Baseband

A baseband signal or lowpass signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not).[2]

A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,[3] for example the upper cut-off frequency of a low-pass filter. By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.

A baseband channel or lowpass channel (or system, or network) is a communication channel that can transfer frequencies that are very near zero.[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. Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls.

Digital baseband transmission, also known as line coding,[5] aims at transferring a digital bit stream over baseband channel, typically an unfiltered wire, contrary to passband transmission, also known as carrier-modulated transmission.[6] Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.[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.[7]

IQ modulation and demodulation

An equivalent baseband signal or equivalent lowpass signal is—in analog and digital modulation methods for (passband) signals with constant or varying carrier frequency (for example ASK, PSK QAM, and FSK)—a complex valued representation of the modulated physical signal (the so-called passband signal or RF signal). The equivalent baseband signal is ${\displaystyle Z(t)=I(t)+jQ(t)\,}$ where ${\displaystyle I(t)}$ is the inphase signal, ${\displaystyle Q(t)}$ the quadrature phase signal, and ${\displaystyle j}$ the imaginary unit. In a digital modulation method, the ${\displaystyle I(t)}$ and ${\displaystyle Q(t)}$ signals of each modulation symbol are evident from the constellation diagram. The frequency spectrum of this signal includes negative as well as positive frequencies. The physical passband signal corresponds to

${\displaystyle I(t)\cos(\omega t)-Q(t)\sin(\omega t)=\mathrm {Re} \{Z(t)e^{j\omega t}\}\,}$

where ${\displaystyle \omega }$ is the carrier angular frequency in rad/s.[10]