1、 Rec. ITU-R F.1705 1 RECOMMENDATION ITU-R F.1705 Analysis and optimization of the error performance of digital fixed wireless systems for the purpose of bringing into service and maintenance (Question ITU-R 235/9) (2005) Scope This Recommendation provides analysis for optimization of the error perfo
2、rmance of digital fixed wireless systems (FWSs) for the purpose of practical maintenance work prior to bringing into service. Annex 1 presents the guidance and systematic methods for the maintenance of both point-to-point (P-P) and point-to-multipoint P-MP systems. The ITU Radiocommunication Assembl
3、y, considering a) that many factors including fading due to multipath effects may distort and attenuate received signals on line-of-sight paths and thereby impair the performance of FWSs; b) that countermeasures such as diversity reception and adaptive equalization are available to reduce the effect
4、s of multipath fading on system performance; c) that methods for the analysis and optimization of the error performance degradation of digital FWSs due to many factors are needed for the bringing into service (BIS) and/or maintenance of FWSs and for radio equipment development; d) that the performan
5、ce limits and the technical guidance on BIS or maintenance for FWSs are given in Recommendations ITU-R F.1330 (Performance limits for bringing into service of the parts of international plesiochronous digital hierarchy and synchronous digital hierarchy paths and sections implemented by digital radio
6、-relay systems) and ITU-R F.1566 (Performance limits for maintenance of digital fixed wireless systems operating in plesiochronous and synchronous digital hierarchy-based international paths and sections), respectively; e) that it is often necessary to optimize the performance of FWSs after they are
7、 installed, in particular during the maintenance work before BIS, recommends 1 that the guidance and the systematic method for the analysis and optimization of digital FWS error performance detailed in Annex 1 should be applied in cases of sub-optimal performance. Annex 1 Analysis and optimization o
8、f the error performance of digital FWSs for the purpose of bringing into service and maintenance 1 Introduction The purpose of this Annex is to provide guidance on the availability of methods for fault analysis and optimization of the performance of digital FWSs. Detailed information on propagation
9、aspects of the design of fixed wireless systems is contained in Recommendation ITU-R P.530 (Propagation data and prediction methods required for the design of terrestrial line-of-sight systems). 2 Rec. ITU-R F.1705 ITU-R error-performance Recommendations (e.g. Recommendation ITU-R F.1668 Error perfo
10、rmance objectives for real digital fixed wireless links used in 27 500 km hypothetical reference paths and connections) place requirements on the design and operation of FWSs to meet the performance objectives of these Recommendations. In view of these requirements, this Recommendation seeks to prov
11、ide guidance for commissioning and operating FWSs in situations where difficulties are experienced due to poor error performance. Guidance on performance limits for systems BIS is given in Recommendation ITU-R F.1330. This Recommendation may also serve to provide maintenance trigger points for opera
12、tional systems. Furthermore it is noted that error cause assessment can basically be triggered by the performance limits given in Recommenda-tion ITU-R F.1566. There has been a great deal of development of systems and techniques to provide high quality FWSs. Measurement technologies and software are
13、 now available for the analysis of digital FWSs in the laboratory and in field operation. In view of these developments it is recommended that where system analysis for fault rectification or optimization for the purpose of meeting the required performance objectives is necessary, then measuring ins
14、truments and software may be applied to achieve the above aims. More detailed general information on FWS operations can be found in the ITU-R Handbook on digital radio-relay systems. 2 Link analysis There are many potential causes of errors on FWSs. The analysis of system errors is thus complex and
15、potentially time-consuming. To progress the analysis and optimization of systems it is necessary to adopt a systematic approach to understanding the nature of the errors and their relationship to the environment in which the FWS operates. In the absence of a systematic approach to error performance
16、analysis, it is easy for the system operator to ascribe all errors to fading and do nothing to rectify the problem. The purpose of this guide is to simplify the process of system analysis and optimization by giving directions that assist in the improvement of system performance. Measurement and anal
17、ysis tools are available to assist this process even in the face of seemingly severe error-performance problems. A basic flow chart is shown in Fig. 1 for assessing the potential causes of system errors, which are observed in P-P FWSs with multi-hop connection. There are many branches in this flow c
18、hart each of which requires some special knowledge about the system and its environment. To gain the necessary knowledge, specific measurements or analyses are required at each branch of Fig. 1. Following such measurements, further application software may be required to assist in evaluating the mea
19、surements and to derive an optimum solution. For FWSs used in the access portion, in particular for P-MP systems operating in different propagation environments, another flow chart in Fig. 2 could be used. The following sections set out guidance for each decision point, the numbering of each section
20、 corresponding to a decision in the flow charts of Figs. 1 and 2. Rec. ITU-R F.1705 3 1705-013.13.23.33.43.54.14.24.34.4433Occasional errors correlatedwith meteorologicalconditions?See Rec. ITU-R P.530Link errorsContinuous errors?Flat fade marginunchanged?Rain fading?Multipath fading?Adjust path or
21、usediversity or equalizerN-hop over-reach?See Rec. ITU-R P.452Other interference?Find interferenceSystem instabilityInterference* orsystem alignment?System alignmentSite intermodulation?Find intermodulationCo-channelover-reach?Change system designAdjacent-channelover-reach?Change system designRadar
22、or other sources?Radar or otherinterferenceYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoFIGURE 1Flow chart for assessing the causes of link errors observedin P-P FWS wih multi-hop connection* Interference may come from services sharing the same band as well as unwanted emissions from
23、other bands.4 Rec. ITU-R F.1705 1705-025.135.25.33.23.44.24.145.44.4Occasional errors correlatedwith meteorologicalconditions?See Rec. ITU-R P.530Link errorsIdentificationof errored hopsContinuous errors?Rain fading?Multipath fading?Adjust path or usediversity or equalizerReflection wave with propog
24、ation path?(building reflection, automobile, ship, etc.)Antenna directionadjustmentOther interference(ISM equipments etc.)Find interferenceSystem instabilitySystem alignment,propagation orinterference*?System alignmentor improvementof installationconditionUnexpected obstructionof propagation?Change
25、system designSite intermodulation?Find intermodulationRadar or other sources?Radar or otherinterferenceYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNoYesNo* Interference may come from services sharing the same band as well as unwanted emissions from other bands.FIGURE 2Flow chart for assessing th
26、e causes of link errors observed in the P-MP FWS3 Link error analysis for P-P FWS in multi-hop connection The initial decision is to determine how often the errors are occurring. The errors may be at a low level but always be there, for example a system may have in excess of 100 errored seconds (ESs
27、) every month, these would be classed as “continuous errors”. If the system has less ESs, e.g. 50 even including severely errored seconds (SESs), in most months, they would be classed as “occasional errors”. Rec. ITU-R F.1705 5 Even when occasional errors are observed, there may be a case that these
28、 errors are due to certain reduction of flat fade margin caused by impairment of the equipment or the feeder system. If the flat fade margin of the system is reduced by several dB (or more), a small effect of multipath fading which would not normally produce any bit errors may result in apparent occ
29、asional errors. Therefore, by inspecting flat fade margin, the cause of the occasional errors may be found in system alignment or site intermodulation and so on, which are generally considered as factors relating to continuous errors. 3.1 Interference or system alignment To make this decision it is
30、necessary to determine whether the errors are caused by interference, arriving via the antenna, or if there is a fault internal to the system. An effective method to see whether interference is present is to attach a spectrum analyser to the RF branching filter output to the receiver and closely exa
31、mine the spectrum. This approach works well for high-level interferers but can be compromised for lower-level wideband interference that is below the spectrum analyser noise floor (see the next paragraph for an alternative approach). For this measurement to be most effective the far end transmitter
32、should be turned off. If there is no interference evident then the errors are most likely due to system alignment. This assumes that the errors are continuous and that a measurement of the received spectrum will show the offending interferer. The received spectrum can also be measured at the output
33、of the receiver IF amplifier as the system receiver has sufficient gain and excellent noise figure to ensure that any interference will be evident. This alleviates the need for microwave spectrum analysis tools and yields much greater sensitivity for measuring low-level interference. Generally, inte
34、rference should be preventable through prudent frequency planning. In practice, interference from unknown sources does occur. Interference can also be aggravated by abnormal propagation conditions and should not be dismissed as a source of errors without appropriate system level investigation. The s
35、ystem alignment issues are complex and diverse. Key issues include antenna alignment, fade margin, wave-guide and branching filter loss, group delay and amplitude distortion, diversity equalization, channel frequency, cabling and connector quality and of course basic equipment faults. Each item requ
36、ires detailed checking to ensure that the system is correctly aligned and potential error sources minimized. 3.2 Site intermodulation Local site intermodulation products can arise from the mixing products of co-located transmitters. Such mixing products can result from poor mechanical connections; f
37、or example the elements of an antenna tower can act as a non-linear “diode” mixer to produce in-band interference products. The potential cure for these problems is to shift the frequency of the offending transmitter or to locate and correct the mechanical/electrical structure that produces the mixi
38、ng. Spectrum analysis of the received signal should be made with the far end transmitter turned off. The received spectrum analysis can easily be facilitated at the output of the receiver IF amplifier. Mixing products can be evaluated by calculation. 3.3 Co-channel over-reach interference Co-channel
39、 over-reach under normal propagation conditions will propagate typically over three hops or two hops further away than the wanted transmitter for standard frequency arrangements. In design it is normal to ensure that paths are correctly offset so that the antenna discrimination will minimize any ove
40、r-reach interference. If over-reach interference is suspected, then it may be detectable by spectrum analysis of the received signal with the wanted transmitter turned off. It is 6 Rec. ITU-R F.1705 also possible that the next hop transmitter facing away from the affected receiver may be the cause o
41、f co-channel interference. This can occur if the next hop antenna front-to-back ratio is poor or the angle of the next hop is such that a sidelobe of the next path antenna directs sufficient co-channel signal to cause interference (see Recommendations ITU-R F.1096 (Methods of calculating line-of-sig
42、ht interference into radio-relay systems to account for terrain scattering) and ITU-R F.1095 (A procedure for determining coordination area between radio-relay stations of the fixed service). 3.4 Adjacent-channel over-reach interference Adjacent-channel over-reach interference will have similar issu
43、es to those of co-channel interference. Spectrum analysis at RF or IF with the wanted transmitter off is required to determine the amount of interference (see Recommendations ITU-R F.1191 (Bandwidths and unwanted emissions of digital fixed service systems) and ITU-R SM.328 (Spectra and bandwidth of
44、emissions). 3.5 Radar or other interference Pulse-type interference can rise from a variety of sources such as radar, electrical arcs and motor vehicle ignition systems. The radar signature can usually be determined from an analysis of the error bursts. As a qualitative estimate, a radar that causes
45、 continuous interference will generally be located within line-of-sight of the affected receiver. It may also be close to the boresight of the antenna unless the radar is very close (see Recommendations ITU-R F.1097 (Interference mitigation options to enhance compatibility between radar systems and
46、digital radio-relay systems) and ITU-R F.1190 (Protection criteria for digital radio-relay systems to ensure compatibility with radar systems in the radiodetermination service) for detailed guidance on radar interference). Pulse interference can occur from other sources located close to the affected
47、 receiver. Comprehensive error analysis tools are required to evaluate the effects of pulse interference. Information and guidance concerning interference from other services sharing the same frequency bands is contained in several F-Series Recommendations and the ITU-R Handbook on digital radio-rel
48、ay systems. 4 Occasional errors correlated with meteorological conditions Meteorological conditions can play a significant role in causing errors on FWSs. In many cases, errors can be correlated with stable air in high-pressure systems, fronts, cold, hot or even windy conditions. To determine which
49、meteorological conditions are related to poor error performance may take some effort in meteorological observation. Having determined that the errors are related to particular meteorological conditions further effort is required to isolate the particular issue. If occasional errors are not correlated with meteorological conditions then the errors could be due to other interference sources (see 4.4). 4.1 Rain attenuation Loss of signal due to rain attenuation will clearly be correlated with higher rainfall rates. Poorly designed systems will fade out at low rain rates. In such cases Recom
