Diversity scheme
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In telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by utilizing two or more communication channels with different characteristics. Diversity plays an important role in combating fading 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 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 transferred using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading. Middle-late 20th century microwave radio relay lines often used several regular wideband radio channels, and one protection channel for automatic use by any faded channel. Later examples include:
- OFDM modulation in combination with subcarrier interleaving and forward error correction
- Spread spectrum, for example frequency hopping or DS-CDMA.
- Space diversity: The signal is transferred 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). In the latter case, a diversity combining 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. If the antennas are at a distance in the order of one wavelength, this is called microdiversity. A special case is phased antenna arrays, which also can be utilized for beamforming, MIMO channels and Space–time coding (STC).
- Polarisation diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combining 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 diversity: Achieves antenna diversity gain by utilizing the cooperation of distributed antennas belonging to each node.
[edit] See also
- Space–time coding (STC)
- Antenna diversity
- Macrodiversity
- Diversity combining
- Aperture synthesis
- Cooperative diversity
- Channel access method
- Fresnel zone
- Tropospheric scatter
[edit] External links
- Diversity reception - Background information of the development of Diversity reception devices.
