1、 Rep. ITU-R M.760-3 1 REPORT ITU-R M.760-3 Link power budgets for a maritime mobile-satellite service (1978-1982-1990-2004) 1 Introduction The purpose of this Report is basically twofold: to list the parameters which must be taken into account when determining link power budgets for a future maritim
2、e mobile-satellite system, and the conditions under which these parameters apply; to provide examples of link power budgets to illustrate the application of these parameters. Link power budgets are required in order to determine the likely power requirements and physical characteristics of the space
3、 sector (satellite) and the Earth sector (coast earth stations and ship earth stations). The overall design for a future system will rely to a great extent on the experience acquired in the development and operation of existing and planned maritime satellite systems, and hence considerable guidance
4、on the choice of suitable power budget parameters can be obtained by reference to the INMARSAT system Inmarsat, 1978; MARISAT, 1977. The link budget examples given in this Report relate to analogue systems with global coverage by a satellite system utilizing a shaped beam. The example mobile standar
5、ds used are derived from those of the analogue INMARSAT Standard A system. Examples of link budgets for digital systems are presented in Report ITU-R M.921. 2 Power budget philosophy The philosophy adopted in this Report is to assume a “reference” system configuration from which corresponding parame
6、ters for other configurations may be derived. From the propagation viewpoint, the channel quality obtained with a maritime satellite system will be strongly dependent on the satellite elevation angle, and so it would appear reasonable to adopt the elevation angle as one of the parameters on which to
7、 base the reference system. The reference configuration for the power budgets is assumed here to apply at satellite elevation angles of 5 for coast earth stations and 10 for ship earth stations; corresponding parameters and resultant channel qualities for elevation angles other than these may then b
8、e derived. A further criterion adopted here is to assume that parameters for the Earth sector are the same as for existing, or planned, systems: in particular, INMARSAT standard-A ship earth stations are assumed. 3 Telephony channel parameters Telephony is expected to be the dominant service in term
9、s of system power requirements and hence the power budgets are based on a telephony channel. For the purpose of presenting information on link budget examples in this Report, as a reference it has been assumed that an overall carrier-to-noise density ratio (C/N0) of 53 dB/Hz should be achieved for a
10、t least 80% of the time in both directions of transmission, with satellite elevation 2 Rep. ITU-R M.760-3 angles of 5 at coast earth stations and 10 at ship earth stations and with no multipath fading on ship/satellite links. This objective has been used as the reference configuration for the power
11、budgets, as shown in the extreme right-hand column of Tables 1 and 2. The current ITU-R telephone channel quality objective for telephone channels in the maritime mobile-satellite service corresponds to an overall C/N0of about 52 to 53 dB/Hz (see Recommendation ITU-R M.547). In addition to the refer
12、ence configuration, example power budgets are shown in Tables 1 and 2 for ship elevation angles of 5 and 10 when multipath fading occurs on ship/satellite links. The fading parameters are assumed to be applicable to conditions obtaining for at least 99% of the time. However, insufficient experimenta
13、l data is available to reliably establish required fading margins for any confidence level. Single-channel-per-carrier operation in frequency division multiple access is envisaged for telephony, with narrow-band frequency modulation and speech processing (e.g. 2:1 syllabic companding). With C/N0of 5
14、3 dB/Hz at the demodulator input, a suitable receiver noise bandwidth would be 30 kHz with 50 kHz channel spacing; the corresponding carrier-to-noise ratio (C/N) would be about 8 dB. Threshold extension demodulators, with a nominal threshold of around 50 dB/Hz, would be required to ensure a graceful
15、 degradation of speech quality with reducing signal level. 4 Space sector parameters The space sector configuration assumed here is based on global coverage in both shore-to-ship and ship-to-shore directions of transmission. Assumptions regarding the characteristics of the 1.5/1.6 GHz antenna and th
16、e intermodulation noise performance of the transponders are summarized below. 4.1 1.5/1.6 GHz antenna The critical transmission path, from the viewpoint of satellite power requirements, is that in the satellite-to-ship direction at 1.5 GHz. Efficient use of the available satellite power for Earth co
17、verage may be obtained by assuming a shaped beam antenna Wood and Boswell, 1974; Lancrenon et al., 1976. The trade-off between beam-centre and beam-edge gain with this antenna provides an almost constant received flux density at the Earths surface for all elevation angles, and thus optimizes system
18、performance for those ship earth stations at the edge of the coverage area where propagation effects are at their most severe. Similar compensation may also be provided on the up-path at 1.6 GHz. 4.2 Intermodulation noise performance The satellite amplifier carrier-to-intermodulation-noise density r
19、atio (C/I0) in Table 1 for the shore-to-ship link is shown to be a limiting factor in determining the resultant C/N0and therefore must be optimized against amplifier output power. For typical amplifiers and suitable frequency plans that minimize intermodulation noise in occupied channels, a C/I valu
20、e of about 19 dB in a 30 kHz bandwidth may be reasonably achieved with significant satellite amplifier loading. This results in the assumed C/I0of 63.8 dBHz for the shore-to-ship direction. In the ship-to-shore direction, as given in Table 2, a C/I0of 70 dB/Hz has been assumed as the satellite ampli
21、fier is not expected to be as power limited as in the shore-to-ship direction. Rep. ITU-R M.760-3 3 5 Earth sector parameters The following ship earth station and coast earth station radio-frequency characteristics have been adopted as the basis for the power budgets: Ship earth station G/T: 4 dB(K1
22、) (Standard-A) Ship earth station e.i.r.p.: 37 dBW Coast earth station G/T: 32 dB(K1) Coast earth station e.i.r.p.: 60 dBW. 5.1 Ship earth station The assumed G/T of 4 dB(K1) is a net value which includes allowances for power losses due to pointing accuracy, misaligned polarization ellipses, diplexe
23、r and dry radome; an additional allowance is shown in the link budgets for loss due to a wet radome. This sensitivity may be achieved with an antenna of 23 dBi gain and a transistorized receive amplifier of 500 K total system noise temperature. An appropriate antenna would be a paraboloid of 1.2 m d
24、iameter, with 60% illumination efficiency, and a beamwidth of approximately 10 to the 3 dB points; an axial ratio of about 2 dB has been assumed. For the purposes of the link budget calculations, the pointing and polarization losses have been considered separately. No allowance has been made in the
25、power budgets for blockage effects from the ships superstructure; these would depend on the individual ship and could, in theory, be eliminated by choosing an unobstructed site. 5.2 Coast earth station For a clear-sky G/T of 32 dB(Kl) at 5 elevation, a total system clear-sky noise temperature of 100
26、 K has been assumed for the calculation of noise temperature degradation under fading conditions. Fading parameters for 4/6 GHz links are assumed to be applicable to propagation conditions obtaining for 99.99% of the time; the channel quality of the overall links between coast earth stations and shi
27、p earth stations is thus mainly determined by the loss parameters on ship/satellite links at 1.5/1.6 GHz. An appropriate coast earth stations antenna diameter would be 13 m, assuming an illumination efficiency of 60%. The antenna axial ratio has been assumed to be 0.5 dB; for the purposes of the lin
28、k budget calculations, polarization and pointing tosses have been considered separately. The uplink intermodulation noise from all coast earth station transmit amplifiers has been assumed to result in a total uplink C/I greater than 30 dB in a 30 kHz bandwidth. This assumption is consistent with the
29、 current Inmarsat Coast Earth Station Technical Requirements Document and the expected performance of typical frequency plans that could be employed. This results in the C/I0value of 75 dB/Hz used in Table 1. 6 Derivation of link margins The margins necessary to achieve the desired objectives for th
30、e satellite links are dependent upon many factors including the satellite characteristics, ship earth station and coast earth station characteristics, weather, sea states and satellite elevation angles. The overall system design should include link margins consistent with the following factors: mini
31、mum elevation angle considered for service; tolerable performance degradation for elevation angles lower than the minimum design point. 4 Rep. ITU-R M.760-3 The choice of link margins in relation to elevation angle () and the system design will affect the performance of the entire system (as seen fr
32、om the terrestrial network). A system designed to achieve a specified quality for a stated percentage of time, with margins to allow for degradations occurring at low elevation angles, will achieve this quality for much higher percentages of time when calls to and from all ships within the coverage
33、area are considered. The margins required to compensate for losses other than the basic free space path loss can be grouped into categories of long term (fixed and random losses) and short term. 6.1 Long-term losses Long-term losses may be defined as those which would tend to last for a period of at
34、 least a minute per occurrence. 6.1.1 Fixed long-term losses Long-term losses which occur with certainty should be included in the path loss for all calculations. The composite margin for these losses (dB) is their sum: =jjffLL,(1) 6.1.2 Random long-term losses Several of the long-term losses tend t
35、o occur randomly and are generally independent of each other. The cumulative mean loss (lL ) and variance (2l ) may be obtained by summing the individual means and variances: =jjllLL,(2) =jjll2,2(3) 6.2 Short-term losses Short-term losses may be defined as those which would tend to last for a period
36、 of less than a minute per occurrence. Since the occurrence of a significant short-term fading period may be infrequent, uncertain, or may only occur at certain times of the day, consideration should be given to the fading statistics when establishing fade margins. Short-term losses tend to be indep
37、endent of each other and to have different statistical distributions. The cumulative margins needed for these losses may be strongly dependent on elevation angle (), since the elevation angle independent losses are relatively small or infrequent. The cumulative mean loss (sL ) and variance (2s ) for
38、 the cumulative short-term losses may be obtained by summing the individual mean losses and variances: =jjssLL,(4) =jjss2,2(5) Rep. ITU-R M.760-3 5 6.3 Cumulative margins for all losses Since the long-term and short-term losses are reasonably numerous and independent, the total margin requirements c
39、an be approximated by a normal loss distribution having a mean (dB) given by: slfLLLL +(6) and standard deviation (dB) given by: 22sl+ (7) The appropriate cumulative margin (dB), corresponding to time-percentages of 80%, 99% and 99.99%, would then be given by: +=+=+=72.3 %)99,99(33.2 %)99(85.0 %)80(
40、LMLMLM(8) 7 Margins at 1.5/1.6 GHz for ship/satellite links 7.1 Long-term losses Long-term losses for which margins may be needed are as follows: La(): atmospheric absorption (dB): a function of the ship-to-satellite elevation angle () Le(): excess attenuation (dB), exceeded for not more than 20% or
41、 1% of the time; this is considered to be negligible at 1.5/1.6 GHz Lr: radome (if used) loss (dB); the losses due to a dry radome (0.2 dB) may already be accounted for in the definition of ship earth station antenna gain and receive sensitivity (G/T), as is assumed here Lp: polarization coupling lo
42、ss (dB): a function of ship earth station antenna and satellite antenna axial ratio (2 dB and 3 dB respectively); the worst case is misaligned polarization ellipses Lb: ship structure blockage loss (dB): a function of antenna placement, size and shape of the obstruction, direction and ship earth sta
43、tion antenna beamwidth. Additional degradations due to an increase in system noise temperature caused by various losses are as follows: DTr: ratio (dB) of the system noise temperature, when a radome is fitted to the system noise temperature without the radome; this may already be accounted for in th
44、e G/T value, as has been assumed here, but an allowance is required for the degradation in noise temperature due to a wet radome Ta: temperature increase due to atmospheric absorption; this is considered negligible for a system noise temperature of 500 K. 7.1.1 Fixed long-term losses The composite m
45、argin for fixed long-term losses is given by: )dry()()(rafLLL += (9) 6 Rep. ITU-R M.760-3 7.1.2 Random long-term losses Cumulative statistics for random long-term losses would include, for example, Lpand Lr (wet); typical means and standard deviations for these loss factors might be as follows: Loss
46、 factor Mean loss (dB) Standard deviation (dB) Lp0.2 0.09 Lr(wet) 0.2 0.13 DTr(wet) 0.1 0.09 (reference noise temperature = 500 K) The inclusion of blockage loss (Lb) in the margin calculations may not be consistent with ship earth station installation practice and operational objectives. Normal pra
47、ctice may be to install the ship earth station antenna with a completely unobstructed view for elevation angles above the system design point and not to provide a system-wide margin for blockage, as has been assumed here. 7.2 Short-term losses Short-term losses for which margins may be needed are as
48、 follows: Lt:ship earth station transmitter power level fluctuations: are assumed here to be 0.5 dB for about 95% of the time, corresponding to a mean loss of 0 dB and a standard deviation of 0.25 dB G: ship earth station antenna pointing loss (dB): a function of the ship antenna pointing system per
49、formance which tends to be worse in high sea states; some designs may exhibit high tracking errors at certain orientations such as in the zenith direction Li: ionospheric fading loss (dB) due to scintillation; a function of ship location, season and time of day Lm(): multipath fading loss (dB) due to reflections from the sea and ship structure: the magnitude of the loss for a given percentage of time, and the rate at which the loss varies, will depend upon the sea state, elevation angle (), link polarization, ship antenna pattern
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