In CCIR Recommendation 393 it is recommended: that the noise power at a point of zero relative level in any telephone channel on a 2500-km hypothetical reference circuit for frequency-division multiplex radio-relay systems should not exceed the values given below, which have been chosen to take account of fading.
ITU-T G.441 SPANISH-1993 history
1993ITU-T G.441 SPANISH-1993 PERMISSIBLE CIRCUIT NOISE ON FREQUENCY-DIVISION MULTIPLEX RADIO-RELAY SYSTEMS