Mobile TV
Apr 1, 2008 12:00 PM, BY DAVID AUSTERBERRY
Interest is growing in mobile television services. However, for many countries there is no spare UHF spectrum to start transmissions. Although analog switch-off will free spectrum, that is two to five years off, depending on where you live. There is also no certainty as to what that freed spectrum will be used for. It could be for terrestrial HDTV or it could be for mobile data service, so there is much competition with a possible mobile TV operation in that spectrum.
One solution to this problem is to look to different areas of the electromagnetic spectrum. Most mobile terrestrial services use UHF with L-band as an alternative. The advantage of these frequencies is that the propagation characteristics match the needs of mobile use. These bands are both crowded, but higher frequencies in S-band adjacent to existing 3G services do have available bandwidth. However, S-band requires a denser transmitter network for good reception in urban areas.
The EU has approved the use of a section of S-band adjacent to the existing 3G terrestrial services at 2.2GHz for use with satellite or terrestrial transmissions. Geo-stationary satellite transmission for broadcast has the advantage that one transmitter can cover a large territory, whereas a terrestrial service requires a considerable investment in a transmitter network.
This could be a big advantage for rural coverage without the expense of setting up a terrestrial network for a business that has yet to be proven as a sure-fire commercial success. But S-band satellite transmission does not penetrate buildings in the same way as UHF. The expectation for a mobile service is that it will work in shopping malls, offices and the home. These realities have led to the development of a hybrid satellite/terrestrial system to give much more reliable delivery of content to mobile receivers.
The cost of setting up a terrestrial mobile service is not the only barrier to success. Experience has shown that the public must have a wide range of handsets to choose from, especially the tier-one suppliers. This means that receiver chips and antenna designs must be attractive to the mass-market handset manufacturer.
Combine opportunity with a business case, and the service is possible. Add standardization, and it becomes a real possibility. In 2007, the DVB approved a standard for a hybrid satellite-mobile service, designated DVB-SH.
DVB-SH is primarily designed to deliver IP-based multimedia, much like DVB-H. This would include broadcast TV and radio, plus data for EPGs, news services and interactive services.
DVB-SH is based on DVB-H, but adds enhancements to improve the robustness of the signal. The same time-slicing is used to conserve battery power. The primary modulation is orthogonal frequency division multiplex (OFDM), but a second scheme, time division multiplex (TDM), which is partially derived from DVB-S2, is added as an option for the satellite downlink. The provision of two options allows more flexibility for optimization of the satellite performance.
If a single DVB-SH multiplex is used per HPA in the satellite, then TDM allows the optimum HPA performance by operating close to saturation. If several multiplexes are used per HPA, then the amplifier must be backed off. In this case either, TDM or OFDM can be used with little difference in performance.
A new FEC called 3GPP turbo code, plus a highly flexible channel interleaver to address typical satellite propagation impairments like fading, is used. Tall buildings obscure signals, as do bridges, and a mobile service should be designed to cope with short interruptions from such shading of the signal. The interleaving provides for time diversity from about 100ms (for terrestrial) to several seconds (for satellite) depending on the targeted service level.
Transmission and distribution
The hybrid model uses satellite as the primary means of transmission. In urban areas, gap fillers can use dedicated circuits or repeat the satellite transmissions. These repeaters could well be co-sited with 3G transmitters, especially as they share the 2.2GHz band. The gap fillers can be configured as an SFN along with the satellite transmissions. Both QPSK and 16-QAM are supported in the standard, so the operator can make the usual trade-offs between capacity and QoS.
Figure 1 on page 22 shows the distribution network with the primary satellite transmitter. Local gap fillers (A) could be fed via a fiber broadcast transmission network and could inset local content. Personal gap fillers (B) could be used in apartment blocks, dorms and so on, but without content insertion. And mobile repeaters (C) could be used in buses, ferries or trains, wherever the satellite signal cannot be guaranteed.
Handsets
It is proposed that handsets would use diversity receivers with two antennae a few centimeters apart. The receiver module could incorporate the two tuners and operate at UHF (for DVB-H) and 2.2GHz S-band. The combination of SFN transmission and diversity reception should counteract the drawbacks of S-band against UHF in urban areas.
Services
Proposed services could support nine 256kb/s channels in a 5MHz carrier. The urban repeaters could add further channels, between 18 and 36 channels of 256kb/s. Combine this with unicast VOD 3G, and that represents a compelling service offering for mobile video.
Summary
There are several standards for the delivery of TV to handheld devices. The business model is the unknown. Is the public prepared to pay for the service? As the only way is to offer a service, the decision to invest is being made cautiously. Some countries use DVB-H, FLO and DMB; others wait for spectrum. DVB-SH offers an alternative where L-band or UHF is not available, but provides common standards to DVB-H and 3G so that handsets can be manufactured for the mass market, and 3G transmitter equipment and locations can be used for the terrestrial coverage.
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