Revert to Section 7.1

7.2.3 Digital service possibilities

Terrestrial digital television services offer both advantages and disadvantages compared with analogue television services and in some ways these are linked. The abrupt failure characteristic of digital systems, as compared with the gradual failure typical of analogue systems, is a disadvantage as it means that more care needs to be taken to ensure that a high percentage of viewers can receive a satisfactory service. In practice, this means that coverage boundaries need to be defined for a high percentage of locations, both in terms of the minimum signal levels needed for satisfactory reception and in protection against interference. On the other hand, the full quality of the digital system is retained out to the coverage boundary, and indeed at many locations well beyond it.

Considering the transmission of television and sound having a particular information content, in principle digital systems can provide a higher quality of reception than can analogue systems for the same propagation conditions, system bandwidth and effective radiated power. However, some of this potential extra reception quality may be given up in order to provide a larger transmission capacity in a given bandwidth. This greater capacity may be used to provide higher-definition standards, more programmes, or additional features (for example, more data or sound information) with an individual programme. An alternative approach would be to trade-off both service quality and quantity in order to provide a more rugged system, for example, a service which is intended to be received on portable receivers with attached or built-in antennas.

The inherent flexibility of digital transmissions has many advantages compared to that of transmission using a "fixed-format" analogue system. However, the number of digital system configurations possible makes it difficult to provide a direct comparison between the capabilities of analogue and digital systems which are designed to occupy the same channel-width. These difficulties are compounded by the fact that some digital systems permit changes of configuration on a dynamic basis to suit broadcasters' varying needs. Nevertheless, there are some features which seem to be quite general. Digital systems:

Even so, it is necessary to qualify some of these features. The better spectrum utilization and the lower radiated powers are the result of C/N and protection ratio values which are lower than those for analogue systems. The use of precision offset with analogue transmissions can give protection ratios comparable with those for digital systems which are intended to provide high quality and in the latter case, the saving on transmitter power may not be very high if an attempt is made to provide coverage to a very high percentage of locations. Similarly, the use of ghost-cancellation schemes can reduce the sensitivity of analogue systems to that particular type of impairment. Nonetheless, the overall balance is that the use of digital television systems offers significant advantages over their analogue equivalents.

7.2.4 Digital system techniques

7.2.4.1 Single-carrier techniques

In the case of single-carrier systems, which are described in detail in 4, the factors for the planning process are affected by choices made in connection with the frequency planning for the area. These choices include coverage, graceful degradation, service availability, picture quality, expected receiving conditions and effects on current analogue services. For single-carrier systems, the new digital service will be introduced as an enhancement or as a simulcasting replacement to the existing analogue TV service and will need to coexist with that service over a lengthy introduction time period. Additionally, the digital service must fit into the existing spectrum allocated to TV broadcasting.

In the present environment to coexist with the present analogue TV services, factors are influenced by the implementation choices based on population and spectrum usage density, i.e. in dense environments coverage is likely to be interference-limited, in less dense environments coverage may extend to match the protected contours of the existing analogue service.

7.2.4.2 Multicarrier techniques

As described in detail in 4, the approach taken is to modulate the data onto a large number of carriers at closely-spaced frequencies. The symbol rate of each carrier is thus very low, giving it a narrow bandwidth. The key point is that the carriers are orthogonally spaced in frequency - that is, they are spaced such that there is no mutual interference. Benefits occurs when using this system in the presence of interference and multipath, as only some of the carriers will be affected at any one time.

Three additional features can be added to improve performance:

The result is a system with an enhanced performance in the presence of interference and multipath.

Key benefits occur when COFDM systems are used for wide-area broadcasting (in which the same service is broadcast from many transmitters or gap-fillers). With conventional systems, interference is prevented by operating the transmitters on different frequencies. With COFDM, however, the simultaneous reception of signals from several transmitters or gap-fillers appear at the receiver to be multipath propagation with delays. COFDM systems can be designed to allow relatively long delayed echoes, and so all the transmitters or gap-fillers in a network can be operated on the same frequency. This concept is known as a single frequency network (SFN). In planning SFN systems, the power in a delayed signal can be considered to be composed of a constructive and interfering component. The relative amounts of each component depend on the signal delay.

7.2.4.3 The proposals outlined in 7.2.7 are examples of the application of both types of the above digital techniques.

7.2.5 Digital network possibilities

The full range of possibilities for digital television networks will only become available when it is no longer necessary for digital and analogue services to share spectrum (see 7.2.2 and 7.3.1) and the remainder of this section assumes that digital television services have exclusive use of a given spectrum allocation.

The inherent flexibility and better spectrum utilization of terrestrial digital television systems (as compared with analogue systems) makes it possible to consider a much greater range of network configurations than is available with analogue television. One obvious difference is that Single Frequency Networks (SFNs) may become possible under some circumstances. This leads to an initial division of networks into Conventional and SFN types, although there are significant similarities and overlaps in such a division.

Conventional networks imply similar planning concepts to those used at present for analogue networks, whether these are intended to provide individual station, regional or even national coverage. It is likely that transmitter sites similar to those used at present would continue to be used in order to maintain existing coverage patterns. The major differences from the existing analogue networks would be the smaller distances between co-channel transmitters and the reduced set of constraints on the channel relationships between overlapping coverages (whether the transmitters are nominally co-located or not). In practice, these apparently small differences will have major consequences because of the potentially large increase in the capacity of the available spectrum. This will lead either to a significant increase in the number of programmes available or to a reduction in the amount of spectrum allocated to television.

SFNs on a large scale imply the use of a multicarrier digital system (such as OFDM). In addition, the basic planning concepts have major differences from those used for analogue networks. If medium or large areas require to be served with exactly the same programme material, then a complete network may have all of its transmitters on exactly the same frequency, although there are significant constraints on the timing requirements for the programme material to be transmitted. Clearly, the use of a single frequency for large area coverage of a programme leads to significant spectrum savings. In the case where multiple programmes are carried within a single channel, the savings may be even greater, although such usage implies that higher C/N and protection ratios are required and this to some extent offsets the apparent gains. In addition, it is necessary to consider carefully the symbol length and guard interval requirements if the full benefits of an SFN are to be achieved.

Several variants of SFN for providing large area coverage exist, although these differ more in appearance than in reality. The primary difference lies in the spacing between transmitter sites. At one extreme would be a network based on the sites used currently for analogue services, which can be up to some 80 km apart. At the other extreme would be a dense network with transmitter spacings of only 10 or 20 km. In practice, any real network is likely to consist of some elements of both of these cases. Even a network based primarily on the existing analogue station sites would be likely to need a number of relay stations and these would have relatively small spacings. Conversely, a dense network is likely to have some "gaps" where the population density is too low to make it economically justifiable to build some stations.

It cannot be assumed that SFN usage implies that large areas are to be covered. An alternative usage would be confined to urban areas in order to provide the high signal levels needed for portable reception. In this case, there could be an SFN for each urban area, with a conventional planning approach used to provide different services in separate, individual urban areas.

One aspect of SFN usage may not be confined to multicarrier systems. If delay equalizers are used with a single carrier system, then it is possible to use a single frequency for a main station and its geographically nearby relays in order to provide for coverage extensions. However, one normal requirement of a delay equalizer is that there should be a significant difference in amplitude between the main signal and any delayed component. If this is the case then there can be little or no coverage overlap between the service area of the main station and that of any of its relays, or between the coverage areas of the individual relay stations.

7.2.6 Planning Factor Considerations

Protection Ratios

For the digital/analogue coexistence environment, protection ratios are dependent on a number of factors which for both services may include modulation schemes, error correction used, synchronization, picture quality and interference protection. For digital interference to the analogue service, the digital interference resembles a noise-like signal and the co-channel protection ratios fall in the range of 35 to 45 dB. For the digital service, interference may result from either an existing analogue service or a newly established digital service. For this case, the co-channel protection ratios may range from 5 dB (digital interferer) to 20 dB (analogue interferer). Additionally, in the case of the analogue interferer the protection ratio may vary depending on the picture content of the analogue signal. Other protection ratios required for planning include those for adjacent and image channel.

Receiving System

Receiver noise figure, receiver C/N ratio, antenna gain, and feeder line loss are factors used to establish the required field strength for satisfactory receiver operation (Grade 3 picture quality for analogue service in 1 to 5% of the time and available picture for digital). An example of receiving system parameters currently being considered is given in Table 15.

Table 15

Receiving system parameters

Receiver Noise Figure

10 dB

Receiver C/N Ratio

16 dB

Receiver Bandwidth

6 MHz

Frequency

615 MHz

Antenna Gain (dipole)

10 dB

Line Loss

4 dB

Minimum Field Strength

44.5 dB

 

In practice, the performance of any receiving system may be improved by simple methods such as using a low noise amplifier or an antenna with higher gain.

Polarization

The use of orthogonal, horizontal and/or vertical polarization of the radiated signal may be beneficial in reducing mutual interference, particularly in densely congested areas.

Service availability

The abrupt failure characteristics of digital services requires close attention to the coverage or service availability. A location and time availability of 90% is generally assumed to be necessary.

Portable indoor reception

Portable indoor reception is not possible throughout the full coverage area derived by planning with external antennas. Indoor reception has a loss penalty in the order of 20 to 30 dB due to lower antenna gain and height and to building penetration which results in a considerable reduction in the service area. One solution to this problem is to accept the coverage where it is possible.

7.2.7 Digital proposals

Consideration of the above planning factors leads to a range of implementation strategies which are described in 7.3.1.

 

Continue to Section 7.3

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