Symbol (communications engineering)

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In digital transmission technology and communications technology , symbols are used to denote the various character units for transmitting the information content . The number of symbols transmitted per period is the symbol rate and is expressed in the unit of baud .


The finite and discrete set of symbols with the index that can be used in a particular application is called the symbol alphabet and the symbol alphabet is mapped into a constellation for transmission .

The mapping of a certain number of bits onto a specific symbol takes place within the framework of line coding . A symbol is mapped to specific physical parameters of a carrier signal as a function of the modulation .

The simplest and most important symbolic alphabets include unipolar symbols with the set and the bipolar symbols with the set . In these cases the information content of a bit can be assigned to a symbol. Only in this case is the bit rate , expressed in bits per second, equal to the symbol rate, expressed in baud . For example, with phase modulation (BPSK) there are two symbols in the form of the phase position 0 ° and 180 ° of the carrier signal. In the field of digital technology , the two symbols are represented by two different electrical voltages of 0 V and 5 V, as is common in transistor-transistor logic .

In the case of symbolic alphabets with more than two symbols, a higher information content than one bit per symbol can be achieved. For example, with four symbols, two bits per symbol can be represented, as is the case with quadrature phase shift keying . Non-integer bits per symbol result in modulations (NIBS), the constellations of which have numbers of symbols that deviate from the form .

The more symbols a symbol alphabet comprises, the more difficult it is to be able to reliably differentiate between adjacent symbols in real transmission systems, represented in different amplitude values ​​and phase positions of a carrier signal, since physically induced interference such as noise is also added. In order to achieve a specific bit rate on a transmission channel , either the symbol rate can be changed, this is equivalent to changing the bandwidth of the transmission channel, or the number of symbols used can be changed. The channel capacity describes the relationship .

Icon representation

Depending on the transmission medium and transmission method, a symbol can be expressed using different physical quantities. Common sizes for the representation of symbols in communications engineering are:

The actual transmission of information usually takes place using digital modulation techniques .

supporting documents

  1. Franklin, Daniel; Burnett, Ian: On Non-Integer Bits-per-Symbol Modulation in DMT Modems ; University of Wollongong; 2002 Online (PDF; 98 kB)


  • Karl Dirk Kammeyer, Volker Kühn: MATLAB in communications engineering . J. Schlembach Fachverlag, 2001, ISBN 3-935340-05-2 .