Editing Unipolar encoding (section)

{{Short description|Binary line code in telecommunication}}
{{More citations needed|date=March 2022}}

'''Unipolar encoding''' is a [[line code]]. A positive voltage represents a [[Principle of bivalence|binary]] 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to [[on-off keying]] in modulation.<ref>{{Cite book |last=K. |first=Prasad, K. V. K. |url=http://worldcat.org/oclc/443732841 |title=Principles of digital communication systems and computer networks |date=2004 |publisher=Charles River Media |isbn=1-58450-329-7 |oclc=443732841}}</ref>

Its drawbacks are that it is not [[self-clocking signal|self-clocking]] and it has a significant [[DC component]], which can be halved by using [[return-to-zero]], where the signal returns to zero in the middle of the bit period.  With a 50% [[duty cycle]] each rectangular pulse is only at a positive voltage for half of the [[bit period]]. This is ideal if one symbol is sent much more often than the other and power considerations are necessary, and also makes the signal self-clocking.

'''NRZ (Non-Return-to-Zero)''' -
Traditionally, a unipolar scheme was designed as a [[non-return-to-zero]] (NRZ) scheme, in which the positive voltage defines bit 1 and the zero voltage defines bit 0.  It is called NRZ because the signal does not return to zero at the middle of the bit, as instead happens in other line coding schemes, such as [[Manchester code]]. Compared with its polar counterpart, polar NRZ, this scheme applies a DC bias to the line and unnecessarily wastes power – The normalized power (power required to send 1 bit per unit line resistance) is double that for polar NRZ.  For this reason, unipolar encoding is not normally used in data communications today.

An '''Optical Orthogonal Code (OOC)''' is a family of (0,1) sequences with good [[Autocorrelation|auto]]- and [[cross-correlation]] properties for ''unipolar'' environments.<ref>{{Cite journal |last1=Chung |first1=Fan R.K. |last2=Salehi |first2=Jawad A. |last3=Wei |first3=Victor K. |date=May 1989 |title=Optical orthogonal codes: design, analysis and applications |url=https://mathweb.ucsd.edu/~fan/mypaps/fanpap/97_opticalorthocodes.pdf |journal=IEEE Transactions on Information Theory |volume=35 |issue=3 |pages=595–604 |doi=10.1109/18.30982 |url-status=live |archive-url=https://web.archive.org/web/20231225160424/https://mathweb.ucsd.edu/~fan/mypaps/fanpap/97_opticalorthocodes.pdf |archive-date=2023-12-25}}</ref> They differ from codes developed for electrical communication which are usually ''bipolar''. i.e. (&minus;1,1) sequences.  They are used in optical communications to enable [[CDMA]] in optical fiber transmission.<ref>{{Cite journal |last1=Maric |first1=Svetislav V. |last2=Hahm |first2=Mark D. |last3=Titlebaum |first3=Edward L. |date=February 1995 |title=Construction and performance analysis of a new family of optical orthogonal codes for CDMA fiber-optic networks |url=https://ieeexplore.ieee.org/document/380066 |url-access=subscription |journal=IEEE Transactions on Communications |volume=43 |issue=2/3/4 |pages=485–489 |doi=10.1109/26.380066 |issn=0090-6778}}</ref>