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Coverage Aspects of Digital Terrestrial Television Broadcasting

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Coverage Aspects of Digital Terrestrial Television Broadcasting Coverage aspects of digital terrestrial television broadcasting C. Weck (IRT) 1. Introduction The specification for digital terrestrial television, DVB-T, offers a wide range The intention is to make terrestrial broadcasting of of potential applications: single trans- television programmes a more attractive proposi- mitter and single-frequency networks, tion to meet competition from cable and satellite prohibited channel operation, port- transmission. The new specification for digital terrestrial television, DVB-T [1], will make it pos- able reception, hierarchical trans- sible to provide television services which can hold mission, etc. The network operator their own, even when digital transmission via sat- can select technical parameters such ellite and cable is introduced. as the number of OFDM carriers, the length of guard interval, the degree of The chief advantages of terrestrial broadcasting error protection and the modulation are to be found in regional and local services method. The last two parameters in (pictures, sound and data). A further plus point particular allow the operator to reach for digital systems is the marked improvement in a compromise between the number portable reception when ordinary antennas are of programmes carried and their used. It is also possible with digital systems to transmission reliability. This raises transmit within a standard (7 or 8 MHz) television channel either (i) more programmes concurrently the question of which results, with or (ii) programmes with a higher image resolution regard to coverage, need to be (EDTV, HDTV). achieved in each case. The main applications have been As far as coverage planning is concerned, there studied at the IRT using Monte Carlo are important advantages in the design of the simulations of regular network transmitter network structure and the energy bal- ance. An important aspect in this respect is the situations. The results are docu- possibility of using single-frequency networks mented here in the form of predic- (SFNs) combined with the possible use of “gap- tions for the coverage probabilities. fillers” (local relay transmitters) on the same car- The reasons for the choice of para- rier frequency in regions where the service is poor. meters for the terrestrial specification With a digital system it is also possible to use are dealt with specifically. Ways to domestic gap-fillers, at the same transmission fre- restrict the basically very wide choice quency as the main transmitter, inside houses or of system states (modes) can also be flats where the signal attenuation is frequently high, in order to supply adequate field strengths derived from these simulations. Original language: German for portable appliances. Manuscript received 25/9/96. EBU Technical Review Winter 1996 19 Weck 2. Digital terrestrial television fact that the introduction of DVB-T will generate broadcasting instant demand for several digital television pro- grammes or additional multimedia services. Because of their high data-rate (ITU-R Recom- mendation BT.601 [2] specifies a rate of 216 Mbit/s A transmission method for digital video signals in the studio), digital video signals can only be with a high spectral efficiency is therefore re- transmitted via existing television channels after quired which, at the same time, has a high resist- the data-rate has been effectively reduced. The ance to the many different types of interference on MPEG-2 source-coding method envisaged for the channel (e.g. interference due to multipath re- DVB achieves a data-rate of 5-6 Mbit/s for digital ception, signal interference from other radio or video signals of a quality at least comparable with broadcasting services, high signal attenuation in PAL. This data-rate is assumed for a standard- the case of portable reception in buildings, etc.). definition programme (SDTV) although an ade- quate image quality (comparable to that of the In the development and definition phase of the VHS tape format) could be achieved with lower DVB-T specification, the advantages of the data-rates, depending on the picture content; higher COFDM transmission method [3][4] outweighed data-rates (8-11 Mbit/s) are also of interest for those of single-carrier methods in terms of broad- highly-critical image presentations (e.g. sport). casting requirements (as was also the case with DAB). The crucial factor in favour of COFDM is There are still bottlenecks with the available trans- the ability to set up single-frequency networks mission channels for DVB-T, despite the large which offer network planners a higher network data reduction achieved by source coding. The efficiency. However, a digital system also allows frequency resources in the VHF and UHF ranges greater planning leeway for regional broadcasting are already largely exhausted by analogue ser- services in the existing transmitter network (e.g. vices (e.g. in Germany alone, there are currently possible use of prohibited channels such as adja- 578 analogue television transmitters and 8707 cent and image channels) because, with an ap- gap-fillers). The situation is exacerbated by the propriate choice of parameters, networks can also be planned with lower signal-to-noise ratios. Abbreviations agl Above ground level 3. Coverage probability C/I Carrier-to-interference ratio The abrupt degradation of a DVB system at the C/N Carrier-to-noise ratio coverage periphery makes it easy to establish whether or not reception is possible at a location. CCETT Centre Commun d’Etudes de Télé- diffusion et de Télécommunications A single location can be deemed to be covered if (France) the required signal-to-noise ratio is achieved for 99 % of the time. On the other hand, it is not COFDM Coded orthogonal frequency practicable to demand a similar coverage for all division multiplex locations in a “sub-area” (e.g. 100 m x 100 m). DLR Deutsche Forschungsanstalt für The coverage criterion for a sub-area is also in- Luftund Raumfahrt (German fluenced by the extent to which the signal-to-noise aerospace research establishment) ratio varies as a function of the location (“log- DVB Digital Video Broadcasting normal fading” component). A high probability of EDTV Enhanced (extended) definition interference-free reception within a particular sub- television area results in high costs for the network operator FFT Fast Fourier transform (higher transmission power, additional main trans- mitters and gap-fillers). In an initial approach by HDTV High-definition television the EBU, the coverage was described as “good” if MPEG Moving Picture Experts Group at least 95 % of the sub-area is served for 99 % of OFDM Orthogonal frequency division the time and as “acceptable” if at least 70 % of the multiplex sub-area is served for 99 % of the time [5]. PSK Phase shift keying Another quantity of interest for the planning of QAM Quadradure amplitude modulation DVB-T is the global coverage probability within SDTV Standard-definition television the target service area, which is derived from the coverage probability of all the sub-areas con- SFN Single-frequency network tained within the target area. 20 EBU Technical Review Winter 1996 Weck 4. Reception situations 50 m); in other words, the actual terrain of the coverage areas is not taken into account. It is thus A distinction is drawn between two particular re- possible to simulate the main reception situations ception conditions for digital terrestrial television with a “Monte Carlo” simulation (explicit study of broadcasting: stationary and portable reception. random events). First and foremost, this allows In both cases, typical conditions found in practice the fundamental characteristics of the new are assumed, not the worst conditions; for exam- technology to be identified, whereas practical use ple, it is assumed that the position of an antenna calls for the planning of coverage for specific can be optimized over a range of ± 50 cm. cases. Simulations such as these are based on propagation models whose parameters are not A directional antenna 10 m above ground level clearly established. The influence of critical prop- (agl) is assumed for stationary reception. For por- agation parameters therefore have to be taken into table reception, a non-directional antenna on top of account. In the case of single-frequency net- or inside the equipment – positioned 1.5 m above works, the statistical propagation processes from the ground or above floor level – is assumed. The all the active transmitters are superimposed. least favourable case, on the ground floor of a building, is generally considered separately. The fading component at a specific distance from the transmitter exhibits a log-normal distribution. When investigating the coverage probabilities by The data in the DVB-T specification which relates means of Monte Carlo simulations, the basic stan- to the efficiency of the system are based on simula- dard deviation (σ) for the fading component must tion results using a Rayleigh channel model for be selected carefully. The use of a mean variation portable reception (omnidirectional reception in a of σ = 9.5 dB is recommended for the planning of multipath channel) and a Rice channel model for analogue services in the UHF range. However, stationary reception with a directional antenna. In propagation measurements with digital signals in the latter case, a Rice factor of K = 10 dB is app- a bandwidth of 1.5-8 MHz show a significantly lied, i.e. the received field strength of the direct lower variation (e.g. 5 dB) in the measured field signal is 10 dB higher than that of all the reflected strengths across the location [7][8]. This means signal components. The DVB-T specification thus that the value on which local planning calcula- uses known values for the theoretically required tions are based could also be lower. The variation signal-to-noise ratio for various operating modes. of the field strength prediction error also plays a However, during coverage analyses an imple- crucial role in the calculation of coverage proba- mentation loss of at least 3-4 dB must be expected.
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