How to Calculate Satellite Bandwidth

Satellites have a limited amount of bandwidth and they must use this space efficiently. The amount of bandwidth available on a satellite is easily calculated by adding together the bandwidth of each amplifier (transponder). Satellites typically have several carriers. Calculating the bandwidth for each individual carrier on the satellite is much more complicated and varies greatly from carrier to carrier. This is one of several calculations needed to effectively plan a satellite's carriers

1. Calculate satellite carrier bandwidth using the basic formula for carrier bandwidth: SR = DR / (m x CRv x CRrs), where SR = symbol rate, DR = data rate, m = modulation factor, CRv = Viterbi FEC rate and CRrs = Reed Solomon FEC rate.

2. Replace "DR" in the equation with the carrier's data rate in Mbp/s (megabits per second). The data rate should be available on the carrier parameter sheet and represents the amount of data being passed on the carrier. The data rate is also known as the information rate.

3. Substitute "m" in the equation with the carrier's modulation factor. The modulation factor is the transmit scheme used to lessen the bandwidth while still passing the necessary data. There are several different modulation schemes, including BPSK, QPSK, 8PSK, 8QAM and 16QAM. For the equation listed above, the following modulation factors can be used: BPSK = 1, QPSK = 2, 8PSK = 3, 8QAM = 3 and 16QAM = 4. The modulation scheme being used should be available on the carrier parameter sheet.

4. Swap the carrier's Viterbi FEC for the "CRv" in the equation. The Viterbi Forward Error Correction (FEC) rate is adjustable and adds bits to the data stream to allow for error correction. Typical Viterbi FEC settings are 1/2, 2/3, 3/4 or 7/8. The first number is the number of information bits, while the second number is the number of information bits plus added error correction bits. This setting also should be listed on the carrier parameter sheet.

5. Interchange 188/204 for "CRrs" in the equation if the carrier is using Reed Solomon FEC. It has the same purpose as Viterbi FEC. However, it cannot be changed, it is simply on or off. This information is also listed on the carrier parameter sheet.