Unipolar encoding is a

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained ...

. A positive voltage represents a 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. Its drawbacks are that it is not

self-clocking In telecommunications and electronics, a self-clocking signal is one that can be decoded without the need for a separate clock signal or other source of synchronization. This is usually done by including embedded synchronization information withi ...

and it has a significant

DC component DC, D.C., D/C, Dc, or dc may refer to: Places * Washington, D.C. (District of Columbia), the capital and the federal territory of the United States * Bogotá, Distrito Capital, the capital city of Colombia * Dubai City, as distinct from t ...

, 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 A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a form ...

each rectangular pulse is only at a positive voltage for half of the

bit period The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented ...

. 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 In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negat ...

(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 In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no D ...

. 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,l) sequences with good auto- and

cross-correlation In signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also known as a ''sliding dot product'' or ''sliding inner-product''. It is commonly used f ...

properties for unipolar environments. They are used in optical communications to enable

CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...

in optical fiber transmission.

References

{{Bit-encoding Encodings Line codes

See also

References