Error in example comm/Decis​ionFeedbac​kEqualizeB​PSKModulat​edSignalEx​ample

In the example comm/DecisionFeedbackEqualizeBPSKModulatedSignalExample the following line:

eqdfe_lms = comm.DecisionFeedbackEqualizer('Algorithm','LMS', ...
    'NumForwardTaps',8,'NumFeedbackTaps',5,'StepSize',0.03);

Needs to read:

eqdfe_lms = comm.DecisionFeedbackEqualizer('Algorithm','LMS', ...
    'NumForwardTaps',8,'NumFeedbackTaps',5,'StepSize',0.03, ...
    'Constellation', pskmod(0:1, 2));

This is because the default for comm.DecisionFeedbackEqualizer seems to be QPSK not BPSK.

Also the line above it:

bpsk = comm.BPSKModulator;

Is deprecated, it can be changed to:

bpsk = @(x) pskmod(x, 2);

Answer 1

[Vinay]

Hi @Nicholas Destefano,

The "Decision Feedback Equalizer" uses the QPSK as the default constellation and can be seen by the constellation pattern containing 4 signals defined as

Constellation: [0.7071 + 0.7071i -0.7071 + 0.7071i -0.7071 - 0.7071i 0.7071 - 0.7071i ]

The BPSK signal can be created by providing the argument as

% Real axis constellation BPSK diagram
'Constellation' = pskmod(0:1,2);

The function "comm.BPSKModulator" can be replaced by the "pskmod" as per the modulation technique.

Kindly refer to the below documentaton of "Decision Feedback Equalizer" for more details

https://www.mathworks.com/help/comm/ref/comm.decisionfeedbackequalizer-system-object.html

I hope this resolves the query!

[Nicholas]

Apologies if I wasn't clear. I understand how to add the constellation to the equalizer, as I mention it in my comment.

It appears that the MATLAB example code itself is incorrect, specifically comm/DecisionFeedbackEqualizeBPSKModulatedSignalExample. Since it's a small example, with no plots, it's not easy to notice.

I don't know how to make suggestions to improve MATLAB's examples. Is there another place to do that?

[Voss]

"I don't know how to make suggestions to improve MATLAB's examples. Is there another place to do that?"

@Nicholas Destefano: At the bottom of the documentation page, there is a text prompt ("How useful was this information?") with five stars you can click on to provide feedback.