Engineering Tutorials

The major issues with microwave link engineering are rain fade, multi path and interference. An RF engineer begins a design by doing a link budget analysis. A given radio system has a system gain that depends on the design on the radio and the modulation used. The gains from the antenna at each end are added to this gain. Larger antennas provide higher gain. The free space loss of the radio signal as it travels over the air is then subtracted from the system: the longer the link the higher the loss. These calculations result in a “fade margin” for the link. Anything that affects the radio signal within this margin will be overcome by the radio; if the margin is exceeded, then the link could go down. The next step, then, is to analyze impediments that could potentially affect the radio signal . With good understanding of the potential affects on the signal, the RF engineer can design links with availability and performance equal to or better than a wire line link. A link budget is a rough calculation of all known elements of the link to determine if the signal will have the proper strength when it reaches the other end of the link. To make this calculation, the following information should be available:

The amount of extra RF power radiated to overcome this phenomenon is referred to as fade margin. The exact amount of fade margin required depends on the desired reliability of the link, but a good rule-of-thumb is 20dB to 30dB.

Fade Margin = SG + AG - LC – LP

Where
SG = System gain (depend on modem)
AG = Antenna gain
LC = Cable loss
LP = Path loss

These parameters are also responsible for gain loss.

System Gain = Radiated Power of radio Equipment - (Receiver Sensitivity)