MATLAB code for Constellation Diagram of QPSK

• 📘 Overview
• 🧮 MATLAB Code
• 🧮 Theory
• 🧮 BER performance of QPSK with BPSK, 4-QAM, 16-QAM, 64-QAM, 256-QAM, etc
• 📚 Further Reading

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;

    

Output

Fig 1: QPSK Modulation and Demodulation

Fig 2: Constellation Diagram of QPSK

Further Reading

1. Quadrature Phase Shift Keying (QPSK) (Theory)
2. MATLAB Code for QPSK Modulation and Demodulation
3. Constellation Diagrams of ASK, FSK, and PSK
4. MATLAB Code for Constellation Diagrams of ASK, FSK, and PSK