Technical overview of DVB-T2As a fi

Technical overview of DVB-T2
As a first step in developing the DVB-T2 specification, the DVB Project developed
the key commercial requirements for the proposed specification. These commercial
requirements placed some limitations on the technology that could be used but also
ensured that the new specification could meet with the needs of the existing
broadcast market.
Among the 21 commercial requirements approved by the DVB Project, it was
necessary that the new specification could make use of existing domestic receiving
antennas and transmitter infrastructure, provide a minimum of 30% capacity
increase compared with the DVB-T standard in similar reception conditions, and
meet with the interference levels and spectrum mask requirements of the Geneva
2006 Agreement. The specification is also designed to target fixed rooftop antennas.
Based on these commercial requirements, the DVB Project established a group to
develop the technical features of the proposed specification.
Key technical features
Building on the success of DVB-T, the DVB-T2 specification incorporates the latest
developments in modulation and error-protection to increase the bit-rate capacity
and improve signal robustness. To achieve these improvements, detailed changes
have been made to the physical layer features, to the network configuration, and to
optimize performance to match the propagation characteristics of the frequency
channel.
The DVB-T2 commercial requirements called for capacity increase of 30%
compared with DVB-T in equivalent reception conditions. However, current field
testing suggests that the capacity gain may be closer to 65%. However, it is not until
widespread experience is gained with DVB-T2 in all application circumstances that
the full extent of its beneficial gain will be known.
Physical layer features
Like the DVB-T standard, the DVB-T2 specification uses OFDM (Orthogonal
Frequency Division Multiplex) modulation. The availability of a large number of
modes allows for the same level of flexibility to suit the specific area of application
as with the DVB-T standard. However, the addition of the 256 QAM mode in the
DVB-T2 specification allows for the ability to increase the number of bits carried
per data cell and benefit from improved FEC (forward error correction) which gives
a major capacity boost.
Like the DVB-S2 standard, the DVB-T2 specification makes use of LDPC (Lowdensity
parity-check) codes in combination with BCH (Bose-Chaudhuri-
Hocquengham) to protect against high noise levels and interference. In comparison,
the DVB-T standard, which makes use of convolutional coding and Reed-Solomon,
two further code rates have been added.
As with the DVB-T standard, the DVB-T2 specification makes use of scattered pilot
patterns for use by receivers to compensate for changes in channels as a result of
time and frequency. The DVB-T2 specification has the additional flexibility
provided by the choice of eight scattered pilot patterns that can be selected based
upon the FFT size and Guard Interval fraction adopted to maximize the data
payload.
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The DVB-T2 specification offers a choice of various robustness and protection
levels for each service separately within a transport stream carried by a signal in a
given channel. This allows each service to have a unique modulation mode
depending on the required signal robustness through the use of Physical Layer Pipes
(PLP).
Network configuration
The DVB-T2 specification allows for the possibility of maximizing the performance
in single frequency network applications. Compared with the DVB-T standard, new
carrier modes have been added to improve the performance of SFNs and increase the
symbol period.
This increase in the symbol period, in turn, allows for a reduction in the proportional
size of the guard interval while still handling multipath reflections. An additional
Alamouti coding mode is also available in option for simple SFNs where a receiver
can benefit from signals simultaneously received from more than one transmitter.
Using these features, it has been estimated that the use of a SFNs could allow for a
potential capacity gain of up to 67% compared with a DVB-T mode of similar
robustness.
Compared with the DVB-T standard, the DVB-T2 specification allows for a
reduction in the peak to average power used in the transmitter station. The peak
amplifier power rating can be reduced by 25% which can significantly reduce the
total amount of power that must be made available for the functionality of high
power transmission stations. This is achieved through the use of tone reservation and
ACE (active constellation extension) techniques.
The DVB-T2 specification defines a single profile which incorporates time-slicing
but not time-frequency-slicing (TFS). Features which would allow a possible future
implementation of TFS (for receivers with two tuners/front-ends) can be found in
annex E (ETSI EN302755). TFS might in future make it possible for a large
multiplex of signals to be spread across several linked frequencies and thus benefit
from a potentially significant gain in capacity as a result of statistical multiplexing
and a gain in network planning as a result of increased frequency diversity.
Preliminary analysis within DVB suggests that TFS may allow a capacity gain of
approximately 20% and a network planning gain of 3-4 dBs.
Comparison of available modes in DVB-T and DVB-T2
DVB-T DVB-T2
FEC Convolutional Coding + Reed
Solomon 1/2, 2/3, 3/4, 5/6, 7/8
LPDC + BCH 1/2, 3/5, 2/3,
3/4, 4/5, 5/6
Modes QPSK, 16QAM, 64QAM QPSK, 16QAM, 64QAM,
256QAM
Guard Interval 1/4, 1/8, 1/16, 1/32 1/4, 19/256, 1/8, 19/128, 1/16,
1/32, 1/128
FFT size 2k, 8k 1k, 2k, 4k, 8k, 16k, 32k
Scattered Pilots 8% of total 1%, 2%, 4%, 8% of total
Continual Pilots 2.6% of total 0.35% of total
Source: DVB Project
Optimised performance to match frequency channel propagation characteristics
The DVB-T2 specification provides for improved signal robustness against external
influences such as the impact cause by geography, weather, and buildings. This is
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achieved through the use of the rotated constellations technique and time and
frequency interleaving.
Rotated constellations provide significantly improved robustness against loss of data
cells by ensuring that loss of information from one channel component can be
recovered in another channel component. This is achieved by mapping data on
normal QAM (x,y axis) which is then rotated in the “I-Q” plane so that each axis on
its own (u1, u2) carries sufficient information. The I and Q components are sent at
different times using different cells to ensure information recovery if necessary.
Time interleaving provides further signal robustness against disturbances such as
impulsive noise over a given period of time and disturbances over a limited
frequency span.
Expected spectrum efficiency gain
The exact capacity gain that can be achieved using the DVB-T2 specification in
comparison with the DVB-T standard is not yet fully known. Commercial
requirements called for a capacity gain of 30% in comparison with DVB-T in
equivalent reception conditions. However, the current transmission mode selected by
the United Kingdom shows that the capacity is as much as 66%.
Example of MFN mode in the United Kingdom
Current UK DVB-T
mode
Selected UK DVB-T2
mode
Modulation 64 QAM 256 QAM
FFT size 2k 32k
Guard Interval 1/32 1/128
FEC 2/3CC + RS 2/3 LDPC + BCH
Carrier mode Standard Extended
Capacity 24.1 Mbit/s 40.2 Mbit/s
Source: OFCOM
The use of national SFN can allow for greater spectrum efficiency. However,
national SFNs limit the ability for broadcasters to provide regional and local
services. The full extent of the beneficial capacity gain of DVB-T standard will not
be known until further experience is acquired.
Status (pilots, announced launches)
Currently, the United Kingdom and Finland have announced plans to launch HDTV
services on the terrestrial platform using the DVB-T2 specification. DVB-T2 trials
are either underway or have been completed in Finland, Germany, Italy, Spain,
Sweden, and the United Kingdom.
In the United Kingdom, the communications regulator OFCOM has decided to
allocate one of its six DTT multiplexes (Multiplex B) in the UHF frequency band for
the provision of HD services using the DVB-T2 specification in combination with
MPEG-4 AVC compression technology. It has allocated 4 HD television programme
service slots to broadcasters with services on the current analogue terrestrial
television platform (BBC, ITV, Channel 4/S4C, and Five).
The release of Multiplex B is possible due to the increased capacity available in the
other DTT multiplexes after the completion of analogue switch-off. The SD services
in Multiplex B will be transferred to the other multiplexes so that no SD programme
services will need to be sacrificed. As a result, the launch of HD services using
DVB-T2 will correspond with the calendar for analogue switch-off. As a given
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