1、 Rec. ITU-R SA.1805 1 RECOMMENDATION ITU-R SA.1805 Technical and operational characteristics of space-to-space telecommunication systems operating around 354 THz*and 366 THz*(Question ITU-R 235/7) (2007) Scope This Recommendation specifies technical parameters (frequencies, link direction, signal an
2、d data characteristics, antenna parameters, etc.) and operational characteristics of telecommunication systems operating in the space-to-space direction around 354 THz and 366 THz, which could be used in sharing studies. The ITU Radiocommunication Assembly, considering a) that telecommunication link
3、s are planned for use on some satellite systems for inter-orbit telecommunication at frequencies in the region of 354 THz and 366 THz; b) that using recent technological developments, astronomers are making a concerted effort to build telescopes and make observation in this segment of the spectrum;
4、c) that this segment of the spectrum is also being used for other terrestrial and space services; d) that this segment of the spectrum is also being used for scientific and industrial purposes other than telecommunication, recommends 1 that sharing studies considering space research satellites opera
5、ting in the space-to-space direction around 354 THz and 366 THz should take into account the technical and operational parameters presented in Annex 1. Annex 1 1 Introduction Due to increased pressure for use of the electromagnetic spectrum and the advancement of technology, there is more attention
6、being given to the use of frequencies above 3 000 GHz for free-space telecommunications. Free-space telecommunication at frequencies above 3 000 GHz has the ability to support higher data rates with less mass than traditional RF systems as well as meet gain and directivity requirements of beams used
7、 for space-to-space applications. *1 THz = 1 000 GHz. *This Recommendation should be brought to the attention of Radiocommunication Study Group 1. 2 Rec. ITU-R SA.1805 1.1 Frequency considerations Currently, most of the interest in free-space telecommunications links above 3 000 GHz is focused aroun
8、d the frequencies 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 most widely used for telecommunications in optical fibres. For inter-orbit telecommunication, attention is being focused on the use of
9、high power semiconductor lasers operating around 0.850 m or the use of a semiconductor laser beam amplified by an Erbium-doped (Er) fibre-optical amplifier (EDFA) at the wavelength of 1.5 m. The system with semiconductor lasers operating around 0.85 m is superior to that with EDFA in reliability and
10、 power consumption for relatively low data-rate applications which do not require high transmitter power. 1.2 Generic mission parameters Technical parameters suitable for interference analyses should be based on generic inter-orbit telecommunication links near the Earth. Therefore, link distances wi
11、ll be between a few to several hundred thousand km. A summary of the fundamental technical parameters around 354 THz and 366 THz near-Earth inter-orbit telecommunication link is provided in Table 1. TABLE 1 Technical parameters of a reference inter-orbit telecommunication system operating around 354
12、 THz and 366 THz in the space-to-space direction Parameter Forward link Return link Transmitter power (mW) 10 40 Transmitter aperture (cm) 25 26 Transmitter frequency (wavelength) (THz) Comm: 366 (0.819 m) Beacon: 374 (0.801 m) 354 (0.847 m) Modulation 2PPM NRZ Pointing accuracy (rad) 2.6 (3) Range
13、in free space (km) up to 40 000 Data rate (Mbit/s) 2.048 49.3724 Receiver aperture (cm) 26 25 Detector type APD detector APD detector APD: avalanche plid-o-diode NRZ: non-return to zero PPM: parts per million 2 Link considerations Inter-orbit links are established between a goesynchronous Earth orbi
14、t (GEO) satellite and a low-Earth orbit (LEO) satellite in the space-to-space direction, operating around 366 THz for the Rec. ITU-R SA.1805 3 forward link and 354 THz for the return link. A beacon signal at 374 THz is emitted to assist with telescope pointing and tracking. 2.1 Link performance Like
15、 a space-to-space system operating in the traditional RF spectrum, performance of a link operating around 354 THz and 366 THz is measured in terms of data rate andBER. Performance is calculated as a function of power, telescope quality, propagation considerations, noise and receiver sensitivity. Eac
16、h of these parameters is a function of additional variables. 2.1.1 BER Frames of data must have a BER of less than 106after error correction in order to be retained. A link must retain 99% of data frames. 2.1.2 Margin requirement The typical margin requirement of an inter-satellite link operating ar
17、ound 354 THz and 366 THz is on the order of 1 to 3 dB. 2.2 Modulation The return link operating around 354 THz utilizes NRZ. The forward link operating around 366 THz utilizes 2PPM. This modulation technique allows for direct detection by the receiver rather than implementing coherent receivers. 2.3
18、 Received signal The general method for calculating the signal level around 354 THz and 366 THz received by the space-to-space station is the same as that used with traditional RF systems. SprtrttSLLLLGGPP += dBW (1) where: PS: received signal power (dBW) Pt: average laser output power (dBW) Gt: tra
19、nsmitter antenna gain (dBi) Gr: receiving antenna gain (dBi) Lt: transmitter losses (dB) Lr: receiver losses (dB) Lp: pointing losses (dB) Ls: free-space loss (dB). 2.4 Link losses Ltincludes the effects of absorption, scattering and reflection losses in the optical system of the transmitter; Lrincl
20、udes the effects of absorption, scattering, and reflection losses in the optical train of the receiver; Lpincludes the effects of antenna or satellite jitter and mispointing of the transmitting antenna; Lsis due to the physical separation between the transmitter and receiver. 4 Rec. ITU-R SA.1805 Va
21、lues of each source of loss vary with hardware design, hardware age, mission requirements and the phase of the mission. Suggested values of losses to be used in generic interference analyses are provided in Table 2. TABLE 2 Link losses of a reference inter-orbit telecommunication system operating ar
22、ound 354 THz and 366 THz in the space-to-space direction Mechanism of loss Typical value Transmitter losses, Lt0.63 (= 2 dB) Receiver losses, Lr0.5 (= 3 dB) Pointing losses, Lp0.5 (= 3 dB) Free-space loss, Ls, is calculated around 354 THz and 366 THz in the same manner as with traditional radio-freq
23、uency systems. 2244=fRcRLs(2) where: R: distance between the transmitter and receiver (m) : wavelength (m) f: optical frequency (Hz) c: speed of light (m/s). 2.5 Transmit/receive telescope parameters Telecommunication links operating around 354 THz and 366 THz utilize telescopes as transmitting and
24、receiving antennas. The transmitter and receiver antenna patterns are also different since the transmitter optics are usually fed by a Gaussian distributed beam while the receiver optics use a planar detector. For an envelope of the antenna gain patterns of transmitting and receiving antennae operat
25、ing around 354 THz and 366 THz see Annex 2 of Recommendation ITU-R SA.1742. A reference antenna gain pattern for space-to-Earth optical systems operating at 283 THz is provided in Recommendation ITU-R SA.1742. This pattern is also applicable for space-to-space systems operating around 354 THz and 36
26、6 THz. 2.5.1 Diameter For the purposes of interference analyses, the diameter of the optical antenna should be assumed to be 26/25 cm. The aperture will either be unobstructed or have a 5 cm obscuration. 2.5.2 Transmitting gain pattern The transmitter utilizes a telescope that is fed by a laser. Suc
27、h 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 such that as the intensity of the beam falls off in amplitude with angular separat
28、ion from the axis of transmission, no more than a few per cent of the beam power is wasted. Two points of reference are the angles at which the beam amplitude falls off to either 37% or 13% of the amplitude on axis. These points are called the 1/e and 1/e2points respectively and are referred to freq
29、uently in the characterization of emitted laser energy patterns. Rec. ITU-R SA.1805 5 The full-angle beamwidth at the 1/e2point is approximated by: D=42e/1rad (3) where: 2e/1 : angular width of the beam at the 1/e2point (rad) D: diameter of the aperture (m). In the case of a 354 THz Gaussian beam tr
30、ansmitted from a 26 cm aperture, the beamwidth at the 1/e2point is approximately 4.1 106rad. For the transmitting terminal, the following equations can be used to calculate the far field radiation pattern of a laser with a Gaussian amplitude plane wave feeding a telescope. Use of these equations mak
31、es the following basic assumptions: the laser source is characterized as single mode Gaussian emission; the antenna gain patterns are measured in the far field; and the aperture is circular. The gain pattern of a transmitting telescope of radius, a, fed with a Gaussian amplitude plane wave having a
32、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 (4) below. The term, G0, is the upper limit on antenna gain which is obtained for a uniformly illuminated unobscured circ
33、ular aperture. The second term, gt(, , X), is a gain efficiency term which accounts for obscuration, truncation, off-axis intensity, and defocusing effects. () ()XgGXGtt,0= (4) where: 22024=aAG (5) () ()2102de2,22uuXJXgut= (6) ab= (7) A: area of the telescope aperture (m2) a: radius of the telescope
34、 mirror (m) b: radius of the secondary mirror (m) gt: gain efficiency J0: Bessel function of the first kind of order zero : the ratio, a/ : obscuration ratio u: the variable of integration 6 Rec. ITU-R SA.1805 X: ()sin2a : angle of the optical axis (rad). For the on-axis, X = 0 and the gain efficien
35、cy term in equation (6) becomes: ()=22222ee20,tg (8) Then the on-axis maximum main beam gain in equation (4) becomes: ()=222222ee240,AGt(9) Any obscuration (b) will reduce the main beam gain, fill in the nulls, and increase the side-lobes. 2.5.3 Receiving gain pattern The size of the field of view i
36、s related to the physical size of the detector and the focal length of the telescope. It may be determined by the equation: Fd= (10) where: : field of view of the detector (rad) d: diameter of the detector (m) (typically 104to 103m) F: focal length of the telescope (m). The pattern of a receiving an
37、tenna is typically matched to the detector. The detector is isolated from unwanted energy with the use of field stops and exposed only to the portion of the main beam within radians of the axis of the main beam. Therefore, unwanted energy received in the side lobes of the receiving antenna pattern d
38、oes not arrive at the detector and may be neglected in the course of interference analyses. Assuming the receiving aperture is in the far-field of the transmitting antenna, the received energy is normally treated as a plane wave. The receiving system may use a common or separate aperture from the tr
39、ansmitting 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: ()+=221log104log10AGRdBi (11) where: A: area of the telescope aperture (m2) : wavelength (m) : losses due to energy spi
40、lling over the edge of the detector (dB) Rec. ITU-R SA.1805 7 and: ab= (12) where: a: radius of the telescope mirror (m) b: radius of the secondary mirror (m). The gain calculated in equation (11) represents the quantity of energy incident on the detector. The term GRassumes that the receiving anten
41、na is located in the far-field of the transmitter, and the aperture and the detector are round. The first term of equation (11) 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 by the secondary mirror
42、 of a Cassegrain system. In the case of systems without secondary mirrors, the value of b in equation (12) becomes zero and the second term of equation (11) may be neglected. The third term, , of equation (11) accounts for losses (dB), due to spill over of the signal energy beyond the edge of the de
43、tector. For direct detection systems such as PPM, 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. 2.6 Pointing and tracking The narrow beamwidth and long range of a space-to-space link operating at around 3
44、54 THz and 366 THz impose strict pointing and tracking requirements on a system. Typical pointing requirements are determined by the divergence of the telecommunication beam. For the reference system outlined in Table 1, this equates to 2.6 rad and a pointing loss of no more than 3 dB. 3 Signal-to-n
45、oise ratio (S/N) The performance of space-to-space telecommunication links operating around 354 THz and 366 THz depends directly on achieving a high S/N at the receiver. The higher the S/N, the lower the BER. In general: tsNPRS =/ (13) where: Ps: received signal power as given by equation (1) Nt: no
46、ise power from all sources. Noise comes from two independent sources, detector noise and the background signal. The background signal is due to extraneous energy of albedo and the light from the Sun, planets or stars reaching the detector. Detector noise, discussed in 3.1, is due to the inherent noi
47、se within the detector. The basic equations describing the performance of a laser optical crosslink can be simplified by the following basic assumptions: Optical transmitting and receiving antennas have no central obstructions. 8 Rec. ITU-R SA.1805 Transmitted waveforms are Gaussian and are truncate
48、d at the 1/e2points. Received waves are plane waves. Airy disks are truncated at the first null of the airy disk pattern. 3.1 Detector noise A direct detection receiver with an APD is used for 354 THz and 366 THz telecommunication systems. APD detectors normally operate in one of two noise-limited d
49、etection regions. Detectors receiving high input power levels are generally limited by photon shot noise. However, detectors receiving low input power levels are detector noise limited. The S/N for the commonly used APD followed by a next-stage amplifier in a direct detection system is developed below. Calculate the excess noise factor, NE, by: ()kGGkNE+= 112 (14) where: NE: excess nois
