Diversity combining
Encyclopedia
Diversity combining is the technique applied to combine the multiple received signals of a diversity
reception device into a single improved signal.
Sometimes more than one combining technique is used – for example, lucky imaging
uses selection combining to choose (typically) the best 10% images, followed by equal-gain combining of the selected images.
Other signal combination techniques have been designed for noise reduction and have found applications in single molecule biophysics
, chemometrics
among other disciplines.
, where vehicle-mounted
and hand-held radios communicate with a base station
radio over a single frequency, space diversity is achieved by having several receivers at different sites. Diversity combining, or voting, in two-way radio systems is a method for improving talk-back range from walkie-talkie
and vehicular mobile radios.
The receivers are connected to a device referred to as a voting comparator or voter.
The voting comparator performs an evaluation of all received signals and picks the most usable received signal. In repeater
systems, the voted signal is retransmitted. In simplex systems, it goes to the console speaker at the base station. Audio from a receiver that is not voted is ignored. Voting comparators in analog FM
systems can switch between receivers in tenths- or hundredths-of-a-second, (faster than one syllable). So long as an intelligible signal gets to a single receiver in the system, the repeated audio, or audio sent to the console speaker, would be intelligible.
In this arrangement, receivers at remote sites are connected to the voting comparator by private telephone lines, a channel in a D4 channel bank on a DS1
, or an analog microwave baseband channel.
, or FM receiver limiter voltage, at the remote receiver. This worked poorly because it did not account for microwave baseband noise or noisy telephone company circuits.
Newer voting comparators compare signal-to-noise ratio at the voting comparator, accounting for end-to-end noise, bad phone lines, poor level discipline, as well as the best diversity reception path.
s), base stations generally talk out further than they can receive. Voting among several receivers at different sites increases the probability that one of the receivers will acquire a usable signal from two-way radios in a system..
A potential problem with receivers located at high-elevation receiver sites is that they may acquire signals from distant counties, prefectures, or other provinces. These unwanted, distant signals can be stronger than desired signals from local walkie-talkies. The distant signals may block local weak signals in some cases. Having several receive sites increases the probability that one of the sites will receive the local signal in the presence of a distant, undesired one. Selective calling
can eliminate users having to listen to the audio of distant signals even though the distant signals are within receive range of one or more receivers.
In some installations, diversity combining equipment is configured to lock on a receiver. For example, in some rural, regional coverage systems, the receivers each cover a unique geographic area. There is not much overlap. If the system consisted of two sites, north and south, it would pick the better of the two and remain locked on that receiver until the transmission ended. This works better with mobile radios because their signal strengths tend to be steady.
In some cases this vote-and-hold is used to steer transmitter selection. Consider the case of a regional system with two base stations: north and south. If the diversity combining equipment votes "north," the next time the dispatcher presses the transmit button, the north transmitter will key. Called transmitter steering, this is supposed to automate transmitter selection in systems where more than one transmitter site is available. In some instances it doesn't work well.
In mobile data systems, the vote lock option is preferred because constant switching between receivers causes lost data packets. The diversity combining equipment switches fast enough that syllables are not lost but not fast enough that bits are not lost. Mobile data systems usually originate with modems in mobile radios. Mobile radios usually produce solid signals into more than one receive site, so the signal strengths are strong enough for vote locking to work well.
Diversity scheme
In telecommunications, 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 plays an important role in combatting fading and co-channel interference and avoiding error bursts...
reception device into a single improved signal.
Various techniques
Various diversity combining techniques can be distinguished:- Selection combining: Of the N received signals, the strongest signal is selected. When the N signals are independent and Rayleigh distributed, the expected diversity gainDiversity gainIn 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...
has been shown to be
, expressed as a power ratio. Therefore, any additional gain diminishes rapidly with the increasing number of channels. - Switched combining: The receiver switches to another signal when the currently selected signal drops below a predefined threshold. This is a less efficient technique than selection combining.
- Equal-gain combining: All the received signals are summed coherently.
- Maximal-ratio combiningMaximal-ratio combiningIn telecommunications, maximal-ratio combining is a method of diversity combining in which: the signals from each channel are added together, the gain of each channel is made proportional to the rms signal level and inversely proportional to the mean square noise level in that channel. different...
is often used in large phased-array systems: The received signals are weighted with respect to their SNRSignal-to-noise ratioSignal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
and then summed. The resulting SNR yields
where 
is SNR of the received signal 
.
Sometimes more than one combining technique is used – for example, lucky imaging
Lucky imaging
Lucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...
uses selection combining to choose (typically) the best 10% images, followed by equal-gain combining of the selected images.
Other signal combination techniques have been designed for noise reduction and have found applications in single molecule biophysics
Biophysics
Biophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...
, chemometrics
Chemometrics
Chemometrics is the science of extracting information from chemical systems by data-driven means. It is a highly interfacial discipline, using methods frequently employed in core data-analytic disciplines such as multivariate statistics, applied mathematics, and computer science, in order to...
among other disciplines.
Switched combining two-way radio example
In land-mobile radioTwo-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
, where vehicle-mounted
Mobile Radio
This article is about professional equipment. For mobile radios used in amateur radio, see amateur radio mobile operation. Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either...
and hand-held radios communicate with a base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
radio over a single frequency, space diversity is achieved by having several receivers at different sites. Diversity combining, or voting, in two-way radio systems is a method for improving talk-back range from walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
and vehicular mobile radios.
The voting comparator performs an evaluation of all received signals and picks the most usable received signal. In repeater
Radio repeater
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. This article refers to professional, commercial, and...
systems, the voted signal is retransmitted. In simplex systems, it goes to the console speaker at the base station. Audio from a receiver that is not voted is ignored. Voting comparators in analog FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
systems can switch between receivers in tenths- or hundredths-of-a-second, (faster than one syllable). So long as an intelligible signal gets to a single receiver in the system, the repeated audio, or audio sent to the console speaker, would be intelligible.
In this arrangement, receivers at remote sites are connected to the voting comparator by private telephone lines, a channel in a D4 channel bank on a DS1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
, or an analog microwave baseband channel.
How signals are evaluated
The earliest voting comparators relied on a tone encoded on a separate audio path, requiring each receiver site to have a 4-wire circuit or two audio paths. The pitch of the tone changed to represent the received signal levelSignal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
, or FM receiver limiter voltage, at the remote receiver. This worked poorly because it did not account for microwave baseband noise or noisy telephone company circuits.
Newer voting comparators compare signal-to-noise ratio at the voting comparator, accounting for end-to-end noise, bad phone lines, poor level discipline, as well as the best diversity reception path.
Walkie-talkie talk-back range
When communicating with hand-held radios (walkie-talkieWalkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
s), base stations generally talk out further than they can receive. Voting among several receivers at different sites increases the probability that one of the receivers will acquire a usable signal from two-way radios in a system..
Interference reduction
Diversity combining reduces one possible single-point failure: any single receiver failure, or local interference to a single receiver, will not block reception on the entire system. Equipment sites can host many radio transmitters and receivers. A single site is subject to local, site-specific interfering signals. These interfering signals may come and go as transmitters switch on and off.A potential problem with receivers located at high-elevation receiver sites is that they may acquire signals from distant counties, prefectures, or other provinces. These unwanted, distant signals can be stronger than desired signals from local walkie-talkies. The distant signals may block local weak signals in some cases. Having several receive sites increases the probability that one of the sites will receive the local signal in the presence of a distant, undesired one. Selective calling
Selective calling
In a conventional, analog two-way radio system, a standard radio has noise squelch or carrier squelch which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel...
can eliminate users having to listen to the audio of distant signals even though the distant signals are within receive range of one or more receivers.
Coverage
A minimum of 95% coverage is cited in literature for critical or emergency service two-way radio systems. One definition of system coverage is Telecommunications Industry Association (TIA) TSB-88A standard.Vote-lock or vote-and-hold option
The majority of installations using diversity combining equipment continually evaluate the best received signal against all other signals. Throughout the length of a received transmission, the comparator may switch receivers as often as every few tenths of a second. As a walkie-talkie user causes a signal fade by turning their head, or a passing tractor-trailer rig blocks their signal at the voted receive site, the combining unit rapidly changes to a different receiver.In some installations, diversity combining equipment is configured to lock on a receiver. For example, in some rural, regional coverage systems, the receivers each cover a unique geographic area. There is not much overlap. If the system consisted of two sites, north and south, it would pick the better of the two and remain locked on that receiver until the transmission ended. This works better with mobile radios because their signal strengths tend to be steady.
In some cases this vote-and-hold is used to steer transmitter selection. Consider the case of a regional system with two base stations: north and south. If the diversity combining equipment votes "north," the next time the dispatcher presses the transmit button, the north transmitter will key. Called transmitter steering, this is supposed to automate transmitter selection in systems where more than one transmitter site is available. In some instances it doesn't work well.
In mobile data systems, the vote lock option is preferred because constant switching between receivers causes lost data packets. The diversity combining equipment switches fast enough that syllables are not lost but not fast enough that bits are not lost. Mobile data systems usually originate with modems in mobile radios. Mobile radios usually produce solid signals into more than one receive site, so the signal strengths are strong enough for vote locking to work well.
See also
- Diversity schemeDiversity schemeIn telecommunications, 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 plays an important role in combatting fading and co-channel interference and avoiding error bursts...
s - 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....
- 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...
- 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...
- smart antennaSmart antennaSmart antennas are antenna arrays with smart signal processing algorithms used to identify spatial signal signature such as the direction of arrival of the signal, and use it to calculate beamforming vectors, to track and locate the antenna beam on the mobile/target...