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
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.)
The philosophy of mobile
Broadcasting to mobile devices injects additional complexity to network planning. This is primarily because mobile television users expect quality reception virtually anywhere on devices with small receive antennas at variable orientation. To address challenges such as reduced antenna height, building penetration, reduced receive antenna gain and higher required location availability, trials have shown that for effective mobile reception, the field strength needs to be more than 30dB higher than fixed DTT services. Of course, this depends upon the data-throughput requirement of the network operator.
It is impractical, however, to simply increase the ERP of the centralized transmission site in order to achieve the required field strength. Consider a site broadcasting at 50kW ERP. For that site to deliver a field strength that is increased by 30dB, the required ERP would be a massive 50MW. (See Figure 2A.) Generating a 50MW ERP would be a huge price to pay for good mobile coverage, assuming that the channel would even be permitted to operate at that level. Clearly, an alternative transmission network philosophy is required.
Figure 2B on page 14 demonstrates the principles of a mobile network that instead consists of multiple low-power (1kW ERP) sites, based on theoretical field strengths. The main site broadcasts at 50kW ERP, but as the field strength falls with distance below the minimum requirement for a mobile network (indicated by the gray dashed horizontal line), the coverage is supplemented by the low-power sites.
Not only does this approach provide a consistent signal level across the entire area of interest, but also it delivers signals from multiple directions, thereby improving location availability and reducing the impact of building clutter.
Adjacent channel challenges
Despite the myriad of advantages, multisite broadcast networks can offer their share of challenges. One possible scenario occurs in regions with low-level analog signals where residents use fringe area receive systems with masthead amplifiers. If a mobile TV repeater is deployed in this area, it could lead to receiver overload. To address this, suitable filtering would need to be implemented in the receive system.
Adjacent channel interference can also become an issue. If the mobile TV channel is adjacent to an analog service, the establishment of a mobile TV transmission site where no analog transposer exists will cause adjacent channel interference to the analog signal in the region surrounding the mobile TV site. (See Figure 3A.) This is not easily addressed in a viable manner, because it would require establishing a new analog transposer — assuming spectrum could even be found — and redirecting antennas away from the main site.
If there is an adjacent channel DTT service, on the other hand, there is still the potential for adjacent channel interference caused by a mobile TV repeater, but the area affected is much reduced owing to the robustness of the digital signal. (See Figure 3B.) Moreover, in digital systems, the interference is more easily addressed.
The establishment of a DTT repeater facility along with the mobile TV facility is effective in this case. This is because the DTT receiver would see the DTT repeater signal (operating in SFN mode) as a nondestructive signal, provided the timing is established correctly. The repeater signal therefore supplements the main signal. It is important to maintain the required protection ratio between the mobile TV and digital terrestrial signals. (See Figure 3C.)
Planning imperative
In the face of excitement surrounding mobile TV, it is undeniable that broadcast mobile TV network planning is a complex process, requiring a great deal of technical expertise. The use of SFNs is essential, not only due to lack of spectral resources, but also to allow deployment of a multisite network that delivers the required high minimum field strength with realistic ERPs. Another consideration is transmission handover in the handset, the need for which is eliminated using an SFN.
When it comes to the early stages of planning, regulators need to consider the big picture when allocating spectrum, particularly if mobile TV networks are to be ultimately deployed. Moreover, network planners and designers now face a much broader range of issues than in the days of analog only. The introduction of digital broadcasting added one layer of complexity; mobile TV has raised it a notch higher. It is no longer a matter of just operating within the regulator's limits; real technical expertise and forethought are required for an optimal service. Rigorous service planning is imperative.
Thien Tran is engineering manager at Broadcast Australia.
