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Copyright © Digital Video Broadcasting Project DVB
The Magic of DVB-TA Technology overview
Seminar Venezuela
August 2005
John [email protected]
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Agenda
Frequency Planning
DVB-T modulation process – COFDM
Single Frequency Networks
DVB-T in 6MHz Channels
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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
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Frequency Planning
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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??
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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
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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)
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“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
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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
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UHF Usage: London
Photograph by courtesy and © BBC R&D
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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
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COFDM Principles
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COFDM
Coded [FEC]
Orthogonal
Frequency
Division
Multiplex
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Terrestrial Propagation Realities
Transmitter
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Multipath
Time
Amplitude
Delay
Delayed and Attenuated Echo
Delay
And another
Number, delay and amplitude of echoes varies continually
Wanted Signal
Composite Signal
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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
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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
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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
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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
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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
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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
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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
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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.
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COFDM – Guard Interval
Symbol period
Main Signal
Echo 1
Echo 2
Co-channel
Resulting Signal from Receiver
Guard Interval Rx active period
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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
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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
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Single Frequency Networks
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slide 34
Single Frequency Network Relays
Active Deflector on same frequency
MainRelay
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Single Frequency Networks
Multi TX Single freq network providing a more distributed coverage
Field Strength Profile of SFN
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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
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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
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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
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DVB-T in 6MHz
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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
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COFDM in RF ChannelsAmplitude, dB8 MHz
FrequencyFcentre
6 MHz
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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
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OFDM Parameters
8MHz
7MHz
6MHz
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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
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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
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Thank You
HTTP://WWW.DVB.ORG