extensible while maintaining compatibility
• Support for private extensions
• Can provide standard EPG data-stream
• Look and Feel determined by receiver software
• Resident or Downloaded

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SI Features NIT

• Network Information Table
• Identification of transmission as a member of a
  group of multiplexes - Network
• Network Name
• Tuning parameters with support for various
  delivery media
• List of additional frequencies for terrestrial
  transmission
• Designed for simple transcoding of transport
  streams

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SI Features SDT BAT

• Service Description Table
• Identifies all Service names and Service types in
  TS
• Linked Services
• Pointer to MPEG Program in PSI
• Service_id MPEG Program Number
• Bouquet Association Table (Optional)
• Groupings of Services
• May convey logical channel number

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SI Features EIT

• Present/Following
• Information on current and next events
• Schedule (optional)
• Up to 64 days ahead - ordered by service and time
• Event Information
• Title, short description
• Start time duration
• Content classification parental rating
• Longer text description
• Information on components

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SI Features TDT,TOT, RST

• Time and Date Table
• Transmission of current time for automatic
  setting of receiver clock
• Time Offset Table (optional)
• Transmission of time offset by zone - both
  current, and next offset values, with date at
  which next occurs
• Running Status Table (optional)
• Mechanism for signalling status transitions with
  greater timing precision

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Electronic Program Guide - EPG

• EPG (Electronic Program Guide)
• Combining primarily the EIT and the SDT, both the
  time and description is provided to the viewer
  via some form of EPG ranging from vanilla
  EPGs, simple eye plate style displays to full
  blown EPGs, either from Receiver manufacturers
  designs or downloaded EPGs with GUI interfaces
  designed by the Networks.
• This information so constructed and displayed can
  be used to provide a Parental Guidance lock
  function through PIN number access.

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Example EPG using DVB SI
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Example Event Details
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Example Selection by Genre
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Example Selection by Genre
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Application Program Interface API

• API (Application Programming Interface)
• some form of API must be used to allow the
  control by the viewer or installer of the decoder
  / receiver. The API software provides the
  connection between the applications (eg. EPG) and
  the hardware.
• Some APIs may employ MHEG-5 multimedia support
  and Java programming language for EPG generation.
• CA in practice is reliant upon EPGs and the
  APIs.

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DVB - Conditional Access

• CA (Conditional Access)
• Access to the EMM (Entitlement Management
  Message) is provided by the CAT.
• The EMM allows a single decoder to view the
  program material which is scrambled via a DVB
  common scrambling algorithm by providing the
  key to the code word which is involved in the
  scrambling. The code word is sent via the ECM
  (Entitlement Control Message).

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Typical Conditional Access System
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A Future Digital System Concept
MMDS
HypermediaIntegrated ReceiverDecoder (IRD)
Satellite
Terrestrial
Cable
Broadcast
Interactivity
B-ISDNXDSL
CD, DVDDVC
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DTTB Systems Doppler Performance Limits
for currentimplementations
300
250
UHF
200
VHF - Band III
DOPPLERSHIFT(?Hz)
COFDM 2K, 3dB degrade
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COFDM 2K
100
50
0
0
1000
500
100
200
300
400
600
700
800
900
ATSC see separate curves
SPEED (Km/Hr)
AIRCRAFT
Vehicles
Over Cities
COFDM implementations will inherently handle post
and pre-ghosts equally within the selected guard
interval.
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Main Results - Lab Tests

• C/N ATSC 4 dB better than DVB-T.This Advantage
  offset by Poor Noise Figure
• DVB-T is better than ATSC for Multipath
• ATSC is better than DVB-T for Impulse Noise
• ATSC cannot handle Flutter or Doppler Echoes
• ATSC is very sensitive to Transmission system
  impairments and IF translation
• DVB-T is better at handling Co-channel PAL
• DVB-T is better rejecting on channel interference
  (CW)

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