I am taking a graduate level course in Networking theory and applications and I am still a bit shaky on the difference between baseband and pass band scheming. The book I am using is Computer Networks 5th edition -- Andrew S Tanenbaum

To my knowledge, passband scheming uses the changing of frequency, amplitude, and phase to modulate a analog signal. And base band uses encoding techniques like NRZ, NRZi, and Manchester encoding. How does this work along side, frequency multiplexing and CDMA for multiple device access?

And can someone give me a layman explanation of carrier signals? And how I can maybe think of them as offsets like in assembly languages. Thanks for helping!


1 Answer 1


There is no unique definition, but in general baseband signals have a spectrum that starts close to DC. Most analog information sources that are considered in communications are baseband: audio, video, signals from sensors, images, etc. Also, any signal that is a sum of pulses of different shapes is a baseband signal; examples are NRZ and Manchester-encoded signals. Yet another definition is that a baseband signal has not been modulated, or its spectrum has not been shifted, for the purpose of matching it to a particular communication channel.

Passband signals were originally baseband, but were modulated, or frequency-shifted (or up-converted) for some purpose, usually to make them fit in a passband channel. Consider the case of AM radio. An audio signal is low-pass filtered to 10 kHz, resulting in a baseband signal with a spectrum from 0 Hz (or strictly, 20 Hz) to 10 kHz. Then, this signal is shifted up in frequency (upconverted) to make it fit into the 20 kHz channel assigned to the AM station. The receiver down-converts the signal back to baseband and plays it over the loudspeakers.

As you say, the upconversion process involves changing the amplitude, frequency and/or phase of a high-frequency carrier signal proportionally to the information-bearing baseband signal. The receiver uses the amplitude, frequency and/or phase of the carrier to reconstruct the baseband signal.


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