How can I modify this to receive the same data after demodulation?

clc;

clear all;

close all;

no_of_data_bits = 64%Number of bits per channel

M =4 %Number of carriers

n=256;%Total number of bits 64*4

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

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

%Transmitter

data = randsrc(1, no_of_data_bits, 0:M-1);

figure(1),stem(data); grid on; xlabel('Data Points'); ylabel('Amplitude')

title('Randomly Generated Original Data ')

% Perform QPSK modulation on the input source data

qpsk_modulated_data = pskmod(data, M);

figure(2),stem(qpsk_modulated_data);title('QPSK Modulation ')

% Converting the series data stream into four parallel data stream to form

% four sub carriers

S2P = reshape(qpsk_modulated_data, no_of_data_bits/M,M)

Sub_carrier1 = S2P(:,1)

Sub_carrier2 = S2P(:,2)

Sub_carrier3 = S2P(:,3)

Sub_carrier4 = S2P(:,4)

figure(3), subplot(4,1,1),stem(Sub_carrier1),title('Subcarrier1'),grid on;%plots bits 1-16

subplot(4,1,2),stem(Sub_carrier2),title('Subcarrier2'),grid on;%plots bits 17-32

subplot(4,1,3),stem(Sub_carrier3),title('Subcarrier3'),grid on;%plots bits 33-48

subplot(4,1,4),stem(Sub_carrier4),title('Subcarrier4'),grid on;%plots bits 49-64

% Inverse Fourier Transofrm for each subcarrier

number_of_subcarriers=4;

cp_start=block_size-cp_len;

ifft_Subcarrier1 = ifft(Sub_carrier1)

ifft_Subcarrier2 = ifft(Sub_carrier2)

ifft_Subcarrier3 = ifft(Sub_carrier3)

ifft_Subcarrier4 = ifft(Sub_carrier4)

figure(4), subplot(4,1,1),plot(real(ifft_Subcarrier1),'r'),

title('Inverse Fourier Transform on all the sub-carriers')

subplot(4,1,2),plot(real(ifft_Subcarrier2),'y')

subplot(4,1,3),plot(real(ifft_Subcarrier3),'g')

subplot(4,1,4),plot(real(ifft_Subcarrier4),'b')

%Add Cyclic Prefix

%A cyclic prefix is created to prevent intersymbol interference (ISI)

%when an OFDM signal is transmitted in a dispersive channel.

%The cyclic prefix (CP) is essentially an identical copy of the last

%portion of the OFDM symbol appended before the OFDM symbol.

%This CP preserves the orthogonality of the subcarriers and prevents

%ISI between successive OFDM symbols.

for i=1:number_of_subcarriers,

ifft_Subcarrier(:,i) = ifft((S2P(:,i)),16)% 16 is the ifft point

for j=1:cp_len,

cyclic_prefix(j,i) = ifft_Subcarrier(j+cp_start,i)

end

Append_prefix(:,i) = vertcat( cyclic_prefix(:,i), ifft_Subcarrier(:,i))

% Appends prefix to each subcarriers

end

App1=Append_prefix(:,1);

App2=Append_prefix(:,2);

App3=Append_prefix(:,3);

App4=Append_prefix(:,4);

figure(5), subplot(4,1,1),plot(real(App1),'r'),title('Cyclic prefix added to all the sub-carriers')

subplot(4,1,2),plot(real(App2),'y')

subplot(4,1,3),plot(real(App3),'g')

subplot(4,1,4),plot(real(App4),'b')

figure(6),plot((real(App1)),'r'),title('Orthogonality'),hold on ,plot((real(App2)),'y'),hold on ,

plot((real(App3)),'g'),hold on ,plot((real(App4)),'b'),hold on ,grid on

%Convert to serial from parallel

[rows_Append_prefix cols_Append_prefix]=size(Append_prefix)

len_ofdm_data = rows_Append_prefix*cols_Append_prefix

% OFDM signal to be transmitted

ofdm_signal = reshape(Append_prefix, 1, len_ofdm_data);

figure(7),plot(real(ofdm_signal)); xlabel('Time'); ylabel('Amplitude');

title('OFDM Signal');grid on;

%Serial stream passing through the channel

channel = randn(1,2) + sqrt(-1)*randn(1,2);

after_channel = filter(channel, 1, ofdm_signal);

awgn_noise = awgn(zeros(1,length(after_channel)),0);

recvd_signal = awgn_noise+after_channel; % With AWGN noise

%Converts from serial back to parallel

figure(8),plot(real(recvd_signal)),xlabel('Time'); ylabel('Amplitude');

title('OFDM Signal after passing through channel');grid on;

recvd_signal_paralleled = reshape(recvd_signal,rows_Append_prefix, cols_Append_prefix);

%now that the signal has passed through the channel we begin to work

%backawards, but we are first going to remove the CP

% Remove cyclic Prefix

%Now that the signal has already passed through the channel, we can get rid

%of the CP

recvd_signal_paralleled(1:cp_len,:)=[];

R1=recvd_signal_paralleled(:,1);

R2=recvd_signal_paralleled(:,2);

R3=recvd_signal_paralleled(:,3);

R4=recvd_signal_paralleled(:,4);

figure(9),plot((imag(R1)),'r'),subplot(4,1,1),plot(real(R1),'r'),

title('Cyclic prefix removed from the four sub-carriers')

subplot(4,1,2),plot(real(R2),'y')

subplot(4,1,3),plot(real(R3),'g')

subplot(4,1,4),plot(real(R4),'b')

% Fourier Transofrm of the recievied signal

%Similarly to how we took the IFFT of the original signal, as we are back

%tracking we are now taking the FFT

for i=1:number_of_subcarriers,

fft_data(:,i) = fft(recvd_signal_paralleled(:,i),16);

end

F1=fft_data(:,1);

F2=fft_data(:,2);

F3=fft_data(:,3);

F4=fft_data(:,4);

figure(10), subplot(4,1,1),plot(real(F1),'r'),title('Fourier Transform of all the four sub-carriers')

subplot(4,1,2),plot(real(F2),'y')

subplot(4,1,3),plot(real(F3),'g')

subplot(4,1,4),plot(real(F4),'b')

% Conversion to serial and demodulation

%The orignal data was in series, so we are taking the demodulated data

%which is still in parallel form and we are converting back into series to

%hopefully show the same data

recvd_serial_data = reshape(fft_data, 1,(16*4));

qpsk_demodulated_data = pskdemod(recvd_serial_data,4);

figure(11), subplot(2,1,1), stem(data)

grid on;xlabel('Data Points');ylabel('Amplitude');

title('Original Data')

subplot (2,1,2), stem(qpsk_demodulated_data,'bo');

grid on;xlabel('Data Points');ylabel('Amplitude');

title('Recieved Data')

Answer 1

[Kaashyap Pappu]

You could look into this example to carry out the same operation. Just modify the channel coefficient values to (1,1) and should work similarly.

Hope this helps!