What is DVB-SH?
- an overview, information or tutorial about the basics of what is DVB-SH,
satellite services to handheld devices, and an overview of the DVB-SH standard
and DVB-SH specification.
DVB-SH, Digital Video Broadcast - Satellite services to
Handheld devices is a standard or specification that is likely to be widely used
for Mobile TV services. The DVB-SH standard has been developed to deliver video,
audio and data services to small handheld devices including mobile phones and
PDAs and using frequencies typically within S band but in any case below 3 GHz
from either satellite or terrestrial networks. DVB-SH has also been designed to
complement DVB-H which is focussed on delivering mobile video from terrestrial
networks at frequencies within the UHF TV bands.
One of the key features of DVB-SH is that it is aimed for use
for both satellite and terrestrial delivery. This is a significant advantage
because it allows satellite delivery to achieve coverage of large areas of a
country and then terrestrial coverage can be used for gap fillers for example in
built up areas in cities where tall buildings may shield the satellite signal.
In view of its specifications, DVB-SH is will be used
alongside other forms of cellular technology as it is estimated that most
applications will be in small mobile devices such as cell phones, PDAs, etc.
DVB-SH background
The first DVB standard for mobile TV was DVB-H which was
published in 2004 and some services using this standard are now being deployed.
DVB-H is aimed at terrestrial networks using frequencies at the low end of the
UHF spectrum. Seeing the requirement for additional satellite delivery, work was
started on the DVB-SH specification in November 2006 and the DVB organisation
approving the DVB-SH standard on 14th February 2007.
Basics of DVB-SH standard
In order that the DVB-SH standard is able to deliver the
required performance it offers schemes suitable for both satellite and
terrestrial delivery. To provide the required performance both OFDM and TDM
signal techniques are used. As a result there are two architectures for DVB-SH:
- SH-A uses OFDM on both the satellite and terrestrial links
- SH-B uses TDM on the satellite link and OFDM on the
terrestrial link
While the OFDM (Orthogonal Frequency Division Multiplex)
provides excellent characteristics for terrestrial delivery, the TDM (Time
Division Multiplex) scheme has advantages for satellite delivery.
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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. |
The two variants require different architectures in the
transmitters and receivers in terms of the demodulators and modulators as well
as some of the signal encoding areas. A choice between SH-A and SH-B can then be
made according to the satellite characteristics and regulatory considerations.
Signal modulation within the OFDM and TDM signal formats can
also be varied. Possible choices can be QPSK, 8PSK and 16APSK are available
within the TDM mode using a variety of roll-off factors (0.15, 0.25, or 0.35).
For OFDM QPSK, 16 QAM and non-uniform 16QAM are available within the OFDM
transmission mode.
Flexibility is also provided within DVB-SH for different
bandwidths. This allows service providers to tailor the bandwidth of the
transmission according to their constraints. The DVB-SH standard allows a choice
of transmission bandwidths between: 8 MHz, 7 MHz, 6 MHZ, 5 MHz, 1.7 MHz. The FFT
length choice is between 8k, 4k, 2k, and an additional 1k scaled directly from
the 2k mode.
Using DVB-SH it is possible to provide seamless reception of
satellite and terrestrial signals using signal diversity either via a Single
Frequency Network (SFN) - SH-A only; Maximal Ratio Combining (MRC) - both SH-A
and SH-B; or code diversity (complementary puncturing) - SH-B only. The latter
scheme is possible via a common frame structure shared between OFDM and TDM
modes.
FEC and turbo-coding
The fact that DVB-SH is focussed on frequencies up to 3 GHz,
and expected to operate on frequencies around 2.2 GHz means that the performance
requirements for the system are more exacting than those for DVB-H where
frequencies up to 900MHz are typically used. Typically it is found that the
signal to noise ratios are inferior, and this could result in high levels of bit
error rate and poor performance unless power levels were raised and dense
terrestrial networks employed. As these options are not desirable, enhancements
have been included in the signal processing areas of the DVB-SH standard. A
state of the art forward error correction system has been included in the form
of the 3GPP2 Turbocode. In addition to this, the standard includes a highly
effective channel interleaver. This offers time diversity of between 100 ms up
to several seconds dependent upon the targeted service levels and also the
amount of memory available in the target receivers. By interleaving, the effects
of interference can be minimised, and the longer the period of interleaving, the
greater the interference duration that can be tolerated.
In addition to this, pilot symbols are used to provide a
robust form of signal estimation and fast re-acquisition. This provides
considerable performance improvements when there are long shadowing or signal
blockages. This scheme is used for both TDM and OFDM modes.
DVB-SH summary
The DVB-SH standard complements the other DVB standards that
are in existence and many of which are very well established. With satellite
technology relatively commonplace, and satellite distribution an ideal way of
broadcasting video media, DVB-SH fills a hole in the marketplace. It also has
the advantage that it recognises the limitations of satellite technology for
broadcasting to handheld devices by also including a terrestrial method of
delivery. In this way the DVB-SH specification provides a complete solution.
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