Link budgets
The fundamental basis for assessing whether a satellite link will work is the calculation of the link budget. This determines how much signal transmitted from the uplink will arrive at the downlink, and hopefully with enough in hand to give some degree of margin for physical factors that can vary or cannot be calculated in a theoretical link budget.
What are we trying to measure with the calculation of a link budget?
Fundamentally, whether enough signal transmitted from the uplink can be received at the downlink to convey the information carried accurately. This encompasses the uplink and the downlink equipment, the uplink and the downlink paths, as well as the effects of the reception and transmission through the satellite.
The link budget is similar to a financial budget, with contributions (income) and deductions (expenditure) that define the overall performance, where the currency is the decibel (dB). As with a financial budget calculation, depending on the level of detail, the link budget can be a relatively simple one, involving perhaps only a page of calculations, or it can be extremely detailed, and spread over up to ten pages.
Digital link budgets are complex, due to the potential effects of interference from both adjacent signals and natural sources of interference.
The significant results of the link budget will be ratios that are a measure of the predicted quality of the link.
First there is the Eb/No (pronounced ‘ebbno’) – which defines how much energy has been received in each bit compared to the background noise. It describes how good the link is.
A ‘good’ link will have an Eb/No of at least 7 or 8, and hopefully as high as 12 or 13. On the other hand, an Eb/No of 5 or 6 is marginal, and the link is likely to fail.
Secondly, there is the bit error rate (BER – pronounced ‘berr’), which is the ratio of bits received in error compared to the total number of bits sent.
Again, it is an indicator of how good the link is, so a BER of 10−6 is good (i.e. there is only one error in every million bits), whereas a BER of 10−3 is very marginal (where there is an error in every 1000 bits).
Both the Eb/No and the BER are interrelated, so if the Eb/No is too low (and hence the BER is too high) the decoder at the downlink will suddenly stop working, giving a frozen or black picture.
The calculation of a link budget enables us to predict to a given degree how good (or bad) the link will actually perform. In practice, the results will be disappointing, if:
• the uplink does not produce enough power
• the satellite does not have a sensitive enough receive characteristic
• the satellite does not have a powerful enough transmit characteristic
• the downlink antenna is not large enough (and therefore does not have enough gain)
• there is more noise and interference than expected.
With a digital link, anything apart from total success results in just nothing at all. Digital signals suffer from what is known as the ‘cliff-edge’ effect – that is, either perfect results, or no pictures and sound. What prevents the signal being clearly received?
Assuming that the uplink and the downlink equipment are working correctly, then it is predominantly the factors of interference, noise and losses, such as the scattering of the signal as it travels through space and the atmosphere on both the uplink and the downlink paths.
Calculating link budgets manually is extremely time consuming, and it is common for uplink operators to use software packages such as Arrowe Satmaster Pro, which calculates not only the link budget but also produces pointing data (azimuth, elevation, etc.).
Fade margin
As we saw in our discussion in terrestrial microwave links, the difference between the operating level of the link and the point at which it fails because the power level reaching the receiver is too low is called the fade margin. This is usually calculated as part of the link budget. The fade margin is expressed in dB.