-While calculating the uplink link budget, if we only considered the received power we would miss a very important factor. In order to explain this, we will consider a specific situation. You are cooking a perfect dinner. You are listening to music and set the volume of the radio to a certain power so it is comfortable to hear. At some point, you need to start the cooker hood. The noise of the latter will prevent you from hearing the radio show. This is the important element. The noise created by the cooker hood is called noise because it interferes with the signal you are interested in, the radio show. It is exactly the same for satellite communications. They are impacted by ambient noise. But the satellite communication noise is not audible. It simply characterizes an uncoordinated agitation of the different particles that allow communications to take place, that is to say to transmit a signal. I have very bad news. Unfortunately, noise is everywhere. To take into account the impact of noise in the link budget, we will define a reference point in our system. We will measure, at this reference point, the noise amount called T. This noise amount is expressed in kelvins, represented by the unit K. We talk about noise temperature. At 0 K, there is no noise at all. Let us get back to our T measuring point. It is fictional, but it allows all noise contributions to be grouped in a single value. What are these contributions to ambient noise? There are several. They are listed here. Some are natural, such as the noise coming from the Earth, others are caused by imperfections in the electronic circuits used. To tell the truth, we are not really interested in the noise temperature but rather in its power spectral density. How many noise watts per spectrum hertz? How can we go to noise power spectral density from noise temperature? It is very simple. We multiply it by the Boltzmann constant, k. When using the dB scale, we do not multiply by this constant but we add it. Here it is expressed in dBW/Hz/K. Eventually, it will change in dBW/Hz, expressing a power spectral density. On the one hand we have a received power expressed in dBW, represented by PRX, and on the other hand, a noise density expressed in dBW/Hz. With these two values, we can calculate the relation between signal and noise. This relation is really interesting. In the example of the kitchen and the radio we were listening to, we easily understand that if the volume of the radio is higher than the noise of the cooker hood, we can easily listen to the show. This is what we call the C/N0 value, expressed in dB/Hz. It will divide, thus subtract in dB, the received power and the noise spectral density. It can be called a signal-to-noise ratio. It is not exactly true because we do not compare two values with the same unit. But this semantic approximation is acceptable. This C/N0 value represents our link budget for the uplink. We have now gone through this first step.