Multiple input multiple output (MIMO) is to use multiple antennas at both the transmitter and receiver to form multiple channels between the transmitter and receiver to greatly improve the channel capacityantenna system。An obvious feature of MIMO system is that it has high spectrum utilization efficiency. On the basis of making full use of existing spectrum resources, it uses space resources to obtain gains in reliability and effectiveness, at the cost of increasing the processing complexity of the sender and receiver.large-scaleMIMO technologyUsing a large number of antennas to serve a relatively small number of users can effectively improvespectrumEfficiency.
Chinese name
Multiple in and multiple out
English name
multiple-in multipleout;MIMO
definition
In order to greatly improve the channel capacity, multiple antennas are used at both the transmitter and receiver, and an antenna system with multiple channels is formed between the transmitter and receiver.
Applied discipline
Communication technology (first level discipline), mobile communication (second level discipline)
Multiple input multiple output (MIMO) is to greatly improvechannel capacity , multiple antennas are used at both the transmitting end and the receiving end to form an antenna system with multiple channels between the transmitting and receiving ends.
Multiple input multiple output (MIMO) is a very complex antenna diversity technology.The multipath effect will affect the signal quality, so the traditional antenna systems are always thinking about how to eliminate the multipath effect.On the contrary, MIMO system uses multipath effect to improve communication quality.In MIMO system, the transmitter and receiver use multiple antennas that can work simultaneously to communicate.MIMO systems often use complex signal processing techniques to significantly enhance reliability, transmission range andthroughput。Transmitters use these techniques to send multiple channels simultaneouslyRF signalThe receiver recovers the data from these signals.[1]
MIMO array
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A good way to improve the system throughput is to increase the channel capacity.MIMO can double the capacity of fading channels.According to the latest achievements of information theory, assuming that the number of transmitting antennas is m and the number of receiving antennas is n, the following channel capacity formula can be obtained when the transmission signal of each antenna can be separated:
According to this tool, for systems using multiple antenna array transmission and reception technology, the channel capacity will ideally increase with the m-line, thus providing capacity potential that other technologies cannot achieve at present.Secondly, because the multi antenna array transmission and reception technology is essentially the combination of space diversity and time diversity technology, it has good anti-interference ability;Further integrate the multi antenna transmission and reception technology withChannel coding technologyThe combination can greatly improve the performance of the system, which results inSpace-time codingThe production of technology.Space time coding technology really realizesSpace Division Multiple Access, is one of the inevitable technologies in the future wireless communication.[2]
MIMO antennaArray is an open-loop MIMO technology. There are m transmit antennas. The code reuse technology is used to reuse each code of the same code set for m times. Each code is used to modulate different data streams, which improves the transmission rate of original data without increasing code resources.In order to distinguish m data sub streams, multiple antennas and spatial signal processing are also used at the receiving end.[2]
MIMO technologyThere are mainly two forms of expression, namely spatial reuse and space-time coding.Both forms have been applied in WiMAX technology.WiMAX technology also gives the form of using spatial multiplexing and space-time coding at the same time.At present, MIMO technology is being developed and applied to various high-speed wireless communication systems.[2]
In order to make full use of the capacity of MIMO, different space-time processing schemes have been proposed.Bell Labs has proposed a layered space-time structure, which divides the source data into several sub data streams and independently performs coding and modulation, so it is not based on transmit diversity.AT&T formally proposed space-time coding based on transmit delay diversity.At the same time, a simple transmission diversity scheme is also proposed, and it is further promotedSpace-time block codingBecause of its low decoding complexity, it has been formally listed in the WCDMA standard.Space time coding is a new technology that combines coding, modulation and spatial diversity. It has also become an important part of post 3G technology.[2]
MIMO system
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MIMO wireless communication system is one of the key technologies of future mobile and wireless communication systems.An obvious feature of MIMO system is that it has high spectrum utilization efficiency. On the basis of making full use of existing spectrum resources, it uses space resources to obtain gains in reliability and effectiveness, at the cost of increasing the processing complexity of the sender and receiver.
Key modules
1. MIMO system channel model modeling
The performance of MIMO system depends on the channel model to a large extent. Although there are standardized wireless propagation models and many MIMO channel models have been provided on the basis of a large number of measured and theoretical research work, there is no standardized MIMO channel model recognized by ITU so far (3GPP has developed a channel model standard for MIMO).Therefore, understand and master wireless in indoor and outdoor environmentsMIMO channelThe establishment of static model and specific dynamic model of MIMO channel is crucial to the selection of appropriate system structure and the design of excellent signal processing algorithm to realize the potential huge channel capacity of MIMO system and achieve the expected performance.[3]
2. Capacity of MIMO system
Compared with the traditional single antenna system, the MIMO system has greatly improved both in performance and data transmission rate. First, the channel capacity of the MIMO system is deeply analyzed by Telestar and FoschiniGaussian noise The research on the capacity of MIMO system under the assumption that each antenna is independent of each other shows that the multi antenna system is significantly improved than the single antenna system. Considering the wireless transmission system with M sub transmit antenna and N receive antenna, Foschini's research shows that when M=N, the channel capacity increases proportionally to N.Under the same transmission power and transmission bandwidth, the channel capacity of the system is about 40 times higher than that of the single input single output (SISO) system.[3]
3. Design of MIMO antenna array
In general, the base station antenna is set high, and the near-field scattering around the antenna array is relatively weak. Therefore, in order to obtain uncorrelated signals on different array elements, it is often necessary to maintain at least 10 times the wavelength spacing between the array elements.When the number of antennas is large, there may be obstacles in the erection of base station line array.For mobile terminals, due to the rich near-field scatterers, it is generally believed that the signal correlation is weak enough when the antenna array element spacing is more than 1/2 wavelength.Polarized antenna array can use mutually orthogonal polarization states to realize uncorrelation of array elements at the same spatial position, so the size of antenna array can be relatively reduced.[3]
4. Signal processing of MIMO system
Array antenna communication systems in fading environments face co channel interference and inter symbol interference.In order to approach the capacity of multi antenna systems, good signal processing techniques are needed.High performance and low complexity signal detection methods or joint detection methods have always been the hot topics of researchers.[3]
5. Complexity of MIMO system
Since the signal in MIMO system is extended to space-time two-dimensional, compared with single antenna system, the complexity of channel estimation, channel equalization, decoding and detection will increase sharply with the increase of antenna number or signal modulation order, and the amount of algorithm calculation will directly affect the processing delay, equipment power consumption and standby time.At the same time, in practical applications, a key factor limiting MIMO systems is the high cost of multiple RF links.In order to reduce the computational complexity of "software", more simple and effective signal processing methods and various space-time coding and decoding schemes are provided for MIMO systems.For reducing the "hardware" cost, antenna selection is a very critical technology. It can maintain the advantages of MIMO technology while greatly reducing the processing complexity and hardware cost. It is a research focus to push MIMO systems to practical use.
6. Diversity and multiplexing of MIMO system
The essence of MIMO system is to providediversityGain andmultiplexingGain.The former ensures the transmission reliability of the system, while the latter improves the transmission rate of the system.Most of the early literature focused on the use of transmit diversity and spatial multiplexing alone or in combination with coding alone. Research shows that multiple antenna systems can provide diversity and spatial multiplexing at the same time, and there is a tradeoff between the two.It is worth discussing how to maximize the system gain by reasonably utilizing the diversity and multiplexing modes of MIMO systems.
7. (Multi cell) Multi user MIMO system
Theoretically, the capacity domain of multi-user MIMO system has been solved, but how to make the capacity domain meet the requirements of various users for transmission rate is still not well solved.Moreover, in the broadcast channel, since MIMO systems have inter antenna and inter user interference, how to design transmission vectors to eliminate co channel interference between users, how to optimize the system capacity and the power control of each user's specific QoS when power is limited, and the related technologies existing in multi cell multi-user systems are still the focus of research.[3]
Basic Principles of MIMO Technology
MIMO technology refers to the use of multiple transmit antennas and receive antennas at the transmitter and receiver, so that signals can be transmitted and received through multiple antennas at the transmitter and receiver, thus improving communication quality.It can make full use of space resources to achieve multiple transmissions and multiple receivers through multiple antennas. Without increasing spectrum resources and antenna transmission power, it can double the system channel capacity, showing obvious advantages, and is regarded as the core technology of next-generation mobile communication.The essence of MIMO technology is to provide space for the systemDiversity gainAnd spacemultiplexingGain.[3]
The transmitting end maps the data signals to be sent to multiple antennas through space-time mapping, and the receiving end decodes the signals received by each antenna to recover the data signals sent by the transmitting end.According to different space-time mapping methods, MIMO technology can be roughly divided into two categories: spatial diversity and spatial reuse.Spatial diversity refers to the use of multiple transmit antennas to send signals with the same information through different paths, and at the same time obtain multiple independent fading signals of the same data symbol at the receiver, so as to obtain diversity enhanced reception reliability.For example, in a slow Rayleigh fading channel, one transmit antenna and n receive antennas are used, and the transmitted signal passes through n different paths.If the fading between the antennas is independent, the maximum diversity gain can be n.For transmit diversity technology, the gain of multiple paths is also used to improve the reliability of the system.In a system with m transmit antennas and n receive antennas, if the path gain between antenna pairs is Rayleigh fading with independent and uniform distribution, the maximum diversity gain can be mn.At present, space diversity technologies commonly used in MIMO systems mainly include Space Time Block Code (STBC) and beamforming technology.STBC is an important coding form based on transmit diversity, and the most basic one is the Alamouti scheme designed for two antennas.
The most important part of STBC method is to make the signal vectors to be transmitted on multiple antennas orthogonal to each other.With STBC technology, full diversity can be achieved, that is, the maximum diversity gain is MN when STBC technology is used in a system with M transmit antennas and N receive antennas.Beamforming technology is to transmit the same data through different transmit antennas to form a shaped beam pointing to some users, thus effectively improving the antenna gain.In order to maximize the signal strength of the beam pointing to the user, beam forming technology usually needs to calculate the phase and power of the data transmitted on each transmission antenna, also known as the Wei beam forming vector.Common beamforming vector calculation methods include maximum eigenvalue vector, MUSIC algorithm, etc.The maximum transmit diversity gain of M transmit antennas using beamforming technology is M.Spatial multiplexing technology is to divide the data to be transmitted into several data streams, and then transmit them on different antennas, so as to improve the transmission rate of the system.The common spatial reuse method is the vertical reuse method proposed by Bell LaboratoriesLayered space-time codeV-BLAST technology.
Large scale MIMO
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Intelligent terminalThe rapid popularization has pushed the demand for mobile data services to a higher level, resulting in a serious shortage of spectrum resources.The main challenges of large-scale technology include pilot pollution, beam shaping, precoding and high-performance signal detection.[4]
Technical advantages
Large scale MIMO technology uses a large number of antennas to serve a relatively small number of users, which can effectively improvespectrumEfficiency.Research institutions at home and abroad have conducted in-depth research on large-scale technologies around throughput, transmission power efficiency, precoding and receiver design.On the other hand, energy efficiency is anotherKey performance indicators。Large scale MIMO can not only effectively improve the system channel capacity, but also significantly improve the energy efficiency of wireless systems.[4]
Performance comparison between large-scale MIMO and traditional MIMO
Large scale MIMO has many characteristics compared with traditional MIMO.Pilot pollutionThis is a key limiting factor for large-scale MIMO technology. This is because with the increase of the number of base station antennas, users in adjacent cells use the same group (or non orthogonal) training sequence in uplink estimation, which results in the channel estimation result at the base station end not being the channel between the local user and the base station, but being polluted by the training sequence sent by users in other cells.In addition, problems such as channel measurement, modeling and estimation, beam shaping/precoding and detection design, and hardware complexity will also limit the implementation of large-scale MIMO systems.[4]
