What is multilevel constellations

[Shruti01]

Hello all,

Can anybody explain me the meaning of multilevel constellations? For BPSK, in the constellation diagram, we have 2 symbols i.e. 1 symbol per bit is transmitted. For QPSK, we have 2 symbols per bit. But I am not able to understand what is multilevel constellations.

Regards,
Shruti

 

[WimRFP]

I assume that they mean that you can also modulate the amplitude together with the phase resulting in more symbols with a larger distance.

with QPSK or higher level PSK, all symbols are on a circle (and close together when you want large bitrate/symbol rate). By also using the amplitude you can get constellations everywhere within the maximum amplitude/power. This is mostly done with I & Q mixers that upconvert the I and Q signals. They call it QAM (Quadrature Amplitude Modulation).

You can have (for example) 16 constellations (4*4 square) resulting in 4 bits/symbol.