Diversity scheme
Encyclopedia
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
with different characteristics. Diversity plays an important role in combatting fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
and co-channel interference
Co-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
and avoiding error burst
Error burst
In telecommunication, a burst error or error burst is a contiguous sequence of symbols, received over a data transmission channel, such that the first and last symbols are in error and there exists no contiguous subsequence of m correctly received symbols within the error burst.The integer...
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
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
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
Diversity gain
In wireless communications, diversity gain is the increase in signal-to-interference ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in decibel, and sometimes...
, often measured in decibels.
The following classes of diversity schemes can be identified:
- Time diversityTime diversityTime Diversity is used in digital communication systems to combat that the transmissions channel may suffer from error bursts due to time-varying channel conditions...
: 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 burstError burstIn telecommunication, a burst error or error burst is a contiguous sequence of symbols, received over a data transmission channel, such that the first and last symbols are in error and there exists no contiguous subsequence of m correctly received symbols within the error burst.The integer...
s 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 fadingFadingIn wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. Middle-late 20th century microwave radio relay lines often used several regular widebandWidebandIn communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....
radio channels, and one protection channel for automatic use by any faded channel. Later examples include:- OFDM modulation in combination with subcarrier interleavingInterleavingIn computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory....
and forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels.... - Spread spectrumSpread spectrumSpread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
, for example frequency hopping or DS-CDMA.
- OFDM modulation in combination with subcarrier interleaving
- 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 diversityAntenna diversityAntenna diversity, also known as space 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 indoor environments, there is no clear line-of-sight between transmitter and...
using multiple transmitter antennas (transmit diversityTransmit diversityTransmit 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 instant....
) and/or multiple receiving antennas (reception diversity). In the latter case, a diversity combiningDiversity combiningDiversity 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:...
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 macrodiversityMacrodiversityIn the field of wireless communication, Macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal...
or site diversitySite diversitySite diversity is a technique to limit the effect of rain fade in satellite communications, mainly in the Ka band. The downlink transmissions of satellites cover very large areas, that will have different weather...
. If the antennas are at a distance in the order of one wavelengthWavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, this is called microdiversity. A special case is phased antenna arrayAntenna arrayAntenna array may refer to:*Antenna array a group of isotropic radiators such that the currents running through them are of different amplitudes and phases*Interferometric array of radio telescopes used in radio astronomy...
s, which also can be used for beamformingBeamformingBeamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
, MIMOMIMOIn radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology...
channels and Space–time coding (STC).
- Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combiningDiversity combiningDiversity 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:...
technique is applied on the receiver side.
- Multiuser diversity: Multiuser diversity is obtained by opportunistic user scheduling at either the transmitter or the receiver. Opportunistic user scheduling is as follows: the transmit selects the best user among candidate receivers according to the qualities of each channel between the transmitter and each receiver. In FDD systems, a receiver must feed back the channel quality information to the transmitter with the limited level of resolution.
- Cooperative diversityCooperative diversityCooperative 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 the direct signal in wireless multihop networks...
: Achieves antenna diversity gain by using the cooperation of distributed antennas belonging to each node.
See also
- Space–time coding (STC)
- Antenna diversityAntenna diversityAntenna diversity, also known as space 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 indoor environments, there is no clear line-of-sight between transmitter and...
- MacrodiversityMacrodiversityIn the field of wireless communication, Macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal...
- Diversity combiningDiversity combiningDiversity 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:...
- Aperture synthesisAperture synthesisAperture 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...
- Cooperative diversityCooperative diversityCooperative 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 the direct signal in wireless multihop networks...
- Channel access methodChannel access methodIn telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...
- Fresnel zoneFresnel zoneIn optics and radio communications , a Fresnel zone , named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids which define volumes in the radiation pattern of a circular aperture...
- Tropospheric scatterTropospheric scatterTropospheric scatter is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km...
External links
- Diversity reception - Background information of the development of Diversity reception devices.
- Antenna Diversity Tutorial - www.Antenna-Theory.com
- K. T. Wong http://www.eie.polyu.edu.hk/~enktwong/ & A. K.-Y. Lai, “Inexpensive Upgrade of Base-Station Dumb-Antennas by Two Magnetic Loops for ‘Blind’ Adaptive Downlink Beamforming,” IEEE Antennas & Propagation Magazine, vol. 47, no. 1, pp. 189-193, February 2005.http://www.eie.polyu.edu.hk/~enktwong/ktw/WongKT_APM0205.pdf
- for an example of field measured benefits of reception diversity 2 in mobile handsets technologies , see Christian Le Floc’h, Regis Duval,Gerard Pousset,Gaël Scott,"S-UMTS band radio propagation performances evaluation: reception diversity 2 field measurements methodology and results", June 18, 2008, https://sites.google.com/site/dvbsh4real/articles-1