Theoretical model of path loss

[nafah]

hi all
I am working now in Path Loss in WiMax link
most papers which related with Path loss calculations
present the empirical path loss models

Is there any theoretical model to compute the path loss for fixed WiMax system ?

thanks a lot

 

[WimRFP]

Hello,

This depends on your situation.

If you have line of sight (that means first Fresnel zone sufficiently clear), there are no strong reflections from objects out of the first Fresnel zone, you can calculate the path loss using the Friis formula.

If you have one dominant reflection (for example via earth), you can calculate the direct en reflected fieldstrength and add them (phasors) manually to find the path loss. When the number of reflected, diffracted and refracted waves becomes large, it is no longer possible to do this by hand. If you have 3D terrain and building data (inclusieve properties), a computer can solve your problem via ray tracing or GTD. One person or a van in the propagation path will change the path loss!

In most cases, you don't have a building database and 3D terrain data with sufficient resolution, so you can't solve your problem by number crunching. In such cases empirical models are used. You may know that due to multipath effects, the fieldstrength may vary signifficantly over less then 1 lambda. This means you need to add sufficient margin. A diversity scheme or very wide band scheme may require less margin.