Baseband

For example, the electronic output of a microphone is a baseband signal that is analogous to the applied voice audio.

A baseband signal may have frequency components going all the way down to the DC bias, or at least it will have a high ratio bandwidth.

This occupies a higher range of frequencies and has a lower ratio and fractional bandwidth.

[4] Examples are serial cables and local area networks (LANs), as opposed to passband channels such as radio frequency channels and passband filtered wires of the analog telephone network.

[7] The word "BASE" in Ethernet physical layer standards, for example 10BASE5, 100BASE-TX and 1000BASE-SX, implies baseband digital transmission (i.e. that a line code and an unfiltered wire are used).

[8][9] A baseband processor also known as BP or BBP is used to process the down-converted digital signal to retrieve essential data for a wireless digital system.

The baseband processing block in GNSS receivers is responsible for providing observable data: that is, code pseudo-ranges and carrier phase measurements, as well as navigation data.

It is a concept within analog and digital modulation methods for (passband) signals with constant or varying carrier frequency (for example ASK, PSK QAM, and FSK).

signals of each modulation symbol are evident from the constellation diagram.

A key consequence of the usual double-sideband amplitude modulation (AM) is that the range of frequencies the signal spans (its spectral bandwidth) is doubled.

Conversely, some transmission schemes such as frequency modulation use even more bandwidth.

Spectrum of a baseband signal , energy E per unit frequency as a function of frequency f . The total energy is the area under the curve.
On the left is a part of the transmitter, which will take in a stream of baseband IQ data , and use this to amplitude modulate a Local Oscillator's signal, both the standard sine wave from the LO, and also a version which phase shifted by 90° (in-phase and quadrature) - these modulated signals are combined, to form the Intermediate frequency IF representation. In a typical transmitter, the IF would get up-converted, filtered, amplified, then transmitted from an antenna. (These are not shown)
On the right we see an aspect of the receiver. After some low-noise amplification, filtering and down-conversion (not shown) to an IF, the signal is mixed with the in-phase sine from the LO, and also the quadrature version of the LO, giving a complex (or 2-dimensional) representation of the signal. This IQ data could then be supplied to a digital signal processor to extract symbols or data.
Comparison of the equivalent baseband version of a signal and its AM-modulated (double- sideband ) RF version, showing the typical doubling of the occupied bandwidth.