1、-1- Rep. ITU-R BS.1203-I REPORT ITU-R BS.1203-1 DIGITAL SOUND BROADCASTING TO VEHICULAR, PORTABLE AND FIXED RECEIVERS USING TERRESTRIAL TRANSMITTERS IN THE UHFNHF BANDS (1 990- 1994) This Report contains background and descriptive material for Recommendation ITU-R BS.774 entitled “Digital sound broa
2、dcasting to vehicular, portable and fixed receivers using terrestrial transmitters in the UHFNHF bands“. 1. Introduction Digital techniques have been used in sound programme production and transmission by some broadcasters for many years now, and more recently have become cheap enough to be introduc
3、ed into the domestic consumer market, leading to wider public appreciation of high-quality sound, albeit via non-broadcast digital media. At the same time, there is rapidly growing congestion in the VHF/FM radio bands in many countries; thus the FM broadcasting services, which can deliver unimpaired
4、 sound quality into the home, are under threat of erosion of the quality deliverable. Overcrowding will inevitably increase the levels of interference which must be tolerated, particularly for vehicular and portable receivers, which do not benefit from elevated, directional, receiving antennas, usua
5、lly assumed in planning service coverage. receivers, the solution for the future development of sound broadcasting is to provide an entirely new digital sound broadcasting service, designed at the outset to meet all the reception requirements of the diverse listening audiences. Also, a complete digi
6、tal sound programme chain can be established from studio to domestic receiver. In contrast to existing sound broadcasting systems, the new system has to provide unimpaired sound broadcast reception to fixed, portable and mobile receivers. The requirement for mobile reception necessitates entirely ne
7、w transmission methods which have been defined and are described below. Although VHF/FM services can still provide excellent service to properly-installed fixed This Report describes the requirements for digital sound broadcasting to vehicular, portable and fixed receivers using terrestrial transmit
8、ters, the techniques employed in the digital sound broadcasting system, and considers relevant planning parameters and sharing considerations. The Report also makes reference to the terrestrial part and common system characteristics of the mixed satellite/terrestrial digital sound broadcasting servi
9、ce concept as well as the hybrid delivery concept. The mixed satellite/terrestrial service concept is based on the use of the same frequency band by both satellite and terrestrial broadcasting services to the same receiver. The hybrid delivery concept is based on the use of low power terrestrial “ga
10、p-filler“ to improve the satellite coverage. Both concepts are made possible by the techniques employed in the digital sound broadcasting system and are described in more detail in Report ITU-R B0.955. -2- Rep. ITU-R BS.1203-I Most of the information included in this Report is based on studies and t
11、ests undertaken in Canada, Germany, France, the Netherlands, United Kingdom, Sweden and by the EBU in association with the Eureka-147 Project using the system concept described in Annex 1-A. systems (see Annex 2). Recent efforts have been undertaken to explore other possible digital sound broadcasti
12、ng 2. Service and system requirements into account. The system shall be intended for fixed as well as portable and vehicular reception. The list of system requirements applies to terrestrial, cable, satellite as well as mixedhybrid satellite/terrestrial delivery concepts. The requirements are: In de
13、fining a digital sound broadcasting system, the following requirements shall be taken 2.1 Sound quality levels - High quality stereo sound of two or more channels with subjective quality indistinguishable from high quality consumer digital recorded media (“CD quality“). 2.2 Sound control signals - T
14、ransmission of control information on sound representation (loudness, dynamic range compression, matrixing, etc.). Service configurations - High-quality channel stereophonic sound. - High-quality monophonic sound. - For special applications, the possibility of adding further sound channels to the ba
15、sic system (for the universal multi-channel stereophonic sound system as defined in Recommendation ITU-R BS.775). Value added services with different data capacities and delivery time (e.g. traffic message channel, business data, paging, still picture/graphics, 1.5 Mbit/s video/sound multiplex, futu
16、re Integrated Services Digital Broadcasting (ISDB). Flexible allocation/reallocation of services, without affecting continuing services. - - 2.4 Service delivery Use of common signal processing in receivers for: a) local, sub-national and national terrestrial VHFLJHF networks; b) mixed use of terres
17、trial and national/supernational UHF satellite services; c) cable. It would be advantageous, in some countries, to design the system and plan the service in such a way that a common receiver could be used for all the above delivery concepts. -3- Rep. ITU-R BS.1203-I 2.5 2.6 2.7 2.8 2.9 3. 3.1 Servic
18、e information - Radio programme data related to each programme signal (programme labelling, programme delivery control, copyright control, conditional access, dynamic programme linking, services for the hearing-impaired). Multiplex system information (simple programme or service identification, sele
19、ction and linking). - Interface - Recording capability of sound signals (in bit-rate reduced form) and related data. This implies recording the complete programme signal including its programme-related data, and the ability to access small blocks of data in the encoded signal. Data interface capabil
20、ity to information technology equipment (ITE) and communication networks. - Service availability - - - Subjectively acceptable failure characteristics. - Vehicular, portable and fixed reception. High-coverage availability in location and time. High immunity to multipath (long and short delay) and Do
21、ppler effect (for mobile receivers). Trade off between extent of coverage for a given emission power, service quality and number of sound programmes and data services. - Spectrum efficiency - High spectrum utilization efficiency (better than FM, maximize frequency reuse and single frequency networki
22、ng, minimize sharing constraints with other services) Multiple programme service provision within a contiguous frequency band. - Complexity - Low-cost basic receiver configuration - System design considerations Use simple, non-directional receiving antenna appropriate to vehicular and portable recep
23、tion. Channel characteristics The design of both a satellite and a terrestrial digital sound broadcasting system is strongly dependent on the factors affecting the propagation characteristics of the path to the vehicular receiver. The propagation path in the VHFUHF frequencies is subject to attenuat
24、ion by shadowing due to buildings and other obstacles, and to multipath fading due to diffuse scattering from the ground and nearby obstacles such as trees, etc. The shadowing andmultipath effects depend on the operating frequency, the elevation angle to the transmitter and the type of environment i
25、n which the receiver is operating: whether it is an open, rural, wooded suburban or dense urban environment. A mathematical description of a vehicular broadcast channel with multipath propagation is given in Annex 1 -C together with experimental results. -4- Rep. ITU-R BS.1203-I Conventional digital
26、 modulation systems are particularly sensitive to multipath signals since these can create severe inter-symbol interference related to the differences in path delays. The inter- symbol interference cannot be overcome by an increase of the transmit power. frequency selective and also time varying. Fo
27、r a conventional digital modulation scheme, the achievable error performance is then strongly limited by the frequency selectivity and the fast field-strength variations with mobile reception. Studies of the statistical distribution of the field-strength (see Report ITU-R B0.955) have shown that it
28、follows a log-normal distribution over large areas, coupled with a Rayleigh (no line-of-sight path) or Rice distribution (consisting of the direct path and Rayleigh distribution) within small areas (usually of dimensions of the order of a few hundred wavelengths). Therefore, in most respects the Ray
29、leigh channel within dense urban areas is the least favourable. Any new system must be designed to operate in this propagation environment. A certain amount of wideband propagation data is available Cox et al., 19751. Typically, at UHF, the 90% correlation bandwidth is of the order of 30 kHz with in
30、dependence (40% correlation) at frequency separations of the order of 3 MHz. Also the delay spread in urban areas is found to be of the order of 1-2 ps but exceeds 3 ps for about 1% of locations in any localized area. In mountainous and hilly terrain, delay spreads can be many tens of microseconds f
31、or large area coverage. For a fixed receiver, the time delay of each path will typically be fixed, but for a moving receiver, the time delay will vary proportionally to the speed of the receiver parallel to the direction of each received path. Thus, different Doppler shifts are associated with multi
32、ple paths arriving at the receiver from different angles. The Doppler effect is described in terms of the parameters “Doppler spread“ and its Fourier inverse, the correlation (coherence) time. The Doppler spread of the mobile channel depends on the vehicle speed and is typically equal to 2 VA where
33、v = vehicle speed and h = the carrier wavelength. For a vehicle speed of 100 kmh, a 1 500 MHz signal has a Doppler spread of about 275 Hz and a correlation time of 3.6 ms. A stationary vehicle will nominally have a Doppler spread approaching zero and a very long correlation time. However, the presen
34、ce of other moving vehicles in the vicinity will also create a non-stationary multipath field. The correlation time defines the amount of time diversity that can be gained by simple symbol interleaving. If the time interval over which symbols are interleaved is large compared to the correlation time
35、, significant time diversity is achieved. Thus, with respect to achieving time diversity, the stationary or slow moving vehicle defines the worst case and at least several hundreds of milliseconds of time interleaving may be desirable. In relation to the digital channel bandwidth necessary, the mult
36、ipath propagation may be However, no amount of time interleaving can provide usable time diversity for the stationary receiver. Thus, extensive frequency diversity is also needed. For portable receivers usually situated in the indoor urban environment, building penetration loss may be a critical fac
37、tor. In some cases, the attenuation caused by walls and ceilings may be very severe (e.g. exceeding 20 dB), though penetration through apertures will mitigate the overall effects. Nevertheless, it may be uneconomical to provide sufficient link margins from a satellite, and some low power terrestrial
38、 retransmitters (gap-fillers) will be needed to provide a service to listeners located in multi-floor office buildings and apartment buildings located within urban service areas. It should be pointed out that satellite propagation characteristics may substantially differ from those of the terrestria
39、l links. The delay spread of the terrestrial links is generally much longer than that of the satellite link. Similarly, the correlation bandwidth of the terrestrial link is smaller. These differences may cause some divergencies in the parameters of the system design for each -5- Rep. ITU-R BS.1203-I
40、 application. For example, a VHF terrestrial system may use a longer symbol duration than a UHF satellite or hybrid satellite/terrestrial system (see Annex 1 -A). 3.2 Basic system characteristics 3.2.1 Modulation and channel coding spread spectrum techniques. Its main weakness is however the low spe
41、ctrum utilization attainable, usually below 0.25 bit/s/Hz, and this is unacceptable for use in broadcasting where there are severe restrictions on the available spectrum. One way to overcome the selective fading effects of the Rayleigh channel is to employ 3.2.1.1 COFDM (coded orthogonal frequency d
42、ivision multiplex) A new scheme, known as coded orthogonal frequency division multiplex (COFDM) Alard and Lassalle, 1987; Le Floch, 19891 has therefore been devised which is well suited to a selective Rayleigh channel and, despite being broadband, provides for efficient frequency utilization. This m
43、odulation approach is used in the Digital System A described in Annex 1-A. COFDM, the RF transmission technique employed by the Digital System A system, was developed to meet the exacting requirements of high bit-rate transmission to vehicular, portable and fixed receivers. Its basic principle consi
44、sts of distributing the information to be transmitted over many carriers each having low bit rates, so that the corresponding symbol duration can be larger than the delay spread of the channel. Then, provided that a temporal guard interval is inserted between successive symbols, the channel frequenc
45、y selectivity will not be a cause of intersymbol interference, it does not suppress frequency selective fading; i.e. some of the carriers may be enhanced by constructive interference, while others may suffer from destructive interference (i.e. frequency- selective fading). To correct this problem, C
46、OFDM provides for multiple linkage of the elementary signals (information modulating a given carrier during a given symbol time) received at distant locations of the time-frequency domain. The linkage is achieved by convolutional encoding with time and frequency interleaving at the source, in conjun
47、ction with a maximum-likelihood Viterbi- decoding algorithm employed in receivers. The diversity provided by interleaving plays an important role in maximizing Viterbi- decoding efficiency, because successive samples presented at its input are affected only by independent (uncorrelated) fades. Even
48、when a receiver is not moving, the diversity in the frequency domain is sufficient to ensure correct behaviour of the system. Consequently, multipath provides a form of diversity, which, in stark contrast to conventional FM reception, is actually an advantage in COFDM reception. Furthermore, this ad
49、vantage in ruggedness of the system performance improves with increased transmission-channel bandwidth. The signal-to-noise ratio will increase as soon as the received signal power is augmented by echoes that cannot combine destructively: this is the case when the echoes are separated by a minimal delay equal to the inverse of the signal bandwidth. These echoes can be artificial (“active echoes“) which are obtained by retransmitting on the same frequency block. This is a form of space diversity (at the transmitting end). It allows for a number of different network concepts (see 9 4.6): The
