Link Budget

Link Budget

A link budget is accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline and miscellaneous losses. Randomly varying channel gains such as fading are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping. A simple link budget equation looks like this:

Received Power (dB) = Transmitted Power (dB) + Gains (dB) − Losses (dB)

For the FloripaSat project, we have to consider the uplink and downlink budget. The figure below is divided by two columns. The left side is the uplink (telecommand) system and the right side is the downlink (telemetry) system. There are several parameters such as F.E.C encoder/decoder, data demodulator, bit rate, temperature, gain on amplifiers and antennas, frequency and losses on filters, cables and radio links.

The most important parameter is the link margin on Eb/No and S/N methods. They show if the link closes or it does not. The equation below shows the link margin equation:

Link Margin < 0 , “No link”

0 < Link Margin < 6, “Marginal link”

Link Margin > 6, “link closes”

For example, the F.E.C encoder/decoder is the most import parameter to cause a “link closes”. The F.EC. increases the S/N method and decreases the Eb/No. The bit rate parameter is a highly influential parameter, where's the higher the bitrate, lower is the S/N method. For a good link-budget compromise, a biterate of 2400 is highly appropriate.

Note that was calculated only to UHF band. For VHF band, the parameters are similar.