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1Research & Innovation
Developing the DVB-T2 Specification
Nick Wells
BBC Research & Innovation
2Research & Innovation
Introduction
UK are committed to starting T2-based HD service in 2009/2010
Presuming a 30% increase in capacity over DVB-T
Other countries (e.g. Finland, Sweden, Italy) also showing strong early interest in T2
More than 40 companies and 70 people have actively participated in development of T2 specification
Many meetings; many tens of telecons; thousands of emails
Completed T2 specification to be published to DVB Technical Module by end of March
3Research & Innovation
Background to T2 development
Initial motivationDVB-S2 successful in providing 30% more capacityTerrestrial HD will require new receiversWhy not upgrade modulation scheme while upgrading receivers/set-top boxes for HD?
DVB processLaunched Study Mission in February 2006
Conclusion: Capacity of terrestrial network must be increased in order to be a viable platform for HD.
TM-T2 ad-hoc group started August 06: T2 specification to be published to DVB-TM, March 08
4Research & Innovation
T2 Timeline
TM-T2 ad hoc group
2006 2007 2008 2009 2010
T2 Study Mission
T2 spec stable
T2 VLSI development
HD T2 services
FirstPrototype
chipT2 Receiver development
Firstcommercial receivers
T2 lab & field trials
5Research & Innovation
Commercial Requirements for T2
Key requirements includeMust be able to use existing domestic receive antenna and existing transmitter infrastructure
Intended primarily for services to fixed and portable receivers
Should provide minimum of 30% capacity increase over DVB-T
Within same planning constraints as DVB-T
Should provide for improved SFN performance
Should have mechanism for providing service-specific robustness
Should provide for bandwidth and frequency flexibility
Should provide means to reduce peak-to-average power ratio
6Research & Innovation
Key features: Compatibility with S2
General DVB principlesDVB should aim to provide a coherent family of standards (where possible)There should be easy translation between standards
e.g. between S2 and T2Don’t re-invent solutions when they already exist within other DVB standards
T2 adopted two key technologies from S2System Layer architecture
Packaging of data into BaseBand FramesS2 LDPC error correcting codes
7Research & Innovation
Key features: BB Frames and LDPC
Data packaged into BaseBand Frames
BaseBand Frames protected by the S2 LDPC FECWith an additional small BCH code to mop up any residual errors after LDPC decoding
Data LDPC check bits
Header BCH FEC bits
BaseBand Frame
FEC frame = 64800 bits
8Research & Innovation
Comparative Performance of S2 LDPC
S2-LDPC can give up to 5dB improvement in C/N performanceCompared to DVB-T convolution code at output error rate of 1 in 104
This gain can be converted to increased bit rate by changing to higher order modulation (typically 30% gain)
14 16 18 20 22 24 26 2810-8
10-6
10-4
10-2
100
Es/No (dB)
BER
Comparison between Convolutional code and LDPC for rate 2/3 0dB 1μs echo
LDPC 256QAMCC 256QAMCC 64QAMLDPC 64QAM
9Research & Innovation
Key Features: Modulation (1)
T2 will use conventional Guard-Interval OFDM (GI-OFDM)as in DVB-T
Each symbol carries data on a large number of separate carriers
Many options for this number will be available in T2 – e.g.32K: to give improved Single Frequency Network performance1K for bandwidth and frequency flexibility
Increasing the number of carriers increases the symbol period
GI Data GI Data GI Data
Symbol
10Research & Innovation
Key Features: Modulation (2)
Increasing the symbol period Can reduce guard interval overhead for given size of SFNCan increase SFN capability for a given fractional GI
32K symbolGI
GI 8K symbol
~6% overhead
25% overhead
11Research & Innovation
Key Features: Modulation (3)
T2 will include 256 QAM modeCarries 8 bits/ data cell
(c.f max. 6 bits / data cell for 64 QAM in DVB-T)
Enables greater capacity, exploiting improved FEC performance of LDPC
Studies show that typical tuner phase noise should not be a problem
12Research & Innovation
Scattered Pilot Patterns (1)
Scattered pilots are data cells of known amplitude and phase
Receiver uses these to compensate for effects of channel changing in frequency and time.
In DVB-T, 1 in 12 data cells is a scattered pilot8% overheadIndependent of guard-interval fraction
8%
Frequency
Time
13Research & Innovation
Scattered Pilot Patterns (2)
T2 will probably use several different scattered pilot pattern options
Aim: to minimise pilot pattern overhead for a given fractional guard interval; e.g. ….
1%
4%
Frequency
Time
14Research & Innovation
Key features: Service Specific Robustness
Each service can be given its own modulation mode and FEC coding rate
Different applications: roof-top reception/portables
Carriers
Symbols
Each service is given a slice within a frameAlso enables power saving in the receiver
15Research & Innovation
Key features: Frame Structure
Start of frame is signalled by a short OFDM symbol (‘P1’)Based on 1K OFDM symbol with frequency shifted repeats at front and rear of symbol
1K symbol
AC B
Fsh Fsh
This format of P1 symbol providesRapid and robust detection of T2 signal without pattern imitationFast frequency lock mechanismLimited signalling capability (e.g. for FFT size in main frame)
16Research & Innovation
Frame Structure (3)
Typical frame duration: 150 -200 msFrame structure overhead typically less than 1%
T2 Frame
P1 P2 Payload
Frame structure information
17Research & Innovation
Rotated Constellations (1)
Map data onto a normal QAM (x,y)
Rotate constellation
Axes now (u1,u2)
Ensure u1 and u2 travel in different cells
So that they fade independentlyGather together in receiver
Each of u1,u2 carries all of the info of original x,y
So can decode (less ruggedly) if one is erased completely
18Research & Innovation
Rotated Constellations (2)
Rotated constellations provide significantly improved robustness against loss of data cells
Can achieve gains of up to 5 dB on difficult channels
Can translate into increased bit rate by choosing less robust FEC with lower overhead
15 16 17 18 19 20 21 22 23 24 2510-6
10-5
10-4
10-3
10-2
10-1
Es/No (dB)
BER
16 qam no rotation16 qam, atan(1/4)
Comparison of performance for rotated/non-rotated constellations(code rate=4/5; channel = Rayleigh + 15% erasures)
19Research & Innovation
Transmit Diversity
It is proposed that T2 will include an optional Alamouticoding mode for simple SFNs
While Tx1 transmits pair of data cells (S0,S1), Tx2 transmits (-S1
*,S0*)
Also involves modification of pilot patterns to measure h1 and h2This prevents possibility of ‘flat fading’ at receiver
Initial planning studies predict 30% increase in coverage area for simple SFNs
RXTX1TX2
So ,S1 -S1*S0
*h1 h2
20Research & Innovation
Peak to Average Power Reduction
T2 will use a combination of 2 PAPR reduction techniques
Tone reservation1% of carriers reserved to counteract any peaks
‘ACE’Constellation distortion to counteract peaks
Reduction in peak-to-average power ratio allows peak amplifier power rating to be reduced by 20%
21Research & Innovation
Additional Features
Future Expansion Frames (FEFs)Provide a mechanism for future compatible enhancements –e.g. MIMO
P1 P1 P1 P1 P1P1
T2 T2 T2 T2FEF FEF
Time Frequency SlicingIf receiver has 2 tuners, a large multiplex of signals can be spread across several linked frequencies
Can give significant Stat Mux gain (20%) and frequency planning gain (5dB)
Transmitter identificationA low-level signal (-40dB) can be added to transmitted signal to identify individual transmitters within a network
Provides capability for professional receiver to identify faults within SFN
22Research & Innovation
Status of T2 Specification
Huge amount of work has been done in last 8 months
Specification is being drafted and reviewed for publication to DVB Technical Module next week
Some subsequent polishing may be requiredTo fully comply with ETSI guidelinesExpected to be stable enough for VLSI design to start
Lab and field trials to test specification expected before end of 2008
23Research & Innovation
UK Capacity estimate (unverified)
Current UK mode T2
Modulation 64QAM 256QAM
FFT size 2K 32K
Guard Interval 1/32 1/128
FEC 2/3 CC + RS (8%) 3/5LDPC + BCH (0.3%)
Scattered Pilots 8% 1%
Continual Pilots 2.6% 0.35%
Frame structure overhead 0% 0.7%
Bandwidth Standard Enhanced
Capacity 24 Mbit/s 35.4Mbit/s
Capacity = DVB-T + 47%
24Research & Innovation
Conclusion
T2 specification will be completed in time for launch of HD services in 2009/2010
Specification meets requirements set by Commercial Module
Hooks have been included for future extensionse.g. Time Frequency Slicing; Future Expansion Frames
30% improvement in capacity should be easily achieved
Up to ~45% might be possible if all techniques verified
T2 will be a new and exciting specificationStemming from excellent and intense international collaboration involving a large number high-calibre contributions