A key criterion of broadcast mobile TV network design is providing the right level of service coverage. As with mobile phone reception, subscribers will demand a perfect mobile TV picture everywhere. Meeting this coverage expectation will thus be an essential element of any mobile TV business plan.
But what is the "right level" of coverage? Mobile TV coverage requirements will be based on where the main viewer traffic is expected to occur. Moreover, a significant proportion of high-traffic locations will be indoors or underground where a conventional broadcast network signal cannot penetrate. Network design will be heavily influenced by the reception environment - whether outdoor, indoor or in a moving vehicle (such as a car or train).
The commercial challenge is therefore to determine the appropriate balance between a high quality of service, the right level of coverage, and the amount of infrastructure deployed - essentially a balance between factors that will dictate subscriber take-up, and those that will impact network cost.
Where's the traffic?
As is the case for digital terrestrial television (DTT), a broadcast mobile TV network can comprise a mix of high-power transmission sites to provide blanket coverage, supplemented by lower-power "gap-filler" repeater stations. In the case of mobile TV, however, the number of repeater stations can be almost an order of magnitude higher than for DTT, in order to maintain the high signal strengths required for handset reception. The actual number of sites will depend on the required grade of coverage.
A high percentage of peak viewing is expected in homes and office buildings, plus busy public places such as metros, shopping centres and airport terminals. It is possible to design the outdoor network to provide signal penetration into many of these so-called confined structures - especially high-rise apartment blocks and office towers, which usually have large windows.
Invariably, however, network planners must consider how to provide coverage for high-traffic locations that cannot be served by the outdoor network - for example, subway systems and large complexes such as shopping centres and airports. Here, dedicated confined coverage infrastructure solutions are required for distribution of the RF signal indoors. These so-called "micro" or confined-space indoor coverage networks complement the outdoor macro network.
Antennas indoors
Such micro indoor coverage networks typically comprise tailored broadband RF distribution networks founded on broadband radiating cable and/or distributed point-source antennas superimposed on a full band optical transmission backbone. Similar infrastructure is widely used by the mobile telephony, Wi-Fi, wireless data, public safety, private mobile radio and FM radio industries.
With the right expertise, these existing distribution systems can often be re-engineered on a case-by-case basis to support mobile TV. Much depends on the age, specification and configuration of the existing system. Other major considerations will include cost of work, rollout time and electromagnetic compatibility with co-networked radio services.
It is also worth noting that, unlike mobile phone calls, it is likely that mobile TV viewing will take place at a user-determined time and place. This could lead to the deployment of simpler, dedicated mobile TV hot spots. Accurately identifying the key indoor areas to be covered will contribute to the commercial success of a mobile TV service.
Mobile TV networks - both outdoor and indoor - will invariably operate in single-frequency network (SFN) mode, owing to limited spectrum availability and handset operation. Network fine-tuning will be essential to ensure synchronisation between the micro indoor cells and the macro outdoor broadcast network. SFNs demand the tightest control of signal levels and timing to minimize interference and to optimize network coverage.
It is clear that the mobile TV network environment will be complex and dynamic, requiring a unique breed of engineering solution. It will be important to balance the demands of the macro network as a whole, with the precise confined-coverage requirements of individual locations on a micro scale.
Chris Jaeger is managing director of Broadcast Australia International Business Group
Paul Chan is engineering director of Hong Kong-based Radio Frequency Engineering