-We just studied the analog link budget and we now know the impact the antenna size, the transmission power or the quality of electronic components can have. Let us now detail the digital part of the link budget. Remember that this digital part of the link budget allows user requirements in terms of service to be characterized. Remember weeks 3 and 4, too. We had established a link between the energy dedicated to the transmission of one bit and the bit error rate found on this link. This is what we call Eb/N0. N0, here, is nothing else than the noise spectral density. Eb is of course the energy dedicated to transmit a bit. Here we represented three curves. The blue curve represents, for the QPSK and BPSK modulations, the ratio between Eb/N0 and the bit error rate. The green curve is for the 8-PSK modulation and the red curve for the 16-PSK modulation. We consider the 8-PSK modulation, the green curve. We set Eb/N0 to 8 dB. We can expect a 10 to the power of -2 bit error rate, that is to say one erroneous bit every 100 bits. Let us assume that this performance is not satisfying. What can we do? We can increase the energy dedicated to the transmission of one bit or decrease the noise power spectral density. Let us assume that none of these options are satisfying. There is a third option, changing from a 8-PSK modulation to a QPSK modulation. Indeed, on the blue curve, for an equivalent Eb/N0 we have a bit error rate which is twice as good, 10 to the power of -4. You will tell me: "This is magical, why not directly use the QPSK?" No, it is not magical. There is no magic in engineering. There will be a price to pay that is not represented on the curve, that of the bandwidth. Indeed, by using the QPSK, we will use more bandwidth for an identical transmitted information quantity compared to 8-PSK. On one side we have a modulation, on the other a bit error rate. We can deduce Eb/N0 from that. To end the characterization of my service, I need to know the bit error rate, how many bits per second. This is represented by R here. The characterization of my service is over. But how can we establish a link with the analog link budget? The analog link budget was C/N0. It represented the available resource quantity for my service. My service is to transmit bits R times, that is to say the bit rate. So I will compare C/N0 and the product of Eb/N0 and the bit rate. Or, if I use dBs, the sum of Eb/N0 expressed in dB and the bit rate also expressed in dB. I will compare these two values. It will allow me to know if I managed to close my link budget, that is to say if I have enough resources for my service or not. Let us consider the first case. C/N0 is lower than the sum of Eb/N0 and the bit rate expressed in dB. In that case, I did not manage to close my link budget. What can I do? Typically, I will have to increase C. Increasing C means using bigger antennas with a higher gain. To increase C, I can also increase the transmission power in the satellite. It is quite tricky. It must be done during the design or at the level of the terminal. This is easier. Conversely, I can also modify the Eb/N0 part. I can change modulations and choose one with a lower Eb/N0 but that will also be less efficient spectrally-speaking. I can also decide that a higher bit error rate is acceptable. You see that I have several strategies when my link budget is not closed. Let us now consider some good news, my link budget is closed and higher as illustrated here. C/N0 is greater than the sum of Eb/N0 and the bit rate. What does it mean? First, I have some margin and this is a good practice. In a link budget we always try to have some margin. This margin allows us to have a system that can keep on operating even, for example, in the case of strong rains which create much more attenuation than expected. On the other hand, if this margin is too important, over 10-15 dB, I have a potential issue. Why so? Because it means that I have too many resources for my given service. We know that using too many resources in satellite communications is never a good practice. These resources, for example the power onboard the satellite or the spectrum, are extremely rare. In that case, I will review my link budget and, for example, decrease the power onboard the satellite, onboard the terminal or maybe use a more efficient modulation spectrally-speaking. This is what happens when I have a margin. We always need to keep some margin but not too much otherwise we waste resources, and this is not a good practice.
