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.

This Digital Video Broadcasting, DVB tutorial is split into several pages each of which addresses a different aspect of DVB technology and operation:

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.

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.

Further pages from this tutorial
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