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Mobile TV: The Next Killer-App
Factoring in Availability
Korea and China are both deploying commercial T-DMB mobile TV services in VHF Band III, as per DAB services. To-date, there has been no move to deploy DVB-H in VHF Band III; however, since DVB-T services operate in VHF Band III, there seems little reason why DVB-H would not as well. The main obstacle is again one of spectrum availability-of the four considered bands it has the most limited availability in most countries-coupled perhaps with convention.
The other two bands-L Band and the satellite S Band-are emerging as contenders. Both provide reduced terrestrial propagation and in-building coverage compared with the lower frequency bands, but have the advantage of being more readily available.
Irrespective of which frequency band is selected, the signal polarization is also under examination. The FLO systems being deployed use circular polarization (CP), which is a combination of vertical (VP) and horizontal (HP) components. It has been speculated that a CP signal may facilitate reception at the mobile handset regardless of orientation. This may, however, be a moot point, since the multiple reflections experienced by HP and VP signals can alter the polarization, effectively producing a mixture of polarization components by the time the signal reaches the handset.
Vertical polarization is favored at present by both DVB-H trials and T-DMB deployments. In the latter case, this probably harks back to the DAB convention, since radio signals are often VP to enhance reception by car antennas. Use of VP also enhances isolation from HP television signals at similar frequencies. Most DVB-H trials are using VP, although at least one utilizes a HP signal. Ultimately, the selection of polarization will depend upon the receiver performance when faced with multiple signals from reflections, plus the indoor penetration of the signal.
Which Way Forward?
The future of mobile TV depends on many factors; but if it is proved that consumers want mobile TV-and are prepared to pay for it-then half the battle is won. The network model will then be determined by how cost-effectively networks can be deployed and the availability of frequencies and licenses. This is likely to differ on a case-by-case basis.
Utilizing existing infrastructure will be a key element. It is not difficult to incorporate mobile TV services into existing broadband terrestrial broadcast systems, particularly if the systems were initially designed to accommodate additional services or channels. The most significant capital outlay would come with the deployment of additional repeater stations.
If, on the other hand, a mobile TV network is deployed as a cellular overlay, this will involve a significant shift in broadcast infrastructure philosophy. The quest to deploy television antennas at existing mobile base stations (hundreds, perhaps thousands, of sites) will encounter the same challenges as experienced by mobile phone carriers-the demand for low-profile, environmentally friendly antennas; the mandate for low emissions; site-by-site negotiations; and the trade-off between capex and opex. It could also promote utilization of the higher-frequency L Band and its inherently more compact infrastructure.
Co-location interference issues also need to be considered when overlaying mobile TV and wireless communications services. With UHF frequencies so close to the GSM 900MHz receive band (usually 890 to 915MHz) and the CDMA 800MHz receive band (usually 824 to 849MHz), careful frequency planning and coordination will be required. Moreover, if the broadcast signal is too high in power, it could cause 'blocking' in the sensitive GSM or CDMA receivers, unless RF filtering is deployed. Similar situations arise with both the L Band and S Band frequencies, which are all in the vicinity of high-band GSM, CDMA and UMTS services.
In addition, it is likely that all mobile TV network topologies will ultimately need to incorporate dedicated wireless indoor solutions (WINS) to provide coverage inside multi-level buildings, large campuses and underground road tunnels and metro systems. These could be integrated with existing broadband WINS systems for mobile wireless communications.
True Convergence
Clearly, for mobile TV to succeed as a commercial venture, it will involve many players in the wireless sector: mobile phone carriers, broadcasters, handset manufacturers, content providers, infrastructure groups, base station OEMs, government and licensing bodies.
The quest to maximize the bottom line will ultimately reveal which network model, technology platform and frequency band combine to form the most viable option for a specific country or market. And it will be dependent on which provides the most attractive and accessible model for consumer uptake. Whatever the outcome, it will represent a true convergence of multiple technologies. From this will materialize the true meaning of mobile TV.
Mike Dallimore
VP, Broadcast, Towers and Defence Systems, Radio Frequency Systems
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