DVB-H
- an overview of the DVB-H system to be used for mobile video broadcasts.
DVB-H or Digital Video Broadcast - Handheld, is one of the
major systems to be used for mobile video and television for cellular phones and
handsets. DVB-H has been developed from the DVB-T (Terrestrial) television
standard that is used in many countries around the globe including much of
Europe including the UK, and also other countries including the USA. The DVB-T
standard has been shown to be very robust and in view of its widespread
acceptance it forms a good platform for further development for handheld
applications.
DVB-H development requirements
The environment for handheld devices is considerably different to that
experienced by most televisions. Normally domestic televisions have good
directional antenna systems and in addition to this the reception conditions are
fairly constant. Additionally most televisions receiving DVB-T will be powered
by mains supplies. As a result current consumption is not a major issue.
The conditions for handheld receivers are very different. In
the first instance the antennas will be particularly poor because they will need
to be small, and integrated into the handset in such a way that they either
appear fashionable, or they are not visible. Additionally they will obviously be
mobile, and this will entail receiving signals in a variety locations, many of
which will not be particularly suitable for video reception. Not only will be
signal be subject to considerable signal variations and multi-path effects, but
it may also experience high levels of interference. Also some difficulties are
presented by the fact that the handset could be in a vehicle and actually on the
move. The operation of DVB-H has to be sufficiently robust to accommodate all
these requirements.
Note on multi-path effects:
Multi-path effects occur when signals reach the
receiver via several different paths from the transmitter. This occurs
because the signals leave the transmitter in a variety of directions -
typically the transmitter may have an omni-directional radiation pattern
so that it radiates signals equally in all directions. Accordingly some
of the signal may travel directly to the receiver in what is termed the
direct path, but some of the radiated may be reflected off a nearby
hill, building or other object. In fact the received signal will consist
of components reaching the receiver from the transmitter via a large
number of paths. As the path length travelled by each of these
components will be slightly different, each component will arrive at a
slightly different time. If there are significant differences, this can
cause the data being transmitted to be corrupted under some
circumstances, although many modern receiver technologies can
accommodate this and use the different signals travelling over different
paths to reinforce one another. |
While DVB-T proved to be remarkably robust under many
circumstances, one of the major problems was that of current consumption.
Battery life for handsets is a major concern where users anticipated the life
between charges will be several days.
Operation of DVB-H
The DVB-H standard has been adopted by ETSI, European Telecom Standards
Institute, and in this way the system can be truly international, and this will
prevent compatibility problems caused by different countries and operators using
different variants of the same system. The documents for the physical layer were
ratified in 2004, with the upper layers defined in 2005.
DVB-H (Digital Video Broadcast Handheld) is based on the very
successful DVB-T (Digital Video Broadcast Terrestrial) standard that is now used
in many countries for domestic digital television broadcasts. DVB-H has taken
the basic standard and adapted so that it is suitable for use in a mobile
environment, particularly with the electronics incorporated into a mobile phone.
The DVB-H standard like DVB-T uses a form of transmission
called Orthogonal Frequency Division Multiplex (OFDM). This has been adopted
because of its high data capacity and suitability for applications such as
broadcasting. It also offers a high resilience to interference, can tolerate
multi-path effects and is able to offer the possibility of a single frequency
network, SFN.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a
form of transmission that uses a large number of close spaced carriers
that are modulated with low rate data. Normally these signals would be
expected to interfere with each other, but by making the signals
orthogonal to each another there is no mutual interference. This is
achieved by having the carrier spacing equal to the reciprocal of the
symbol period. This means that when the signals are demodulated they
will have a whole number of cycles in the symbol period and their
contribution will sum to zero - in other words there is no interference
contribution. The data to be transmitted is split across all the
carriers and this means that by using error correction techniques, if
some of the carriers are lost due to multi-path effects, then the data
can be reconstructed. Additionally having data carried at a low rate
across all the carriers means that the effects of reflections and
inter-symbol interference can be overcome. It also means that single
frequency networks, where all transmitters can transmit on the same
channel can be implemented.
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There are a variety of modes in which the DVB-H signal can be
configured. These are conform to the same concepts as those used by DVB-T. These
are 2K, 4K, and 8K modes, each having a different number of carriers as defined
in the table below. The 4K mode is a further introduction beyond that which is
available for DVB-T.
| Parameter |
2K mode |
4K mode |
8K mode |
| Number of active carriers |
1705 |
3409 |
6817 |
| Number of data carriers |
1512 |
3024 |
6048 |
| Individual carrier spacing |
4464 Hz |
2232 Hz |
1116 Hz |
| Channel width |
7.61 MHz |
7.61 MHz |
7.61 MHz |
Signal parameters for DVB-H OFDM Signal (8MHz Channel)
The different modes balance the different requirements for
network design, trading mobility for single frequency network size, with the 4K
mode being that which is expected to be most widely used.
The standard will support a variety of different types of
modulation within the OFDM signal. QPSK (Quadrature Phase Shift Keying), 16QAM
(16 point Quadrature Amplitude Modulation), and 64QAM (64 point Quadrature
Amplitude Modulation) will all be supported, chipsets being able to detect the
modulation and receive the incoming signal. The choice of modulation is again a
balance, QPSK offering the best reception under low signal and high noise
conditions, but offering the lowest data rate. 64QAM offers the highest data
rate, but requires the highest signal level to provide sufficiently error free
reception.
Time slicing
One of the key requirements for any mobile TV system is that it should not give
rise to undue battery drain. Mobile handset users are used to battery life times
extending over several days, and although battery technology is improving, the
basic mobile TV technology should ensure that battery drain is minimised.
There is a module within the standard and hence the software
that enables the receiver to decode only the required service and shut off
during the other service bits. It operates in such a way that it enables the
receiver power consumption to be reduced while also offering an uninterrupted
service for the required functions.
The time slicing elements of DVB-H enable the power
consumption of the mobile TV receiver to be reduced by 90% when compared to a
system not using this technique. Although the receiver will add some additional
power drain on the battery, this will not be nearly as much as it would have
been had the TV reception scheme not employed the time slicing techniques.
Interleaving
Interleaving is a technique where sequential data words or packets are spread
across several transmitted data bursts. In this way, if one transmitted burst or
group is lost as a result of noise or some other drop-out, then only a small
proportion of the data in each original word or packet is lost and it can be
reconstructed using the error detection and correction techniques employed.
Further levels of interleaving have been introduced into DVB-H
beyond those used for DVB-T. The basic mode of interleaving used on DVB-T and
which is also available for DVB-H is a native interleaver that interleaves bits
over one OFDM symbol. However DVB-H provides a more in-depth interleaver that
interleaves bits over two OFDM symbols (for the 4K mode) and four bits (for the
2K mode).
Using the in-depth interleaver enables the noise resilience
performance of the 2K and 4K modes to be brought up to the performance of the 8K
mode and it also improves the robustness of the reception of the transmissions
in a mobile environment.
MPE-FEC
In view of the particularly difficult reception conditions that may occur in the
mobile environment, further error correction schemes are included. A scheme
known as MPE-FEC provides additional error correction to that applied in the
physical layer by the interleaving. Tjis is a forward error correction scheme
that is applied to the transmitted data and after reception and demodulation,
allows the errors to be detected and corrected.
Compatibility with DVB-T
DVB-H is a development of DVB-T and as a result it shares many common
components. It has also been designed so that it can be used in 6, 7, and 8 MHz
channel schemes although the 8MHz scheme will be the most widely used. There is
also a 5MHz option that may be used for non-broadcast applications.
In view of the similarities between DVB-H and DVB-T it is
possible for both forms of transmission to exist together on the same multiplex.
In this way a broadcaster may choose to run two DVB-T services and one DVB-H
service on the same multiplex. This feature may be particularly attractive in
the early days of DVB-H when separate spectrum is not available.
Summary
DVB-H has been used in a number of trials and appear to perform well. It ahs
support from a number of the major industry players and is likely to achieve a
considerable degree of acceptance world-wide. Accordingly it is likely to be one
of the major standards, if not the major standard used for mobile video.
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