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This Recommendation provides prediction methods for the propagation effects that should be taken into account in the design of digital fixed line-of-sight links, both in clear-air and rainfall conditions. It also provides link design guidance in clear step-by-step procedures including the use of mitigation techniques to minimize propagation impairments.
The final outage predicted is the base for other ITU-R Recommendations addressing error performance and availability. Different propagation mechanisms, with a variety of effects on the radio links, are addressed in the Recommendation.
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The ranges of application of the prediction methods are not always coincident. A brief description of the implemented prediction methods is given in the following sections. Fading is the most important mechanism that affects the performance of digital radio links. Multipath in the troposphere can cause deep fades, especially in longer paths or at higher frequencies.
The prediction method for all percentages of time is graphically illustrated in figure 1.
For small percentages of time, fading follows a Rayleigh distribution, with an asymptotic variation of 10 dB per probability decade. This can be predicted by the following expression:.
Rain can cause very deep fades, particularly at higher frequencies. The Rec.
An estimate of this factor is given by:. The incidence of this effect is determined by the height of the link in relation to the rain height, which varies with geographic location.
A detailed procedure is included in the Recommendation . The reduction in XPD that occurs during both clear-air and precipitation conditions must be taken into account. The combined effect of multipath propagation and the cross-polarization patterns of the antennas governs the reductions in XPD occurring for small percentages of time in clear-air conditions.
To compute the effect of these reductions in link performance a detailed step-by-step procedure is presented in the Recommendation . The XPD can also be degraded by the presence of intense rain.
For paths on which more detailed predictions or measurements are not available, a rough estimate of the unconditional distribution of XPD can be obtained from a cumulative distribution of the co-polar attenuation CPA for rain see section 3 using the equi-probability relation:. For line-of-sight paths with small elevation angles and horizontal or vertical polarization, these coefficients may be approximated by:.
A step-by-step procedure is given to calculate the outage due to XPD reduction in the presence of rain. The primary cause of distortion on line-of-sight links in the UHF and SHF bands is the frequency dependence of amplitude and group delay during clear-air multipath conditions.
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The propagation channel is most often modeled by assuming that the signal follows several paths, or rays, from the transmitter to the receiver. The method recommended in  for predicting error performance is a signature method. The outage probability is here defined as the probability that BER is larger than a given threshold.
There are a number of techniques available for alleviating the effects of flat and selective fading, most of which alleviate both at the same time. The same techniques often alleviate the reductions in cross-polarization discrimination also.
Diversity techniques include space, angle and frequency diversity.
Whenever space diversity is used, angle diversity should also be employed by tilting the antennas at different upward angles. Angle diversity can be used in situations in which adequate space diversity is not possible or to reduce tower heights. The degree of improvement afforded by all of these techniques depends on the extent to which the signals in the diversity branches of the system are uncorrelated.
The diversity improvement factor for digital systems is defined by the ratio of the exceedance times for a given BER with and without diversity. The outage due to clear-air effects is apportioned mostly to performance and the outage due to precipitation, predominantly to availability. Skip to content. ITU-R P. Fading due to multipath and related mechanisms Fading is the most important mechanism that affects the performance of digital radio links.
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