Diversity technology is used to compensate for fading channel loss, which is usually realized by two or more receiving antennas.Like equalizer, it improves the transmission quality of wireless communication channel without increasing transmission power and bandwidth.In mobile communication, the receiver of base station and mobile station can adopt diversity technology.
Diversity is a method that the receiver independently processes the fading characteristics it receives to reduce the fluctuation of signal level.Diversity refers to decentralized transmission and centralized reception.Decentralized transmission enables the receiver to obtain multiple statistically independent fading signals with the same information.Centralized reception is the combination (selection and combination) of multiple statistically independent fading signals received by the receiver to reduce the impact of fading.
According to the principle of signal theory, if a copy of the original transmitted signal with other attenuation degrees is provided to the receiver, it will help to determine the received signal correctly.This method of improving the correct decision rate of the received signal by providing multiple copies of the transmission signal is called diversity.Diversity technology is used to compensate for fading channel loss. It usually uses the characteristics of uncorrelation between independent samples of the same signal in the wireless propagation environment, and uses certain signal combining technology to improve the received signal to resist the adverse effects caused by fading.Spatial diversity can overcome spatial selective fading, but the distance between diversity receivers must meet the basic condition of more than three times the wavelength.
Chinese name
Diversity technology
Foreign name
Diversity
According to
Signal Theory Principle
influence factor
Fading effect
Role
Helps to make correct decisions on received signals
In practical mobile communication systems, mobile stations often work in urban buildings or other complex geographical environments, and the speed and direction of movement are arbitrary.After the transmitted signal passes through the propagation path of reflection, scattering, etc., the signal arriving at the receiving end is often the superposition of multiple signals with different amplitude and phase, which makes the amplitude of the received signal fluctuate randomly, forming multi-path fading.The signal components of different paths have different propagation delay, phase and amplitude, and are added with channel noise. Their superposition will make the composite signals cancel or enhance each other, leading to serious fading.This fading will reduce the available useful signal power and increase the impact of interference, causing distortion, waveform broadening, waveform overlap and distortion of the received signal of the receiver, and even causing a large number of errors in the output of the demodulator of the communication system, so that communication is completely impossible.In addition, if the transmitter or receiver is in a moving state, or the channel environment changes, the channel characteristics will change randomly with time, and the received signal will have more serious distortion due to the Doppler effect.In practical mobile communication, there is shadow fading in addition to multi-path fading.When the signal is blocked by tall buildings (for example, the mobile station moves in front of the building that is away from the base station) or terrain fluctuations, the amplitude of the received signal will be reduced.In addition, changes in meteorological conditions also affect the propagation of signals, resulting in changes in the amplitude and phase of received signals.These are the unique characteristics of mobile channels, which bring adverse effects to mobile communications.
Figure 1 Diversity technology in mobile communication
In order to improve the performance of mobile communication systems, diversity, equalization and channel coding can be used to improve the quality of received signals. They can be used alone or in combination.
Fundamentals
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The basic principle of diversity is to receive multiple copies of the same information through multiple channels (time, frequency or space). Because the transmission characteristics of multiple channels are different, the fading of multiple copies of the signal will not be the same.The receiver can correctly recover the original transmission signal by using the information contained in multiple copies.If diversity technology is not used, the transmitter must send higher power under the condition of limited noise to ensure normal link connection when the channel condition is poor.In the mobile wireless environment, the battery capacity of the handheld terminal is very limited, so the power available in the reverse link is also very limited. The diversity method can reduce the transmission power, which is very important in mobile communication.
Figure 2 Diversity technology
Diversity technology includes two aspects: one is decentralized transmission, which enables the receiver to obtain multiple statistically independent fading signals carrying the same information;The second is centralized processing, that is, combining multiple statistically independent fading signals received by the receiver to reduce the impact of fading.Therefore, the most important condition to obtain diversity effect is that each signal should be "uncorrelated".
Diversity is a technical measure that transmits the same information through two or more channels to mitigate the impact of fading.Diversity technology includes diversity transmission technology anddiversity reception From the type of diversity, space diversity and frequency diversity are used more frequently. In addition, there are the following:
The combination of spatial diversity and frequency diversity, that is, the sending station sends the same information with two frequencies, and the receiving station receives signals of different frequencies with two vertically separated antennas, and then synthesizes or selects them, is called hybrid diversity.In addition, there are station diversity, time diversity, angle diversity, etc.
Due to the harsh propagation environment, microwave signals will generate deep fading and Doppler frequency shift, which will reduce the reception level to near the thermal noise level, and the phase will also change randomly with time, leading to the decline of communication quality.In this regard, diversity reception technology is used to reduce the impact of fading, obtain diversity gain and improve reception sensitivity.
Technical classification
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There are two common diversity methods: macro diversity and micro diversity.
Macro diversity
Macro diversity, also known as "multi base diversity", is mainly used in the diversity technology of cellular systems.In macro diversity, multiple base stations are set in different geographical locations and directions, and simultaneously communicate with a mobile station in the cell.As long as the signal propagation in all directions is not affected by shadow effect or terrain at the same time and serious slow fading occurs, this method can ensure that communication will not be interrupted.It is a technology to reduce slow fading.[1]
Differential set
Differential set is a diversity technology to reduce the impact of fast fading, which is often used in various wireless communication systems.The main technologies used in differential set include: space diversity, polarization diversity, frequency diversity, field component diversity, angle diversity, time diversity and other diversity technologies.
(1) Spatial diversity
The basic principle of spatial diversity is to receive the same signal at any two different locations. As long as the distance between the two locations is large to a certain extent, the signal fading received at the two locations is irrelevant, that is, the fast fading has spatial independence.
Spatial diversity, also known as antenna diversity, is the most widely used diversity technology in wireless communication.
Space diversity requires at least two antennas, and the distance between them is d. The separation distance d is related to the working wavelength, ground objects and antenna height. In mobile communication, it is usually taken as: urban d=0.5, suburban d=0.8. The greater the d value, the weaker the correlation.
Figure 3 Spatial Diversity
(2) Frequency diversity
The basic principle of frequency diversity is that the fading of two signals whose frequency spacing is greater than the correlation bandwidth is uncorrelated. Therefore, multiple frequencies can be used to transmit the same information to achieve frequency diversity.
According to the definition of relevant bandwidth, namely:
Where is delay extension.In urban area,=0.3 μ s, Bc=53kHz.
Frequency diversity requires two transmitters to send the same signal and two receivers to receive the same signal.
This diversity technology is mostly used in the frequency division duplex (FDM) line of sight microwave communication.Due to the propagation and refraction of the troposphere, sometimes deep fading occurs in the propagation.
In actual use, it is often called 1 ∶ N protection switching mode.When diversity is required, the corresponding services are switched to a spare channel.Its disadvantage is that not only standby switching is required, but also several receivers equal to the number of channels used in frequency diversity are required.
Figure 4 Frequency Diversity
(3) Polarization diversity
The basic principle of polarization diversity is that two electromagnetic waves with different polarities have independent fading, so the sender and receiver can use two antennas with close positions but different polarities to send and receive signals respectively to obtain diversity effects.
Polarization diversity can be seen as a special case of space diversity. It also uses two pairs of antennas (double diversity case), but only uses the irrelevant fading characteristics of electromagnetic waves at different poles, thus shortening the distance between antennas.
In polarization concentration, the RF power is divided into two different polarization antennas, so the transmission power will be lost about 3dB.
(4) Field component diversity
The E field and H field of electromagnetic wave contain the same message, but the reflection mechanism is different.
The standing wave pattern phase difference between E wave and H wave reflected by a scatterer is 90 °, that is, when E wave is the maximum, H wave is the minimum.
In the mobile channel, multiple E waves and H waves are superimposed, and the components of Ex, Hx, Hy are independent of each other. Therefore, diversity can also be achieved by receiving three field components.
Field component diversity does not require physical separation between antennas, so it is applicable to the lower (100MHz) operating frequency band.When the operating frequency is high (800 ～ 900MHz), spatial diversity is easily realized in structure.
(5) Angular diversity
The method of angle diversity is to make radio waves pass through several different paths and reach the receiver at different angles. The receiver can separate signal components from different directions by using multiple sharp directional receiving antennas. Because these signal components have independent fading characteristics, angle diversity can be achieved and anti fading effect can be obtained.
(6) Time diversity
In addition to space and frequency independence, fast fading also has time independence, that is, the same signal is retransmitted multiple times at different times and intervals. As long as the time interval of each transmission is large enough, the fading of each transmission signal will be independent of each other. The receiver will combine the same signal received repeatedly to reduce the impact of fading.[2]
Time diversity is mainly used to transmit digital signals in fading channels.
Consolidation technology
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Diversity technology is to study how to make full use of multi-path signal energy in transmission to improve transmission reliability. It is also a technology to study how to disperse and collect the basic parameters of signals in time domain, frequency domain and space domain."Diversity" and "set" are a pair of contradictions. After receiving several independent branch signals, the diversity gain can be obtained by combining technology.From the location of the merge, the merge can be performed before the detector, that is, on the IF and RF, and most of them are merged on the IF;Merging can also be performed on the baseband after the detector.The criteria and methods used in the merger are mainly divided into four types: MRC: MaximalRatioCombining, EGC: EqualGainCombining, SC: SelectionCombining, and SwitchingCombining.[3]
Maximum ratio consolidation
At the receiving end, there are multiple diversity branches. After phase adjustment, they are added in phase according to the appropriate gain coefficient, and then sent to the detector for detection.Each uncorrelated diversity branch at the receiving end is sent to the detector for coherent detection after phase correction and addition with appropriate variable gain weighting.When doing this, you can set the variable gain weighting coefficient of the ith branch as the ratio of signal amplitude and noise power of the diversity path.
The maximum ratio combining scheme only needs linear processing of the received signal at the receiving end, and then uses the maximum likelihood detection to restore the original information at the sending end.Its decoding process is simple and easy to implement.The combining gain is proportional to the number of diversity branches N.
Equal gain combining
Equal gain combining principle Equal gain combining is also called phase equalization, which only corrects the phase offset of the channel without correcting the amplitude.Equal gain combining is not the best combining method in any sense. It is the best in the sense of maximizing the signal-to-noise ratio only if the signal-to-noise ratio of each signal is the same.Its output result is the superposition of various signal amplitudes.For CDMA systems, it maintains the orthogonality between user signals in the received signal, that is, it recognizes the difference caused by fading among channels, and does not affect the system's signal-to-noise ratio.EGC is selected when it is inconvenient to measure the amplitude of the received signal in some systems.
Figure 5 Equal gain combining technology
When N (diversity multiplicity) is large, the difference between equal gain combining and maximum ratio combining is not much, about 1dB.The implementation of equal gain combining is relatively simple, and its equipment is also simple.
Selective merge
When the selective combining system adopts the selective combining technology, the output signals of N receivers are sent to the selection logic first, and then the selection logic selects the baseband signal with the highest baseband signal-to-noise ratio from the N received signals as the output.Each additional diversity branch contributes only a reciprocal number of the total diversity branches to the output signal-to-noise ratio of selective diversity.
Figure 6 Selective Consolidation System
Toggle Merge
The schematic diagram of switching and merging is as follows:
Figure 7 Schematic diagram of switching and merging
The receiver scans all diversity branches and selects a specific branch above a specific preset threshold for SNR.Select the signal as the output signal before the SNR of the signal drops below the set threshold value.When the SNR is lower than the set threshold, the receiver starts to rescan and switch to another branch. This scheme is also called scan merging.Because switching merge is not the best instantaneous signal for continuous selection, it may be worse than selecting merge.However, since handoff merging does not require continuous monitoring of all diversity branches at the same time, this method is much simpler.
For selection combining and switching combining, their output signals are only equal to one signal in all diversity branches.In addition, they do not need to know the channel state information.Therefore, these two schemes can be used for both coherent and incoherent modulation.
Performance comparison between diversity technology and combination mode:
The comparison here is mainly about the maximum ratio combination and equal gain combination selective combination.
Merge Performance Comparison
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It can be seen that among the three consolidation methods, the performance of maximum ratio consolidation is the best, and the performance of selective consolidation is the worst.When N is large, the combining gain of equal gain combining is close to that of maximum ratio combining.
Diversity technology and diversity improvement effect:
Diversity improvement effect refers to the effect (benefit) of reducing the impact of deep fading by using diversity technology compared with not using diversity technology.In order to quantitatively measure the improvement degree of diversity, the nominal improvement effect is often used, that is, diversity gain and diversity improvement degree are two indicators to describe.
Diversity improvement effect
Diversity gain refers to the difference of reception level between diversity reception and single reception within a certain cumulative time percentage.The greater the level difference, the higher the diversity gain, indicating that the better the diversity improvement effect.The smaller the accumulation time percentage, the higher the diversity gain.The diversity gain of 0.1% time percentage is 14dB, which means that when there is no diversitycurveA finds that the attenuation depth at this time is 30dB lower than the free space receiving level;After using diversity technology, it is found from curve B that the fading depth at this time is only 16dB lower than the free space reception level.It can be seen that diversity reception reduces the fading depth by 14dB.
Definition of diversity improvement
Diversity improvement refers to the ratio of the cumulative fading time percentage of single reception and diversity reception at a relative receiving level.When the reception level is 20dB lower than the free space reception level, for the same reception level, the cumulative time percentage of fading for single reception and diversity reception is 1% and 0.01% respectively, and the ratio of the two is 100, that is, diversity is improved to 100.In the digital microwave system, no matter which diversity reception mode is used, the effective fading reserve of the system will be increased, that is, anti fadingfrequency selective fading The ability of.It can also improve in band distortion and cross polarization discrimination to varying degrees.The digital mobile microwave based on its technology is mainly imported products, such as Hitachi, Thomson, etc. Our research in this area is accelerating and will soon be available. At that time, the mobile digital microwave will provide high-quality digital signals and better serve the radio and television industry.
