MATLAB Script
% QPSK Modulation and Demodulation
% Define input data
data = [1 1 0 1 0 1 0 0 1 0]; % Information
% Modulation
M = 4; % Number of symbols
k = log2(M); % Number of bits per symbol
bits_per_symbol = length(data) / k;
data_reshaped = reshape(data, k, bits_per_symbol)';
symbol = bi2de(data_reshaped, 'left-msb')';
% Generate QPSK modulation symbols
modulated_signal = pskmod(symbol, M, pi/4); % Phase offset pi/4 for QPSK
% AWGN Channel (Additive White Gaussian Noise)
EbNo = 10; % Energy per bit to noise power spectral density ratio (dB)
SNR = EbNo + 10*log10(k); % Signal to Noise Ratio (dB)
rx_signal = awgn(modulated_signal, SNR, 'measured');
% Demodulation
demodulated_signal = pskdemod(rx_signal, M, pi/4); % Phase offset pi/4 for QPSK
% Convert symbols to bits
demodulated_bits = de2bi(demodulated_signal, k, 'left-msb')';
received_data = reshape(demodulated_bits', 1, []);
% Plot original and received data
figure;
subplot(2,1,1);
stem(data, 'linewidth', 2);
title('Original Data');
xlabel('Bit');
ylabel('Amplitude');
axis([0 length(data) 0 1.5]);
subplot(2,1,2);
stem(received_data, 'linewidth', 2);
title('Received Data');
xlabel('Bit');
ylabel('Amplitude');
axis([0 length(received_data) 0 1.5]);
% Scatter plot
demodulated_symbols = demodulated_signal; % Replace with your demodulated symbols
% Map demodulated symbols to complex constellation points
constellation_points = [1+1i, -1+1i, -1-1i, 1-1i]; % QPSK constellation points
figure()
scatter(real(constellation_points(demodulated_symbols+1)), imag(constellation_points(demodulated_symbols+1)));
title('QPSK Constellation Diagram');
xlabel('In-phase Component');
ylabel('Quadrature Component');
axis([-2 2 -2 2]); % Adjust axis limits if needed
grid on;
