Mobile TV
May 1, 2009 12:00 PM, By Thien Tran
Implementing a single-frequency network is key.
First introduced to allow spectrally efficient digital broadcast networks, single-frequency networks (SFNs) are now proving essential in the move toward mobile TV. When it comes to deploying a new digital television broadcast network, there is more to consider than a mere upgrade of transmission site infrastructure. Not only does the impact of the new network on existing analog TV services need to be taken into account, but, in today's increasingly mobile-focused society, the ability of the new network to support future mobile TV services must be considered as well. In an increasing number of cases, the network is expected to support both fixed and mobile digital terrestrial television (DTT) services.
This places unprecedented significance on the planning phase of a new network. From the increase in spectrum congestion to the high field strengths required for mobile TV reception, digital broadcast network planners face a far greater number of variables than in the days of analog only. Network planning is now one of the most challenging — and arguably the most important — phases in network deployment.
Assigning spectrum
Figure 1. A single-frequency network performance analysis determines whether co-channel transmission signals will cause interference at receiver site ∑2.
There are two main stages of network planning: spectrum and service. Allocation of spectrum is usually undertaken by the national regulator, and in today's environment is increasingly scarce. Whereas multi-frequency networks (MFNs) were commonly deployed for analog TV services, the spectral efficiency of SFNs are proving essential in many countries when it comes to overlaying digital networks. There is simply not always enough spectrum available to deploy digital repeater sites on different channels.
Most regulators around the world are allocating spectrum for digital services in the same band as analog TV services — in many cases using adjacent channels. This means that the impact on the existing analog services needs to be closely examined. There are typically two deployment scenarios: to provide a simulcast period during which the impact on analog services is minimal, or to assume a short simulcast period allowing a higher but acceptable level of interference to the existing analog audience. Depending on which option is selected, either the digital or analog service will potentially be compromised in some way.
Consider the first option, where disruption to the existing analog TV audience is minimized. Interference to analog services is prevented by limiting the effective radiated power (ERP) of the digital signal relative to the analog transmission to maintain the required protection ratio. Depending on the digital signal modulation scheme used, the number of programs per digital channel may be constrained as a result. The reduced ERP limits the extent of the coverage area. Either way, the full potential of DTT broadcasting is compromised until analog services are switched off.
It follows that if the full potential of digital broadcasting is to be realized immediately, existing analog services may be deteriorated, as per the second deployment option. This option has been adopted in countries with a high cable and satellite penetration and a low proportion of analog terrestrial viewers.
The SFN scenario
Once the spectrum has been allocated and the acceptable level of protection of existing analog services determined, the detailed service planning is carried out by the broadcaster or broadcast service provider. If the new digital network is to be an MFN with reception by fixed outdoor antennas, the planning approach is similar to analog planning principles and consequently well-documented.
When planning an SFN, on the other hand, the situation is more complex. The basic principles of same content, same frequency and synchronized timing at the receiver are all important. Moreover, the issue of whether reception is to be fixed or mobile introduces different challenges.
Consider first a fixed-reception DTT system. In the case of an MFN overlay, it is reasonable to assume that existing consumer receive antennas will be appropriate, assuming the same frequency band and signal polarization as the existing analog service are used. Household antennas are likely to be already pointing in the ideal direction, depending on which site provides the most appropriate transmission signal. This is not necessarily the case with an SFN, where the optimum transmission signal might originate from an entirely different direction.
Computer modeling tools are invaluable in planning, designing and optimizing an SFN. Parameters such as receive antenna orientation, ground cover or clutter, and the details of all transmission sites (such as antenna radiation patterns of the transmit antenna, ERP and synchronization timing) can be used to map coverage signal strength for a given area. Furthermore, the relative signal strengths from each site, and the corresponding time of arrival, can be calculated to determine which of the transmission sites is expected to provide the greatest level of service, or cause interference.
Interference in SFN networks can be caused by co-channel transmission signals arriving at the receiver outside a nominated time of the guard interval. Same-frequency signals that arrive within this time guard interval will be nondestructive. If they arrive outside the guard interval they reduce the carrier-to-noise margin available as if they are co-channel interferers. From experience, as long as they are more than 20dB below the main signal, they do not cause the digital signal to fail. (See Figure 1.)
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