# How to identify type of modulation based on constellation diagram?

The constellation diagrams for a 4-ary modulation using PSK (qpsk),MSK ,QAM and OQPSK look alike. How to differentiate them?

What is the modulation for the constellation diagram in the figure?

In general, any modulation that makes sense to plot on a constellation diagram could be described as a kind of QAM. However, we generally use the term QAM only for modulations that use both degrees of freedom on the plot (amplitude and phase).

• If the modulation varies the phase but not the amplitude — all the constellation points lie on a circle about the origin — then we call it PSK (phase-shift keying).

• If the modulation varies the amplitude, but not the phase — all the constellation points lie on a ray from the origin — then we call it ASK (amplitude-shift keying).

So you can tell QAM from PSK because QAM displays multiple amplitudes as well as multiple phases, whereas PSK has exactly one amplitude. MSK and OQPSK are subtypes of PSK and you are right to think that you cannot tell them apart on a constellation diagram, but the question you show is not asking you to do so — only to distinguish between ASK, PSK, and QAM.

• ♦ :In the pic the points lie on same ray so,Is it ASK? – Aparna B Jun 19 '16 at 15:08
• @aparna Yes, exactly. – Kevin Reid AG6YO Jun 19 '16 at 17:19
• Kevin Reid AG6YO♦ : Thank you! – Aparna B Jun 20 '16 at 2:52

The distance of a point in the constellation diagram from the origin gives the amplitude while the angle each point makes with the horizontal axis gives the phase. PSK can't be the right option here because the amplitudes are different and we have the same phases with respect to the horizontal axis. However, in the case of PSK, we have the same amplitude and different phases(this depends on the order of modulation). For 8-PSK, we have a single amplitude and eight different phases. This means that the constellation points lie on a circle. This also eliminates option c.) This also eliminates QAM because QAM has multiple amplitudes as well as phases which is not the case here.

• Hello and welcome to ham.stackexchange.com! – rclocher3 Dec 14 '20 at 14:53
• So the answer is ASK? – Aparna B Dec 18 '20 at 0:23
• @AparnaBhuvanagiri ASK is the correct answer. This is because, for ASK, we have different amplitudes and use only in-phase carrier. – Kaswechiha Dec 18 '20 at 2:54
• Got it. The answer given in the options was wrong. – Aparna B Dec 18 '20 at 5:20