real-valued signal constellation has hidden imaginary part

Hi guys!

I found something which isn't so nice in Matlab. When you create a demodulator object with

h = modem.pammod('M',2,'InputType', 'Bit','SymbolOrder','Gray');

the signal constellation should be real-valued. If you look at h.constellation you see no imaginary part, but isreal(h.constellation) gives back a zero, which means the constellation points do have a imaginary part. For me it was a little bit annoying, because I cannot use mxIsComplex to determine whether a signal constellation is complex or real-valued. Do you have any clever suggestions for a workaround.

best regards, Bernhard

Answer 1

[Bernhard Schmidt]

Wow, thanks, Walter! Why does this work?

[Walter Roberson]

When Matlab does a plain assignment, it just creates a new variable header pointing to the old information. But when you do any arithmetic operation on a complex array, Matlab checks for the case of all-zero complex part and drops the complex part in that case. Adding 0 preserves any complex part that is really there, but triggers the check on the complex part.

Interestingly, double(h.constellation) does *not* trigger the check for all-zero complex part.

[Bernhard Schmidt]

Thanks again

Answer 2

[Walter Roberson]

isreal(h.constellation + 0)

I don't know a fast way to do that at the mex level though.