1、 Rec. ITU-R SF.1572 1 RECOMMENDATION ITU-R SF.1572*Methodology to evaluate the impact of space-to-Earth interference from the fixed-satellite service to the fixed service in frequency bands where precipitation is the predominant fade mechanism (Questions ITU-R 250/4 and ITU-R 217/9) (2002) The ITU R
2、adiocommunication Assembly, considering a) that emissions from space stations in the fixed-satellite service (FSS) operating in geo-stationary orbit (GSO) and sharing the same spectrum as the fixed service (FS) may produce interference in receiving stations of the FS; b) results obtained using a sta
3、tistical approach compared to a worst-case analysis may lead to a more efficient use of the spectrum than results from criteria developed using worst-case analysis; c) that sharing methodologies should take into consideration the performance requirements and deployment characteristics of FS systems
4、being used and planned for use in these frequency bands; d) that in spectrum where precipitation is the predominant fade mechanism it is desirable to have an interference evaluation tool using C/N, C/I and C/(N + I) statistics to determine impact on availability; e) that such an interference evaluat
5、ion tool may have application in bands above about 17 GHz to assist administrations in performing sharing studies, recommends 1 that the methodology as described in Annex 1 can be used for developing computer simulation tools which evaluate the impact of interference from FSS systems to digital FS s
6、ystems operating in frequency bands above 17 GHz. _ *The methodology in Annex 1 concentrates on interference from GSO FSS systems. Further studies are required in order to ensure that this methodology is generally applicable to interference from non-GSO FSS systems. 2 Rec. ITU-R SF.1572 ANNEX 1 1 In
7、troduction The methodology described in this Annex provides a model for an analysis of all of the FS system parameters and local geoclimatic parameters of both point-to-point (P-P) and point-to-multipoint (P-MP) systems, which may contribute to the susceptibility of FS receivers to interference from
8、 FSS downlinks. 1.1 Definitions In the frequency bands where precipitation is the predominant fade mechanism, FS design objectives are determined by availability performance rather than error performance. For the purpose of this Recommendation, the term “designed availability” is considered based up
9、on severely errored second (SES) threshold taking into account that in these frequency bands the percentage of events with consecutive SESs less than 10 s is negligible. Hereafter the designed availability for any FS link is estimated by the percentage of time in an average year that a receiver sign
10、al, C/N, falls below the threshold, C/Nth, which corresponds to SES events. Throughout this Annex, unavailability (100% availability) is indicated by the symbol pD(%). Designed availability for P-MP system The design availability is the percentage of time in an average year that the carrier-to-total
11、 noise plus interference, C/(N + I ), into a reference subscriber located at the maximum radius of a P-MP cell will receive at or above the threshold C/Nth. Designed availability for P-P The design availability is the percentage of time in an average year that the carrier-to-total noise plus interfe
12、rence, C/(N + I ), into the receiver will receive at or above the threshold C/Nth. Reference subscriber for a P-MP system The receiver which is located at the maximum distance from the transmitting hub antenna which is used to calculate the transmit power necessary to achieve the designed availabili
13、ty. In P-MP systems, which are modelled with subscriber antennas that have heights, which follow a statistical distribution, the height of the reference receiver is the most probable height. In such a P-MP system model, a hub antenna to which just sufficient transmitter power is delivered to achieve
14、 the designed availability on a link to the reference receiver, will not have sufficient transmit power to meet the designed availability of 100% of all possible subscribers. This is due to a combination of lesser hub antenna gain and greater free space loss in the direction of subscribers also at o
15、r near the maximum distance from the hub antenna. Additional power delivered to the hub antenna would be required for all possible subscribers to achieve their designed availability in a P-MP system characterized by subscriber antennas that have heights, which follow a statistical distribution. Four
16、 types of modulations employed by FS systems are referred to in this Annex. These types of modulation are: quadrature phase shift keying (QPSK) and three different types of quadrature amplitude modulation (16-QAM, 64-QAM and 256-QAM). Rec. ITU-R SF.1572 3 2 Types of FS systems analysed for susceptib
17、ility to GSO FSS interference Based on the systems described in Recommendation ITU-R F.758, there are two distinctly different implementations of FS systems. 2.1 P-MP system The P-MP system is characterized by a central or “hub” transmitting antenna that radiates omnidirectionally in the horizontal
18、plane (azimuth) and directionally in the vertical plane (elevation). The hub antenna radiation pattern is accomplished by combining a number of sector antennas together and may be given a negative elevation angle bias in order to maximize coverage from a high point on the top of a tall building or t
19、ower. The user or “subscriber” antennas, in contrast, are directive and for computational purposes may be assumed to be axially symmetric. The distribution of subscribers over a specified range of possible values of hop length may be statistically modelled or user defined. Some of todays modern, thi
20、rd generation P-MP systems operate on up to three modulations simultaneously such as: QPSK, 16-QAM, 64-QAM and 256-QAM. Such a configuration allows a higher traffic capacity per sector, which is essential to make the networks more economical. The result is that in every cell site, up to three concen
21、tric rings may exist where the same minimum availability objective is encountered. This means that a much larger number of subscribers that receive higher capacities have higher average elevation angles than in P-MP cells and are modelled using a single modulation scheme throughout the whole cell. S
22、ubscribers operating at the higher modulation levels in the inner-most rings are inherently subject to higher power flux-density (pfd) levels from satellites operating in the GSO due to having higher elevation angles than those subscribers in the outer rings. The overall impact from FSS interference
23、 on the availability of such a P-MP system may be statistically modelled by weighting either the number of subscribers (independent of the capacity) in each of the rings or by weighting the number of subscribers of equivalent capacity by post processing the availability data obtained from analysing
24、each of the rings individually. Applying the methodology over a large range of possible deployment scenarios allows the sensitivity of P-MP systems to a variety of FS system parameters, including FS receiver antenna diameter, receiver system noise figure, P-MP cell characteristics and geoclimatic fa
25、ctors, to be assessed parametrically. 2.2 P-P system This type of system is characterized by randomly oriented microwave links having a wide range of hop lengths and elevation angles. When considering the impact of interference from the FSS systems into a P-P FS network, the impact of the FSS interf
26、erence is examined into both ends of the FS link. A subset of P-P systems is that of an implementation of a P-MP system or a “star” configured network where the traffic may or may not be asymmetrical (i.e. higher capacity from centrally located transmitter to the subscriber). In this case, a higher
27、proportion of P-P receivers will have higher elevation angles, much like that of the receivers in a P-MP network and thus have an increased susceptibility to interference from space-to-Earth links. In the “star” type configuration of a P-P network, the distribution of subscribers over a specified ra
28、nge of possible values of hop length may be specified in the same way as for a P-MP network. 4 Rec. ITU-R SF.1572 In some FS deployments, it may be possible to encounter hybrid developments consisting of P-P (both random and star configurations), and P-MP deployments that are optimized based on netw
29、ork efficiency considerations. In all such cases, the dominant interference scenario is the FSS inter-ference into the FS subscriber receiver. 3 General considerations 3.1 P-MP systems The P-MP cell geometry and the hub and subscriber antenna patterns affect the statistical distribution of the level
30、s of interference over the population of possible receivers as well as their susceptibility to interference. Also, in frequency bands above 17 GHz, long-term attenuation by atmospheric gases, the effects of scintillation and short-term attenuation due to rain, which are in-turn affected by geoclimat
31、ic factors related to latitude, are important in determining the susceptibility of subscriber terminals to space-to-Earth interference from FSS satellites. The effect of each of these factors on the susceptibility of subscriber terminals to interference from GSO FSS satellites can be parametrically
32、determined. 3.2 Fixed P-P systems (including star configuration) In P-P systems, in the frequency bands above 17 GHz, a number of factors including receive antenna diameter, elevation angle and system fade margins contribute to the susceptibility of receiving terminals to external interference. Syst
33、em fade margin in a P-P system is dependent on the hop distance and by the same geoclimatic factors and in a similar manner as for P-MP systems. 4 Assumptions The methodology presented in this Annex makes certain assumptions. 4.1 Basic assumptions The following basic assumptions, summarized below, a
34、re common to both P-MP and to P-P FS systems and are important considerations in the implementation of the methodology: a) The path design takes into account any performance objectives and the fade margin required to comply with the applicable recommended or desired short-term performance objectives
35、 b) The impact of the interference to an FS network may be assessed, for instance as a percentage of the total possible FS receivers that achieve an availability at or above a given level of degraded availability. c) The portion of the GSO arc above the horizontal plane is visible to all FS receive
36、 terminals and no part of the arc in any direction is blocked from the view of any FS receive antenna. d) The height above mean sea level (amsl) of the entire wanted signal path is assumed to be the same for the purpose of calculating the attenuation due to atmospheric gaseous (when the option of im
37、plementing Recommendation ITU-R P.676 is used) and the long-term Rec. ITU-R SF.1572 5 attenuation due to rain (Recommendation ITU-R P.530). The height used is the average of heights of the transmitting and the receiving antenna in a P-P system and the minimum antenna height of the subscriber in a P-
38、MP system. e) The maximum equivalent isotropically radiated power (e.i.r.p.) value of 55 dBW is observed. The channel bandwidth for the FS system under consideration in conjunction with the 55 dBW e.i.r.p. value will establish the maximum transmit power density limit for that FS system. f) FS charac
39、teristics used to model any P-P or P-MP link should be representative of typically deployed networks. g) FS availability is always defined based on an annual average. 4.2 P-MP assumptions 4.2.1 Intra-service (including intra-system) interference In the most basic system model, the FS system could be
40、 assumed to have been allocated a level of intra-service interference for a reference subscriber terminal located at the edge of the P-MP cell at a specified height above ground level (agl). This assumption results in the total noise, thermal plus intra-service, being a specified level above the the
41、rmal noise level alone. Actual levels of intra-service interference can be considered, for example, if it is desired to assess the change in the impact from space-to-Earth interference (e.g. if all systems in a given region were to increase their modulation to a higher order to achieve greater capac
42、ities). Also, the methodology allows for a specific model of the FS deployment characteristics to compute the levels of intra-service interference. 4.2.2 Omni hub In most cases, the hub antenna gain pattern envelope is circularly symmetrical in the horizontal plane and the gain of the antenna is ind
43、ependent of azimuth for a given angle in the vertical plane (i.e. declination angle). This assumption applies whether the hub antenna is a single omnidirectional antenna or composed of multiple sector antennas. In the case that a single sector antenna is used, the off-axis pattern in both the horizo
44、ntal and the vertical plane would be required as an input to permit calculation of the gain at any given point. 4.2.3 Subscriber distribution There are many possible ways to model the distribution of the subscribers throughout a P-MP cell including statistically described distributions such as unifo
45、rm or Rayleigh, or other user defined distributions. Where possible, the selection of a model may be made through validation based on statistical data concerning actual subscriber locations. Actual data giving the relative position of a subscriber in a P-MP cell may be used when examining real links
46、 A combination of statistical distribution and actual data can also be used. 4.2.4 Level ground The ground elevation, amsl, throughout the entire cell may be considered to be the same as the ground elevation for the hub. Alternatively, actual data giving the ground elevation for each subscriber pos
47、ition is located in a P-MP cell may be used when examining links on a case-by-case basis. 6 Rec. ITU-R SF.1572 4.2.5 Subscriber antenna heights The height of subscriber antennas, agl, may be statistically modelled per Recommendation ITU-R P.1410 if the Rayleigh, , height for the city being modelled
48、is known. In the case of a Rayleigh antenna height model, practical considerations should be used in truncating the height at some maximum and minimum values. Actual data giving the height of each subscriber terminal may be used when examining links on a case-by-case basis. A combination of statisti
49、cal distribution and actual data can also be used. 4.2.6 0% blockage The hub antenna is assumed to be visible to every subscriber antenna in a P-MP cell and is not blocked from the view by any building upon which other subscriber antennas may be situated even if the randomization employed by a given antenna height model would result in a closer building blocking the view of a more distant subscriber antenna. 4.2.7 No diversity Every remote subscriber terminal is assumed to be pointed toward the same point and thus no account of a worst-case azimuth exposure is made
