1、 STD=ITU-R RECMN P-bBO-3-ENGL 3999 m 4855232 0537395 958 Rec. ITU-R P.680-3 209 RECOMMENDATION ITU-R P.680-3 PROPAGATION DATA REQUIRED FOR THE DESIGN TELECOMMUNICATION SYSTEMS OF EARTH-SPACE MARITIME MOBILE (Question ITU-R 207/3) (1990-1992-1997-1999) The ITU Radiocommunication Assembly, considering
2、 a) that for the proper planning of Earth-space maritime mobile systems it is necessary to have appropriate propagation data and prediction methods; b) that the methods of Recommendation ITU-R P.618 are recommended for the planning of Earth-space telecom- munication systems; c) that further developm
3、ent of prediction methods for specific application to maritime mobile-satellite systems is required to give adequate accuracy for all operational conditions; d) that, however, methods are available which yield sufficient accuracy for many applications, recommends 1 that the current methods set out i
4、n Annex 1 be adopted for use in the planning of Earth-space maritime mobile telecommunication systems, in addition to the methods recommended in Recommendation ITU-R P.618. ANNEX 1 1 Introduction Telecommunications over Earth-space links for maritime mobile-satellite systems lead to propagation prob
5、lems that are substantially different from those arising in the fixed-satellite service. For instance, the effects of reflections and scattering by the sea surface can be quite severe, in particular where antennas with wide beamwidths are used. Furthermore, maritime mobile-satellite systems may oper
6、ate on a world-wide basis, including paths with low elevation angles. This Annex deals with data and models specifically needed to characterize the sea-space path impairments which include: tropospheric effects, including rain attenuation, gaseous absorption, refraction, scintillation and anomalous
7、propagation occumng at low elevation angles; ionospheric effects such as scintillation and Faraday rotation; surface reflection effects (multipath due to secondary paths arising from the reflection of radio waves from the sea surface); local environment effects (ship motion and sea conditions); inte
8、rference effects due to differential fading between a desired signal and an interference signal, both affected by multipath fading. STD-ITU-R RECMN P.bBO-3-ENGL 1999 m 4855212 0537L9b 894 m 210 Rec. ITU-R P.680-3 2 Tropospheric effects 2.1 Attenuation Signal losses in the troposphere are caused by a
9、tmospheric gases, rain, fog and clouds. Except at low elevation angles, tropospheric attenuation is negligible at frequencies below about 1 GHz, and is generally small at frequencies up to about 10 GHz. Above 10 GHz, the attenuation can be large for significant percentages of the time on many paths.
10、 Prediction methods are available for estimating gaseous absorption (see Recommendation ITU-R P.676) and rain attenuation (see Recommendation ITU-R P.618). Fog and cloud attenuation is usually negligible for frequencies up to 10 GHz. 2.2 Scintillation Irregular variations in received signal level an
11、d in angle of arrival are caused by both tropospheric turbulence and atmospheric multipath. The magnitudes of these effects increase with increasing frequency and decreasing path elevation angle, except that angle of arrival fluctuations caused by turbulence are independent of frequency. Antenna bea
12、mwidth also affects the magnitude of these scintillations. These effects are observed to be at a maximum in the summer season. A prediction method is given in Recommendation ITU-R P.618. 3 Ionospheric effects Ionospheric effects (see Recommendation ITU-R PS3 1) may be important, particularly at freq
13、uencies below 1 GHz. For convenience these have been quantified for frequencies of 0.1, 0.25, 0.5, 1, 3 and 10 GHz in Table 1 for a high value of total electron content (TEC). 3.1 Ionospheric scintillation Inhomogeneities of electron density in the ionosphere cause refractive focusing or defocusing
14、of radio waves and lead to amplitude fluctuations termed scintillations. Ionospheric scintillation is maximum near the geomagnetic equator and smallest in the mid-latitude regions. The auroral zones are also regions of large scintillation. Strong scintillation is Rayleigh distributed in amplitude; w
15、eaker scintillation is nearly log-normal. These fluctuations decrease with increasing frequency and depend upon path geometry, location, season, solar activity and local time. Table 2 tabulates fade depth data for VHF and UHF in mid-latitudes, based on data in Recommendation ITU-R P.531. Accompanyin
16、g the amplitude fluctuation is also a phase fluctuation. The spectral density of the phase fluctuation is proportional to l/f3, where f is the Fourier frequency of the fluctuation. This spectral characteristic is similar to that arising from flicker of frequency in oscillators and can cause signific
17、ant degradation to the performance of receiver hardware. 3.2 Faraday rotation A linearly polarized wave propagating through the ionosphere undergoes a progressive rotation of the plane of polarization. Effects are summarized in Table 1. The axial ratio of an incident elliptically polarized wave may
18、be increased or decreased upon reflection (particularly at small angles) since Faraday rotation varies the orientation of the principal polarization axis of the incident wave. This results from the difference in reflection coefficient to be expected between vertical and horizontal components in most
19、 multipath situations. The effects of Faraday rotation on wideband signals can be of significance to system performance. The differential rotation effects cannot be fully corrected at VHF by reorientation of the antenna axis of a linearly polarized antenna. On circularly polarized antennas, the effe
20、ct is to introduce differential phase shifts of signal components across the band. Thus, signal components separated in frequency may be expected to be subject to frequency and phase selective distortion. i 3 “ 0000 “c? 211 4 Fading due to sea reflection 4.1 Fading depth The following simple method
21、provides approximate estimates of multipath power or fading depth suitable for many engineering applications. Applicable conditions: Frequency range: 0.8-8 GHz Elevation angle: 5“ I 8i I 20“ where G(8) is the antenna radiation pattern of the main lobe given by: where: G, : value of the maximum anten
22、na gain (Bi) 8 : angle measured from boresight (degrees) Polarization: circular Sea condition: wave height of 1-3 m (incoherent component fully developed). Step I: Find the relative antenna gain G in the direction of the point of specular reflection. The relative antenna gain is approximated by equa
23、tion (1) where 8 = 2 8i (degrees). Step 2: Calculate the Fresnel reflection coefficient of the sea for circular polarization, Rc: RH -F RV Rc = 2 where: (circular polarization) (horizontal polarization) (vertical polarization) and q = &,.(f) - j 60 ho(f) where: E,. (f) : relative permittivity of the
24、 surface at frequencyf (from Recommendation ITU-R P.527) o(f) : conductivity /m) of the surface at frequencyf (from Recommendation ITU-R P.527) h : free space wavelength (m). A set of curves is given in Fig. 1 for the magnitude of the Fresnel reflection coefficient of sea for circular polarization f
25、or five frequencies between 0.8 GHz and 8 GHz. The curves are obtained from equation (2) with the electrical parameters corresponding to average salinity sea water. Step 3: Find the normalized diffuse coefficient (ratio of diffuse component of reflection-to-reflection coefficient for calm sea condit
26、ion), (dB), from Fig. 2. FIGURE 1 Magnitude of Fresnel reflection coefficient, R, of sea of average salinity for circular polarization lene: ! Hz) = ! 1 - 0.8 - 1.5 - 3- 5- 8- c T 10 90 Elevation angle (degrees) 214 FIGURE 2 Average normalized diffuse coefficients in the range 0.8 to 8 GHz G,= 18 17
27、 16 15 6.0 5 .O 4.0 3.0 S 3 2.0 F 1 .o 0.0 - 1.0 P 14 / 13 12 11 j9 O I 7 7 /+ I l I I l l ! t- -2.0 L 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Elevation angle (degrees) WO step 4: The mean incoherent power of sea reflected waves relative to the direct wave, P, is given by: Pr=G+R+q dB where: R =
28、20log lRcl dB with Rc from equation (2). Step 5: Assuming the Nakagami-Rice distribution, fading depth is estimated from: Pr/ 10 where A is the amplitude (dB) read from the ordinate of Fig. 3. STDiITU-R RECMN P.bBO-3-ENGL 3999 m 4855232 0537203 T81 Rec. ITU-R P.680-3 10 O - 10 - 30 - 40 - 50 O FIGUR
29、E 3 Nakagami-Rice distribution for a constant total power with the parameter a 0.0 1 o1 50 r t I 1 80 90 95 98 99 99.9 99.99 99.999 58 58 58 Percentage probability that ordinate will be exceeded multipath power total power a= 10 Pr110 For this case, a = 1 + 10 4.2 Frequency spectrum and fade duratio
30、n statistics 215 In general, spectral bandwidth increases with increasing wave height, elevation angle, ship velocity, and the relative motion of the shipborne antenna (rolling/pitching). The dependence of the spectral shape on antenna polarization is small, and the dependence on antenna gain is wea
31、k for gains less than about 10 dB. The -10 dB spectral bandwidth,f-lo, is defined as the bandwidth over which the power density decays to - 10 dB relative to the peak power density. Figure 4 shows the probable range of the -10 dB spectral bandwidth of 1.5 GHz multipath fading obtained by a theoretic
32、al fading model as a function of the elevation angle under conditions typical of maritime satellite communications (significant wave height of 1-5 m, ship speed of 0-20 knots and rolling of 0-30). FIGURE 4 -10 dB spectral bandwidth of 15 GHz multipath fading due to sea reflection as a function of th
33、e elevation angle h N 3 10 1 o. 1 I I I I I I 111 I 1 i I i i l Wave height: 5 m Ship velocity: 20 knots Rolling: 30“ Wave height: 1 m Ship velocity: O knots Rolling: O“ 5 10 15 Elevation angle (degrees) Average values of fade duration, defined in Fig. 5 can be obtained by the following procedure by
34、 using the -10 dB spectral bandwidth, f-10: TI (P) = = 4 / f-10 m = 2.33 - 0.847 a - 0.144 a2 - 0.0657 a3 a = log (100 - p) for 70% and for 99% of the time at elevation angles from 5“ to 10 are 0.05 to 0.4 S for and 5 to 40 S for . The probability density function of TD and TI at any time percentage
35、 ranging from 50% to 99% is approximately an exponential distribution. STD-ITU-R RECMN P-bBO-3-ENGL 3999 4855232 0537203 854 m Rec. ITU-R P.680-3 217 5 Interference from adjacent satellite systems 5.1 General In mobile-satellite communication systems, amplitudes of the desired signal from the satell
36、ite and an interfering signal from an adjacent satellite experience independent level fluctuations due to multipath fading, requiring a different treatment from that of fixed-satellite systems. A main point to be considered is the statistics of differential fading, which is the difference between am
37、plitudes of the direct wave and interference wave, both affected by multipath fading. FIGURE 5 Fade duration and fade Occurrence interval Signal level Rp: signal level for a given percentage of the time A practical prediction method for the statistics of signal-to-interference ratio where the effect
38、 of thermal noise and time- variant interference is taken into account is provided below. 5.2 Prediction method In general there are two kinds of interference between adjacent satellite systems. One is “down-link interference” on the mobile earth station side, and the other is “up-link interference”
39、 on the satellite side. Another situation is interference between beams in multi-spot-beam operation, where the same frequency is allocated repeatedly. The method is applicable to such cases. Input parameters (in units of power, not dB) are: D : power of the direct wave component of desired signal M
40、 : average power of the reflected component (i.e. incoherent component) of desired signal N: average power of system noise ZD : power of the direct wave component of interference signal ZM : average power of reflected component of interference signal (I: average power of interference: Z = ID + ZM) O
41、utput parameters (in units of power, not dB) are: c/nI(p) : ratio of desired signal power to system noise power as a function of time percentage p c/i(p) : ratio of desired signal power to interfering signal power c/ (i + n) (P) : ratio of desired signal power to system noise plus interfering signal
42、 power. Carrier-to-noise ratio as a function of p is given by: where qc is the normalized time-percentage-dependent factor of desired signal power having a probability density function of a Nakagami-Rice distribution with constant direct power given in Fig. 3, in which: 20 log Tc = A + 1Olog (D + M)
43、/D) (6) where A is amplitude (dB) read from the ordinate of Fig. 3. The parameter in the figure for this application is M/(D -+M). The signal-to-interference ratio as a function of p is given by: where 150 is the median value (i.e. value for 50% of the time) of power variations of the interference s
44、ignal: and qc-i is the normalized time-percentage-dependent factor of cli variations approximately given by: where qi is the normalized time-percentage-dependent factor of interference signal power. A solution where qcli 1. By setting 1lZ = b, q1,50 and qi (both in dB) as a function of b are given i
45、n Table 3. Finally, Prediction accuracy of the method for cli and cl(n + i) is within 1 dB for all cases within the following parameter range: where all quantities are relative to D. TABLE 3 Values of qi and q130 as functions of time percentage ) and b = ZD/ (ZD + ZM) I I I I I I I 1 .o 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.00 -OD
