How does TAP™ calculate composite fade margin in microwave link reliability predictions?

The TAP fixed facility database includes fields for digital fade margin values. These values can be used in addition to the computed thermal fade margin to compute a composite fade margin value.

The additional fade margins included are:

  • Dispersive Fade Margin (DFM) is the “contribution to outage that accounts for in-band distortion that can at times cause a digital system to fail when the AGC or flat fade is less than that required to reach the thermal noise threshold.” (“Digital Radio Path Fade Margin Calculations”, p.32, MDR-2000 Series Product Description, Rockwell International)
  • Adjacent Channel Interference Fade Margin (AIFM) is the contribution to system outage resulting “from the broad transmit spectra of digital systems that have sufficient energy that spills over into adjacent channel digital receivers.” (“Digital Radio Path Fade Margin Calculations”, p.32, MDR-2000 Series Product Description, Rockwell International)
  • External Interference Fade Margin (EIFM) is the “contribution to system outage from intersystem (foreign route) cochannel interference. Such interference can come from transmitters on the desired route (overreach, for example) or from transmitters from other routes in the vicinity.” (“Digital Radio Path Fade Margin Calculations”, p.32, MDR-2000 Series Product Description, Rockwell International).

EIFM can also be computed by pressing the Calculate button next to the EIFM field. EIFM is computed using the following equation:

EIFM = RSL - T +(T/I) - (C/I)

where:

  • RSL is the unfaded received signal level in dBm. Since the RSL is not known until later in the program, the computed EIFM value will not be displayed on the Receive Site screen.
  • T is the receiver threshold for 10-6 BER in dBm.
  • (T/I) is the threshold to interference ratio in dB.
  • (C/I) is the carrier to interference ratio in dB.

The fade margin values are entered in the fixed facility lookup or fixed facility database editor. If you want to exclude any of the fade margin values from the composite fade margin calculation, enter a value of zero (0) or 99.9 for that fade margin value.

The composite fade margin is computed using the equation:

CFM = -10 LOG (10-DFM/10 + 10-TFM/10 + 10-AIFM/10 + 10-EIFM/10)

where:

  • CFM is the Composite Fade Margin.
  • DFM is the Dispersive Fade Margin.
  • TFM is the computed Thermal Fade Margin.
  • AIFM is the Adjacent Channel Interference Fade Margin.
  • EIFM is the External Interference Fade Margin.