RADIO SYSTEMS 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 satisfy the Eb/No (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 the fading. We have to determine what percentage of the time the link meets BER performance requirements. 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) 99.99 99.999 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): 0.0005/9=0.0000555 Now we convert this valu