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Mobile TV: The Next Killer-App
Coverage Adjustments
The broadcast industry approaches mobile TV coverage from the other direction. Conventional free-to-air TV is typically broadcast from centralized high-power transmission sites, supported by supplementary repeater or 'gap filling' stations. It is relatively straightforward to deploy a mobile TV service in the same manner; however, there do need to be adjustments to coverage planning.
Research indicates that the 'high-power terrestrial broadcast' model for mobile TV will require more repeater sites than for conventional television. One reason is because, owing to an increase in reflections at ground level, the forward error correction applied to the signal is increased, resulting in a trade-off in signal-strength that needs to be addressed.
Additionally, consumers have also come to expect their handsets to work indoors and in moving vehicles. The provision of indoor coverage is considered one of the main challenges of mobile TV networks.
A third infrastructure model, incorporating satellite blanket coverage supported by low-power terrestrial repeaters, has been proposed. The repeaters would be co-located at mobile base stations to supplement urban and provide indoor coverage.
A unifying element in all three network models is the convergence of industries that have been hitherto quite separate. Mobile carriers will need to embrace broadcast technology and content; broadcasters (or infrastructure/service providers) will need to team up with carriers, who already have the subscriber base. In fact, it seems logical for mobile TV systems to be intrinsically linked with mobile phone services, which can provide a one-to-one back-channel for interactivity. This could even prove to be a driver for consumer take-up.
The Band Debate
From a technical and practical standpoint, the other major delivery option pertains to frequency band, of which several are being considered: VHF (170 to 240MHz), UHF (470 to 860MHz), L Band (variable depending on region, but generally falls somewhere between UHF and S Band) and S Band (2170 to 2200MHz).
Most popular globally for digital terrestrial television is the UHF band, which has also seen the most mobile TV activity to-date. It has good propagation characteristics and, if deployed using the terrestrial broadcast model, should be capable of providing coverage of a large city using 20 to 50 repeater sites. Qualcomm in the USA is using this model for its commercial MediaFLO service (using the FLO platform), but, as other trials have shown, it is also ideal for deploying DVB-H.
The UHF band is also suitable for networks deployed using the cellular overlay model, since UHF frequencies are just below conventional global standard for mobile communications (GSM) or US 'Cellular' code division multiple access (CDMA) frequencies. This type of network is being trialled in many countries across Europe.
One of the main challenges associated with the UHF band is the limited availability of spectrum in most parts of the world, but especially Europe. Some governments are considering assigning two or three UHF frequencies for DVB-H mobile TV services, which can be deployed as single frequency networks (SFN). Although it makes network configuration more complex, an SFN is a highly efficient use of spectrum, and a network of two or three overlapping SFNs could be a promising option.
The VHF Band III has even better RF propagation characteristics than UHF. It is not suitable for the cellular overlay model, since the antennas would be too large for existing base stations; but it is an ideal candidate for the terrestrial broadcast model, where city coverage could be achieved with just a handful of repeaters. From a network deployment perspective, VHF would appear to offer the lowest roll-out costs coupled with the best indoor coverage.
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