-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.