In
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 tha ...
, 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 transm ...
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 ...
and
co-channel interference and avoiding
error bursts. 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 code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the
multipath propagation, 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 or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a pow ...
.
Diversity techniques
The following classes of diversity schemes can be identified:
*
Time diversity: Multiple versions of the same signal are transmitted at different time instants. Alternatively, a redundant
forward error correction code is added and the message is spread in time by means of
bit-interleaving before it is transmitted. Thus,
error bursts are avoided, which simplifies the error correction.
*
Frequency diversity: 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 ...
. Later examples include:
**
OFDM modulation in combination with subcarrier
interleaving
Interleaving may refer to:
* Interleaving, a technique for making forward error correction more robust with respect to burst errors
* An optical interleaver, a fiber-optic device to combine two sets of dense wavelength-division multiplexing (DW ...
and
forward error correction
**
Spread spectrum, for example
frequency hopping or
DS-CDMA.
*
Space diversity: 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 using multiple transmitter antennas (
transmit diversity) and/or multiple receiving antennas (
reception diversity 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 com ...
). 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 or
site diversity. 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, tr ...
, this is called
microdiversity. A special case is phased
antenna arrays, which also can be used for
beamforming,
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 w ...
channels and
space–time coding (STC).
*
Polarization
Polarization or polarisation may refer to:
Mathematics
*Polarization of an Abelian variety, in the mathematics of complex manifolds
*Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
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
Multi-user software is computer software that allows access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leavin ...
: 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 ...
: 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 (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 and ...
*
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 ...
*
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 ...
*
Fresnel zone
*
Tropospheric scatter
*
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