Mobile TV
May 1, 2009 12:00 PM, By Thien Tran
Implementing a single-frequency network is key.
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.
Figure 2. To achieve the minimum field strength of 90dBµV/m required for mobile TV networks, new network philosophies are needed. A) Whereas it is impractical to generate a 50MW ERP to achieve this, B) a more viable option would be to deploy multiple low-power sites.
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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.
Figure 3. Mobile TV services can cause A) adjacent channel interference to analog TV and B) fixed DTV services. In the latter case, this interference can be addressed by the establishment of a C) co-located DTV repeater facility.
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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.
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