The performance calculation is always performed for both directions of transmission. This is considered in the performance report.


Multipath occurrence factor
 The following parameters are considered:

• r.f. (centre frequency) in GHz
• the beam path length in km
• the point refractivity gradient in the lowest 65 m of the atmosphere not exceeded for 1 % of an average year. The figure can be obtained on a 1.5 degree grid resolution in latitude and longitude from a database available from ITU-R. This database is incorporated in RLTool
• the area terrain roughness, defined as the standard deviation in m of the terrain heights (in m) within a 110 km x 110 km area with a 30” resolution. Terrain data are available from Internet, eg the Globe gtopo30 data.
• the antenna altitudes, ie the antenna heights above seal level (asl) in m
• the hop inclination in milliradians, see the next section.

The parameters have to be set individually for each hop.


Hop inclination
 The hop inclination is the angle between the radio beam and the horizontal. It describes the ability for the radio beam to pass through air layers without loosing energy by reflection at the boundaries between air layers. The higher the inclination, the less is the loss.
No input data are necessary, all necessary data for the calculation are already known.


Point refractivity gradient and area terrain roughness
To obtain these parameters from the corresponding database, the geographical position of the hop concerned is necessary. If the network layout is based upon co-ordinates, RLTool calculates the hop’s geographical position. If no co-ordinates are given, an approximate position of the network has to be given by the operator. In this case, the refractivity gradient and the area terrain roughness for this position will be valid for all hops.


Performance prediction considering multipath fading and related mechanisms
 The following parameters are considered:   

• multipath occurrence factor as calculated above
• fading margin in dB according to section 2.6.2.2 below.

These parameters have to be known for each hop, RLTool will calculate them from other input data.


Fading margin
This parameter is based on the receiver's threshold level for the bit-error ratio (BER) concerned, ie based on an equipment parameter. The fading margin has to be considered for two different bit-error ratios:

• applying ITU Recommendation G.821 : BER = 10-3 and10-6
• applying ITU Recommendation G.826:  BER = BERSES and RBER

RLTool feature:

• The typical receiver threshold level in dBm can be obtained from the equipment database or given by the operator.

Performance prediction considering distortions caused by selective multipath fading and related mechanisms
The following parameters are considered:

• multipath occurrence factor (from above)
• mean value of the width of the signature in MHz
• mean value of the signature (or notch) depth in dB
• reference delay in ns used when the signature has been obtained
• hop length in km

Signature and reference delay figures
These figures are equipment parameters. They have to be selected for the BER concerned.

RLTool feature:

• The typical signature data and the co-ordinated reference delay can be obtained from the equipment database or given by the operator.

Unavailability caused by multipath fading and distortions
The practice to consider the exceeding of BERSES or BER = 10-3 due to the above mechanism as performance degradation is based on the assumption that the fading activities responsible for that excess lasts shorter than 10 consecutive seconds. However, measurements have shown, that this is not always true. During a certain percentage of time, Y, the exceeding of BERSES or BER = 10-3 lasts longer than 10 consecutive seconds and should, consequently, be treated as unavailability. However, ITU has so far not published any figures for the parameter Y. For the moment Y should be set to 0 %.

RLTool feature:

• RLTool offers the facility to select a value for Y other than 0 %. The program considers this percentage of the SES as unavailability and combines it with the unavailability figures due to rain and hardware faults to a total unavailability figure.
• The default value for Y is 0 %.

Small-time-percentage prediction considering attenuation caused by precipitation
The following parameters are considered:

• radio frequency in GHz and its polarisation
• rain attenuation coefficient in dB/km
• hop length in km
• the hop length in km influenced by rain - the effective path length
• clock-minute average annual rainfall rate (or rainfall intensity) in mm/h exceeded for 0.01 % of the time
• fading margin

The radio frequency is again the midband frequency. The hop length is the geodetic one. Rain attenuation will only be considered for r.f. > 7.1 GHz.


Clock-minute average annual rainfall intensity
This parameter can be obtained from meteorological data and inserted by the operator as input data. It can be set to be valid for the entire project or individually for each hop.

However, the rainfall intensity can also be calculated from WMO data with a 1.5o grid resolution in latitude and longitude. The necessary databases are available from ITU-R. They are incorporated in RLTool. Again, the knowledge of the co-ordinates is the precondition for this calculation.


Rain attenuation coefficient
The program calculates this parameter. For that, two parameters, which are measured figures, have to be obtained from a table. The figures are frequency and polarisation dependent.

RLTool feature:

• The parameters necessary to determine the attenuation coefficient are included in RLTool as an internal database. RLTool picks up the correct figures with respect to r.f. and polarisation.

Effective hop length
This parameter considers that the dimension of a rain cell may be shorter than the length of a radio hop, and that a rain cell can cross a radio path in different directions. It considers also, that a the rain cell dimension decreases with increasing rainfall intensity. However, a limit of 100 mm/h has been set for this consideration.

RLTool feature:

• If the rainfall rate during 0.01 % of the average year exceeds 100 mm/h, RLTool limits it to 100 mm/h  (but only for the calculation of the effective hop length).

Calculation of the small-time percentage
For very low rainfall rates combined with large fading margins, the formula used cannot be solved. This is the case, if the relationship between rain attenuation and fading margin will be < 0.154024. But then, the risk for rain fading will be less than 8x10-7 %, ie negligible.

RLTool feature:

• If the relationship between rain attenuation and fading margin is < 0.154024, the program sets the small-time percentage for rain fading to 0.000001 % or 10-8.

Performance and unavailability
The excess of the receiver threshold level for the BERSES or BER = 10-3 respectively due to rain is considered as unavailability. The calculation acc. to § 2.6.5.5 above results in an unavailability figure. If, however, the calculation is carried out for a threshold level < BERSES  (< 10-3 resp), the excess percentage has no longer to be treated as unavailability, but as performance. As the performance is referred to the average worst month, the result from the calculation has to be transferred from an average year to a worst month value.

RLTool feature:

• RLTool takes care of the above transformation.

The practice to consider the exceeding of BERSES or BER = 10-3 as unavailability is based on the assumption that the rain intensity responsible for that excess lasts at least 10 consecutive seconds. However, measurements have shown, that this is not always true. Only during a certain percentage of time, X, the exceeding of BERSES or BER = 10-3 lasts at least 10 consecutive seconds. The remaining percentage should, consequently, be treated as performance degradation. However, ITU has so far not published any figures for that parameter X. For the moment X should be set to 100 %.

RLTool feature:

• RLTool offers the facility to select a value for X other than 100 %. The program considers the (100 – X) percentage of the SES as performance degradation and combines it with the performance degradations due to multipath fading and distortion. The default value for X is 100 %.

Calculation with passive repeaters
Passive repeaters divide the radio path into two legs, each crossing its type of terrain, normally with different lengths and different inclinations. This, of course, gives different propagation conditions for these two legs, both concerning fading, distortions and rain.

RLTool feature:

• RLTool considers the different propagation conditions on paths with passive repeaters.

Improvement of the performance by frequency diversity reception
The prediction method used has the following limitation:

• The maximum separation considered between the two r.f. is 0.5 GHz. If the separation is larger, no further improvement is achieved.

This limitations is not included in the formulae, it has to be observed by the operator or the program.

RLTool feature:

• RLTool tests the above condition and limits the frequency separation to 0.5 GHz.

Improvement of the performance by space diversity reception
RLTool performs this calculation, provided that the relevant control commands are activated.


Improvement by combined frequency and space diversity
Two diversity configurations are possible:

• combined diversity with 2 Rx
• combined diversity with 4 Rx

RLTool feature:

• RLTool supports both configurations, incl the limitation for frequency diversity.