I am getting amplitude zero, But it is impossible

Question

Dipsikha Roy 2021-3-3

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/761996-i-am-getting-amplitude-zero-but-it-is-impossible

评论： Dipsikha Roy 2021-4-24

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I am getting a graph where amplitude is zero,How it possible?

I am attchimg here code which generate this graphs

clc;
M =8; %here we initialize the number of constellation point of qam
no_of_data_bits=1024;
Fm=10^6;%Max freq
block_size = 16; %Size of each OFDM block to add cyclic prefix
cp_len = floor(0.1 * block_size); %Length of the cyclic prefix
data_source= abs(round(randn(1,no_of_data_bits)));%here we take random normal function
figure(1);
x=1:no_of_data_bits;
stem (x*(1/Fm),data_source);
grid minor;
xlabel('time(Microsecond)','Fontsize',14);
ylabel('amplitude','Fontsize',14);
title('Transmitted Data','Fontsize',14);%here we plot that transmitted data
qam_modulated_data = qammod(data_source, M);%here we perform 8bit qam on the random normal function
nqdata = length(qam_modulated_data);
data = 0:M-1;
scatterplot(qam_modulated_data,1,0,'r*');
[udata, uidx] = unique(qam_modulated_data);
nudata = length(udata);
grid minor
for k=1:nudata
   text(real(udata(k))-0.4,imag(udata(k))+0.4,num2str(data_source(uidx(k))));
end
axis([-4 4 -2 2])
qm = abs(qam_modulated_data);
figure(3);
stem(qm)
title('MODULATED TRANSMITTED DATA','Fontsize',14);%here we plot those constellation point
y=ifft(qam_modulated_data);
figure(4);
x=1:nqdata;
stem(x*Fm,abs(y));
grid minor;
 xlabel('freq(Mhz)');
 ylabel('amplitude of ifft');
 title('without hermitian ifft','Fontsize',14);

I dont know why zero amplitude is coming,please help.

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Cris LaPierre 2021-3-3

One observation - you take the ifft of a time-domain signal, and label the x axis as frequency. Did you mean to take the fft instead?

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Answer 1

Walter Roberson 2021-3-3

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/761996-i-am-getting-amplitude-zero-but-it-is-impossible#answer_638436

在 MATLAB Online 中打开

Not 0. The value in the first location is enough larger than the rest of the data, that the rest of the data appears close to the axes in comparison. Below I skip plotting the first point.

Remember that fft() and ifft() are close to being the same operation, so when you ifft() your time domain signal, that is close to being the same as doing fft() of the time domain signal. And, just like if you had done fft(), the first output bin will be the sum of the time domain signal, so if your time domain signal has a non-zero mean then the first output bin can end up being much higher in magnitude than the other outputs.

M =8; %here we initialize the number of constellation point of qam

no_of_data_bits=1024;

Fm=10^6;%Max freq

block_size = 16; %Size of each OFDM block to add cyclic prefix

cp_len = floor(0.1 * block_size); %Length of the cyclic prefix

data_source= abs(round(randn(1,no_of_data_bits)));%here we take random normal function

figure(1);

x=1:no_of_data_bits;

stem (x*(1/Fm),data_source);

grid minor;

xlabel('time(Microsecond)','Fontsize',14);

ylabel('amplitude','Fontsize',14);

title('Transmitted Data','Fontsize',14);%here we plot that transmitted data

qam_modulated_data = qammod(data_source, M);%here we perform 8bit qam on the random normal function

nqdata = length(qam_modulated_data);

data = 0:M-1;

scatterplot(qam_modulated_data,1,0,'r*');

[udata, uidx] = unique(qam_modulated_data);

nudata = length(udata);

grid minor

for k=1:nudata

text(real(udata(k))-0.4,imag(udata(k))+0.4,num2str(data_source(uidx(k))));

end

axis([-4 4 -2 2])

qm = abs(qam_modulated_data);

figure(3);

stem(qm)

title('MODULATED TRANSMITTED DATA','Fontsize',14);%here we plot those constellation point

y=ifft(qam_modulated_data);

figure(4);

x=1:nqdata;

stem(x(2:end)*Fm,abs(y(2:end)));

grid minor;

xlabel('freq(Mhz)');

ylabel('amplitude of ifft');

title('without hermitian ifft','Fontsize',14);

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Dipsikha Roy 2021-3-4

在 MATLAB Online 中打开

Sir you are great,I have written code

A1=Append_prefix(:,1);
A2=Append_prefix(:,2);
A3=Append_prefix(:,3);
A4=Append_prefix(:,4);
A5=Append_prefix(:,5);
A6=Append_prefix(:,6);
A7=Append_prefix(:,7);
A8=Append_prefix(:,8);
figure(12), subplot(8,1,1),plot(real(A1),'r'),title('Cyclic prefix added to all the  odd sub-carriers')
subplot(8,1,2),plot(real(A2),'y')
subplot(8,1,3),plot(real(A3),'b')
subplot(8,1,4),plot(real(A4),'g')
subplot(8,1,5),plot(real(A5),'m')
subplot(8,1,6),plot(real(A6),'k')
subplot(8,1,7),plot(real(A7),'c')
subplot(8,1,8),plot(real(A8),'g')