A link budget is one of the main guiding factors in designing any communications system, and has three main components.
Once a system’s requirements have been set out, the link budget can be used to define parameters such as the output power of the CubeSat transceiver, and the antenna gain required on the ground station and CubeSat.
Often the first budget to be developed, a downlink budget gives an indication of the received signal strength at the ground station. To produce it involves listing all the gains and losses in a system, including path loss (the loss due to the signal travelling from space to Earth, antenna gains, atmospheric losses, etc.
An uplink budget is often nearly identical to the downlink one, but differs in that it calculates the received signal strength at the CubeSat instead of the ground station. This means that the transmit power figure comes from the ground station hardware. As a result, the received power tends to be a lot higher than the figure in the downlink budget. The downlink parameters are therefore often the driving factor, deeming the uplink budget less important.
A throughput budget defines the amount of data that the satellite can transfer to the ground station and vice versa (there is both an uplink and downlink throughput). This is calculated based on the amount of time the satellite spends per day over the ground station, the bitrate, and the bit error rate (which comes from uplink/downlink budgets).