1、 Rec. ITU-R F.1093-2 1 RECOMMENDATION ITU-R F.1093-2*Effects of multipath propagation on the design and operation of line-of-sight digital fixed wireless systems (Question ITU-R 122/9) (1994-1997-2006) Scope This Recommendation provides an introduction to propagation-related aspects of the design an
2、d operation of digital radio-relay systems, drawing on information from Radiocommunication Study Group 3 texts and measurements conducted by administrations. Annex 1 explains the role of multipath fading as the dominant propagation factor for digital radio-relay systems operating at frequencies belo
3、w about 10 GHz. Further material discusses the roles of diversity techniques and adaptive equalization in reducing channel degradations. The Radiocommunication Assembly, considering a) that fading due to multipath propagation may distort and attenuate received signals on line-of-sight paths and ther
4、eby impair the performance of fixed wireless systems (FWSs); b) that Recommendation ITU-R P.530 provides data and methods for FWS propagation prediction and path planning; c) that countermeasures to reduce the effects of multipath fading on system performance, such as diversity reception and adaptiv
5、e equalization, are available; d) that methods of analysing the effects of multipath fading on the error performance of FWS are needed for comparing alternative designs, recommends 1 that multipath fading countermeasures should be incorporated in radio system design, as needed, to improve error perf
6、ormance; 2 that the methods in Annex 1 should be used for guidance in radio link planning. *This Recommendation should be brought to the attention of Radiocommunication Study Group 3. 2 Rec. ITU-R F.1093-2 Annex 1 Effects of multipath propagation on design and operation of line-of-sight digital FWSs
7、 1 Introduction The purpose of the present Annex is to furnish an introduction to propagation-related aspects of the design and operation of digital radio-relay systems, drawing on information from Radiocommunication Study Group 3 texts and measurements conducted by administrations. The first part o
8、f the Annex explains the role of multipath fading as the dominant propagation factor for digital radio-relay systems operating at frequencies below about 10 GHz. The following sections discuss the roles of diversity techniques and adaptive equalization in reducing channel degradations. Finally, the
9、prediction of system performance depending on the foregoing factors is treated. More detailed information on the application of the guidance contained here can be found in the Handbook on digital radio-relay systems. 2 Propagation considerations The texts established by Radiocommunication Study Grou
10、p 3 contain a wealth of information on the propagation phenomena to be taken into account in the design and operation of radio-relay systems. In particular, Recommendation ITU-R P.530 is especially concerned with “propagation data and prediction methods required for line-of-sight radio-relay systems
11、”. In that Recommendation, the information is arranged according to the propagation effects that must be considered. The relevant meteorological information concerning the propagation mechanisms is given in other Recommendations of the P series, notably Recommendations ITU-R P.834 and ITU-R P.676. P
12、ropagation conditions vary from month to month and from year to year, and the probability of occurrence of these conditions may vary by as much as several orders of magnitude. It may therefore take some three to five years before drawing a proper conclusion on the results of a propagation experiment
13、. However, for system application requirements, this time is often not available and models of this variability for some parameters have been examined in Recom-mendation ITU-R P.841. From propagation data it was concluded that for a well-designed path which is not subject to diffraction fading or su
14、rface reflections, multipath propagation is the dominant factor in fading below 10 GHz. Above this frequency, the effects of precipitation tend increasingly to determine the permissible path length through the system availability objectives. The necessary reduction in path length with increase in fr
15、equency, reduces the severity of multipath fading. These two principal causes of fading are normally mutually exclusive. Given the split between availability and error performance objectives, precipitation effects contribute mainly to unavailability and multipath propagation mainly to error performa
16、nce. Another influence of precipitation, e.g. back-scatter from rain, may influence the choice of radio-frequency channel arrangements. Propagation effects due to various forms of precipitation tend not to be frequency dispersive, while multipath propagation caused by tropospheric layers can be, and
17、 this may cause severe distortion of information-bearing signals. The rapid development of digital communication systems has required an improved understanding of these effects and the means to overcome them. Rec. ITU-R F.1093-2 3 3 Countermeasures to propagation effects There are two countermeasure
18、s to propagation distortion commonly used: diversity techniques and adaptive channel equalizers, which attempt to combat attenuation and distortion caused by the propagation medium. The effectiveness of a fading countermeasure is usually expressed in terms of an improvement factor. On a single test
19、path, the improvement factor is the ratio of the outage time observed for a system without the countermeasure, to that observed when the countermeasure is operative (see Note 1). The improvement factor depends on the outage threshold chosen. NOTE 1 Outage time is a general term to indicate the time
20、duration over which the system exceeds a chosen bit-error ratio (BER) threshold. 3.1 Diversity techniques The most commonly used diversity techniques are frequency diversity and space diversity. For others, see Recommendations ITU-R F.752, ITU-R P.530 and the ITU-R Handbook on Digital Radio-Relay Sy
21、stems (edition 1996). 3.1.1 Space diversity Space diversity is one of the most effective methods of combating multipath fading. For digital radio systems, where the performance objectives can be difficult to meet owing to waveform distortions caused by multipath effects, system designs must often be
22、 based on the use of space diversity. In space diversity systems the signals received by two vertically separated receiving antennas rarely fade simultaneously when the fades are deep. The improvement factor that a system may achieve by using these two signals depends on both propagation factors and
23、 the radio system implementation, that is, its vulnerability to the power loss and multipath distortion of signals and its method of processing them. In evaluating the improvements achievable with space diversity, the accepted practice has been to use the single frequency fading improvement factor f
24、ormulation in Recommen-dation ITU-R P.530, or similar formulations verified for regional application, particularly for thermal noise considerations in calculating outage probabilities (see 4). By reducing the effective incidence of deep fading, space diversity can reduce the effects of various types
25、 of interference. In particular, it can reduce the short-term interference effects from cross-polar channels on the same or adjacent channel frequencies, the interference from other systems, and from within the same system. Linear amplitude dispersion (LAD) is an important component of waveform dist
26、ortion and quadrature cross-talk effects, and can be reduced by the use of space diversity. Diversity combining designed specifically to minimize LAD (see Recommendation ITU-R F.752) is among the methods that are particularly effective in combating this distortion. The improvement derived from space
27、 diversity will depend upon how the two signals are processed at the receiver. Two examples of techniques are “hitless” switching and variable phase combining (see Recommendation ITU-R F.752). The “hitless” switch selects the receiver which has the greater eye opening or the lower error ratio and th
28、e combiners utilize either co-phase or various types of dispersion-minimizing control algorithms. “Hitless” switching and co-phase combining provide very similar improvement factors. 4 Rec. ITU-R F.1093-2 3.1.2 Frequency diversity The frequency diversity improvement in a digital radio hop with a 1 +
29、 1 configuration depends on the correlation of degradations (for example, fade depth, amplitude and group-delay dispersion) in the two radio-frequency (RF) channels. Experimental results show a low correlation of amplitude dispersion between two 30 MHz wide channels separated by 60 MHz. The largest
30、frequency diversity improvement can be achieved usually using cross-band frequency diversity. In N + 1 systems the frequency diversity improvement applicable to a working channel decreases as the number of working channels increases. In considering the use of frequency diversity with a multi-hop swi
31、tching section, it must be taken into account that the frequency diversity improvement depends both on the correlation of the degradation between RF channels within one hop and at the same time on the other hops of the same switching section. In order to achieve the predicted frequency diversity imp
32、rovement in digital radio systems, the switching system must operate in a “hitless” mode. Furthermore, the overall switching procedure has to be completed before significant degradation of the traffic channel occurs. A response time of about 10 ms or less is suitable for this purpose. 3.2 Adaptive c
33、hannel equalization Some form of receiver equalization is usually necessary in the radio channel. The equalizer must be adaptively controlled to follow variations in transmission characteristics as propagation conditions vary. The equalization techniques employed can be classified into two groups, d
34、epending on whether their mode of operation is more naturally described in the frequency or time domain: “frequency domain equalization” and “time domain equalization”. 3.2.1 Frequency domain equalization This type of equalizer comprises one or more linear networks that are designed to produce ampli
35、tude and group delay responses, to compensate for the transmission impairments considered most likely to cause a degradation of system performance during periods of multipath fading. 3.2.2 Time domain equalization For digital systems, time-domain signal processing can be considered as the most natur
36、al equalization technique, since it attempts to combat intersymbol interference directly. Control information is derived by correlating the interference that appears at the decision instant with the various adjacent symbols producing it, and is used to adjust tapped delay line networks to provide ap
37、propriate cancellation signals. This type of equalizer has the ability to handle simultaneously and independently the distortions which arise from amplitude and group delay deviations in the faded channel, thereby providing compensation for either minimum-phase or non-minimum phase characteristics.
38、In systems employing quadrature modulation, important destructive effects of fading are known to be associated with cross-talk generated by channel asymmetries. Consequently, to be of value, a time-domain equalizer must be capable of providing the means for quadrature distortion compensation. 3.2.3
39、Performance improvement factors Digital radio system outages are caused by a combination of three main degradations: interference, thermal noise and waveform distortion. Equalization is generally only effective against the last of these. Consequently, in considering the performance improvements asso
40、ciated with the use of Rec. ITU-R F.1093-2 5 adaptive equalizers, it is clear that the largest reductions in outage time will occur on hops where signal distortion is known to be the prime cause of system failure. 3.3 Adaptive equalization in combination with space-diversity combining Dramatic reduc
41、tions in the incidence of multipath outage can be achieved when adaptive channel equalization is combined with space diversity. Measured total outage time improvement usually exceeds the product of the corresponding individual improvements obtained from diversity and equalization separately, showing
42、 that an important synergistic interaction is taking place. The improvement for space diversity together with equalization is approximately equal to the product of the space-diversity improvement and the square of the equalizer improvement. This seems most accurate for the switched diversity case. 3
43、.4 System design considerations in the presence of propagation ducts Ducts are known to exist in certain geographical areas at elevations up to and exceeding 1 000 m. In locations where ducts are known to exist, and digital microwave radio-relay systems are to be operated, attention should be given
44、to the following factors in system design: the antenna pointing and position, the antenna beamwidth, required to minimize the amount of energy radiated towards or received from reflection layers and from the ground, the modulation scheme used, in order to increase the symbol duration, the path geome
45、try, required to minimize the probability of destructive reflections. 4 Calculation of outage probabilities In digital systems, outage times are caused by waveform distortion due to frequency selective fading, interference and thermal noise. The total outage time will be dependent on these three con
46、tributors. There are various methods for calculating the outage time of digital systems which will be discussed briefly in this section. Typical input parameters for these methods include: path length, operating frequency, antenna radiation pattern, diversity parameters, surface roughness, path clea
47、rance, climatic zone. The conventional method for calculating outage times for analogue systems is based on the concept of single-frequency fades and is therefore not directly applicable to high-capacity digital radio-relay systems. An increase in the fade margin, which in analogue systems will tend
48、 to reduce the effect of thermal noise, will not improve the performance of digital systems if multipath fading has already collapsed the eye-diagram amplitude to zero. It follows that increasing the transmitter power cannot be employed as the only means of making digital radio systems meet their ou
49、tage requirements. Three general approaches have been used in the development of outage prediction methods: fade margin methods, signature curve methods, and methods using the LAD. As yet, there are insufficient data to conclude that one of these approaches is clearly superior than the others. Nevertheless, a set of methods for unprotected and protected systems (space, frequency, and angle 6 Rec. ITU-R F.1093-2 diversity), including dual polarization co-channel systems, are given in step-by-step form in Recommendation ITU-R P.530. The performance reduction due to dist
