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1Research & Innovation

Developing the DVB-T2 Specification

Nick Wells

BBC Research & 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


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


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


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


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


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


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


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



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



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


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


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


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


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)


Frame Structure (3)

Typical frame duration: 150 -200 msFrame structure overhead typically less than 1%

T2 Frame

P1 P2 Payload

Frame structure information


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


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)


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


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%


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


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


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%


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

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