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to get the proper scatter plot

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ABDUL
ABDUL on 25 Mar 2021
Commented: ABDUL on 4 Apr 2021
Hi...
How do i map the random symbols after binary to symbol mapping to get the constellation diagram of M-ary QAM correctly . i am getting the incorrect constellation diagram.
clc;
close all;
clear all;
M=64; % QAM Signal Constellation
fft_size=64;
fspacing=15000;
fs=15000*64;
Ts = 1/fs; % Sampling period of channel
Fd = 0; % Max Doppler frequency shift
k = log2(M); % Number of bits per symbol
nbits=k*fft_size;
nsym = fft_size*2; %1000
cp_length=round(0.06*fft_size); % length of cyclic prefix
xbits = randi([0 1],1,nbits); % Random binary data stream
% mappedData = wlanConstellationMap(xbits,k);
xsym = bi2de(reshape(xbits,k,(length(xbits)/k)).','left-msb');
tx=[];
% QAM Modulation
QAM_Modulation=qammod(xsym,M,'PlotConstellation',true);
% scatterplot(QAM_Modulation);

Accepted Answer

Sriram Tadavarty
Sriram Tadavarty on 1 Apr 2021
Hi Abdul,
The scatterplot usage is correct itself. May i know why do you feel the constellation is incorrect?
Since, xbits are random, possibly, same number of bits might make a symbol, leading to partial constellation while looking from scatterplot.
Try this:
M=64; % QAM Signal Constellation
fft_size=64;
fspacing=15000;
fs=15000*64;
Ts = 1/fs; % Sampling period of channel
Fd = 0; % Max Doppler frequency shift
k = log2(M); % Number of bits per symbol
nbits=k*fft_size;
nsym = fft_size*2; %1000
cp_length=round(0.06*fft_size); % length of cyclic prefix
xbits = randi([0 1],1,nbits*10); % Random binary data stream (Updated with 10 times the previous length)
% mappedData = wlanConstellationMap(xbits,k);
xsym = bi2de(reshape(xbits,k,(length(xbits)/k)).','left-msb');
tx=[];
% QAM Modulation
QAM_Modulation=qammod(xsym,M,'PlotConstellation',true);
scatterplot(QAM_Modulation)
Hope this helps.
Regards,
Sriram

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