Copyright © Digital Video Broadcasting Project DVB

The Magic of DVB-TA Technology overview

Seminar Venezuela

August 2005

John Bigenibigeni@dvb.org

Agenda

Frequency Planning

DVB-T modulation process – COFDM

Single Frequency Networks

DVB-T in 6MHz Channels

Why Digital

Better quality transmissions (error free)

Higher Efficiency = More services within the one broadcast channel

Services beyond video & audioHigh DefinitionMultiple servicesData information servicesInteractive servicesMobile services etc etc

Services can be provided with less power

Spectrum efficiency using SFN enables less spectrum requirements – other applications

Frequency Planning

Frequency Planning

Fundamental Matter - Scarce Resource

Analogue Rules set limit to more Services

No NEW Spectrum in most countries

Digital Transmission changes Rules0Signals have different behaviour

Digital Signals can occupy unused space -“Taboos”

International Co-Ordination Needed

Eventually Digital Only - but long wait??

Analogue Frequency Planning

r2r

F2

F2 F2

F3

F3 F3

F3

F1

F1

F4 F4

F4

Each frequency is used many times in a network

Most spectrum is used by low power repeaters

Main problem is Interferance

Cannot use same frequency

Frequency Re-Use Distance

The 6 main Taboo Frequencies are:

[N+1] adjacent, [N+5] IFs and [N+9] Images

Digital Planning RulesDigital services can co-exist with Analogue existing services in the VHF/UHF bands

Digital has good Spectral efficiency and there is no interference between A & D with proper planning.

COFDM enables single Frequency Networks

Digital transmitters must be located at same transmission location for maximum efficiency

Digital Pwr = - 13db Analogue pwr (Approx)

“Taboo” ChannelsAMPLITUDE

FREQUENCY

12 -20 dB8 MHz

CHANNEL N N + 2N + 1 N + 3 N + 4 N + 5 N + 6 N + 7 N + 8

VISION CARRIERSOUND CARRIER

DIGITAL SIGNAL INUNUSED CHANNEL

Channel CombiningVISION

CARRIER

20dBNom.

AUDIOCARRIER

DIGITALCHANNEL

CHANNEL [N - 1]LOWER ADJACENT

6 MHz

CHANNEL [N + 1]UPPER ADJACENT

6 MHz 6 MHz

6 MHz

6.552 MHz

PAL/SECAM[NTSC]

PAL/SECAM[NTSC]

1.25MHz

10 dB

20dB

Colour

Sub Carrier

Colour

Sub Carrier

VISIONCARRIER

UHF Usage: London

Photograph by courtesy and © BBC R&D

Service Area PlanningPICTUREQUALITY

C/N

ANALOGUE

DIGITAL

THRESHOLD OFACCEPTABILITY

5

4

3

2

1

10 15 20 25 30 40 45

ANALOGUEFAILURE

35

ModulationDependentVariation

TypicalChoice of

C/N

0

COFDM Principles

COFDM

Coded [FEC]

Orthogonal

Frequency

Division

Multiplex

Terrestrial Propagation Realities

Transmitter

Multipath

Time

Amplitude

Delay

Delayed and Attenuated Echo

Delay

And another

Number, delay and amplitude of echoes varies continually

Wanted Signal

Composite Signal

COFDM : HOW ?

1 1 -- Organize time & frequency partitions in the RF channelOrganize time & frequency partitions in the RF channel

time

frequency

RFRFChannelChannel

bandwidthbandwidth

frequencyfrequencysubsub--bandband

timetimesegmentsegment

To consider stabilized channel characteristics inside To consider stabilized channel characteristics inside ““time time vsvsfrequencyfrequency”” cellscells

With courtesy of Mr Gerard Faria Teamcast

Make subMake sub--carriers orthogonal (carriers orthogonal (dfdf=1/=1/dtdt) to avoid ) to avoid ““interinter--carrierscarriers””interference interference

COFDM : HOW ?2 2 -- Spread subSpread sub--carriers over carriers over ““time time vs vs frequencyfrequency”” cellscells

time

frequency

OFDMOFDMsymbolsymbol

8K system8K system 2K system2K system

With courtesy of Mr Gerard Faria Teamcast

COFDM : HOW ?

3 3 -- Insert Guard Interval to avoid Insert Guard Interval to avoid ““interinter--symbolsymbol”” interferenceinterference

time

frequency

Useful symbolUseful symboldurationduration

Guard Guard IntervalIntervaldurationduration

OFDMOFDMsymbolsymbol

BUT : Guard interval introduces a first loss in transport capaciBUT : Guard interval introduces a first loss in transport capacity ty

With courtesy of Mr Gerard Faria Teamcast

COFDM : HOW ?

4 4 -- Insert Insert ““Synchronization PilotsSynchronization Pilots”” to help Receivers to lock onto the useful to help Receivers to lock onto the useful signalsignal

FFTFFTtime windowstime windowsfor receiversfor receivers

OFDM FrameOFDM Frame(68 OFDM symbols)(68 OFDM symbols)

time

frequency

BUT : Synchronization markers introduce a second loss in transpBUT : Synchronization markers introduce a second loss in transport capacity ort capacity With courtesy of Mr Gerard Faria Teamcast

COFDM : HOW ?

5 5 -- Prepare data to be carried on OFDM symbolsPrepare data to be carried on OFDM symbols

time

frequency

DATA to broadcast

Protected DATA (convolutionnal coding for error protection)

BUT : Protection codes introduces a further loss in transport caBUT : Protection codes introduces a further loss in transport capacity pacity With courtesy of Mr Gerard Faria Teamcast

COFDM : HOW ?

6 6 -- Map bits onto OFDM : spread contiguous data bits over distant sMap bits onto OFDM : spread contiguous data bits over distant subub--carrierscarriers

time

frequency

DATA to broadcast

Protected DATA

0011

0000

11

Create frequency diversity to improve robustness against selectiCreate frequency diversity to improve robustness against selective fadingve fadingWith courtesy of Mr Gerard Faria Teamcast

DVB-T : Regular constellations

4QAM4QAM00

16QAM16QAM0000

64QAM64QAM000000

Each constellation gives to each subEach constellation gives to each sub--carrier a specific data transport carrier a specific data transport capacitycapacity

2 bitsper carrier 4 bits

per carrier6 bits

per carrier

Data mapping is realised by applying one physical modulation to Data mapping is realised by applying one physical modulation to each subeach sub--carriercarrier

Channel Response due to Terrestrial propagation

Selective and timeSelective and time--variant frequency fading can be corrected byvariant frequency fading can be corrected by

COFDM. COFDM.

COFDM – Guard Interval

Symbol period

Main Signal

Echo 1

Echo 2

Co-channel

Resulting Signal from Receiver

Guard Interval Rx active period

OFDM - Features

Multicarrier - many carriers sharing

Uses Fast Fourier Transform [FFT]

Low symbol rate per carrier0 KBaud: Long Symbol Period, can Extend with Guard

Interval

Resistant to echoes, Interference etc

Single Frequency Networks [SFN]

With FEC becomes COFDM

COFDM Advantages

Performance in RF Channel0Multipath: withstands 0dB echo, no ghosting0 Interference: can be interleaved with high power

analogue signals0 C/N requirement: much better than analogue

Spectrally efficient - can use SFN

Power efficient - low power transmitters

Makes possible mobile services

ATSC cannot provide these features

Single Frequency Networks

slide 34

Single Frequency Network Relays

Active Deflector on same frequency

MainRelay

Single Frequency Networks

Multi TX Single freq network providing a more distributed coverage

Field Strength Profile of SFN

SFN PRINCIPLEIN ORDER TO AVOID INTERFERENCE IN THE OVERLAPPING AREAS, EACH TRANSMITTER MUST BROADCAST :

At the same Frequency

At the same Time

The same Symbol

All the TX are synchronized on the same Frequency reference : 10 MHz GPS

All the TX are tuned on the same temporal Reference : 1 PPS (*) GPS

All the TX must Broadcast the same Multiplex (« Bouquet ») => no local insertions allowed

(*) 1 Pulse Per Second

EXAMPLE OF LARGE SFN NETWORK

Optical link Production

Center (example)

OpticalReceiver

DVB-T MOD

uW TX

Sources and MPEG Coders/Multiplexers

OpticalTransmitter

F1DVB-T MOD

µW RX DVB-T MOD

F1

F1

µW TX

µW RX

GPS

10 MHz1PPS

GPS 10

MH

z1P

PS

GPS

10 MHz1PPS

SFN Inserter

GPS

10 MHz1PPS

Regional / National Networks

TX1

TX2 TX3

GI USEFUL SYMBOL

1 2 3

Satellite Distribution

EXAMPLE OF LOCAL SFN NETWORK

Head End

NETWORK

HP SFN TX

MP SFN TX MP

SFN TX

MP SFN TX

SFNINSERTER

GPS

10 MHz1PPS

GPS

10 MHz1PPS

Gap Filler

Gap Filler

Gap Filler

GPS

10 MHz1PPS

GPS

10 MHz1PPSGPS

10 MHz1PPS

Quick coverage of the Main Zone

Easy extension of the Coverage

DVB-T in 6MHz

DVB -T in 6Mhz

DVB T specifications include 6Mhz

Tested at 6Mhz in many locations many times and demonstrated at NAB2000 with real transmissions

Payloads scale with Bandwidth (6:8)

Bandwidth change by clock frequency0 Same Modulator/Demodulator for all bandwidths0 Same Tuner as for any 6 MHz system

C/Ns stay the same

COFDM in RF ChannelsAmplitude, dB8 MHz

FrequencyFcentre

6 MHz

SNR & Bit RatesDVB-T [6 MHz]

Minimum Data Rate: 4.5 Mbit/s @ <3 dB*

Minimum Data Rate: 3.7 Mbit/s @ <3 dB#

Maximum Data Rate: 24 Mbit/s @ <20 dB*

ATSC/8VSB

Original Specification for only one Bit Rate

19.3 MBit/s @ 15.5 dB*Using 7/8 Rate Code, 1/32* [1/4#] Guard Interval in 6 MHz Gaussian

Channel

+3-7 dB for Rayleigh

OFDM Parameters

8MHz

7MHz

6MHz

DVB-T - Bit Rates 6 MHzCodeRate

QPSK 16 -QAM

64 -QAM

QPSK QPSK16 -QAM

64 -QAM

64 -QAM

16 -QAM

D/Tu = 1/4 D/Tu = 1/8 D/Tu = 1/32

1/2

2/3

3/4

5/6

7/8

3.73

4.97

5.59

6.22

6.53

7.46

9.95

11.19

12.44

13.0619.59

18.66

16.79

14.92

11.19 4.14

5.52

6.22

6.91

7.25 14.51

13.82

12.44

11.05

8.29 12.44

16.58

18.66

20.73

21.77

4.52

6.03

6.78

7.54

7.91 15.83

15.08

13.57

12.06

9.08 13.57

18.09

20.35

22.62

23.75

224 microsecs 112 microsecs 28 microsecs

DVB-T in 6 MHz

11.3% Carrier pilots in EVERY symbol0 Channel bandwidth explored in detail

1.36 ms [8k]/340 µsec [2k] symbol period (Better than 8Mhz performance)

0 Measures channel >70 times more often than 8VSB

ATSC/8VSB

Training Sequence only 0.3% of signal

Repeats only every 24.2 msecs

Channel Estimation

Thank You

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