1、 Rec. ITU-R P.1814 1 RECOMMENDATION ITU-R P.1814*Prediction methods required for the design of terrestrial free-space optical links (Question ITU-R 228/3) (2007) Scope This Recommendation provides propagation prediction methods for planning terrestrial free-space optical systems. It includes methods
2、 to estimate attenuation in clear air, fog, and rain and snow precipitation. It also covers scintillation and impairments by sunlight. The ITU Radiocommunication Assembly, considering a) that the visible optical and infrared spectrum is available for radiocommunications in the Earths environments; b
3、) that for the proper planning of free-space optical (FSO) radiocommunication systems operating in visible optical and infrared spectrum, it is necessary to have appropriate propagation data; c) that methods have been developed that allow the calculation of the most important propagation parameters
4、needed in planning free-space optical systems operating in the visible optical and infrared spectrum; d) that these methods have been tested against available data and have been shown to yield an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for
5、most present applications in the planning of systems operating in visible optical and infrared spectrum, recognizing a) that No. 78 of article 12 of the ITU Constitution states that a function of the Radiocommunication Sector includes, “. carrying out studies without limit of frequency range and ado
6、pting recommendations .”, recommends 1 that the methods for predicting the propagation parameters given in Annex 1 should be adopted for planning free-space optical systems, in the respective ranges of validity indicated in the Annex. NOTE 1 Supplementary information related to propagation predictio
7、n methods in visible optical and infrared spectrum may be found in Recommendation ITU-R P.1817 Propagation data required for the design of terrestrial free-space optical links. *This Recommendation should be brought to the attention of Radiocommunication Study Groups 1 and 9. 2 Rec. ITU-R P.1814 Ann
8、ex 1 1 Introduction In the design of FSO links several effects must be considered including the losses due to atmospheric absorption, scattering and turbulence, microclimate environment and localized effects, link distance and link misalignment. The selection of wave length, data rate, eye-safety is
9、sues, and ambient solar radiation must also be considered. FSO system operation requires line-of-sight (LOS). When testing for LOS, as FSO systems use beam expansion and a collimated beam, the clearance needed between the centre of the beam and any obstructions is essentially equal to the beam radiu
10、s. This is in contrast to RF systems where Fresnel zone clearance is needed. The primary disadvantage of FSO systems are their vulnerability to atmospheric effects such as attenuation and scintillation, which can reduce link availability. The narrow beam also makes alignment of the laser communicati
11、ons terminal more critical than is usual for RF systems. A key parameter in the design of FSO links is the consideration of the power budget. The link margin, linkM (dB), which is the power available above the sensitivity of the receiver, can be found from equation (1): systemionscintillatatmogeorel
12、inkAAAASPM = (1) where: Pe(dBm): total power of the emitter Sr(dBm): sensitivity of the receiver which also depends on the bandwidth (Data rate) Ageo(dB): link geometrical attenuation due to transmit beam spreading with increasing range Aatmo(dB): atmospheric attenuation due to absorption and scatte
13、ring Ascintillation(dB): attenuation due to atmospheric turbulence Asystem(dB): represents all other system dependent losses including misalignment of the beam direction, receiver optical losses, loss due to beam wander, reduction in sensitivity due to ambient light (solar radiation), etc. The defin
14、ition and computation of these terms and the initial consideration for planning an FSO link are given in the following sections. 2 Initial considerations in designing an FSO link The choice of a suitable link location is an important issue for a successful operation of an FSO system. The installatio
15、n of FSO links has to account for prevailing weather conditions, physical obstructions and surface types along the path, and transceiver mounting arrangements to ensure optimum link performance. 2.1 Weather Weather conditions, and in particular the local climate, in the vicinity of the chosen link p
16、ath will influence the occurrence of snow, rain, drizzle fog, haze, aerosol and dust/sand that will lead to absorption and scattering of the transmitted signal. Rec. ITU-R P.1814 3 2.2 Path characteristics Physical obstructions to the path between emitter and receiver are clearly to be avoided. It i
17、s well worth noting that mature trees can increase in height by between 0.5 and 1 m in one year and vary in foliage density over the year. Links between buildings should account for thermal vents that could result in hot air rising through the link path, and the resulting turbulence could lead to si
18、gnificant scintillation at the receiver. The topography and the type of surface beneath an FSO line-of-sight path can significantly impact the performance of the link. FSO links across river valleys, or across areas of open sea, will often experience increased incidents of fog. Building structures b
19、eneath the link may cause additional thermal activity in the air above them that may then lead to increased scintillation on the received signal. 2.3 Transceiver mounting Most FSO systems have very narrow beam widths, and, as a result, the accurate alignment of the emitter and receiver is critical;
20、any misalignment causes significant signal loss. The telescope mounts must be stable and direct mounting to substantial walls, or to the top of a single column, is considered essential for reliable performance over a period of time. Movement as the result of differential thermal expansion, or buffet
21、ing by wind, should be minimized. 3 Geometrical attenuation Even in clear weather conditions, the beam diverges and, as a result, the detector receives less signal power. The attenuation due to transmit beam spreading with increasing range is called geometrical attenuation and is given by the formul
22、a (2): =capturedgeoSSA10log10)dB( (2) where: Scapture: receiver capture surface (m2) Sd: surface area of transmit beam at range d, which is approximated by: 2)(4= dSdwhere: : beam divergence (mrad) d: emitter-receiver distance (km). It is possible on short links for the capture area to be greater th
23、an the beam area. In these cases the value of Ageoshould be set to zero as all of the beam energy is collected. 4 Rec. ITU-R P.1814 4 Specific atmospheric attenuation due to absorption and scatteringatmo The specific atmospheric attenuation atmo(dB/km) can be written as the sum of two terms: atmo= c
24、lear_air+ excess(3) where: clear_air: specific attenuation under clear air (due to the presence of gaseous molecules) excess: specific attenuation due to the occasional presence of fog, mist, haze, drizzle, rain, snow, hail, etc. The atmosphere is a time-varying transmission medium and as a result a
25、tmo is a stochastic process. However, as shown in equation (1), imposing limits on system availability and its effects are generally treated statistically. Link margin, Mlink, represents the amount of attenuation which can be tolerated by a given system at a given range. 4.1 Specific clear-air atten
26、uation clear_airAttenuation under clear-air conditions is mainly the attenuation due to the absorption by gaseous molecules. Atmospheric absorption at specific optical wavelengths results from the interaction between photons and atoms or molecules (N2, O2, H2, H2O, CO2, O2, etc,) which leads to the
27、absorption of the incident photon and an elevation of the temperature. The absorption coefficient depends on: the type of gas molecules; and their concentration. Molecular absorption is a wavelength-selective phenomenon which results in atmospheric transmission windows, and atmospheric absorbing reg
28、ions. The important atmospheric molecules that have high absorption in the IR band include water, CO2, O3and O2. Because the size of the gaseous molecules is much smaller than the wavelength, scattering attenuation from the gaseous molecules is negligible. Usually the laser wavelengths are selected
29、to fall inside atmospheric transmission windows, so clear_airis negligible. The wavelengths generally used in FSO systems are near 690, 780, 850, and 1 550 nm. However, in comparison to relatively unpolluted suburban locations, applications in dense urban areas with high aerosol contents might benef
30、it from a different wavelength. 4.2 Specific excess attenuation Excess attenuation is the attenuation caused by the occasional presence of fog, mist, haze, drizzle, rain and snow particles. The presence of these particles causes an angular redistribution of the incident flux, known as scattering, an
31、d reduces the flux propagation in the original direction. However, there is no loss of energy similar to absorption. The physical size of the scatterers with respect to the transmission laser wavelength determines the type of scattering. Table 1 shows the three different scattering regimes depending
32、 on the scatters size and the approximate relationship between wavelength and scatters attenuation coefficient (effective-cross section). Also shown in Table 1 are the type of scatters in each regime for the visible and IR wavelengths. Rec. ITU-R P.1814 5 TABLE 1 Scattering regimes depending on the
33、scatters size r with respect to the transmission laser wavelength . Also shown is the approximate relationship between wavelength and scatters attenuation coefficient )(Q Rayleigh scattering Mie scattering Non-selective or geometrical scattering r 0)(Q Type of scatter Air molecules Haze Haze Fog Aer
34、osol Fog Rain Snow Hail Because of the 4)(Q relationship of the Rayleigh regime the air molecular scattering contribution to the total attenuation coefficient is negligible. For particles that are much larger than the wavelength, scattering can be described by geometric optics which is independent o
35、f laser wavelength. Rain drops, snow, hail, cloud droplets and heavy fog will geometrically scatter laser light. For particles whose size is comparable to the laser wavelength, Mie scattering theory can be applied. Fog and aerosol particles are the major contributors to the Mie scattering process. A
36、n analytical approach can be used in which computation predictions of the specific attenuation are made based on the theoretically derived effective cross sections of atmospheric particles with assumed particle size distributions. However, the particle size distributions of either aerosol or fog, wh
37、ich is a key parameter to determine their physical and optical properties, are difficult to model and measure. 4.2.1 Estimation of specific attenuation due to fog fog (Mie scattering) Since an analytical approach is often not practical to compute the attenuation due to Mie scattering, empirical meth
38、ods have been adopted by the FSO community. In these methods, the attenuation coefficient due to Mie scattering is related to visibility. The technical definition of visibility or visual range is the distance that light decreases to 2% of the original power or qualitatively visibility is the distanc
39、e at which it is just possible to distinguish a dark object against the horizon. The visibility parameter is easily measured and stored in meteorological stations or airports databases, which allows geo-local performance evaluation of these telecommunication systems using the distribution of this pa
40、rameter. However, the visibility data collected at airports may not necessarily represent conditions found in either urban or rural environments, which can be very different in terms of topography and proximity to water. An empirical simplified formula, which has been used in the FSO community to ca
41、lculate the specific attenuation due to fog, )(fog(dB/km), is: qfogV=nm55091.3)( (4) 6 Rec. ITU-R P.1814 where: V : visibility (km) : wavelength (nm) q : a coefficient dependent on the size distribution of the scattering particles. It has been determined from experimental data and given by: q = 1.6
42、V 50 km 3.1= 6 km 1 m) and away from a side wall if the installation takes place in a desert-like environment. Margins allocated to compensate for fog or rain attenuation can compensate also for scintillation effects. 6 Ambient light effect Solar conjunction occurs when the sun or a reflected image
43、of the sun is in or near the instantaneous field of view (IFOV) of an optical receiver. The receive IFOV is generally at least as large as the transmit divergence. The problem becomes severe when the sun position is parallel to the optical link and the sun power penetrating inside the receiver is gr
44、eater than the power received from the emitter. Solar interference is usually reduced by arranging for the receiver to be positioned so that the sun is always off-axis. Figure 4 represents the geometry of the sun path in the sky with regard to a free-space optical link (A is the receiver, B the emit
45、ter). FIGURE 4 Schematic sun path with regard to a free-space optical link The power radiated by the sun,radiatedP (W/m2) is defined by the following relation: =lsradiatedEP2cos2001 (9) where lsE is the sun height (rad). The received power is given by: 100/receivercaptureradiatedsolarsolarWSPFP = (1
46、0) Rec. ITU-R P.1814 11 where: Fsolar: solar spectral power as a function of wavelength Scapture: receiver capture surface area (m2) Wreceiver: receiver bandwidth (nm) Fsolar: modelled by the following curve fit: 70.505.410067.91037.91065.41097.8233649513+=solarF (11) where: : wavelength (nm). 7 Lin
47、k margin calculation The link fade margin for an FSO system with a receiver at a distance d (km) from the emitter can be estimated using the following steps: Step 1: The geometrical attenuation geoA can be obtained from equation (1). Step 2: Laser wavelengths are usually selected to fall inside atmo
48、spheric transmission windows so airclear _ can be considered negligible. However, estimates of the specific clear-air attenuation can be obtained from Recommendation ITU-R P.1817. Step 3: The specific attenuation due to fog fog can be obtained from equations (4) and (5). In the absence of local data
49、 typical values of visibility can be found in Recommendation ITU-R P.1817. Step 4: The specific attenuation due to rain rain can be obtained from equation (6) and Table 2. Step 5: The specific attenuation due to snow can be obtained from equation (7) and Table 3. Step 6: The fade margin linkM (dB) is given by: ddddAASPMsnowrainfogaircleargeosystemrelink= _where: eP (dBm): total power of the emitter rS (dBm): sensitivity of the receiver systemA (dB): represents all oth
