ITU-R P.530 RECOMMENDATION

ITU-R P.530 RECOMMENDATION

1. Description

The ITU-R Recommendation P.530, “Propagation data and prediction methods required for the design of terrestrial line-of-sight systems” provides a number of propagation models useful for the evaluation of propagation effects in microwave radiocommunications systems.

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. 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.

2. Fading due to multipath and related mechanisms

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:

(1)
(2)

(3)
  • K : geoclimatic factor
  • dN: point refractivity gradient in the lowest 65 m of the atmosphere not exceeded for 1% of an average year
  • sa : area terrain roughness, defined as the standard deviation of terrain heights (m) within a 110 km x 110 km area with a 30 s resolution
  • d : Link path distance (km)
  • f : Link frequency (GHz)
  • hL : altitude of the lower antenna above sea level (m)
  • |εp| : absolute value of the path inclination (mrad)
  • p0 : multipath occurrence factor
  • pw : percentage of time fade depth A is exceeded in average worst month

Figure 1: Percentage of time, pw, fade depth, A, exceeded in average worst month, with p0 ranging from 0.01 to 1 000

If is made equal to the receiver margin, the probability of link outage due to multipath propagation is equal to pw /100. For a link with hops, the probability of outage PT takes into account the possibility of a small correlation between fades in consecutive hops.

(4)

In (4), , for most practical cases. Pi is the outage probability predicted for the i-th hop, and di its distance. C = 1 if A exceeds 40 km or the sum of the distances exceeds 120 km.

3. Attenuation due to hydrometeors

Rain can cause very deep fades, particularly at higher frequencies. The Rec. P. 530 includes the following simple technique that may be used for estimating the long-term statistics of rain attenuation:

Step 1:  Obtain the rain rate R0.01 exceeded for 0.01% of the time (with an integration time of 1 min).

Step 2:  Compute the specific attenuation, γR (dB/km) for the frequency, polarization and rain rate of interest using Recommendation ITU-R P.838.

Step 3:  Compute the effective path length, deff, of the link by multiplying the actual path length by a distance factor r. An estimate of this factor is given by:

(5)

where, for R0.01 ≤ 100 mm/h:

(6)

For R0.01 > 100 mm/h, use the value 100 mm/h in place of R0.01.

Step 4:  An estimate of the path attenuation exceeded for 0.01% of the time is given by:

A0.01 = γR deff = γR d
(7)

Step 5:  For radio links located in latitudes equal to or greater than 30° (North or South), the attenuation exceeded for other percentages of time in the range 0.001% to 1% may be deduced from the following power law:

(8)

Step 6:  For radio links located at latitudes below 30° (North or South), the attenuation exceeded for other percentages of time in the range 0.001% to 1% may be deduced from the following power law.

(9)

The formulas (8) and (9) are valid within the range 0.001% – 1%.

For high latitudes or high link altitudes, higher values of attenuation may be exceeded for time percentage due to the effect of melting ice particles or wet snow in the melting layer. 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 [1].

The probability of outage due to rain is calculated as p / 100, where is the percentage of time rain attenuation exceeds the link margin.

4. Reduction of cross-polar discrimination (XPD)

The XPD can deteriorate sufficiently to cause co‑channel interference and, to a lesser extent, adjacent channel interference. 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 [1].

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:

(10)

The coefficients and V(f) are in general dependent on a number of variables and empirical parameters, including frequency, f. For line-of-sight paths with small elevation angles and horizontal or vertical polarization, these coefficients may be approximated by:

(11)
(12)

An average value of U0 of about 15 dB, with a lower bound of 9 dB for all measurements, has been obtained for attenuations greater than 15 dB.

A step-by-step procedure is given to calculate the outage due to XPD reduction in the presence of rain.

5. Distortion due to propagation effects

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.

The propagation channel is most often modeled by assuming that the signal follows several paths, or rays, from the transmitter to the receiver. Performance prediction methods make use of such a multi-ray model by integrating the various variables such as delay (time difference between the first arrived ray and the others) and amplitude distributions along with a proper model of equipment elements such as modulators, equalizer, forward‑error correction (FEC) schemes, etc.. The method recommended in [1] 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.

Step 1:  Calculate the mean time delay from:

(13)

where is the path length (km).

Step 2:  Calculate the multipath activity parameter η as:

(14)

Step 3:  Calculate the selective outage probability from:

(15)

where:

  • Wx : signature width (GHz)
  • Bx : signature depth (dB)
  • τr,x : the reference delay (ns) used to obtain the signature, with denoting either minimum phase (M) or non-minimum phase (NM) fades.

If only the normalized system parameter Kn is available, the selective outage probability in equation (15) can be calculated by:

(16)

where:

  • T : system baud period (ns)
  • Kn,x : the normalized system parameter, with denoting either minimum phase (M) or non-minimum phase (NM) fades.

6. Diversity techniques

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. Space diversity helps to combat flat fading (such as caused by beam spreading loss, or by atmospheric multipath with short relative delay) as well as frequency selective fading, whereas frequency diversity only helps to combat frequency selective fading (such as caused by surface multipath and/or atmospheric multipath).

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, I, for fade depth, A, is defined by:

I = p(A) / pd(A)
(17)

where pd(A) is the percentage of time in the combined diversity signal branch with fade depth larger than and p(A) is the percentage for the unprotected path. 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 improvement due to the following diversity techniques can be calculated:

  • Space diversity.
  • Frequency diversity.
  • Angle diversity.
  • Space and frequency diversity (two receivers)
  • Space and frequency diversity (four receivers)

The detailed calculations can be found in [1].

7. Prediction of total outage

The total outage probability due to clear-air effects is calculated as:

(18)
  • Pns : Outage probability due to non-selective clear-air fading (Section 2).
  • Ps : Outage probability due to selective fading (Section 5)
    PXP : Outage probability due XPD degradation in clear-air (Section 4).
  • Pd : Outage probability for a protected system (Section 6).

The total outage probability due to rain is calculated from taking the larger of Prain and PXPR.

  • Prain : Outage probability due to rain fading (Section 3).
  • PXPR : Outage probability due XPD degradation associated to rain (Section 4).

The outage due to clear-air effects is apportioned mostly to performance and the outage due to precipitation, predominantly to availability.

8. References

[1] ITU-R Recommendation P.530-13, “Propagation data and prediction methods required for the design of terrestrial line-of-sight systems”, ITU, Geneva, Switzerland, 2009.

 

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Microwave Rain Fade Planning ITU-R P.837-6

RECOMMENDATION ITU-R P.837-6

ITU-R P.837-6 P.0837-01  – Characteristics of precipitation for propagation modelling Radiowave propagation for Terrestrial Microwave Links and Radio Links for Point to Point (P2P, PTP) and Point to Multipoint (P2MP, PTMP) deployments.

Calculations can be made for Link Availability (%) for all frequency bands, to take into account link budgets, transmit power, receive sensitivity, antenna gain, target availability and other factors.  Typical Link Availability Targets are 99.99%, 99.999% and higher.

ITU-R P.837-6 P.0837-01

ITU-R P.837-6 P.0837-01
ITU-R P.837-6 P.0837-01

Recommendation ITU-R P.837 contains maps of meteorological parameters that have been obtained using the European Centre for Medium-Range Weather Forecast (ECMWF) ERA-40 re-analysis database, which are recommended for the prediction of rainfall rate statistics with a 1-min integration time, when local measurements are missing.
Rainfall rate statistics with a 1-min integration time are required for the prediction of rain attenuation in terrestrial and satellite links. Data of long-term measurements of rainfall rate may be available from local sources, but only with higher integration times. This Recommendation provides a method for the conversion of rainfall rate statistics with a higher integration time to rainfall rate statistics with a 1-min integration time.

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OFCOM Channel Plans for 31GHz and 38GHz

OFCOM Channel Plans for 31GHz and 38GHz

Here is a chart showing channel plans for the UK

OFCOM - 31GHz 38GHz
OFCOM – 31GHz 38GHz

Uses & Applications

31GHz and 38GHz bands are used for Point to Point (P2P) Microwave Radio Links

Sources of Data and Graphics

All contents (C) OFCOM and taken from:

OfW48 UK Frequency Allocations for Fixed (Point-to-Point) Wireless Services and Scanning Telemetry This document shows the current bands managed by Ofcom that are available for fixed terrestrial (point to point) links and scanning telemetry in the UK.

Technical regulations

The Radio Equipment and Telecommunications Terminal Equipment Directive
99/5/EC (R&TTED) has been implemented in ‘The Radio Equipment and Telecommunications Terminal Equipment Regulations 2000, Statutory
Instrument (SI) 730. In accordance with Articles 4.1 and 7.2 of the R&TTED
the:
• IR2000: The UK Interface Requirement 2000 contains the requirements for the licensing and use of fixed (point-to-point) wireless services in the UK.
• IR2037: The UK Interface Requirement 2037 applies for scanning telemetry services.
• IR2078: The UK Interface Requirement 2078 applies for the 60 GHz band

Notes specific to the frequency charts

The first column describes each available frequency band, represented by a diagram (not to scale). The frequency band limits are listed below the diagram; frequencies below 10 GHz are represented in MHz, while those above 10 GHz are in GHz. The width of each guard band is shown above the diagram, and is always specified in MHz.
The channel arrangements in some bands are staggered, so that the width and position of the guard band vary for different channel spacings. In these cases, a table underneath gives details of the guard bands for different spacings (with all frequencies in MHz).
The first column also includes the title of the relevant international recommendations for each band, produced by the European Conference of Postal and Telecommunications (CEPT) or the International Telecommunication Union (ITU). CEPT recommendations are available at http://www.cept.org/ecc/ and ITU Recommendations at http://www.itu.int.
The final column contains the channel spacing for duplex operation in each frequency band except for bands above 60 GHz. Details of standard systems assigned in the UK are shown in the relevant technical frequency assignment criteria.

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OFCOM Channel Plans 450MHz and 1.4GHz

OFCOM Channel Plans for 450MHz and 1.4GHz

Here is a chart showing channel plans for the UK

OFCOM 450MHz & 1.4GHz
OFCOM 450MHz & 1.4GHz

Uses & Applications

450MHz and 1.4GHz bands are used for Point to Point (P2P) Microwave Radio Links

Sources of Data and Graphics

All contents (C) OFCOM and taken from:

OfW48 UK Frequency Allocations for Fixed (Point-to-Point) Wireless Services and Scanning Telemetry This document shows the current bands managed by Ofcom that are available for fixed terrestrial (point to point) links and scanning telemetry in the UK.

Technical regulations

The Radio Equipment and Telecommunications Terminal Equipment Directive
99/5/EC (R&TTED) has been implemented in ‘The Radio Equipment and Telecommunications Terminal Equipment Regulations 2000, Statutory
Instrument (SI) 730. In accordance with Articles 4.1 and 7.2 of the R&TTED
the:
• IR2000: The UK Interface Requirement 2000 contains the requirements for the licensing and use of fixed (point-to-point) wireless services in the UK.
• IR2037: The UK Interface Requirement 2037 applies for scanning telemetry services.
• IR2078: The UK Interface Requirement 2078 applies for the 60 GHz band

Notes specific to the frequency charts

The first column describes each available frequency band, represented by a diagram (not to scale). The frequency band limits are listed below the diagram; frequencies below 10 GHz are represented in MHz, while those above 10 GHz are in GHz. The width of each guard band is shown above the diagram, and is always specified in MHz.
The channel arrangements in some bands are staggered, so that the width and position of the guard band vary for different channel spacings. In these cases, a table underneath gives details of the guard bands for different spacings (with all frequencies in MHz).
The first column also includes the title of the relevant international recommendations for each band, produced by the European Conference of Postal and Telecommunications (CEPT) or the International Telecommunication Union (ITU). CEPT recommendations are available at http://www.cept.org/ecc/ and ITU Recommendations at http://www.itu.int.
The final column contains the channel spacing for duplex operation in each frequency band except for bands above 60 GHz. Details of standard systems assigned in the UK are shown in the relevant technical frequency assignment criteria.

For Further Information

For More Information on Microwave Planning, Please Contact Us

Fresnel Zone – Microwave Planning

In radio communications, a Fresnel zone (/freɪˈnɛl/ fray-nel), , is one of a (theoretically infinite) number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture. The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first.    The Fresnel Zone is named after the physicist Augustin-Jean Fresnel.

Importance of Fresnel zones

CableFree Microwave and Radio Fresnel Zone
Microwave and Radio Fresnel Zone

If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are reflective surfaces along the path, such as bodies of water or smooth terrain, the radio waves reflecting off those surfaces may arrive either out of phase or in phase with the signals that travel directly to the receiver. Waves that reflect off of surfaces within an even Fresnel zone are out of phase with the direct-path wave and reduce the power of the received signal. Waves that reflect off of surfaces within an odd Fresnel zone are in phase with the direct-path wave and can enhance the power of the received signal. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the signal-to-noise ratio.

Fresnel provided a means to calculate where the zones are–where a given obstacle will cause mostly in phase or mostly out of phase reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone they will be 180 to 360 degrees out of phase, and so on. Even numbered zones have the maximum phase cancelling effect and odd numbered zones may actually add to the signal power.

To maximize receiver strength, one needs to minimize the effect of obstruction loss by removing obstacles from the radio frequency line of sight (RF LOS). The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone.

Determining Fresnel zone clearance

Microwave and Radio Fresnel Zone
Microwave and Radio Fresnel Zone

The concept of Fresnel zone clearance may be used to analyse interference by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated. As a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.

For establishing Fresnel zones, first determine the RF Line of Sight (RF LOS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF Line of Sight is said to be the Fresnel zone.