1、222 RECOMMENDATION ITU-R BT. 1 123 PLANNING METHODS FOR 625-LZNE TERRESTRIAL TELEVISION IN VHF/UHF BANDS (Question ITU-R 43/11) ( 1994) The IT Radiocommunication Assembly, considering a) is provided using the minimum number of frequencies; that broadcasting transmitter networks should be planned in
2、such a way that the required coverage of the area b) remodelling existing ones; c) line systems, that the theory of uniform transmitter networks is useful for designing new transmitter networks or that while well-established networks exist, there is still a need for guidance on planning methods for
3、new 625- recommends 1. planning of new 625-line terrestrial television networks in the VHF/UHF bands. that the methods set out in Annexes 1 and 2 be used for the design of transmitter networks for the preliminary ANNEX 1 Planning methods for terrestrial television in VHFAJHF bands 1. General Broadca
4、sting-transmitter networks should be planned in such a way that the required coverage of the area is provided using the minimum number of frequencies. The coverage area of each transmitter depends upon a number of technical factors, for example: transmitter power, minimum usable field-strength, radi
5、o-frequency protection ratio, the distance between transmitters sharing the same or adjacent channels, channel spacing, bandwidth of emission and factors influencing wave propagation. It may also depend on the channel distribution scheme. When a large number of channels is to be planned or replanned
6、 for a particular Ah4 or FM sound or television service, it has been found that utilizing the spectrum efficiently can prove difficult when only empirical methods are employed. For this reason, a theory of uniform transmitter networks was developed. This method can be applied with success when some
7、uniformity of standards exists for the services to be planned. Furthermore, the frequency band to be planned should be constrained as little as possible, i.e. there should ideally be complete freedom in assigning any frequency to any transmitter. This theory is not only useful in designing new trans
8、mitter networks or remodelling existing ones, but also provides a powerful tool for determining optimal technical parameters such as channel spacing, transmitter characteristics, etc., and identifying the best attainable coverage. Some countries may prefer to have a complete area coverage with a sma
9、ll number of programmes and others to sacrifice total area coverage in favour of providing more programmes in the more highly populated areas. In these cases, uniform network theory can be used to provide some reference values for attainable coverage. This can help when comparing the differing netwo
10、rks of individual countries which have chosen different methods for achieving their internal coverage. This Annex refers only to 625-line systems. The attention of administrations using other systems is drawn to this fact. Additional data concerning all systems are required. COPYRIGHT International
11、Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*BT. 1123 94 = 4855232 0522024 262 Rec. ITU-R BT.1123 223 2. Theoretical techniques for an international plan 2.1 General Planning techniques resulting from the principie of uniform transmitter networ
12、ks may be considered to comprise two basic elements: - geometrically regular lattices, - linear channel-distribution schemes. Because of the many parameters and effects that may have an impact on frequency planning, e.g. varying propagation conditions, transmitter powers, transmitting-antenna height
13、s and directivities, and terrain irregularities, the problem first requires simplification by assuming all transmitters to have equal powers, to have omnidirectional antennas all at the same height and with the same polarization, and to be situated on an infinitely extended area forming a geometrica
14、lly regular lattice; also that propagation conditions do not exhibit variations throughout the area considered. The development of such regular lattices is discussed in some detail in Appendix 1 and leads to the following basic conclusions: - full area coverage can most economically be provided by a
15、 lattice having equilateral elementary triangles i.e. having equally spaced geographically adjacent transmitters. Some overlap coverage is inevitable if complete area coverage is to be achieved. This can be expressed in terms of a “coverage factor” .e. the sum of individual coveragehotal area to be
16、covered. The reciprocal of the coverage factor is often referred to as the coverage efficiency. This coverage factor has a minimum value of 1.21 for the optimum case of equilateral elementary triangles; because, for television broadcasting, the required co-channel protection ratio predominates over
17、those for other frequency spacings by a large amount, optimum coverage is also likely to be achieved by maximizing the spacing of co-channel transmitters, i.e. by ensuring equilateral co-channel triangles; only particular numbers of channels allow both co-channel and elementary triangles to be equil
18、ateral. These are known as “rhombic numbers” and require that the number of channels, C, is such that: - - C = u2 + ab + b2 where a and b are non-zero integers and without a common divisor. For values of C 80, these numbers are given by: a123344555567778 b1 1 121312341 123 1 C 3 7 13 19 21 37 31 39
19、49 61 43 57 67 79 73 If, however, the total spectrum available for the network does not correspond to a number of channels coincident with a “rhombic number”, a solution using the full available number of channels will still be possible but this will generally mean adopting a lattice formation in wh
20、ich either the co-channel or elementary triangles will not be equilateral. Such a solution may well permit substantially better coverage than that obtainable by restricting spectrum usage to that corresponding to the next lower rhombic number. Exceptionally, other channel numbers can also permit bot
21、h equilateral elementary and co-channel triangles but in such cases linear channel distributions (see Annex 1, 9 3) cannot be used and hence interference levels are not necessarily uniform through the lattice. An example of such a network is given in Fig. 14. If it is considered more important to ha
22、ve the elementary triangles equilateral, this may be achieved by a transformation (e.g. affine) which retains the longest side and rotates and extends the remaining sides to make them equal. An example of such a transformation for an %channel lattice is indicated in Fig. 3b). Having once established
23、 lattices of the type described above, the problem is then to arrange the channels required in such a way as to minimize interference, remembering that every co-channel rhombus forms only part of a lattice extending over the whole planning area. The derivation of linear distribution schemes is discu
24、ssed in some detail in Appendix 1. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*BT- II23 94 m 4855212 0522025 IT9 m 224 Rec. ITU-R BT.1123 Such a linear distribution has the property of having an identical interference s
25、ituation on all channels, except for those on the highest and lowest frequencies (in cases where adjacent-channel interference is relevant). The method can be extended to the case (an example of this is given in 5 2.6) where it is desired to provide n programmes from each site using a total of nC ch
26、annels in contiguous sub-bands each of C channels. In this case the channels assigned to each transmitter will be: c, c + C, c + 2C, etc., where O I c I C - 1 2.2 Implications of applying regular lattice planning principles in specifi terrestrial television bands In the following sections!. the appl
27、ication of these principles to the specific numbers of channels available in each band will be considered, and at the same time these examples will be used to develop further aspects of these planning principles. However, before considering the implications in individual bands, it is appropriate to
28、consider which frequency relationships, additional to Co-channel, need to be taken into account in television network planning (see Annex 2). These are: veiy relationship I Channel difference I I Adjacent channel I 1 I I I Radiation from local oscillator() 4 or 5 I I 8,9 or 10 I Image channel() I Fo
29、r channel spacings and receiver intermediate frequencies in general use. For any lattice based on approximately equilateral elementary triangles, it follows that any transmitter will be spaced almost equidistantly from six other transmitters which, except in the case of lattices with a very small nu
30、mber of channels, will all be on different channels. It follows therefore that unless very distorted elementary triangles are adopted, no lattice having less than eight channels can avoid having adjacent-channel overlaps. Figure 2b) shows an example of a 7-channel lattice which avoids adjacent-chann
31、el overlaps at the cost of distorting the elementary triangles hence requiring a substantial increase in the coverage radius required of individual transmitters. 2.3 Band I The full extent of this band is 21 MHz. Hence, the maximum number of channels available is three for any transmission system cu
32、rrently in use having 7 MHz bandwidth. Only one form of lattice is possible with three channels, which has the following characteristics: a) b) c) d) the three channels are at the apexes of the elementary triangles; equilateral elementary and Co-channel triangles (three is a rhombic number); Co-chan
33、nel spacing =* times the spacing between adjacent transmitters; for complete coverage, the maximum distance between any transmitter and the nearest point on the coverage area of the next Co-channel transmitter (at the centroid of the elementary triangle) is twice the coverage radius. The implication
34、 of a) is that adjacent-channel overlaps are inevitable. The implication of d) is that complete area coverage is only possible (even if any allowance for multiple interference or interference from other than Co-channel transmitters is neglected), when the field strength at the extremity of the cover
35、age radius exceeds that at twice the distance by at least the value of the required Co-channel protection ratio. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMNaBT. 1123 94 m 4855232 O522026 035 m Rec. ITU-R BT.1123 225 Fi
36、gure 1 shows, taking a three-channel lattice as an example, how a systematic use of frequency offsets and use of both horizontal and vertical polarization can reduce Co-channel interference. In the general case, polarization discrimination can also be used to reduce adjacent-channel interference. Ho
37、wever, in the particular case of three channels (as shown in Fig. l), this is of limited advantage since the middle channel (B) is equidistant from two of each adjacent channels and polarization discrimination could only be obtained against one of each pair. The same principles, can of course, be ap
38、plied to any lattice. It may be seen that: the separation distance between Co-channel transmitters without either polarization or offset protection is now three times that between adjacent channel transmitters; ali of the nearest ring of six Co-channel transmitters have offsets but only three also h
39、ave the opposite polarization (this is because while three offset options are possible, there are only two polarizations). - - FIGURE 1 3-channel lattice demonstrating use of frequency offsets and polarization to reduce interference A B C A OH OH A, B. C: channels H, V: polarizations O, +, -: offset
40、s (1/3 line time-base) Offsets and polarizations shown for channel A only. In view of the high values of Co-channel protection required for a television service, even taking account of the reduction in interference possible by frequency offset and polarization discrimination, full area coverage cann
41、ot be provided by a three-channel lattice, i.e. it is not possible to provide complete coverage over an extended area using Band I alone. 2.4 Band Ill The total available spectrum is 56 MHz, permitting either 7 channels of 8 MHz channel width or vice versa. Whereas television systems using both 7 an
42、d 8 MHz nominal bandwidth are in use in ITU Region 1, it should be noted that in the UHF television bands a channelling of 8 MHz has been adopted throughout the Region. It would seem COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Servicesther
43、efore that in any future planning this standard should also be adopted uniformly. Moreover, adoption of 8 MHz channelling would not preclude retention of television systems requiring 7 MHz nominal bandwidth if desired. However, for completeness, the following discussion examines both 7- and 8-channe
44、l lattices. 2.4.1 Seven-channel lattice Superficially, the most appropriate lattice is that indicated in Fig. 2a) which has equilateral elementary and co- channel triangles (possible because 7 is a “rhombic number”). The lattice has the following characteristics: - overlaps between adjacent channels
45、; - a Co-channel distance of $ times the distance between geographically adjacent transmitters. (The ratio of Co-channel distance to the distance between transmitters is NO. ofchannels. This is an intrinsic characteristic of such a lattice.) An alternative form of 7-channel lattice is that shown in
46、Fig. 2b). The preference for this lattice is based on the greater adjacent-channel distances but, as indicated by Figs. 2c) and 2d), this is obtained at the cost of a considerable distortion to the elementary triangles and consequently a need for a 22% greater coverage radius by individual transmitt
47、ers to achieve complete area coverage. FIGURE 2 Examples of 7-channel lattices O O 5 *I 7 - / I / 2 U u a) Equilateral elementary triangles f c) Elementary triangle from a) ri: minimum radius for complete coverage O O b) Avoiding adjacent channel overlaps d) Elementary triangle from b) 0: minimum ra
48、dius for complete coverage Ratio 12 11-1 = 1.22 wz Nore I - Figures c) and d) are scaled to same co-channel distance. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services_ ITU-R RECMN*BT- 1123 94 4855232 0522028 708 M Rec. ITU-R BT.1123 22
49、7 2.4.2 Eight-channel lattice Not being a rhombic number, it is not possible to construct a linear channel distribution with both equilateral elementary and Co-channel triangles. The lattice most closely approximating to this is that shown in Fig. 3a) (equilateral co-channel) and Fig. 3b) (normalized to give equilateral elementary triangles). Figure 3c) indicates an alternative lattice having more distorted elementary triangles for which the smallest distance between transmitters is not equal to that between adjacent channels. This does not, however, preclude some adjace
