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ASK, FSK, and PSK (with MATLAB + Online Simulator)


📘 ASK Theory 📘 FSK Theory 📘 PSK Theory 📊 Comparison 🧮 MATLAB Codes 🎮 Simulator

ASK or OFF ON Keying

ASK is a simple (less complex) Digital Modulation Scheme where we vary the modulation signal's amplitude or voltage by the message signal's amplitude or voltage. We select two levels (two different voltage levels) for transmitting modulated message signals.

Example: "+5 Volt" (upper level) and "0 Volt" (lower level). To transmit binary bit "1", the transmitter sends "+5 Volts", and for bit "0", it sends no power.

The receiver uses filters to detect whether a binary "1" or "0" was transmitted.

Output of ASK, FSK, and PSK modulation using MATLAB
Fig 1: Output of ASK, FSK, and PSK modulation using MATLAB for a data stream "1 1 0 0 1 0 1 0" (Get MATLAB Code)

FSK (Frequency Shift Keying)

In Frequency Shift Keying (FSK), the message signal is modulated using a high-frequency carrier. Binary "1" and "0" are represented by two different frequencies close to the carrier frequency.

For example, using frequencies f1 and f2 (where f1 > f2):

S₁(t) = A cos(2Ï€fc1t) for binary 1
S₂(t) = A cos(2Ï€fc2t) for binary 0

Here, fc1 is different from fc2.

PSK (Phase Shift Keying)

In Phase Shift Keying (PSK), the phase of the carrier signal is changed to represent data bits. Binary "1" is transmitted by shifting the signal’s phase by 180°, while binary "0" keeps the same phase.

s(t) = A cos(2πfct + π) for binary 1
s(t) = A cos(2Ï€fct) for binary 0

Bit Rate Comparison

PSK (Phase Shift Keying)

PSK can use various phase shifts to encode more bits per symbol (e.g., QPSK, 16-PSK). It is the most efficient for high bit rates.

FSK (Frequency Shift Keying)

FSK requires more bandwidth, making it less bit-efficient than PSK for high-speed applications.

ASK (Amplitude Shift Keying)

ASK is highly susceptible to noise and is typically less effective than PSK for high-speed transmission.

Conclusion: PSK can achieve the highest bit rates, especially using higher-order modulation techniques.

MATLAB Code: ASK, FSK, and PSK

This single script generates a random binary sequence and applies all three digital modulation techniques (Amplitude, Frequency, and Phase Shift Keying) for side-by-side comparison.

% DIGITAL MODULATION (ASK, FSK, PSK) 
% Source: SalimWireless.Com 

clc; clear all; close all;

% --- 1. Parameters Configuration ---
Fs = 1000;              % Sampling Frequency
Tb = 1;                 % Bit Duration (seconds)
N_bits = 8;             % Number of bits to transmit
fc = 10;                % Base Carrier Frequency (Hz)
fc2 = 30;               % Secondary Frequency for FSK (Hz)
Ts = 1/Fs;              % Sampling Period
t_bit = 0:Ts:Tb-Ts;     % Time vector for one bit
rng(42);                % For reproducible results

% Generate Random Binary Data
binary_data = randi([0, 1], 1, N_bits);

% --- 2. Signal Generation ---
message_sig = [];
ask_sig = [];
fsk_sig = [];
psk_sig = [];

for bit = binary_data
    % Message Signal (Square wave)
    m_segment = bit * ones(1, length(t_bit));
    message_sig = [message_sig m_segment];
    
    % ASK Logic: Bit 1 = Carrier ON, Bit 0 = Carrier OFF
    ask_seg = bit * sin(2*pi*fc*t_bit);
    ask_sig = [ask_sig ask_seg];
    
    % FSK Logic: Bit 1 = fc2 (High), Bit 0 = fc (Low)
    if bit == 1
        fsk_seg = sin(2*pi*fc2*t_bit);
    else
        fsk_seg = sin(2*pi*fc*t_bit);
    end
    fsk_sig = [fsk_sig fsk_seg];
    
    % PSK Logic: Bit 1 = 180 deg phase, Bit 0 = 0 deg phase
    if bit == 1
        psk_seg = sin(2*pi*fc*t_bit + pi);
    else
        psk_seg = sin(2*pi*fc*t_bit + 0);
    end
    psk_sig = [psk_sig psk_seg];
end

% Total Time Vector for Plotting
t_total = 0:Ts:(N_bits*Tb)-Ts;

% --- 3. Visualization ---
figure('Name', 'Digital Modulation Comparison', 'Color', 'w');

subplot(4,1,1);
plot(t_total, message_sig, 'LineWidth', 2, 'Color', 'k');
title(['Binary Message: ', num2str(binary_data)]); grid on; axis([0 N_bits*Tb -0.5 1.5]);

subplot(4,1,2);
plot(t_total, ask_sig, 'LineWidth', 1, 'Color', 'b');
title('ASK (Amplitude Shift Keying)'); grid on;

subplot(4,1,3);
plot(t_total, fsk_sig, 'LineWidth', 1, 'Color', 'r');
title('FSK (Frequency Shift Keying)'); grid on;

subplot(4,1,4);
plot(t_total, psk_sig, 'LineWidth', 1, 'Color', [0 0.5 0]);
title('PSK (Phase Shift Keying)'); grid on;
xlabel('Time (seconds)');

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