7.3.3 Status around the world
The degree of utilization of terrestrial frequency bands for analogue TV is a factor in the implementation strategies possible in different parts of the world .
In some countries the large geographical separations possible between major cities can lead to much less congested use of terrestrial frequency bands in relevant areas. This is in contrast to most countries which, by their size, or by their population concentration in certain areas, have to make extensive use of these same frequency bands.
The following gives a brief review of the current thinking on implementation strategies in the potentially mixed digital/analogue environments in different ITU Regions of the world.
Region 1
Plans for the introduction of DTTB services are under discussion in a number of countries. In particular the United Kingdom government has proposed legislation for digital terrestrial television broadcasting and plans for the introduction of services scheduled to start at the end of 1997. In many countries in ITU Region 1 the allocated terrestrial frequency bands are heavily congested with analogue transmissions especially where the national allocations are fully taken up and a large number of low-power relay transmitters are used to provide almost complete national or regional coverage. In several countries the utilization of related taboo channels could be a possibility for the implementation of DTTB services. The maximum permitted transmit power of digital services is heavily constrained by the need to avoid interference into the existing services especially those supported by low-power relays.
The limitations thus imposed upon transmission powers constrain the coverage obtainable from a single transmitter for a desired quality level and hence the number of channels of DTTB that will be economically feasible. Furthermore, in a number of countries there is interest in developing services for different types of markets. Services being considered range from those aimed previously at roof-level antennas to others primarily engineered for portable or mobile reception. A particular concern then is to define the requirements for the practical introduction of DTTB services in parallel with the existing analogue services whilst maintaining the option for up-grading such services in the long term (after the older analogue services have been phased out on the assumption that this is possible to obtain a "digital-TV-only" scenario).
An additional factor influencing DTTB strategy is that in some countries in Europe the early introduction of digital services by satellite and cable has been proposed. The introduction of such services might assist or hinder the market opportunities for certain types of DTTB service depending upon the national situation. In either case it is recognized that it will be important to harmonize standards among the different transmission media (satellite, cable and terrestrial), to ensure that the future digital television market will develop satisfactorily. This work is being progressed within the DVB project and ETSI.
Networks for digital television can be planned either in the conventional way, i.e. integrated with the analogue networks or as a single frequency network (SFN). SFNs have the potential for extremely efficient spectrum utilization in that for national coverage only one RF channel is needed for the transmission of one HDTV programme or several standard television programmes. In order to be able to start a nationwide digital television service with a SFN of course one RF channel must be completely freed for digital television. This may be more or less difficult in different countries depending on how much replanning of existing analogue services if any that is needed in order to free one RF channel. The consequence is that digital television may be introduced with integrated networks in some countries and with SFN in others. There is also the possibility that some countries will use both integrated networks and SFNs.
The limiting factor for planning of digital television is the interference to and from analogue television. The interference criterion used today allows interference on analogue television during 1% of the time (5% in some countries). One interesting idea put forward is that if this criterion could be changed to become 5% of the time everywhere, it would in many cases permit increase of the power of the digital transmitter by about 6 dB which would greatly facilitate the planning of digital television.
One proposed idea which has led to considerable interest and discussion in Europe is that the different parts in the transmitted bit stream could be unequally protected and that the more protected parts could correspond to fewer picture quality levels, e.g. SDTV, whereas the less protected levels could correspond to e.g. HDTV. With scalable coding a subset of the HDTV bit stream would correspond to SDTV quality.
Such a scheme with scalability and unequal protection allows for graceful degradation and leads to larger coverage for the lower picture quality part (SDTV) than for a system without this feature. However, this larger coverage is obtained at the expense of a reduction in coverage for the high picture quality part (HDTV) and of some reduction in its highest picture quality which requires further thorough evaluation of this scheme.
Reconfigurability refers to dynamic, static or semi-static modification of the disposition of the channel in terms of services carried. It can be combined with scalability and unequal error protection but can also be used as an alternative to those methods. Also the modulation, including the FEC, can be modified to fit robustness or other requirements. The modulation can be reconfigured to provide different spectrum efficiencies (bit/sec/Hz), with a corresponding trade-off in ruggedness. The following are examples of possible service modes:
Many configurations could be envisaged, like the first one where HDTV is given maximum resources and any SDTV versions of the same programme, with the required degree of ruggedness, could be carried by another RF channel. The final choice of a set of configurations has to be made with respect to both non-technical and technical factors, one of them being receiver complexity and another being the coverage/bit rate trade-off.
If the reconfigurability also applies to FEC and modulation in the same system as the one that would provide maximum HDTV quality and coverage in certain RF channels, it would also be able to provide SDTV or LDTV services with a related degree of ruggedness in others.
The long term spectrum requirements for terrestrial television broadcasting in Europe have been reviewed by the CEPT in the second phase of its Detailed Spectrum investigation, covering the frequency range 29.7 to 960 MHz [2]. Noting the very significant increase in spectrum efficiency that could be realised in a transition from analogue to all-digital operation, the Investigation nevertheless recognises that the phasing out of analogue services is likely to be a very long process, especially bearing in mind their current popularity and recent improvements to service quality offered by system enhancements like NICAM sound or PAL-Plus. While the move to 'all-digital' operation is seen as an inevitable long-term trend it is recognised that any forecast of timescale for this, or for the pattern of services that might emerge, is highly speculative. One long term possibility considered is that satellite broadcasting might emerge as the primary means of providing coverage over large areas. Terrestrial broadcasting might then be primarily aimed at providing regional and local services. Furthermore terrestrial broadcasting networks may be instrumental in providing the portable and mobile outlet of the information highway. However it is recognised that, in order to compete with satellite services mooted to start in 1996, initial DTTB services will most likely be aimed at achieving a rapid build up of coverage to the existing field of roof top antennas using whatever spectrum is available in the related (taboo) channels in the existing terrestrial networks.
While there have been some frequency planning studies related to national Single Frequency Networks most 'introductory phase' planning studies have adopted a 'conventional' planning approach - that is an approach which assumes that the digital transmitters will be co-sited with (or very close to) the existing analogue transmitter, and that the frequency planning techniques are similar to those used for analogue services (albeit that the 'criteria relating to minimum field strengths and protection ratio's would be different').
An advantage of the conventional planning approach is that a large part of the existing analogue network infrastructure may be re-used. This has obvious cost saving implications for the broadcaster but should also provide benefits for the viewer. The latter will arise in any case where it is found possible to use channels for the digital transmissions from a particular site which are close to the channels used for the analogue transmissions from the same site, especially if the same polarisation can be used. This should permit viewers to re-use their existing receiving antenna and feeder system. This will be the case for some countries, while for other countries more difficult planning environments exist. In these cases other planning approaches may have to be used.
However, it may be found desirable to introduce a limited number of channel, or even site changes at some of the low power analogue stations to assist in the efficient introduction of the digital services. The use of SFN's to support low power relay stations for the digital service can also make a significant contribution to providing optimum coverage. Additionally the use of 'notches' in the digital transmitter antenna pattern so as to minimise interference in particularly sensitive directions, can allow the digital transmitter power to be increased to provide an overall benefit in coverage terms. Thus by a detailed consideration of particular interference problems, and techniques to overcome them, the digital network can be tailored to offer considerably more coverage than an initial 'broad brush' study might suggest.
'Broad brush' frequency planning studies, using somewhat simplified methods and conservative planning assumptions, have been made to get a first idea of the coverage achievable with digital techniques in a significant sample of major European cities. In general the results are encouraging and further work at a European level, covering introductory scenarios with the associated frequency planning and co-ordination issues is the responsibilty of the CEPT Project Team FM PT24.
Region 2
The United States and Canada are in the process of choosing a digital advanced television (ATV) standard to be used by terrestrial broadcasters to provide an advanced television service. In the two countries, ATV will need to fit within the 6 MHz channels of the VHF and UHF bands now used for conventional television. Hence there will be the conventional television channels and additional ATV channels which may interfere with each other in the television bands. The ATV channels will be implemented on the basis of one channel for each existing conventional television channel. The basic objective of future ATV service is to provide a service area matching as closely as possible the existing NTSC service area which it will eventually replace. Spectrum allocation and accommodation studies have demonstrated the feasibility of this approach.
Active spectrum accommodation is underway to provide for the terrestrial emission of digital ATV signals in the existing VHF and UHF allocations. The principles under consideration as a basis for ATV allotment planning are:
These general principles form the basis upon which to develop a domestic allotment plan for an ATV service. The objective is to provide an ATV service with coverage areas equivalent to existing NTSC stations or allotments. Selection is accomplished through the use of optimization algorithms which search for combinations of channels in the most congested areas to find available spectrum.
During the past 6 years of development of the ATV system, significant technological advances have occurred in digital encoding/decoding and transmission. This evolution in the technology has resulted in the development of a fully digital television system with significant capabilities beyond those originally envisioned for the advanced television system. Because of these enhanced capabilities, Canada is revisiting the issues regarding Planning Factors and Implementation Strategies for the introduction of digital television in their country.
The Digital Terrestrial Television Broadcasting and Frequency Allocation Planning committees of the Joint Technical Committee on Advanced Broadcasting (JTCAB) are continuing their work and technical studies on issues impacting on planning factors and implementation strategies for the introduction of digital television. Areas of work include:
In addition to the technical committees, the Government has established a Task Force to ensure smooth introduction of digital television in Canada. The Task Force will examine implementation issues such as types of services (HDTV, SDTV, Pay TV, Data services, Sound services, etc.) to be carried in the ATV channel, simulcasting with existing NTSC, and the transition period. Social, economic and technical issues relating to other delivery mediums and services (cable, satellite, information highway, compatibility with computer and other services, etc.) will be examined. At present, the most optimistic start-up date for digital television is mid 1998.
Region 3
Several efforts are now underway in Japan to develop a digital television broadcasting system. An important and difficult consideration is the channel allocation capability in a new digital modulation scheme. In planning the introduction of digital terrestrial television broadcasting (DTTB) to vacant channels within the current NTSC services, the reduction of protection ratios between new DTTB and current NTSC services is a major concern. As the Orthogonal Frequency Division Multiplexing scheme (OFDM) has the capability of spectrum shaping (creation of carrier holes) due to its multicarrier scheme a well designed carrier hole scheme improves protection ratios and results in an extension of digital channel availabilities.
The spectrum is almost occupied by the analogue television broadcasting in Japan. However, the channel allocation capability for digital terrestrial television broadcasting is being analyzed with computer simulations. It offers prospects of implementing terrestrial broadcasting using the OFDM scheme, which coexists with conventional analogue systems.
To investigate the efficiency of Single Frequency Network (SFN) using the existing television transmitting site, the service area that has the required field strength and the allowable multiple echoes has also been calculated by computer simulations. Field experiments using two or three transmitters will be made available to confirm the feasibility of SFN.
To compensate for the disadvantages of the single frequency relay network (SFRN), a double frequency network (DFN) is also being studied. DFN is a radiowave relay method that uses two channels alternately. The DFN does not require a large isolation between the transmitter and the receiver antennas of relay stations, and a long guard interval, which SFRN does.
BST (Band Split Transmission) is one of the candidates for the transmission scheme of the DTTB system under the constraints of a usable frequency spectrum. Programmes are transmitted by one OFDM block or more, each of which forms the basic unit of BST.
One of the features of BST is that the number of OFDM blocks can be set up depending on the total bit rate of services. Hence, BST can flexibly accept a variety of services that require a transmission capacity ranging from relatively low (e.g. a standard TV) to considerably high (e.g. studio-quality HDTV). Moreover, each OFDM block may be located at an arbitrary frequency position when contiguous spectrum cannot be secured.
In Australia the Australian Broadcasting Authority has issue a first report "Digital Terrestrial Broadcasting in Australia" which includes some key issues regarding the introduction of DTTB.
It is expected DTTB will be able to be introduced in Bands III, IV and V. Rechannelling of existing services is not considered feasible because of the difficulties in retuning modern television receivers which means the choice will need to be compatible with 7 MHz analogue channel spacing.
The initial studies indicate the current six analogue services in each area could be replicated with digital channels within the bands identified.
Australia is intending field tests to evaluate DTTB modulation options in 1996.
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