a step-by-step guide to signal modulation and demodulation using MATLAB, including code examples, visualizations, and explanations for common modulation techniques like AM, FM, ASK, FSK, and QAM.

Fs = 1000;           % Sampling frequency (Hz)
t = 0:1/Fs:1;        % Time vector (1 second)
Fc = 100;            % Carrier frequency (Hz)

% Message signal (5 Hz sine wave)
fm = 5;
message = 0.5*sin(2*pi*fm*t);

% AM Modulation
carrier = cos(2*pi*Fc*t);
modulated_AM = (1 + message) .* carrier;

% AM Demodulation (envelope detection)
envelope = abs(hilbert(modulated_AM)) - 1; % Remove DC offset

% Plot
figure;
subplot(3,1,1);
plot(t, message);
title('Message Signal');
subplot(3,1,2);
plot(t, modulated_AM);
title('AM Modulated Signal');
subplot(3,1,3);
plot(t, envelope);
title('Demodulated Signal');

Fs = 1000;
t = 0:1/Fs:1;
Fc = 100;
fm = 5;
message = 0.5*sin(2*pi*fm*t);

% FM Modulation
frequency_deviation = 25; % Hz
modulated_FM = fmmod(message, Fc, Fs, frequency_deviation);

% FM Demodulation
demodulated_FM = fmdemod(modulated_FM, Fc, Fs, frequency_deviation);

% Plot
figure;
subplot(3,1,1);
plot(t, message);
title('Message Signal');
subplot(3,1,2);
plot(t, modulated_FM);
title('FM Modulated Signal');
subplot(3,1,3);
plot(t, demodulated_FM);
title('Demodulated Signal');

Fs = 1000;
t = 0:1/Fs:1;
Fc = 100;

% Generate random bits
bits = randi([0 1], 1, 10); % 10 bits
message = repelem(bits, 100); % Repeat each bit for 100 samples

% ASK Modulation
modulated_ASK = message .* cos(2*pi*Fc*t);

% ASK Demodulation (non-coherent)
envelope = abs(hilbert(modulated_ASK));
demodulated_bits = envelope > 0.5; % Threshold detection

% Plot
figure;
subplot(3,1,1);
stem(bits);
title('Original Bits');
subplot(3,1,2);
plot(t, modulated_ASK);
title('ASK Modulated Signal');
subplot(3,1,3);
plot(t, demodulated_bits);
title('Demodulated Bits');

M = 16; % 16-QAM
n = 1000; % Number of symbols
Fs = 1000;
Fc = 100;

% Generate random symbols
data = randi([0 M-1], 1, n);
modulated_QAM = qammod(data, M);

% Add noise (simulate channel)
SNR = 20; % dB
received = awgn(modulated_QAM, SNR, 'measured');

% Demodulate
demodulated_QAM = qamdemod(received, M);

% Plot constellation
scatterplot(modulated_QAM);
title('16-QAM Constellation');