7

I used a Matlab code of LMS (least mean square algorithm) to equalize the effect of the channel, it is working for a tapped delay channel generated in MATLAB but for optical fiber channel using optisystem program, it doesn’t work well, i think the problem is in optical fiber impulse response, as it is not finite impulse response and the algorithm needs a finite impulse response but how to solve this problem?

the constellation of MATLAB channel and optical fiber channel, it is clear that the second constellation hasn't been equalized well.

constellation:

enter image description here

Here is an example of a noisy channel where LMS algorithm couldn't compensate its effect, is there any way to enhance its performance at this SNR :

% // Set up parameters and signals.

M = 4; % // Alphabet size for modulation

msg = randint(15000,1,M); % // Random message


modmsg = pskmod(msg,M); % // Modulate using QPSK.


trainlen = 5000; % // Length of training sequence


chan = [.986; .845; .237; .123+.31i]; % // Channel coefficients


filtmsg = filter(chan,1,modmsg); % // Introduce channel distortion.

filtmsg=awgn(filtmsg,10,'measured');

% // Equalize the received signal.

eq1 = lineareq(8, lms(0.01)); % // Create an equalizer object.

eq1.SigConst = pskmod([0:M-1],M); % // Set signal constellation.

[symbolest,yd] = equalize(eq1,filtmsg,modmsg(1:trainlen)); % // Equalize.

% // Plot signals.

h = scatterplot(filtmsg,1,trainlen,'bx'); hold on;

scatterplot(symbolest,1,trainlen,'g.',h);
kkuilla
  • 2,226
  • 3
  • 34
  • 37
Mai Fouad
  • 87
  • 2
  • 1
    Unless you precise what "it doesn’t work well" mean, you are very unlikely to get an answer. Please be more precise on the difference between the outpout and the desired output, the error messages, etc and share some code if you can. – Ratbert Jul 04 '15 at 06:59
  • 1
    The constellation of the second channel (optical fiber) shows that the received signal couldn't be equalized as shown in the link, that the problem isn't in the algorithm as it works well for other channel, so is there any condition on the channel that the algorithm can compensate its effect? – Mai Fouad Jul 04 '15 at 19:19
  • What is the impulse response of the channel? – Oliver Charlesworth Jul 05 '15 at 17:45
  • it is a tapped delay channel defined by its coefficient : chan = [.986; .845; .237; .123+.31i]; the algorithm can compensate this channel without adding noise, but it couldn't work when noise exists – Mai Fouad Jul 05 '15 at 21:06

1 Answers1

0

Maybe you should have more samples. The "averaging" effect of the LMS doesnt work when the spectrum is close to the Nyquist criteria.

Other problem could be that the noise is not white (wide-band) enough. So the noise-samples are coherent.

bni i
  • 103
  • 7