Revert to Section 6.3

7 Planning studies and Implementation strategies

7.1 Introduction

The potential advantages of digital terrestrial television broadcasting (DTTB), in terms of service quality, lower costs and programme diversity, are summarized in 1 of this report. In frequency planning terms it is generally agreed that, where new or unused frequency spectrum is available, digital television coverage from individual transmitters, or from networks of transmitters, can be planned to achieve the full potential of DTTB - thus enabling the very considerable benefits to be gained (compared to the current analogue situation), in terms of service provision and spectrum utilization.

However it should be recognized that, in reality, the spectrum situation is far from ideal, and that there are numerous problems involved in finding the required spectrum, and in dealing with its allocation and assignment, that will have to be overcome before DTTB services can become a reality in many parts of the world.

The allocation of frequency spectrum to specific services on a regional or worldwide basis is subject to international treaty drawn up under the auspices of the ITU.

The assignment of allocated spectrum to particular uses is subject to regional treaty and cross-border negotiation, as well as to regulation on a national basis.

In Region 1, for example, the Stockholm Plan of 1961, (based upon the use of analogue television standards) has provided the framework for planning and implementing the extensive terrestrial television networks now in operation. Treaty arrangements, such as those in Region 2, are used to govern the planning and procedures of frequency assignment elsewhere.

Within these regional plans there are many geographic areas where the allocated spectrum has been heavily exploited to provide the maximum number of analogue television services, each service being designed to achieve, where possible, a high population coverage. For these areas then there is little prospect that sufficient spectrum can be found for dedication to DTTB, let alone that sufficient could be found for all the DTTB services that might be required. Thus the alternative option of band sharing with the existing analogue services is being intensively studied, accepting that the DTTB transmitter power constraints that this arrangement necessarily imposes will in turn inevitably limit the performance of the DTTB system. There are of course other geographic areas covered by these regional plans, where the allocated spectrum has not yet been heavily exploited, and where it will therefore be feasible to consider the use of relatively high DTTB transmitter powers to achieve increased performance in terms of service quality levels or ruggedness of transmission.

It can be seen then that the constraints that apply to "frequency planning" will vary from country to country, as well as, in some cases, within national boundaries - the degree of variation being dependent on geographic/population factors as well as upon the exploitation of the national "allocations".

It is against this rather complex "frequency planning" background that strategies for the introduction and subsequent evolution of DTTB services are being considered. Central to these considerations is the concern to find sensible "ways and means" of migrating from an initial phase of DTTB, in which limited capability DTTB services are introduced on a "sharing" basis, to a final phase of DTTB service "domination" which could allow the NTSC, PAL and SECAM services to be "phased-out". If such a migration path can be found and followed, to the point where the "switch over" to fully digital operations had been achieved there will be the opportunity to upgrade the DTTB services to their full potential, possibly releasing some of the allocated spectrum for reallocation to other services.

The factors that must be taken into account in the planning of DTTB service coverage areas are discussed in some detail in 7.2

Differing perspectives on the problems to be faced in achieving a strategy for DTTB service implementation whilst retaining a migratory path to "all-digital" operations in the future are described in 7.3.

7.2 Planning factors

7.2.1 Current analogue systems

The first television systems were developed independently in several parts of the world, and despite the substantial efforts towards standardization that has been made since then, even today analogue terrestrial systems with several significantly different values of key parameters such as channel-width remain in widespread operation. Everywhere that systematic planning has been undertaken, however, it has been based on the principle that it should enable the scarce natural resource consisting of the spectrum to be exploited as fully as possible. Nevertheless, the spectrum available for terrestrial broadcasting, and the extent to which it is in fact exploited, also varies significantly from one part of the world to another. In some cases the latter is due to the high cost of operation; in others it reflects the availability of other distribution media such as cable and satellite services.

One of the most important constraints in planning for current analogue systems is the fact that the spacing between co-channel transmitters must be a significant multiple of the service radius of an individual transmitter. Furthermore, at the time when the planning criteria were established, the characteristics of consumer receivers were assumed to be such that certain other "taboos" on the assignment of transmitters to related channels in that area had to be respected. Although consumer receiver performance has since improved significantly, these "taboos" (such as that preventing the assignment of adjacent channels to two transmitters located at the same site) have so far remained in effect.

Furthermore, because the planning process has been designed to enable one of two main types of service to be provided, in most cases the actual configuration of transmitting stations over the territory tends to correspond to one of two characteristic types. One where the objective is to ensure that satisfactory reception of as many programme services as possible can be achieved virtually throughout a large territory; the other where the objective is to enable as many broadcasting companies as possible to compete with each other fairly in providing services within the area covered by a single high-power transmitting station located near the centre of a metropolitan area. In the former (wide-area-coverage) case, numerous low-power rebroadcast stations are also used, especially where the terrain is hilly; in the latter (local market) there are few such low-power rebroadcast stations.

It should be noted that both types may coexist in the same area, because some types of programme service are inherently of mainly local interest, while others are suitable for distribution within a much larger area. Nevertheless, one such type is generally dominant in any particular case, and this has important consequences for the possibility of reorganizing the usage of the spectrum, and thus for the potential introduction of digital broadcasting there. Specifically, with the wide-area coverage type there is generally much less vacant spectrum to be exploited to provide digital services.

7.2.2 Sharing considerations Digital-to-analogue interference

In considering the introduction of DTTB services on a "sharing" basis with the existing analogue services, it is necessary to define the degree of degradation to the analogue services from co-channel interference (CCI) and adjacent-channel interference (ACI) that will be acceptable. In general, the transmitted digital spectrum has a similar spectral characteristic to Gaussian noise. The effect of CCI is therefore to raise the noise thresholds of analogue receivers which in turn reduces the picture grade (ITU 5-point scale) achievable at the edge of the analogue service area. In general, the planning aim is to limit this loss of grade due to digital-to-analogue CCI, where currently grades of 4.0 (continuous) and 3.0 tropospheric are the norm. For initial implementation of digital systems in the vacant channels between existing analogue channels it is possible to allow some overlap of the digital signal into the adjacent analogue channels. For DTTB power levels 20 to 23 dB below the analogue up to 0.5 MHz extra bandwidth for the digital signal can be used without significant impairment to the analogue services. This may provide an opportunity for later channel rationalisation when analogue services are phased out. Digital-to-digital interference

Given the noise-like nature of the digitally-transmitted spectrum, the susceptibility of digital systems to digital CCI is almost identical to their susceptibility to thermal noise - that is, an increasing susceptibility as the modulation levels are increased from QPSK to higher modulation levels like 16 and 64 QAM (by approximately 7 dB and approximately 13 dB respectively in theory). However, as is shown in 4, the increased transmission capacity of the higher modulation levels allows very sophisticated error-management schemes to be used, that more than compensate for this loss to provide an overall gain in performance. Analogue-to-digital interference

The main sources of "analogue-to-digital" CCI are centred around the vision, sound and colour sub-carrier frequencies of the analogue system. While in principle this relatively high-powered "narrow-band" interference can be very damaging to the digital transmission, the sophisticated error-management schemes described in 4 can deal effectively with this type of interference to ruggedise performance. As for the "digital-to-digital" interference case, final performance will be dependent on the choice of modulation level, the transmission capacity devoted to error-protection, as well as, to some extent, the particular characteristics of the modulation system - whether this be single or multicarrier in nature.


Continue to Section 7.2.3

Return to DTTB Tutorial Table Of Contents

Return to Tutorial Index Page