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telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, a diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity is mainly used in
radio communication Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
and is a common technique for combatting
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
and
co-channel interference Co-channel interference or CCI is crosstalk from two different radio transmitters using the same channel. Co-channel interference can be caused by many factors from weather conditions to administrative and design issues. Co-channel interference ...
and avoiding
error burst In telecommunication, a burst error or error burst is a contiguous sequence of symbols, received over a communication channel, such that the first and last symbols are in error and there exists no contiguous subsequence of ''m'' correctly receive ...
s. It is based on the fact that individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver. Alternatively, a redundant
forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the
multipath propagation In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflec ...
, resulting in a diversity gain, often measured in
decibels The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whose ...
.


Diversity techniques

The following classes of diversity schemes can be identified: *
Time diversity Time diversity is used in digital communication systems to combat that the transmissions channel may suffer from error bursts due to time-varying channel conditions. The error bursts may be caused by fading in combination with a moving receiver, tr ...
: Multiple versions of the same signal are transmitted at different time instants. Alternatively, a redundant
forward error correction code In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
is added and the message is spread in time by means of
bit-interleaving In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
before it is transmitted. Thus,
error burst In telecommunication, a burst error or error burst is a contiguous sequence of symbols, received over a communication channel, such that the first and last symbols are in error and there exists no contiguous subsequence of ''m'' correctly receive ...
s are avoided, which simplifies the error correction. *
Frequency diversity Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
: The signal is transmitted using several frequency channels or spread over a wide spectrum that is affected by frequency-selective
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
. Later examples include: **
OFDM In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital commun ...
modulation in combination with subcarrier interleaving and
forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...
**
Spread spectrum In telecommunication and radio communication, spread-spectrum techniques are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency dom ...
, for example
frequency hopping Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both tra ...
or
DS-CDMA In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. The direct-sequence modulation makes the transmitted signal wider in bandwidth than th ...
. *
Space diversity Antenna diversity, also known as space diversity or spatial diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and ind ...
: The signal is transmitted over several different propagation paths. In the case of wired transmission, this can be achieved by transmitting via multiple wires. In the case of wireless transmission, it can be achieved by
antenna diversity Antenna diversity, also known as space diversity or spatial diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and ind ...
using multiple transmitter antennas (
transmit diversity Transmit diversity is radio communication using signals that originate from two or more independent sources that have been modulated with identical information-bearing signals and that may vary in their transmission characteristics at any given ...
) and/or multiple receiving antennas ( reception diversity). In the latter case, a
diversity combining Diversity combining is the technique applied to combine the multiple received signals of a diversity reception device into a single improved signal. Various techniques Various diversity combining techniques can be distinguished: * Equal-gain co ...
technique is applied before further signal processing takes place. If the antennas are far apart, for example at different cellular base station sites or WLAN access points, this is called
macrodiversity In the field of wireless communication, ''macrodiversity''D. Gesbert, S. Hanly, H. Huang, S. Shamai, O. Simeone, W. YuMulti-cell MIMO cooperative networks: A new look at interferenceIEEE Journal on Selected Areas in Communications, vol. 28, no. 9, p ...
or
site diversity Site diversity is one of six techniques used to improve the reliability of satellite communications by limit atmospheric effects, particularly those caused by rain fade. A diversity scheme is typically required when using frequencies in the Ka, V, ...
. If the antennas are at a distance in the order of one
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, this is called microdiversity. A special case is phased
antenna array An antenna array (or array antenna) is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas (called ''elements'') are usually connected to a single receiver ...
s, which also can be used for
beamforming Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in an antenna array in such a way that signals at particular angles e ...
,
MIMO In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wir ...
channels and
space–time coding In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
(STC). * Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A
diversity combining Diversity combining is the technique applied to combine the multiple received signals of a diversity reception device into a single improved signal. Various techniques Various diversity combining techniques can be distinguished: * Equal-gain co ...
technique is applied on the receiver side. * Multiuser diversity: Multiuser diversity is obtained by opportunistic user scheduling at either the transmitter or the receiver.F. Foukalas and T. Khattab,
Multi-User Diversity with Optimal Power Allocation in Spectrum Sharing under Average Interference Power Constraint
" 2014 IEEE 79th Vehicular Technology Conference (VTC Spring), May 2014
Opportunistic user scheduling is as follows: at any given time, the transmitter selects the best user among candidate receivers according to the qualities of each channel between the transmitter and each receiver. A receiver must feed back the channel quality information to the transmitter using limited levels of resolution, in order for the transmitter to implement Multiuser diversity. *
Cooperative diversity Cooperative diversity is a cooperative multiple antenna technique for improving or maximising total network channel capacities for any given set of bandwidths which exploits user diversity by decoding the combined signal of the relayed signal and t ...
: Achieves antenna diversity gain by using the cooperation of distributed antennas belonging to each node.


Combiner techniques

An important element in communication systems applying diversity schemes is the "Combiner", which processes the redundantly received signals. Combiner technologies are traditionally classified according to Brennan: * ''Maximal-Ratio Combiner'' * ''Equal-Gain Combiner'' * ''Scanning/Switching Combiner'' * ''Selection Combiner'' To combine parallel redundant transmitted longer signal sequences, for example network packets, the principle of a ''Timing Combiner'' was defined in 2012. Similarly working like a ''Selection Combiner'', the first fully received and valid data packet will be immediately further processed, whereas the later arriving redundant packets will be immediately discarded after reception. With this approach, always the faster of the redundant channels "wins", yielding significant performance improvements especially in wireless applications.{{Cite journal, last=Rentschler, first=M., last2=Laukemann, first2=P., date=September 2012, title=Performance analysis of parallel redundant WLAN, journal=Proceedings of 2012 IEEE 17th International Conference on Emerging Technologies Factory Automation (ETFA 2012), pages=1–8, doi=10.1109/ETFA.2012.6489647, isbn=978-1-4673-4737-2


See also

*
Space–time coding In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
(STC) *
Aperture synthesis Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation an ...
*
Cooperative diversity Cooperative diversity is a cooperative multiple antenna technique for improving or maximising total network channel capacities for any given set of bandwidths which exploits user diversity by decoding the combined signal of the relayed signal and t ...
*
Channel access method In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical med ...
*
Fresnel zone A Fresnel zone ( ), named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. The primary wave will travel in a relative straight line fro ...
*
Tropospheric scatter Tropospheric scatter, also known as troposcatter, is a method of communicating with microwave radio signals over considerable distances – often up to and further depending on frequency of operation, equipment type, terrain, and climate facto ...
*
Cyclic delay diversity Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-based telecommunication systems, transforming spatial diversity into frequency diversity and thus avoiding intersymbol interference. CDD was introduced in 2001 and can gain frequency ...


References


External links


Diversity reception
- Background information of the development of Diversity reception devices. Antennas (radio) Radio resource management