How to design baseband filter for OFDM?

I design OFDM, after i add CP, i want to give pulse shaping, or window filter (like hamming, hanning, blackman), but the output seems wrong, the main lobe attenuate as sidelobe. I know my ofdm is baseband signal, and i think it's ok with my code. What i want to ask is, whats wrong with my filter design? how to fix it? Is it same with pulse shaping with rcosine? Thank you

Please help me, i have done 2 weeks reseach on internet and matwork code, i dont really know how to do next.

Here's the part that is wrong. Even i use fvtool(filt_hamm), i think freq response of filter is true, the output must be true, but actually no, I know it's wrong when i find its BER (and its PSD is not good), and really bad compare without filtering, it must be wrong.

filt_hamm=fir1(4095,n_subcarrier/n_fft,hamming(4096));  %cut off at n_subcarrier/n_fft
for y=1:1:n_simbol
Tx_OFDM_filt_hamm(y,:)=filter(filt_hamm,1,Tx_OFDM_cp(y,:));
end

This is my code, it's working except the filter

clc; clear all; close all;
  %%Parameter OFDM
  n_subcarrier=1000;       % Number of subcarrier          
  n_simbol=100;            % Number of symbol
  n_fft=4096;              % Size FFT/IFFT
  M=16;                    % Constellation Order
  Tipe=2;                  % Modulate type, 1=PSK, 2=QAM
  Phase_Offset=0;          
  N=4 ;                    % percentage of cyclic prefix n_fft/N

%%Transmitter
if Tipe == 1
    Tx = modem.pskmod('M',M,'PhaseOffset',Phase_Offset,'SymbolOrder','gray','InputType','bit');
    Rx = modem.pskdemod('M',M,'PhaseOffset',Phase_Offset,'SymbolOrder','gray','OutputType','bit');
else
    Norm_mod=sqrt(3/2/(M-1));
    Tx = modem.qammod('M',M,'PhaseOffset',Phase_Offset,'SymbolOrder','gray','InputType','bit');
    Rx = modem.qamdemod('M',M,'PhaseOffset',Phase_Offset,'SymbolOrder','gray','OutputType','bit');
end

Tx_data=double(rand(n_subcarrier*log2(M),n_simbol)<.5);% Generate bitsream
Tx_modulated=Norm_mod*modulate(Tx,Tx_data);            % Modulating
Tx_Zpad = [Tx_modulated(n_subcarrier/2+1:end,:);...    % Add Zero Padding at the center
           zeros(n_fft-n_subcarrier,n_simbol);...
           Tx_modulated(1:n_subcarrier/2,:)];
Tx_OFDM=ifft(Tx_Zpad,n_fft);                           % IFFT 
Tx_p2s=Tx_OFDM.';                                      % Parallel to serial
CP_part=Tx_p2s(:,end-(n_fft/N)+1:end);                 % Part Of CP
Tx_OFDM_cp=[CP_part Tx_p2s];                           % Add CP to OFDM 

filt_hamm=fir1(4095,n_subcarrier/n_fft,hamming(4096));  %cut off at n_subcarrier/n_fft
for y=1:1:n_simbol
Tx_OFDM_filt_hamm(y,:)=filter(filt_hamm,1,Tx_OFDM_cp(y,:));
end

Channel + Receiver

n_tap=4096; % 
h_rayleigh=(randn(n_simbol,n_tap)+1i*randn(n_simbol,n_tap)).*sqrt(1/2);
h_ray_freq=fft(h_rayleigh,length(Tx_OFDM_cp),2);               % channel impulse response
Tx_OFDM_rayleigh=Tx_OFDM_cp.*h_ray_freq;   

SNR_awal=-10; SNR_tambah=2;SNR_akhir=30;const=0;               % Awal means start (first iteration)
r=zeros(size(SNR_awal:SNR_tambah:SNR_akhir));                  % tambah means increament
for snr=SNR_awal:SNR_tambah:SNR_akhir                          % akhir means end (last interation)
    const=const+1;
    Rx_AWGN=awgn(Tx_OFDM_rayleigh,snr,'measured');                    % Noise AWGN
    Rx_equalization=Rx_AWGN./h_ray_freq;                              % ideal equalization
    Rx_OFDM_cp_remove=Rx_equalization(:,(n_fft/N)+1:n_fft+(n_fft/N)); % release Cyclic prefix
    Rx_s2p=Rx_OFDM_cp_remove.';                                       % Serial to parallel
    Rx_deOFDM=fft(Rx_s2p,n_fft) ;                                     % FFT 
    Rx_Zpad= [Rx_deOFDM(n_fft-n_subcarrier/2+1:n_fft,:);...           % release Zero Padding
                  Rx_deOFDM(1:n_subcarrier/2,:)];   
    Rx_demodulation=demodulate(Rx,Rx_Zpad./Norm_mod);                 % Demodulation
    [n, r(1,const)]=biterr(Tx_data,Rx_demodulation);                  % Calculate BER
end

%%Graph
snr=SNR_awal:SNR_tambah:SNR_akhir;
% Plotting BER vs SNR
figure(1);
if M==2
    semilogy(snr,r(1,:),'-ok','DisplayName','OFDM 2QAM');
elseif M==4
    semilogy(snr,r(1,:),'-om','DisplayName','OFDM 4QAM');
elseif M==8
    semilogy(snr,r(1,:),'-oc','DisplayName','OFDM 8QAM');
elseif M==16
    semilogy(snr,r(1,:),'-ob','DisplayName','OFDM 16QAM');
elseif M==32
    semilogy(snr,r(1,:),'-og','DisplayName','OFDM 32QAM'); 
elseif M==64
    semilogy(snr,r(1,:),'-or','DisplayName','OFDM 64QAM'); 
end
hold on
grid on;
title('OFDM Bit Error Rate vs SNR');
ylabel('Bit Error Rate');
xlabel('SNR [dB]');
legend('show')

figure(2);
hold on
[Pxx1,W1]=pwelch(Tx_OFDM_cp(1,:),[],[],[],'centered','psd');
[Pxx3,W3]=pwelch(Tx_OFDM_filt_hamm(1,:),[],[],[],'centered','psd');
plot(W1,10*log10(Pxx1/max(Pxx1)),'DisplayName','OFDM'); 
 plot(W3,10*log10(Pxx3/max(Pxx3)),'DisplayName','OFDM hamming'); hold on
grid on
xlim([-pi,pi])
set(gca,'xtick',[-pi,-pi/2,0,pi/2,pi])
set(gca,'xticklabel',{' -\pi';'-0.5\pi';' 0';' 0.5\pi';' \pi'})
xlabel('Normalize Frequency (radian/sampel)')
ylabel('Power spectral density (PSD)')
title('OFDM');
legend('show')

[Rishabh Rawat]

Hi,

Any solution to this query? I am having the same doubt.

Answer 1

[Madhumitha Munusamy]

i am having a little problem over here. can someone help me

i have a OFDM matlab code. In that i want to replace cyclic prefix by NPR filter to analyze the performance but it's showing error.