1、 International Telecommunication Union ITU-T G.640TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (03/2006) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Transmission media characteristics Submarine cables Co-location longitudinally compatible interfaces for free space optic
2、al systems ITU-T Recommendation G.640 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299 INDIVIDUAL CHARACTERIST
3、ICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450G.499 TRANSMISSIO
4、N MEDIA CHARACTERISTICS G.600G.699 General G.600G.609 Symmetric cable pairs G.610G.619 Land coaxial cable pairs G.620G.629 Submarine cables G.630G.649 Optical fibre cables G.650G.659 Characteristics of optical components and subsystems G.660G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWOR
5、KS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 ETHERNET OVER TRANSPORT ASPECTS G.8000G.8999 ACCESS NETW
6、ORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. G.640 (03/2006) i ITU-T Recommendation G.640 Co-location longitudinally compatible interfaces for free space optical systems Summary This Recommendation provides a procedure for establishing that two
7、 co-located Free Space Optical (FSO) transmission systems will not interfere with each other. Calculations of the conditions required to be met to prevent interference in some examples of co-located FSO systems are also included. Source ITU-T Recommendation G.640 was approved on 29 March 2006 by ITU
8、-T Study Group 15 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. G.640 (03/2006) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is
9、a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, e
10、stablishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary
11、 standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, th
12、e Recommendation may contain certain mandatory provisions (to ensure e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equival
13、ents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a clai
14、med Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had n
15、ot received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2006 All rights r
16、eserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. G.640 (03/2006) iii CONTENTS Page 1 Scope 1 2 References. 1 2.1 Normative references 1 2.2 Informative references 1 3 Terms and definitions . 1 3.1 Definitions 1 3
17、.2 Terms defined in other Recommendations. 2 4 Abbreviations 2 5 Reference points . 3 6 Co-location longitudinal compatibility. 3 6.1 Crosstalk ratio between two FSO systems . 5 6.2 Effect of the weather on crosstalk ratio 9 6.3 Case A interference between two systems that can be at the same wavelen
18、gth 10 6.4 Case B interference between two systems which cannot be at the same wavelength 11 6.5 Procedure for establishing whether the conditions for co-location longitudinal compatibility are met 12 7 Optical safety considerations 13 Appendix I Example crosstalk calculations 13 I.1 Example 1. 13 I
19、.2 Example 2. 15 I.3 Example 3. 16 ITU-T Rec. G.640 (03/2006) 1 ITU-T Recommendation G.640 Co-location longitudinally compatible interfaces for free space optical systems 1 Scope This Recommendation defines optical interfaces for “co-location longitudinally compatible“ free space optical transmissio
20、n systems thereby enabling interference-free coexistence of more than one point-to-point free space optical system at a location. This Recommendation also includes definitions of parameters that are relevant for the characterization of free space optical systems. Free space optical systems are commo
21、nly referred to as “FSO“ systems. 2 References 2.1 Normative references The following ITU-T Recommendations and other references contain provisions, which through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All
22、Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regu
23、larly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T Recommendation G.957 (2006), Optical interfaces for equipments and systems relating to the synchronous digital hierarchy. IEC 60825-1 (2001), Saf
24、ety of laser products Part 1: Equipment classification, requirements and users guide. IEC 60825-2 (2005), Safety of laser products Part 2: Safety of optical fibre communication systems (OFCS). IEC 60825-12 (2005), Safety of laser products Part 12: Safety of free space optical communication systems u
25、sed for transmission of information. 2.2 Informative references ITU-T G-series Recommendations Supplement 39 (2006), Optical system design and engineering considerations. 3 Terms and definitions 3.1 Definitions This Recommendation defines the following terms: 3.1.1 acceptance angle: The acceptance a
26、ngle (of an FSO receiver) is the angle between the lines at which the power detected by the receiver falls to 1/e2. This parameter is also called the Field of View (FOV) of an FSO receiver and is commonly defined to be where the power density falls to 1/e2, 1/e or 50%. 2 ITU-T Rec. G.640 (03/2006) 3
27、.1.2 beam divergence: The beam divergence is the angle between the lines at which the power density of an FSO beam falls to 1/e2. NOTE 1 This parameter is also commonly defined to be where the power density falls to either 1/e or 50%. NOTE 2 The beam divergence should be measured at a distance at le
28、ast five times the Rayleigh distance from the lens (see 3.1.8) to ensure that it is measured under far field conditions. 3.1.3 inter-channel crosstalk: The ratio of the disturbing optical power to the wanted optical power detected by the receiver where the wanted and disturbing signals are at differ
29、ent wavelengths. 3.1.4 inter-channel crosstalk penalty: The penalty assigned in the system budget to account for inter-channel crosstalk. 3.1.5 interferometric crosstalk: The ratio of the disturbing optical power to the wanted optical power detected by the receiver where the wanted and disturbing si
30、gnals can be at the same wavelength. 3.1.6 interferometric crosstalk penalty: The penalty assigned in the system budget to account for interferometric crosstalk. 3.1.7 transmitter (or receiver) setting error: The maximum angle between the axis of the transmitter (or receiver) and a straight line joi
31、ning the transmitter and receiver together. 3.1.8 Rayleigh distance: This is defined as: =22distanceRayleigh Dwhere: D is the diameter of the transmitter lens is the wavelength 3.2 Terms defined in other Recommendations This Recommendation uses the following term defined in ITU-T Rec. G.957: Extinct
32、ion ratio. 4 Abbreviations This Recommendation uses the following abbreviations: FOV Field Of View FSO Free Space Optical RfsoReference plane just before the optical receiver input lens Rx Receiver SfsoReference plane just after the optical transmitter output lens Tx Transmitter ITU-T Rec. G.640 (03
33、/2006) 3 5 Reference points Figure 5-1/G.640 Free space optical link reference diagram The reference planes in Figure 5-1 are defined as follows: Sfsois a reference plane just after the optical transmitter output lens; Rfsois a reference plane just before the optical receiver input lens. 6 Co-locati
34、on longitudinal compatibility The free space between the Sfsoand Rfsoreference planes in an FSO system is a shared medium employed by many other users for a variety of different purposes. In order to establish criteria for the co-location of FSO systems, the crosstalk ratio C generated by one system
35、 interfering with another is described in 6.1 and the effect of the weather on this crosstalk ratio is considered in 6.2. The optical power penalty caused by this crosstalk is then defined for two cases: Case A where the two systems can be at the same wavelength (see 6.3). Case B where the two syste
36、ms can not be at the same wavelength (see 6.4). 4 ITU-T Rec. G.640 (03/2006) The difference between these two cases is illustrated in Figure 6-1. Figure 6-1/G.640 Illustration of the difference between the cases covered by clauses 6.3 and 6.4 For bidirectional systems the two directions have to be c
37、onsidered separately. NOTE 1 For some FSO systems the source coherence is sufficiently low (especially in the case of LED-based systems) that even when the wavelengths are the same, interferometric crosstalk is not observed. For these co-located FSO systems, case B always applies. NOTE 2 Even for so
38、me laser-based FSO systems, the assumption of interferometric crosstalk may be pessimistic because the laser coherence may be largely destroyed by the atmosphere at some wavelengths. ITU-T Rec. G.640 (03/2006) 5 6.1 Crosstalk ratio between two FSO systems On the assumption that the beam produced by
39、an FSO transmitter can be approximated by a Gaussian beam, Figure 6-2 contains a reference diagram for a general FSO transmitter. Figure 6-2/G.640 Gaussian beam reference diagram for an FSO transmitter The optical power density of this beam at an angle to the beam axis is given by: 228edcOO= (6-1) w
40、here: Ocis the optical power density at the centre of the beam d is the beam divergence (the angle between the lines at which the power density falls to 1/e2) and is the angle between the beam axis and the measurement point If the curve of optical power density vs angle is known for a particular FSO
41、 system, then values from the curve should be used in place of the approximation from Equation 6-1. An example measured curve is shown in Figure 6-3. 6 ITU-T Rec. G.640 (03/2006) Figure 6-3/G.640 Example measured curve of power density vs angle The corresponding reference diagram for the FSO receive
42、r is given in Figure 6-4. Figure 6-4/G.640 Reference diagram for an FSO receiver The characteristic of detected power vs angle for an FSO receiver depends on a number of parameters including the focal length of the lens, the lens quality, and the diameter of the detector. If the diameter of the spot
43、 formed by the lens is smaller than the diameter of the detector, then the characteristic is approximately rectangular. However, if the diameter of the spot is approximately the same as the diameter of the detector, then the characteristic is an approximately Gaussian curve where the optical power (
44、incident at an angle to the receiver axis) detected by the receiver is given by: 228eaaRR= (6-2) ITU-T Rec. G.640 (03/2006) 7 where: Rais the optical power detected by the receiver when the light is incident along the receiver axis a is the acceptance angle (the angle between the lines at which the
45、power detected by the receiver falls to 1/e2) is the angle between the incident light and the axis of the receiver The two cases defined above are illustrated in Figures 6-5 and 6-6. Figure 6-5/G.640 Example measured curve of received power vs angle showing a rectangular shape Figure 6-6/G.640 Examp
46、le measured curve of received power vs angle showing a Gaussian shape 8 ITU-T Rec. G.640 (03/2006) As can be seen from Figure 6-5, the fit between the measured characteristic and the Gaussian curve is not very good for the rectangular case. Consequently, if the curve of optical power detected by the
47、 receiver vs angle is known for a particular FSO system, then values from the curve should be used in place of the approximation from Equation 6-2. For the case where the two systems cannot be at the same wavelength, optical filtering at the receiver may further reduce the amount of interfering powe
48、r detected with respect to the wanted power. This effect is illustrated in Figure 6-7. Figure 6-7/G.640 Illustration of reduction of the interfering power due to optical filtering Figure 6-8 shows the general case of one FSO system interfering with another. Figure 6-8/G.640 FSO system crosstalk refe
49、rence diagram ITU-T Rec. G.640 (03/2006) 9 This leads to an equation for the crosstalk ratio C: 222288eeadWIOOLC= (6-3) where: L is the ratio of optical filter loss between the wanted and interfering wavelength ranges as illustrated in Figure 6-7 (this would be 1 if the ranges overlap (Case A) or if both ranges are within a flat region of the filter characteristic) OWis the minimum power density at the centre of the wanted beam Iis the maximum power density at the centre of the inte
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