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Problem with IFFT time in OFDM

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thavamaran

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Hi guys, Im having problem with on analyzing the time of the symbols or bandwidth of the discrete signals after IFFT in OFDM. Below are my codes that I used.
Code:
nchannel1=128; %Total number of carriers
nchannel  =76;  %Occupied carriers with OFDM symbols
padding=nchannel1-nchannel; %zero padding
nsym        =4; % number of symbols
pilotsym  =2; %pilot symbols

t_data=randint(nchannel*nsym*m,1,2); %raw bits
res=reshape(t_data,m,nchannel*nsym); %reshape into   
                                                       equivalent    
                                                       subcarries

qamdata=bi2de(res.','left-msb');    %symbol levels
maping = bin2gray(qamdata,'qam',m^2);  %Gray coding
mod_data =modulate(M,maping);  %mapping

par_data = reshape(mod_data,nchannel,nsym).'; %S/P

 zero_pad=[zeros(nsym,(padding-2)/2) par_data(:,[1:nchannel/2]) zeros(nsym,2) par_data(:,[nchannel/2+1:nchannel]) zeros(nsym,(padding-2)/2)]; %zero padding, making it 128 subcarriers

pilot_ins_trial=[pilot zero_pad pilot]; %inserting pilot

IFFT_data =ifft(fftshift( pilot_ins_trial.')).';  performing IFFT

 cylic_add_data = [IFFT_data(:,[nchannel1-gilen+1:nchannel1]) IFFT_data]; %adding CP

y_samp=[];

  t=1/Ufs:1/Ufs:Tc;        %Ufs=samping frequency
                                  %Tc=1/fc=20 MHz
  nsamp=length(t);

 for iii=1:length(cyclic_add_data1)
     
y_samp=[y_samp cyclic_add_data1(iii).*square(2*pi*fc*t)]; %upsampling with   
                                                                               %rectagular shape  
                                                                               %filter
                                                     
end

The problem is, I want to fix a low pass filter after y_samp to remove all the harmonics, the spectrum looks fine with signal centered at 20 MHz, but after filtering at the first crossing at 40 MHz, my received signal looks crazy, but without filtering, I could retrieve my signal at the receiver in perfect condition! , but I want to filter it for other purposes, I also compensated the delay that occurred cause of the filter.

So I thought may be I could look at the time or bandwidth after IFFT (IFFT_data in the code) before upsampling it to 20 MHz bandwidth, but the problem is I dont have a sampling frequency that is defined cause IFFT is a default function of matlab.

Can anyone suggest me a way or option to look at the bandwidth.
 

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