% Clear workspace, close figures, clear command window
clear all;
close all;
clc;

% Parameters
n = 10;              % Number of symbols
m = 4;               % QPSK has 4 symbols (0, 1, 2, 3)
snr = 20;            % Signal-to-Noise Ratio (dB)
bit_rate = 10^3;     % Bit rate in bits per second
f = bit_rate;        % Carrier frequency
tb = 1 / bit_rate;   % Bit duration
t = 0:(tb/1000):tb;  % Time vector for one symbol

% Generate random data
data = randi([0 m-1], 1, n); % Random symbols (0, 1, 2, 3)

% Perform QPSK modulation
s = exp(1j * (2 * pi * data / m + pi/4)); % QPSK symbols with phase offset of pi/4

% Generate the transmitted QPSK signal
txsig = []; % Initialize transmitted signal
for l = 1:length(data)
    % Modulate using I and Q components
    tx = real(s(l)) * cos(2 * pi * f * t) - imag(s(l)) * sin(2 * pi * f * t);
    txsig = [txsig tx]; % Append the modulated signal for the current symbol
end

% Add noise to the transmitted signal (Noisy QPSK signal)
rxsig = txsig + sqrt(0.5 / (10^(snr/10))) * randn(size(txsig)); % Add noise to the real part

% Add noise to QPSK symbols for constellation diagram
r = s + sqrt(0.5 / (10^(snr/10))) * (randn(size(s)) + 1j * randn(size(s)));

% Plot results
figure;

% Plot message signal (original symbols)
subplot(3, 1, 1);
stairs(data, 'LineWidth', 1.5); % Staircase plot for message symbols
grid minor;
ylim([-0.5, m-0.5]); 
xlim([0, n]);
title('Message Signal (Symbols)');
xlabel('Symbol Index');
ylabel('Symbol Value');

% Plot QPSK modulated signal
subplot(3, 1, 2);
plot(txsig, 'LineWidth', 1.5);
grid minor;
title('QPSK Modulated Signal');
xlabel('Time (samples)');
ylabel('Amplitude');
ylim([-1.5, 1.5]);
xlim([0, length(txsig)]);

% Plot noisy QPSK modulated signal
subplot(3, 1, 3);
plot(rxsig, 'LineWidth', 1.5);
grid minor;
title('Noisy QPSK Signal (AWGN Added)');
xlabel('Time (samples)');
ylabel('Amplitude');
xlim([0, length(rxsig)]);

% Constellation diagram
figure;
scatter(real(r), imag(r), 'filled'); % Scatter plot of noisy received symbols
grid minor;
title('Constellation Diagram of QPSK');
xlabel('In-phase (I)');
ylabel('Quadrature (Q)');
axis([-2 2 -2 2]); % Set axis limits for clarity