Editing Baseband

{{Short description|Range of frequencies occupied by an unmodulated signal}}

[[File:bandlimited2.svg|thumb|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.]]

In [[telecommunications]] and [[signal processing]], '''baseband''' is the range of frequencies occupied by a [[signal]] that has not been [[modulation|modulated]] to higher frequencies.<ref>Jeff Rutenbeck, ''Tech Terms: What Every Telecommunications and Digital Media Professional Should Know'', [https://books.google.com/books?id=dfFsAAAAQBAJ&pg=PA24 p. 24], CRC Press, 2012 {{ISBN|1136034501}}</ref> Baseband signals typically originate from [[transducer]]s, converting some other variable into an electrical signal.  For example, the electronic output of a microphone is a baseband signal that is analogous to the applied voice audio. In conventional [[analog signal|analog]] [[radio broadcasting]], the baseband [[audio signal]] is used to [[Modulation|modulate]] an [[Radio frequency|RF carrier signal]] of a much higher frequency.

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]].  A modulated baseband signal is called a [[passband signal]].  This occupies a higher range of frequencies and has a lower ratio and [[fractional bandwidth]].

== Various uses ==

=== Baseband signal ===
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).<ref>{{cite book|url=https://books.google.com/books?id=deHQeNxHhyEC&dq=baseband-signal+lowpass-signal&pg=PA65|title=Communication System Design Using Dsp Algorithms: With Laboratory Experiments for the TMS320C30|author=Steven Alan Tretter|publisher=Springer|year=1995|isbn=0-306-45032-1}}</ref>

A ''baseband [[bandwidth (signal processing)|bandwidth]]'' is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,<ref>{{cite book | title = Information, Transmission, Modulation and Noise: A Unified Approach to Communication Systems | author = Mischa Schwartz | publisher = McGraw-Hill | year = 1970| isbn = 9780070557611 | url = https://books.google.com/books?id=-gkjAAAAMAAJ&q=baseband-bandwidth }}</ref> 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.

=== Baseband channel ===
A ''baseband channel'' or ''lowpass channel'' (or ''system'', or ''network'') is a [[communication channel]] that can transfer frequencies that are very near zero.<ref>{{cite book | title = Digital Signal Transmission | author = Chris C. Bissell and David A. Chapman | publisher = Cambridge University Press | year = 1992 | isbn = 0-521-42557-3 | url = https://books.google.com/books?id=cj12nN2uW0AC&dq=called-baseband-channels&pg=PA149}}</ref> Examples are serial cables and [[local area network]]s (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 ===
{{Main article|Line code}}

Digital baseband transmission, also known as [[line coding]],<ref>{{cite book | title = CMOS Data Converters for Communications | author = Mikael Gustavsson and J. Jacob Wikner | publisher = Springer | year = 2000 | isbn = 0-7923-7780-X | url = https://books.google.com/books?id=D_I2XvNOc4wC&dq=passband+baseband&pg=PA28}}</ref> aims at transferring a digital bit stream over baseband channel, typically an unfiltered wire, contrary to [[passband]] transmission, also known as ''carrier-modulated'' transmission.<ref>{{cite book | title = Digital Baseband Transmission and Recording | author = Jan W. M. Bergmans | publisher = Springer | isbn = 0-7923-9775-4 | year = 1996 | url =https://books.google.com/books?id=TN-sTybrCLsC&dq=baseband+carrier-modulated&pg=PR11}}</ref> Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.<ref name=":0">{{Cite web |title=Baseband Processing - Navipedia |url=https://gssc.esa.int/navipedia/index.php/Baseband_Processing |access-date=2022-07-04 |website=gssc.esa.int}}</ref>

==== {{anchor|Baseband_Ethernet}}Baseband transmission in Ethernet ====
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).<ref>IEEE 802.3 ''1.2.3 Physical layer and media notation''</ref><ref name="IEEEGetProgram">{{cite web|title=IEEE Get Program|url=http://standards.ieee.org/about/get/802/802.3.html|archive-url=https://web.archive.org/web/20101125111240/http://standards.ieee.org/about/get/802/802.3.html|url-status=dead|archive-date=November 25, 2010|website=[[IEEE]]|publisher=IEEE|access-date=29 March 2017}}</ref>

=== Baseband processor ===
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 [[Satellite navigation|GNSS]] receivers is responsible for providing observable data: that is, code pseudo-ranges and carrier phase measurements, as well as navigation data.<ref name=":0" />

=== Equivalent baseband signal ===
[[File:IQ Mod Demod block diagram.svg|thumb|right|400px|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)<br />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.]]

An ''equivalent baseband signal'' or ''equivalent lowpass signal'' is a complex valued representation of the modulated physical signal (the so-called [[passband]] signal or [[radio frequency|RF]] signal). It is a concept within analog and digital modulation methods for (passband) signals with constant or varying [[carrier frequency]] (for example [[Amplitude-shift keying|ASK]], [[Phase-shift keying|PSK]] [[Quadrature amplitude modulation|QAM]], and [[Frequency-shift keying|FSK]]).  The equivalent baseband signal is <math>Z(t)=I(t)+jQ(t)\,</math> where <math>I(t)</math> is the inphase signal, <math>Q(t)</math> the quadrature phase signal, and <math>j</math> the [[imaginary unit]]. This signal is sometimes called ''[[IQ data]]''.  In a digital modulation method, the <math>I(t)</math> and <math>Q(t)</math> 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 
:<math>I(t)\cos(\omega t) - Q(t)\sin(\omega t) = \mathrm{Re}\{Z(t)e^{j\omega t}\}\,</math>
where <math>\omega</math> is the carrier [[angular frequency]] in rad/s.<ref name=Proakis>Proakis, John G. ''Digital Communications'', 4th edition. McGraw-Hill, 2001. p150</ref>

== Modulation ==
A signal at baseband is often used to [[modulation|modulate]] a higher frequency [[carrier signal]] in order that it may be transmitted via radio. Modulation results in shifting the signal up to much higher frequencies (radio frequencies, or RF) than it originally spanned. A key consequence of the usual [[double sideband|double-sideband]] [[amplitude modulation]] (AM) is that the range of frequencies the signal spans (its spectral [[Bandwidth (signal processing)|bandwidth]]) is doubled. Thus, the RF bandwidth of a signal (measured from the lowest frequency as opposed to 0&nbsp;Hz) is twice its baseband bandwidth. Steps may be taken to reduce this effect, such as [[single-sideband modulation]]. Conversely, some transmission schemes such as [[frequency modulation]] use even more bandwidth.

The figure below shows AM modulation:

[[File:Baseband to RF.svg|frame|center|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.]]

== See also ==
{{Wiktionary}}
* [[Complex envelope]]
* [[Broadband]]
* [[In-phase and quadrature components]]
* [[Narrowband]]
* [[Wideband]]

== References ==
{{Reflist}}

{{Authority control}}

[[Category:Computer network technology]]
[[Category:Signal processing]]