The concept of "worst month"




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Rec. ITU-R P.581-2

RECOMMENDATION ITU- R P.581-2*

THE CONCEPT OF “WORST MONTH”

(1982-1986-1990)



Rec. 581-2

The ITU Radiocommunication Assembly,



considering

a) that performance criteria for radiocommunication systems often refer to “any month” as the period of reference;

b) that for the design of such systems it is necessary to have statistics of propagation effects that are relevant to the period of reference of the performance criteria;

c) that consequently there is a need for an unambiguous definition for the period of reference,
recommends

1. that the fraction of time during which a preselected threshold is exceeded in the worst month of a year is referred to as “the annual worst-month time fraction of excess”;

2. that the statistic relevant for the performance criteria referring to “any month” is the long-term average of the annual worst-month time fraction of excess;

3. that the worst month of a year for a preselected threshold for any performance degrading mechanism, be that month in a period of twelve consecutive calendar months, during which the threshold is exceeded for the longest time. The worst month is not necessarily the same month for all threshold levels.

Note – Recommendation ITU-R P.841 presents a model for the conversion of the average annual time fraction of excess to the average annual worst-month time fraction of excess. Global values of the parameters of this model are given, as well as more detailed values for several regions of the world.


* Radiocommunication Study Group 3 made editorial amendments to this Recommendation in 2000 in accordance with Resolution ITU-R 44.


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