1、 Rec. ITU-R S.1590 1 RECOMMENDATION ITU-R S.1590 Technical and operational characteristics of satellites operating in the range 20-375 THz (Question ITU-R 264/4) (2002) The ITU Radiocommunication Assembly, considering a) that telecommunication links are being used and planned for use on some satelli
2、te systems for inter-satellite communications at frequencies within the range of 20-375 THz; b) that the viability of fixed-satellite service telecommunication systems operating in the 20-375 THz range of the spectrum using Earth-to-space and space-to-Earth links is currently being investigated; c)
3、that astronomers are making observations in this part of the spectrum; d) that this part of the spectrum is also being used for other science services; e) that this part of the spectrum is also being used for scientific and industrial purposes other than communication; f) that mechanisms of interfer
4、ence between satellites and passive systems such as astronomy operating above 20 THz may differ from those in the radio frequency part of the spectrum, recognizing a) that No. 78 of Article 12 of the ITU Constitution states that the functions of the Radiocommunication Sector include, “ carrying out
5、studies without limit of frequency range and adopting recommendations ”; b) that, under Note 2 of 1005 in the Annex to the ITU Convention, Study Groups may consider “radiocommunication” to include electromagnetic spectrum above 3 000 GHz propagated through space without artificial guide in the cours
6、e of their studies and in the creation of draft new Recommendations; c) that as of the end of 2001, use and sharing of this part of the spectrum has not been thoroughly studied within ITU-R, recommends 1 that sharing studies of satellites operating in the frequency range 20-375 THz take into account
7、 the technical and operational parameters presented in Annex 1. 2 Rec. ITU-R S.1590 ANNEX 1 1 Introduction The term “radio waves” is defined in the Radio Regulations as “Electromagnetic waves of frequencies arbitrarily lower than 3 000 GHz propagated in space without artificial guide”. With the incr
8、eased pressure for use of the radio spectrum and the advancement of technology, more attention is being given to the use of frequencies above 3 000 GHz for free space telecommunications. Telecommunication links have become a reality in the frequency bands above 3 000 GHz as a result of many recent t
9、echnological developments in optical communication devices such as optical fibre, solid state lasers (GaAs, InP)1, modulators (electro-optic modulators) and detectors (photodiodes). Free space telecommunication at frequencies above 3 000 GHz has the ability to support data rates in the tens of Gbit/
10、s as well as meet gain and directivity requirements of beams used for deep space applications. For telecommunications, attention is being focused on frequencies in the band 20-375 THz (15-0.8 m). Though some links have been demonstrated, much of the technology for these links is still in development
11、 but evolving rapidly. Such links can provide telecommunication signals on Earth-to-space, space-to-Earth, and space-to-space paths as well as on terrestrial links. They are being considered for satellites in geostationary orbit (GSO) as well as those in non-geostationary orbit (non-GSO) such as low
12、 Earth orbits. Technical and operational characteristics are required for the free space telecommunication links operating in these spectral regions for use in future sharing studies. The specifications and operations of some systems in this region are described in the following sections. The purpos
13、e of this Recommendation is to identify system parameters required for conducting interference analyses for space applications. 2 Frequency considerations Not all frequencies are equally suitable for free space telecommunications since the transparency of the atmosphere varies strongly with frequenc
14、y. Earth-to-space and space-to-Earth links should operate in a region of low absorption while space-to-space links may want to choose a region of high absorption to achieve isolation from interference on the Earth and reduce the probability of causing interference to astronomical observations. The u
15、se of near infrared lasers (0.850 m) are more efficient in terms of signal-to-noise ratio. Medium infrared detectors near 15 m need strong cooling in order to reduce thermal noise. _ 1Lasers used in telecommunication are manufactured from III-V semiconductor compounds. The materials used are alloys
16、of Gallium Arsenide (GaAs) and Indium Phosphide (InP). The wavelengths of such lasers are respectively 0.85 m and 1.5 m. Rec. ITU-R S.1590 3 Figure 1 shows the frequency dependence of atmospheric absorption of an optical signal along a vertical path. This Figure assumes the path begins at sea level,
17、 and the line of sight continues into space through a standard atmosphere. Below about 15 THz (20 m), the sky is effectively opaque. With increases in frequency toward the visible band, 400-750 THz (0.75-0.40 m), there are numerous bands of various width and transparency that occur due to the presen
18、ce of gaseous components in the atmosphere. These gases are not necessarily uniformly mixed and the proportions can vary as a function of altitude. The strength of the absorption lines is also generally dependent on pressure and temperature. At about 300 THz (1 m), the envelope of the absorption sta
19、rts to increase, due primarily to O2and O3. The absorption increases up through the visible band, to about 1 000 THz (0.3 m) where the atmosphere again becomes effectively opaque due to molecular absorption. Currently, most of the interest in telecommunications links is focused around the frequencie
20、s 200, 283, 311 and 353 THz, whose corresponding wavelengths are approximately 1.5, 1.06, 0.965 and 0.850 m. These frequencies are the same as those that are most widely used for communications in optical fibres. 1590-013025201510504035101102103FIGURE 1Absorption (shaded area) above 10 THz of a stan
21、dard atmosphere along a vertical pathFrequency (THz)Absorption(dB)Table 1 shows some characteristics for typical lasers used in terrestrial optical communications. 4 Rec. ITU-R S.1590 TABLE 1 Typical characteristics of solid-state lasers in the range of 0.75 m to 1.6 m 3 Generic block diagram A gene
22、ric block diagram of a typical optical telecommunications system is provided in Fig. 2. 1590-02FIGURE 2Block diagram of a typical optical telecommunications systemPoint aheadmirrorOptical datatransmitterOptical beacontransmitterOpticalconditionerFinetrackingdetectorAcquisitionand trackmicroprocessor
23、Optical terminalmicroprocessorGimbal driveelectronicsGimbal andencodersCommunicationdetectorpreamp /AGCSignalelectronicsRF/opticalcommunicationssystemsinterfaceTransmitReceiveDataFine steeringmirrorAcquisitiondetectorOpticalantenna- Telemetry- Host data handlingAGC: automatic gain controlLaser mediu
24、m Transition wavelength Typical maximum output power(W) Continuous-wave (CW) or pulsed Single mode (S) Multimode (M) Transition linewidth In1-xGa1-yPy(1) Semiconductors 1.1 m to 1.6 m 1 CW S/M A few GHz up to 10 THz Nd3+:YAG(2) Glass 1.064 m 10 CW S/M 120 GHz InxGaxAs Semiconductors 0.965 m 1 CW S/M
25、 A few GHz up to 10 THz ALxGaxAs Semiconductors 0.75 to 0.87 m 10 CW S/M A few GHz up to 10 THz (1)0 = (7)where: : detector quantum efficiency (0.7 to 1 typical) e : electron charge (1.602 1019C) h : Plancks constant (6.63 1034J/s) f : optical frequency (Hz) PS: received signal power (W) B : filter
26、bandwidth (Hz). Note that the noise power varies directly with the signal power, PS. Then, in the absence of atmospheric effects, the output SNR is: BfhPiSNRSNS2= (8a) or in a power form: BfhPSNRS2= (8b) Rec. ITU-R S.1590 13 6 Antenna considerations Free space laser telecommunication systems use con
27、ventional telescopes as transmitting and receiving antennas. Normally these telescopes are of the classic Cassegrain design, but with the advent of modern telescope manufacturing technology and the dramatic increase in optical power available, the current designs utilize primarily refractive element
28、s or off-axis telescope designs to eliminate obscurations and the resultant reduction in on-axis gain. The transmitter and receiver antenna patterns are different since the transmitter optics are usually fed by a laser with a Gaussian beam while the receiver optics have a planar detector. 6.1 Transm
29、itter antennas The transmitter utilizes a telescope that is fed by a laser. Such lasers normally operate only in the lowest cavity mode, TEM00, which results in a beam that has a Gaussian distribution of energy with a maximum intensity along its axis of transmission. The beam pattern is tailored suc
30、h that as the intensity of the beam falls off in amplitude with angular separation from the axis of transmission, no more than a few percent of the beam power is wasted. Two points of reference are the points at which the beam amplitude falls off to either 37% or 13% of the amplitude on axis. These
31、points are called the 1/e and 1/e2points respectively and are referred to frequently in the characterization of emitted laser energy patterns. An example of this beam pattern is provided in Fig. 3. 1590-031/e = 2 1/e2FIGURE 3Gaussian amplitude curve1/e (4.3 dB downfrom on-axis)1/e2(8.7 dB downfrom o
32、n-axis)Uniform far-field intensityGaussian far-field intensityOff-axis beam angle, 14 Rec. ITU-R S.1590 The beamwidth at the 1/e2point is given by: De=44.2/12(9) where: 2/1 e : beamwidth (rad). Typical beamwidths are on the order of 1 105rad or 5.7 104degrees. For the transmitting terminal, the foll
33、owing equations can be used to calculate the far field radiation pattern of a laser with a Gaussian amplitude plane wave feeding a telescope under the following basic assumptions: the laser source is characterized as single mode Gaussian emission; and the antenna gain patterns are measured in the fa
34、r field. The gain of a telescope of radius “a” fed with a Gaussian amplitude plane wave having a waist radius of “”, where “” is the distance from the central axis of the optical system to the 1/e2intensity point, and having a central obscuration of radius “b” is given by equation (10). The term, G0
35、, is the upper limit on antenna gain which is obtained for a uniformly illuminated unobscured circular aperture. The second term, gt(, , X), is a gain efficiency term which accounts for obscuration, truncation, off-axis intensity, and defocusing effects. ),(),(0XgGXgtt= (10) where: 22024=aAG (11) ()
36、210222deJ2),(= uXguuXt(12)A : area of the telescope aperture (m2) a : radius of the telescope aperture (m) : wavelength (m) J0: Bessel function of the first kind of order zero : the ratio, a/, of the radius of the transmitter aperture, a, to the radius of the Gaussian feed beam waist, , at the 1/e2p
37、oint : the ratio, b/a, of the radius of the central obscuration, b, to the radius of the transmitter aperture, a u : the variable of integration )(sin2= aX : angle off the optical axis (rad). Rec. ITU-R S.1590 15 For the on-axis, X = 0 and the gain efficiency term in equation (12) becomes: =22222ee2
38、)0,(tg (13) Then the on-axis maximum main beam gain in equation (10) becomes: =222222ee24)0,(AGt(14) Any obscurations will reduce the main beam gain, fill in the nulls, broaden the beamwidth and increase the side lobes. 6.1.1 Example For example, performance of an optical antenna with an aperture di
39、ameter of 15 cm, operating at a frequency of 282.8 THz (a wavelength of 1.06 m) and no central obscuration is as follows: = 1.06 m a = 0.075 m b = 0 then: 0=abThe gain efficiency term for the on-axis case then becomes: =221e2)0,0,(2tg (15) It has been shown that for optimum performance when there is
40、 an obscuration, the relationship between feed beam, , and should follow the relationship in equation (16), which is accurate to within 1% for 0.4: 4212.230.112.1 += (16) For the present case with no obscuration, = 0, the equation reduces to = 1.12. The gain efficiency term then becomes: 8145.0)0,0,
41、12.1( =tg (17) The upper limit of the on-axis gain for this uniformly illuminated unobscured circular aperture of radius 15 cm is then: 1122010976.124=aAG (18) The maximum on-axis gain is, using equation (10): 111061.1)0,0,12.1( =tG (19) 16 Rec. ITU-R S.1590 The off-axis radiation pattern (dB) for t
42、his case becomes, using equation (10): ),0,12.1(log10),0,12.1(0XgGXGtt= (20) where: ()21002deJ2),0,12.1(2= uuXXgut(21) with: )(sin2= aX (22) 1590-04120100806020 10 0 10 20XGain(dB)Gt(1.12,0,X)FIGURE 4Gain (dB) for a 15 cm diameter antennaFor the above case, the first null occurs at about X = 4.7 whe
43、re = 10.6 rad or 0.0006. The first side lobe occurs at about X = 5.6 where = 12.6 rad or 0.0007 and is down about 27 dB below the main beam gain. Rec. ITU-R S.1590 17 6.2 Receiver antenna Assuming the receiving aperture is in the far-field of the transmitting antenna, the received energy is normally
44、 treated as a plane wave. The receiving system may use a common or separate aperture from the transmitting system. The beamwidth of the receiving aperture is also typically measured in terms of its 1/e2point. The maximum, on-axis, gain of a receiving antenna, GR, is given by: += )1(log104log1022AGR(
45、24) where: A : area of the primary mirror (m2) : wavelength of the incoming signal (m) and ab= (25) where: a : radius of the primary mirror (m) b : radius of the secondary mirror (m). The gain calculated in equation (24) represents the quantity of energy incident on the detector. The term GRassumes
46、that the receiving antenna is located in the far-field of the transmitter, and the aperture and the detector are round. The first term of equation (24) is the classic antenna gain realized by an ideal unobscured antenna of area A. The second term accounts for losses due to the obscuration introduced
47、 by the secondary mirror of a Cassegrain system. In the case of systems without secondary mirrors, the value of b in equation (25) becomes zero and the second term of equation (24) may be neglected. The third term, , of equation (24) accounts for losses (dB) due to spillover of the signal energy bey
48、ond the edge of the detector. For direct detection systems, reduces as the ratio of the detector size to focal length of the telescope increases. For most practical values, will be no more than 0.5 dB. For heterodyne and homodyne systems, becomes a function of the distribution of the local oscillato
49、r energy across the detector. The detector may receive Gaussian or uniform illumination. For Gaussian and uniform illumination, may be closely approximated by equations (26a) and (26b) respectively. 7621.0452.09114.82= (26a) 4937.11113.05836.92+= (26b) Gaussian illumination results in slightly smaller
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