Amplitude Modulation with Multiple Signals

Amplitude modulation with multiple signals is the process of transmitting multiple information signals over the same carrier frequency by using amplitude modulation (AM) techniques.  In this context, each of the information signals modulates the amplitude of the carrier signal in different ways.  The general principle of Amplitude Modulation (AM) involves varying the amplitude of a carrier signal in proportion to the amplitude of the input message signal.  When dealing with multiple signals, two common scenarios arise:

Multiplexing Multiple Signals  -  In this case, several independent signals (voice, data, music) are transmitted simultaneously over the same medium by using different carrier frequencies (frequency-division multiplexing, FDM).  The signals are multiplexed before being modulated onto the carrier and are later demultiplexed at the receiver.

• Frequency Division Multiplexing (FDM)  -  Each signal is modulated onto a different carrier frequency, allowing multiple signals to occupy different portions of the frequency spectrum.
• Each modulated signal occupies its own unique frequency band, ensuring no overlap.

AM with Multiple Message Signals on a Single Carrier  -  In some cases, multiple baseband signals can be added together to form a composite signal that modulates a single carrier.  This is typically used for stereo audio transmissions.

• In this scenario, the final modulated waveform represents the combined effect of all signals.
• The receiver separates these different signals from the composite AM signal using various decoding techniques.

Key Challenges

• Interference  -  Overlapping signals can interfere with each other, leading to distortion or loss of signal quality.
• Bandwidth Limitations  -  Each signal requires its own bandwidth, and if the signals are too close in frequency, they may interfere with each other.
• Signal Separation  -  The receiver needs to be designed to separate and demodulate the individual signals, especially in multiplexing scenarios.

 

Amplitude Modulation with Multiple Signals formula
\( P_t \;=\; P_c \; [\; 1 + ( m_t^2 \;/\; 2 )\;] \)
Symbol	English	Metric
\( P_t \) = Total Transmitted Power	\(W\)	\(kg-m^2\;/\;s^3\)
\( P_c \) = Unmodulated Carrier Power	\(W\)	\(kg-m^2\;/\;s^3\)
\( m_t \) = Total Modulation Index	\(dimensionless\)	\(dimensionless\)