but we can mitigate their effects. The primary tool we have is to overbuild each link by
increasing the margin.
Link margin is the number of dB we have as surplus in the link design. We could
design and LOS microwave link so we just achieve the RSL at the distant receiver to
(and BER) requirements using free-space loss as the only factor in
link attenuation (besides transmission line loss). Unfortunately we will only meet our
specified requirements about 50% of the time. So we must add margin to compensate for
We have to determine what percentage of the time the link meets BER performance
requirements. We call this time availability.
If a link meets its performance requirements
99% of the time, then it does not meet performance requirements 1% of the time. We call
this latter factor unavailability.
To improve time availability, we must increase the link margin, often called the fade
margin. How many additional dB are necessary? There are several approaches to the
calculation of a required fade margin. One of the simplest and most straightforward
approaches is to assume that the fading follows a Rayleigh distribution, often considered
worst-case fading. If we base our premise on a Rayleigh distribution, then the following
fade margins can be used:
Time Availability (%)
Required Fade Margin (dB)
More often than not, LOS microwave systems consist of multiple hops. Here our
primary interest is the time availability at the far-end receiver in the system after the
signal has progressed across all of the hops. From this time availability value we will
want to assign an availability value for each hop or link.
Suppose a system has nine hops and the system time availability specified is 99.95%,
and we want to calculate the time availability per hop or link. The first step is to calculate
the system time unavailability. This is simply 1
.0000 - 0.9995 = 0.0005. We now divide
this value by 9 (i.e., there are nine hops or links):
.0005/9 = 0.0000555.
Now we convert this value to time availability:
Per-hop time availability
= 1.0000000 - 0.0000555
= 0.9999445 or 99.99445%.
We recommend that for digital links, an additional 2 dB of fade margin be added to
the Rayleigh values to compensate for path anomalies which could not be handled by
The most common cause of fading is called multipath. Refer to Figure 9.14. As the
term implies, signal energy follows multiple paths from the transmit antenna to the receive
Other texts call this "reliability." The use of this term should be deprecated because it is ambiguous and
confusing. In our opinion, reliability should relate to equipment failure rate, not propagation performance.