In t rod uct ion t o t he DVB-T2 syst em and p ract ical im p lem ent at ion

Co m p r eh en siv e u n d er st an d in g

o f t h e DVB-T2 sy st em Novem ber 2010

Jérôm e DAVID

St rat eg ic Market ing Manager

Thomson Broadcast DVB-T2 solutions

Transmitter

- Elite 10/100

- Elite 1000

- Gapfillers / Repeaters

- Paragon

- Transmitter upgrade available for

Affinity, Optimum, Ultimate

Exciter

- Supporting MPLP up to 64 PLP’s

- DVB-T / DVB-T2 dualcast exciter

- Full DVB-T2 code rates / IFFT

sizes

- Full Time interleaver

implementation

- T2 MI SFN

- Dual IP Gigabit pro MPEG FEC

inputs

- Low PAPR using Tone

Reservation

- Easy remote upgrade and

operation

- Full DVB-T2 control via front

panel

- DVB-T2 modes : custom settings

or V&V profile selection

- SNMP v2, HTTP control

- Adaptive non linear correction

- Adaptive linear equalization

- Embedded GPS

- High stability low phase noise

OCXO

2/12

Key facts : Channel robustness close to theoretical Shannon limit

Up to 50% more bandwidth than 1st generation standard

Better Transmission quality/efficiency/reliability with PAPR

Larger and better SFN capabilities with distributed MISO scheme

Mobile / fixed network capabilities with MPLP

Future proof solution with Future Extension Frame concept

Flexible Time interleaver

Robustness adjustable per PLP for one or more services

Native IP based network distribution

DVB-T2 is the world’s most advanced digital terrestrial transmission system offering highest efficiency, robustness and flexibility. It introduces the latest modulation and coding techniques to enable highly efficient use of valuable terrestrial spectrum for the delivery of audio, video and data services to fixed, portable and mobile devices. These new techniques give DVB-T2 a 50% increase in efficiency over any other DTT system in the world. DVB-T2 will coexist for some years with DVB-T transmission.

Similarly to the first generation standard (DVB-T, ISDB-T, DAB), DVB-T2 uses OFDM (Orthogonal Frequency Division Multiplex) modulation, with a large number of sub-carriers delivering a robust signal, the new specification offers a range of different modes making it a very flexible standard.

DVB-T2 uses the LDPC (Low Density Parity Check) codes offering excellent performance in the presence of high noise levels and interference.

Transmission quality and reliability is improved by the introduction of advanced clipping functions, named PAPR techniques.

Service Specific coding based on Multi-PLP (Physical Layer Pipes) mechanism is provided to separately adjust the robustness of each delivered service within a channel to meet the required reception conditions for mobile, indoor and fixed reception in the same channel

Transmission diversity method, known as Alamouti coding, extends the coverage in small-scale single-frequency networks.

Finally, DVB-T2 group has defined a method called FEF (Future Extension Frame) which can be implemented over an existing DVB-T2 network. DVB-T2 offers a higher data rate, more robust signal, flexibility and scalability than any other existing transmission standard. For example, in the UK a DVB-T channel typically has a data rate of 24 MBit/s, whereas a DVB-T2 channel can carry 36 MBit/s, while keeping the robustness equal.

Thomson Broadcast contribution to DVB-T2 standard

Thomson Broadcast has a

continued involvement in DVB

activities since early 90’s.

Thomson Broadcast has

contributed to DVB-T2

standardization from 2006 until

now. Thomson Broadcast

longstanding experience in RF

domain and in broadcast

distribution network was

particularly involved in the process

of selecting techniques for PAPR

reduction and has chaired T2 MI

modulator interface specification

activities.

References

Air Interface Spec……...

EN302755

Network Distrib Spec...

TS102773

DVB blue Book………… DVB

A133

Guides…………………. TR101831

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DVB-T2 Key Concepts

A complex standard?

At a first glance, DVB-T2 is a complex standard. It is made of a lot of new building blocks and all of them must be carefully adjusted. These new advanced signal processing techniques such as rotated constellation, ACE (Active Constellation Extension) and TR (Tone Reservation) PAPR reduction techniques, MISO (Multiple Inputs Single Output) , Flexible time interleaver, FEF, scalable frame structure are good examples of how to bring added value to a broadcast technology.

…3 main guidelines for a comprehensive understanding of DVB-T2 possibilities

Capacity increase, Flexible system design and Efficiency improvements have driven DVB partners in DVB-T2 standardization process.

Thomson Broadcast DVB-T2 highlights

DVB-T2 standard is built upon

more than 30 building blocks. All of

these are extensively simulated

and tested against reference

golden streams issued by V&V

(Validation and Verification)

standardization group.

4/12

Capacity increase

In DVB-T2, benefiting from higher modulation order (256QAM) and more efficient DVB-S2 LDPC FEC, up to 50% capacity gain is achieved compared to DVB-T performances.

Figure 1: 256 QAM modulation

Figure 2: 256 baseband frame Beyond a simple constellation change, the number of pilots used to perform channel

equalization can be adjusted allowing further bitrates optimization. User can expect up to 8% gain.

Pilot ratio DVB-T DVB-T2

Continual pilot

8.5% 4%

Higher FFT size means shorter guard interval ratio for a given SFN cell network.

Guard interval duration

DVB-T 8K DVB-T2 with 32K

224 us 25% 6,25%

112 us 12.5% 3,12%

Bandwidth extension: Less guard band carriers are used enabling a +2.6% bitrate gain

Figure 3: Bandwidth extension

Thomson Broadcast DVB-T2 highlights

Thomson DVB-T2 exciter implements

All SPLP modes basic parameter

configuration:

- All IFFT sizes

- All guard intervals

- All constellation

- All LDPC code rates and code

types

- All time interleaver modes

support : I-JUMP, Static,

Dynamic, FEF

Thomson Broadcast Man Machine

interface is designed to be intuitive.

Thanks to its front panel, user can

select DVB-T2 transmission parameter

directly using the front panel.

Configuration is easy based on preset

modes taken from settings used in

V&V validation group. Automatic

parameter check forbids any

unauthorized configuration. Exciter

provides 3 different network type

configuration: MFN-TS is dedicated to

MPEG TS legacy network, MFN-T2 MI

is used for MPLP via a gateway and

SFN T2 MI is used for SFN operations.

5/12

Efficiency increase with Tone reservation or ACE

Lower PAPR achieved with tone reservation or Active Constellation Extension. The goal of these techniques is to achieve better RF performances and increase overall efficiency of the RF power amplifiers. With this technique, the purpose is to achieve Peak to Average Power Ratio below 10dB.

Figure 4: PAPR reduction More efficient SFN networks with distributed MISO

•

Flexible and robust system based on innovative frame structure New T2 frame structure: A T2 frame is based on P1 preamble, P2 preambles followed by data

symbols. P1 preamble is used for fast parameters detection and raw equalization while P2 preamble symbols are used to discover the subsequent T2 frame parameters.

Figure 5: Frame structure T2 frame is based on the same DVB-T frames using guard interval OFDM data symbols

MPLP mode offers a Service specific robustness while SPLP mode can be used in existing distribution network based on MPEG2-TS

Time interleaver combined with Cell interleaver can be adjusted for each PLP and the depth

of the interleaver can be adjusted on 1 or several consecutive frames. This makes time interleaver really robust against impulse noise.

Rotated constellation combined with cell interleaver improves signal robustness in corner

reception scenarios by de-correlating I & Q components of regular grey mapping scheme.

Figure 6: Rotated constellation

Thomson Broadcast DVB-T2 highlights

PAPR support

Thomson Broadcast exciter

implements PAPR natively improving

Quality / Efficiency / Lifecycle of the

transmission network. For instance,

efficiency is improved by more than

1% and power gain is above 0.4dB.

ACE techniques can be used in other

standards since Thomson Broadcast

proposed this PAPR technique in order

to be backward compatible with

others systems such as DVB-T or DAB.

While Tone Reservation is highly

efficient for higher order QAM

constellations, ACE is particularly

efficient for low order constellation

such as QPSK or 16QAM modulation

schemes. It makes ACE a good

solution for reducing PAPR in existing

mobile networks relying on QPSK or

16QAM modulation scheme.

Beyond PAPR technique power gain,

when using either PAPR Tone

Reservation or Active Constellation

Extension, transistor lifecycle is

dramatically improved by reducing

transistor ageing effect.

6/12

DVB-T2 introduction scenarios

DVB-T2 application is primarily dedicated to fixed HD /3D terrestrial networks over existing DVB-T networks but it contains everything

DVB-T2 and mobile application

MPLP concept is very useful in case of mobile applications allowing embedding mobile services among a fixed network. DVB-T2 is the only standard where you can combine long time interleaver depth for rugged mobile services and short time interleaver for fixed application. This could be particularly useful in deploying simulcast mobile and fixed TV services. New mobile devices (e.g. IPAD, tablet PC) will require more and more bit rate capabilities and will generate new usage. To be successful, a mobile broadcast network needs to be future proof in order to allow return on investment in the future. DVB-T2 allows maximum flexibility for mobile application.

Coexistence with future DVB mobile standard: DVB NGH – 4G LTE advanced

DVB NGH (Next Generation Handheld) will be the next standard within DVB. Thanks to FEF (Future Extension Frame), a DVB-T2 channel can spare time slots (Frames) for handheld application. DVB-NGH standard will be published mid of next year and will probably bring commonalities and convergence with 4G LTE broadcast mode.

DVB-T2 and new services

About the content aspect, DVB-T2 will be introduced for new services such as HD and 3D programs. 3D debate is not yet over but we can expect that higher bandwidth will be required to broadcast 3D services even for frame compatible format. DVB-T2 is designed to carry HD/3D services and offer the best signal efficiency (5.6bit/Hz in UK) even more with SFN diversity transmission (MISO). In order to deploy 3D content efficiently, it is necessary to work with DVB 3D TV specification where 3D TV program and 2D services can share the same broadcast channel at different time of the day. In this case, a 3D TV set must be capable to switch between 2D and 3D application. 3DTV specification defines some scenario and signalization over the MPEG to allow flexible 3D transport. For instance, 3D side by side requires broadcasting a full HD definition service with a minimum rate of 8Mb/s. In the UK, this will lead up to 4-5 3D services or full HD services.

Moving DVB-T to DVB-T2 – Dual Cast Concept

Where legacy DVB-T transmission exists, it is easy to introduce DVB-T2 for new services. Current DVB-T2 set-top boxes are also able to perform DVB-T modulation enabling a smooth transition to new services over a network. A user purchasing a DVB-T2 set top box or TV set can also watch legacy DVB-T transmissions. On the network side, the transition from a DVB-T channel to DVB-T2 is simplified when dual cast operation is available in the exciter because network operator only has to manage a single network instead of managing 2 different networks. DVB-T and DVB-T2 spectrum characteristics are closed because they share the same channel raster and both of them are based on OFDM transmission with guard intervals.

When system is transitioned to DVB-T2 and PAPR is implemented, it is possible to increase the coverage by 0.4dB compared to the same DVB-T coverage for the same performance (MER = 34dB at transmitter output). Combined with signal compression improvement, it is possible to move MPEG2 services to MPEG4 and bundle them in new multiplexes. This can free up new frequencies to deploy new services as VOD or 3D services.

Thomson Broadcast DVB-T2 highlights

Unique PLP solution

Thomson Broadcast is going beyond

common implementation by

providing up to 64 PLP in parallel

where 16 PLP is currently state of the

art. This allows unequalled flexibility

in tailoring different services for an

existing network.

For instance, you could provide in a

single channel 16 mobile services

requiring up to 10Mb/s and allowing 3

to 4 HD services in the same

broadcast channel.

Moreover PLP management in

Thomson exciter is dynamic so it

allows live reconfiguration. This is

supporting future statistical PLP

management in order to optimize

robustness in the channel.

7/12

How is DVB-T2 implemented?

Broadcasters can easily get started with a DVB-T2 transmission. DVB-T2 can be initially implemented on a primary DTV transmitter, leveraging existing capital expenditures on the RF transmitter, filtering, transmission line, and broadcast antenna. In MFN network, DVB-T2 Thomson Broadcast transmitters can be operated with the existing MPEG distribution network over ASI or Gigabit Ethernet using the SMPTE2022-1 pro MPEG FEC standard. When T2 network is operated in SFN network or if service protected MPLP variable is used, it is necessary to use T2 MI interface to send video streams to the transmitter. In this situation, a T2 MI (Modulator Interface gateway is needed at the head-end to parse signals into T2-MI packets. A field-proven transmitter must be tested with several Gateway manufacturers and solution must be highly robust to any kind of jitter (packet jitter or time source jitter) as very large hardware buffers size are available in the exciter to process IP input or MPEG-2 TS.

Figure 7: DVB-T2 Transmission chain example co-existing with DVB-T distribution network

Thomson Broadcast DVB-T2 highlights

Thomson DVB-T2 exciter implements

IP interface using pro MPE FEC. When

used additional large buffers are used

to ensure a robust IP interface.

8/12

How is SFN implemented?

T2 SFN networks setup are similar to SFN DVB-T networks but they require a T2-MI gateway and time reference equipments. All networks implementing SFN or / and MPLP must follow the T2-MI stack protocol.

Figure 8 : T2 Modulator Interface protocol stack

T2 Modulator protocol stack is relying on existing and proved MPEG-2 TS and MPEG2-GSE. MPEG-2 GSE is a generic purpose encapsulation protocol dedicated to efficient IP distribution of video services to the T2-MI gateway. The output interface of the gateway to the distribution network is either IP encapsulated or directly in ASI format.

Thomson Broadcast DVB-T2 highlights

Thomson Broadcast played an

active role in the definition of the

T2-MI (Modulator Interface).

T2 Modulator Interface standard is

part of DVB-T2 validation and

verification group.

The standard describing DVB-T2

Modulator Interface is labeled TS

102773.

9/12

DVB-T2 offers more scalability than a DVB-T network. It offers larger guard intervals. Same infrastructure basis than a legacy network can be re-used provided that equipments in the legacy distribution network are transparent to the transport stream. However it requires a gateway to perform timestamp insertion and Physical Layer Pipe multiplexing.

Figure 9: DVB-T2 Transmitter setup dedicated to SFN

Setting up a DVB-T2 SFN network requires different stages:

Modulator interoperability verification, especially when different modulator brands are used

Modulator – gateway system interoperability Program Feed Transparency: bit-synchronous requirement for SFNs means that program

feeds have to pass transport streams transparently, without any modification to packet order.

Bit rate jitter must remain low. Large Bit rate jitter from distribution network may cause SFN disruption.

Distrib. Network Bit rate Jitter Network Delay

Sat. DVB-S/S2 +/- 200ns 250 ms

Off Air +/-750ns 30 ms / hop

ATM over SDH Up to 200 ns Depending on the size of the network

MPEG over IP +/-20ms 30ms / switch

Figure 10: Typical delays experienced in SFN distribution

Thomson Broadcast DVB-T2 highlights

Thomson DVB-T2 exciter

interoperability has been verified by

BBC R&D.

Beyond V&V verification and

validation, Thomson Broadcast

contributes to European Celtic Engines

project and is leading DVB-T2 SFN

interoperability activities inside this

project.

Thomson Exciter implements SFN test

modes easing interoperability

verification between exciters

10/12

Installing a DVB-T2 SFN network requires having specific SFN test modes to verify timing adjustment between transmitters. Once done, a transmitter in a SFN network can be modeled using a triplet (P: power, d: delay, f: frequency). It is important to determine where the 0delay area is falling. In the following example, we can observe that the 0delay area is falling concomitantly to a 0dB echo area. This will result in destructive interference in the 0delay area.

Figure 11: Example of destructive interference situation in a DVB-T2 SFN One solution is to change the delay of one transmitter to make sure that this situation will not occur.

Figure 12: Non destructive “0delay” situation in a DVB-T2 SFN

Once the SFN network is setup, main issues come from the timing drift or disruption in the GPS signal reception or in transport stream. In SFN networks, transmitters must be carefully monitored and it is important that the exciter has embedded capabilities to detect such events separately in order to take the proper preventive or corrective action. Furthermore, on main strategic sites, it is desirable that the transmitter offers at operator wish permanent transmission in case of time reference loss or perturbation of the distribution network.

11/12

Interest of MISO (Multiple Inputs Single Output)

In this mode, DVB-T2 is broadcasted on 2 different antennas. Hence, DVB-T2 signal is split in main and complimentary components feeding the antennas. Main advantage is that the receiver will benefit from an increased C/N if the receiver is able to see both signals. If the receiver is not able to see one of the 2 signals, its performance remain equal to SISO (Single Input Single Output antenna).

Figure 13: MISO co-sited For instance, current DVB-T2 UK transmission is based on a LDPC code rate of 2/3. With MISO, it might be possible to use LDPC code rate 3/4 leading to a bit rate gain of 5 Mb/s.

0

10

20

30

40

50

DVB-T DVB-T2 DVB-T2 MISO

Bitrate comparison Mb/s

Figure 14: Performance Gain

12/12

Network Constraint and distributed MISO

If both transmissions were to be co-sited, the cost would be prohibitive from the network operator point of view. That is the reason to introduce distributed MISO where complimentary signals are sent from two different existing nearby sites. In this situation, the C/N gain can be up to 3dB and be converted into higher bitrates.

Figure 15: MISO in Distributed transmission