Single sideband Modulation & De-modulation

  1. Definition :

    Single Sideband(SSB) modulation is a type of amplitude modulation (AM) that is used to transmit radio signals. In SSB modulation, the carrier and one of the sidebands are suppressed or removed, leaving only the remaining sideband. This reduces the bandwidth required for transmission and allows more channels to be accommodated in a given frequency band. There are two types of SSB modulation: upper sideband (USB) and lower sideband (LSB). In USB, the carrier and the lower sideband are removed, leaving only the upper sideband. In LSB, the carrier and the upper sideband are removed, leaving only the lower sideband. SSB modulation is commonly used in radio communication systems, particularly in long-distance communications. It is also used in some television broadcasting systems and in amateur radio.

    Single Sideband(SSB) demodulation is the process of extracting the original message signal from a single sideband modulated signal. Demodulation is the inverse process of modulation, which means that the modulated signal is transformed back to the original message signal. In single sideband modulation, one of the sidebands (either upper or lower) and the carrier are suppressed, so the demodulation process is more complex than in conventional AM modulation.

  2. Principle of SSB

    The principle of SSB modulation is to suppress one of the sidebands and the carrier signal to reduce the bandwidth requirements for transmission while retaining the necessary information contained in the signal. This is achieved by filtering out either the upper or lower sideband and the carrier, leaving only the other sideband.
    In radio communications, single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modulation, it uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal the bandwidth of which is twice the maximum frequency of the original baseband signal. Single-sideband modulation avoids this bandwidth increase, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.

  3. Expression for SSB

    Modulation

    The expression for SSB modulation is given by:
    s(t) = Ac[m(t) cos(2πfct) - h(t) sin(2πfct)]
    Ac is the carrier amplitude, m(t) is the message signal, fct is the carrier frequency, h(t) is a Hilbert transform of the message signal m(t).

    Demodulation

    The expression for SSB demodulation is given by:
    Multiply the received SSB signal with a local oscillator (LO) signal that has the same frequency and phase as the carrier wave of the modulated signal:
    s1(t) = s(t) * cos(2πfct)
    Pass the product of the multiplication through a low-pass filter to extract the baseband message signal:
    m(t) = LPF[s1(t)]

    where s(t) is the received SSB signal, fct is the carrier frequency, cos(2πfct) is the local oscillator (LO) signal, s1(t) is the product of the multiplication of the received signal with the LO signal, LPF is a low-pass filter that removes the high-frequency components and leaves the baseband message signal m(t).

  4. Advantages of SSB


    • Improved signal-to-noise ratio (SNR): SSB modulation reduces noise and interference by eliminating one of the sidebands and the carrier wave. This results in an improved SNR, which means that the signal is clearer and easier to understand.
    • Reduced bandwidth requirements: SSB modulation requires less bandwidth than conventional AM modulation since it only transmits one sideband instead of both sidebands and the carrier wave. This allows more channels to be accommodated in a given frequency band.
    • Longer transmission range: SSB modulation has a longer transmission range than conventional AM modulation because it is less affected by atmospheric and man-made noise.
    • Lower power consumption: SSB modulation requires less power than conventional AM modulation since it transmits only one sideband, which reduces the overall power consumption of the system.
    • Higher channel capacity: Due to the reduced bandwidth requirements, SSB modulation can support a higher number of channels in the same frequency band, which increases the channel capacity of the system.

  5. Disdvantages of SSB


    • Complex demodulation: Demodulating an SSB signal is more complex than demodulating an AM signal since it requires a specialized circuit called a product detector or synchronous detector.
    • Carrier and synchronization recovery: In SSB modulation, the carrier and synchronization information is removed from the signal, which makes carrier and synchronization recovery more difficult.
    • Speech distortion: SSB modulation can cause speech distortion if the suppressed sideband contains important speech components. This can affect the intelligibility of the transmitted message.
    • Limited bandwidth utilization: SSB modulation utilizes only one sideband and the carrier wave, which limits the bandwidth utilization of the signal.