How are signals and noise reflected in the constellation diagram? What is the relationship between signal-to-noise ratio and EVM?

Assume that the local oscillator in the receiver chain has an ideal frequency response, as shown in (a), and is lower than the phase noise shown in (d). Now this receiver associates the received signal (wanted signal) with the interference signal (unwanted signal), as shown in (b) and (d). The signal and interference pass through the down converter, and the problem begins here. When the local signal has no phase noise, most of the interference after the down converter (for example, using a filter) can be eliminated because they are separated from each other, as shown in (c). However, when your local oscillator has phase noise and (signal+interference) is reduced by a noisy local oscillator, the result will be as shown in (f).

Because the frequency responses of the signal and the interference signal propagate through the phase noise of the local oscillator, the frequency response of the coherent signal and the frequency response of the signal overlap. In this case, it is almost impossible to completely remove the interference of analog stage (such as RF or IF stage). This is why the phase noise characteristics of local oscillators are so important.

Very clean weak power signals enter the receiver, while strong interference signals enter the receiver. In this case, even if the signal (the signal we need) is clean, it may be buried under the phase noise of the nearby signal and cannot be decoded correctly.

In a word, the phase noise is jitter in time sequence (as seen by oscilloscope) and wide skirt in frequency domain, which is the noise we don't want to see. The appearance of phase noise in the transmitting local oscillator (TX LO) will seriously affect the EVM of the constellation, and the demodulation of the signal will be seriously affected in the receiving local oscillator (RX LO).