Abel,
I most cases multipath fading is caused by changes in the atmosphere along your path which can be temperature/humidity related, these changes or layers of different refractive index causes ducts to form which then channels your signal away from its intended path. The way to overcome this is to use freq difersity but will not help in extreme cases. Space difersity will also improve the situation. There is also a combination of space and freq diversity. You can aslo plan shorther links in areas that are heavy affected such as across swamps and bordedring lake or coastel regions. Ultimately in swampy or delta tropical regions you must actually place the divesity antennae on top (above the expected layer formation and the main antennae at the bottom below the expected layer (duct). If you are experiencing the fading seasonaly then these layers typically only accur during certain seasons and not duing your typical day/night transitions. Towers as high as 120 meters over a 30Km link is not uncommon with one antennae at 60 meters and the other at 120 meters.
I also found that hops across beach areas should be avoided, they are aleays affeceted by the tide changes along the path.
Thus first determine if the fading occurs every day or only during certain seasons. It will save you a lot of money.