MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
17
Where : d = Path length in Km.
In our Example: % T = 0.1 (40) / 2500
or : % T = 0.0016 %
This is a practically negligible time of fading ocurrence, since this type of link is designed for a 24-hour daily
operation, or 8760 hours per year. Assuming this time as 100%, the 0.0016% will be 8760 x 0.0016 / 100 = 0.14
hours in a year, or 8.4 minutes of expected fade time.
d) Noise Considerations. Reference Noise and System Signal-to Noise Ratio:
Reference noise is defined as "the total receiver noise level in absence of signal" and it's obtained adding all
random noise components involved in the appearing of noise at receiver's output terminals.
There are some components in the total noise at receiver's output. The theory of each component can be easily
found in any Text Book and is not of the scope of this Introduction. It's enough to know that, for a standard video &
audio microwave link as shown in our illustrative example, the noise components, and its numerical values, are the
following:
a) Noise Figure .............................................................................................. 5.0 dB
b) Boltzmann's Noise (kT
o
) ....................................................................... -204.0 dBW
c) 10 log BW = 60 + 10 log (4.2) .................................................................. 66.23 dB.
d) Modulation Gain ...................................................................................... - 7.09 dB
e) Wei Pre-emphasis/de-emphasis ............................................... -13.80 dB
_________________________________________________________________________
Reference Noise (N
o
) -153.66 dBW.
That is : -123.66 dBm.
Received signal, as seen before, is at a -40 dBm level in order to keep the minimum Fade Margin of 40 dB. Taking
into account the above calculated Reference Noise, we can find now the Signal-to noise Ratio:
S / N [dB] = - (N
o
- S
rec
.) = - ( -123.66 - ( -40) ) = 83.66 dBm.
If a deep 40 dB fading condition appears, and signal drops to threshold level, Signal-to-Noise Ratio will be kept in
the following value:
S / N
thr
[dB] = - ( -123.66 - (-80) ) = 43.66 dB
e) Link's Propagation Reliability:
For a certain analogic Radio Link, Propagation Reliability is given by the expression:
%R = 100 ( 1 - 10
-FM/10
)
