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

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Charles Kidroske
Charles Kidroske 2019-11-25
回答： Kaashyap Pappu 2019-11-28
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')

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回答（1 个）

Kaashyap Pappu
Kaashyap Pappu 2019-11-28
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!

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