Analog Communication - Practical 4

Aim

To study the SSB (Single Sideband) modulation and demodulation process.

Theory

Single Sideband (SSB) modulation is a refined version of Amplitude Modulation (AM), where only one of the sidebands (upper or lower) is transmitted. This reduces bandwidth usage and power requirements compared to conventional AM or DSB-SC.

The modulation process involves suppressing the carrier and one of the sidebands using filters or mathematical operations. The demodulation process retrieves the original message signal from the transmitted SSB signal by mixing it with a locally generated carrier at the receiver.

Mathematically, the modulated signal is expressed as:

SSB(t) = A_m cos(2πf_ct ± 2πf_mt)

Here, the + sign indicates the upper sideband (USB), while the − sign indicates the lower sideband (LSB). Filters or a phasing method are used to isolate the desired sideband during modulation.

MATLAB Code

% MATLAB Code for SSB Modulation and Demodulation
fc = 1000; % Carrier frequency in Hz
fm = 50; % Message frequency in Hz
fs = 10000; % Sampling frequency in Hz
t = 0:1/fs:0.2; % Time vector

% Message signal
message = cos(2*pi*fm*t); 

% Hilbert transform of the message signal
hilbert_message = imag(hilbert(message));

% Carrier signals
carrier_cos = cos(2*pi*fc*t);
carrier_sin = sin(2*pi*fc*t);

% Upper Sideband (USB) modulation
usb = message .* carrier_cos - hilbert_message .* carrier_sin;

% Demodulation (using a synchronous detector)
demodulated = usb .* carrier_cos;
demodulated = lowpass(demodulated, fm, fs); % Low-pass filter to retrieve message

% Plotting
subplot(3,1,1); plot(t, message); title('Message Signal'); xlabel('Time'); ylabel('Amplitude');
subplot(3,1,2); plot(t, usb); title('SSB Modulated Signal (USB)'); xlabel('Time'); ylabel('Amplitude');
subplot(3,1,3); plot(t, demodulated); title('Demodulated Signal'); xlabel('Time'); ylabel('Amplitude');

Expected Output

The MATLAB code produces three plots that illustrate the SSB modulation and demodulation process:

Message Signal: A sinusoidal wave representing the original information signal.
SSB Modulated Signal: The modulated signal containing only the upper sideband.
Demodulated Signal: The recovered message signal after synchronous detection and low-pass filtering.

The modulated signal shows how the information is encoded within the sideband, while the demodulated signal demonstrates successful recovery of the original message.