I always get confused in the terminology of modulation level (M) and number of bits per symbol (N). Please correct me if I am wrong. Because with these values, my question extends to polarization multiplexed versions of the same.

For BPSK, N=1 and M=2

For QPSK, N=2 and M=4

For 8QAM, N=3 and M=8

For 64QAM, N=6 and M=64

Then, how the N and M values change if they all are polarization multiplexed? I.e. PM-BPSK,PM-QPSK,PM-8QAM,PM-64QAM? I think we can send more bits in PM format now but for will the N,M values change? I need to substitute these in some formula for my work and so I need clarity on this.


1 Answer 1


2^N = M


In an 8QAM constellations you got N = 3 and M = 8.

N = bits per symbol

M = symbols

One of the possible 8QAM constellations

Given a modulation scheme, there are many different constellations.

They all differ from the mutual distance or angles.

Once the media is assigned (fiber, copper, air) you can design the best constallation for that media that minimizes SNR.

  • \$\begingroup\$ So you mean, there will be no difference in N,M values (say) for QPSK and PM-QPSK? \$\endgroup\$ Nov 22, 2021 at 16:44
  • \$\begingroup\$ The two polarization modes will be essentially independent. It is the same as having two separate links. \$\endgroup\$ Nov 22, 2021 at 17:32

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