Length and Reliability of a link combined with required Capacity are the three main considerations when designing a link. Changing the value of one parameter has an impact on the possible values of the other two.
The combined values of Distance, Availability and Capacity as shown in the figure, define the minimum system gain of the chosen equipment, calculated as: 2x antenna gain + available Tx power – Rx sensitivity (actual values for the chosen channel, channel width, modulation and other parameters of the wireless connection).
Reduction in Link Availability can be offset using ACM (Automatic Control of Modulation) available in modern microwave units. This automatically decreases link Capacity (reducing modulation) during periods of lower signal. Use of ACM is an ideal solution for last-mile links (optimum is higher standard capacity + few moments of lower capacity) but not for backbone links (decreased capacity may cause issues for the whole network).