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# I am getting amplitude zero, But it is impossible

조회 수: 23(최근 30일)
Dipsikha Roy 3 Mar 2021
댓글: Dipsikha Roy 4 Mar 2021 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.
##### 댓글 수: 1표시숨기기 없음
Cris LaPierre 3 Mar 2021
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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### 채택된 답변

Walter Roberson 3 Mar 2021
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); ##### 댓글 수: 1표시숨기기 없음
Dipsikha Roy 4 Mar 2021
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')
which gives me graph like what I can write in the above code so that I can show in the graph that x axis is time and y axis is amplitude,
I have used x label and ylabel but the way it was showing time and amplitude i dont like it,Can you please suggest some better ways?

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