After the read, you will learn about what is MIMO technology, and the technology from MIMO to Massive MIMO.
What is MIMO?
MIMO is a Multiple Input Multiple Output system, which refers to a communication system that uses multiple antennas at the transmitter and receiver simultaneously to increase the capacity and spectrum utilization of the communication system exponentially without increasing the broadband.
The history of the development of MIMO technology
In 1908 Marconi proposed the use of MIMO technology to anti-fading;
In the 1970s, someone proposed to use MIMO technology for communication systems;
1995 Teladar gave the MIMO capacity in the case of fading.
In 1996 Foshinia gave a multiple-input multiple-output processing algorithm, the Diagonal-Bell Labs Layered Airtime (D-BLAST) algorithm.
1998 Tarokh et al. discussed space-time codes for MIMO technology.
In 1998 Wolniansky et al. used the vertical-Bell Labs hierarchical space-time (V-BLAST) algorithm to build a MIMO experimental system.
These works have received great attention from scholars in various countries and have led to the rapid development of research work on MIMO technology.
The advantages of MIMO technology
MIMO multiple modes to bring a variety of gains
Transmit diversity gain to improve system reliability, cannot improve the data rate.
Beam assignment gain to improve the effectiveness of the system, can improve the data rate.
Space division multiplexing gain to improve the effectiveness of the system, can improve the data rate.
2. Improve spectrum efficiency
Require the downlink spectrum efficiency of TD-LTE to reach 5bps/Hz (30bps/Hz for Rel-10).
The uplink spectrum efficiency of TD-LTE is required to reach 2.5bps/Hz (15bps/Hz for Rel-10).
The classification of MIMO technology
MIMO technology is mainly divided into three categories: beamforming, transmission diversity, and spatial multiplexing.
What is beamforming?
Beamforming is the use of smaller spacing between the correlation of antenna elements (antenna spacing of 0.5-0.65 λ), through the formation of interference between the waves emitted by the array elements, concentrating energy in a particular direction (or some) to form a beam, so as to achieve greater coverage and interference suppression effect. The figure below shows the schematic diagram of beamforming.
The meaning of beamforming is to give a certain shape to electromagnetic waves that propagate centrally.
As a point light source, the light emitted from a light bulb has no direction and can only be dissipated in all directions; while a flashlight can focus the light to a direction of emission, the energy is more focused, thus shining farther.
Wireless base stations are the same, if the antenna signal omnidirectional emission, these phones can only receive a limited signal, most of the energy is wasted.
If the signal can be focused into several beams by beamforming, and specifically pointing at each cell phone, the electromagnetic energy carrying the signal can spread farther, and the signal received by the cell phone will be stronger.
Depending on where and how beamforming is done, there are three types of beamforming: digital beamforming, analog beamforming, and hybrid beamforming.
Analog beamforming is through the processing of RF signal weights, through the phase shifter to complete the antenna phase adjustment, the processing position is relatively back.
The analog beamforming point is that the number of channels of baseband processing is much smaller than the number of antenna units, so the capacity is limited, and the antenna assignment is completely built by hardware and will be affected by the accuracy of the device so that the performance is somewhat limited.
Digital beamforming, on the other hand, is performed in the baseband module when the antenna weights are processed, and the number of channels processed in the baseband is equal to the number of antenna units, so a set of RF links needs to be configured for each data channel.
The advantages of digital beamforming are high accuracy, flexible implementation, and timely response of antenna weight transformation; the disadvantages are high baseband processing capability, complex system, large equipment size, and high cost.
Hybrid beamforming combines digital beamforming and analog beamforming so that the beamforming can be amplitude and phase-modulated at the analog side, combined with digital beamforming in the baseband.
Hybrid beamforming digital and analog combines the advantages of both, the number of channels in baseband processing is significantly smaller than the number of analog antenna units, the complexity is significantly reduced, the cost is reduced, the system performance is close to the full digital beamforming, very suitable for high-frequency systems.
Beamforming can also be divided into single-stream beamforming and dual-stream beamforming.
Single-stream beamforming (corresponding to TM7): LTE R8 only supports Single-stream beamforming technology based on a dedicated frequency guide.
During transmission, the UE needs to estimate the equivalent channel after beam assignment by measuring the dedicated guide frequency and performing coherence detection.
In order to be able to estimate the channel experienced by the transmission after beamforming, the base station must send a beamforming reference signal that is transmitted simultaneously with the data, which is user-specific and corresponds to the transmission of the user-specific reference signal called transmission using antenna port 5.
Dual-stream beamforming (corresponding to TM8): TD-LTE R9 extends beamforming to dual-stream transmission and realizes the combination of beamforming and spatial multiplexing technology.
To support dual-stream beamforming, new dual-port dedicated guide frequencies (ports 7 and 8) are defined in LTE R9, and new control signaling is introduced.
In dual-stream beamforming, the UE estimates the equivalent channel after beam focussing based on the measurement of the dedicated guide frequencies, whereas the Precoding module does not perform any pre-processing operation.
Transmission diversity of MIMO technology
Transmission diversity of MIMO technology is the use of uncorrelated between antenna arrays with larger spacing or between focussed beams (antenna spacing above 10λ) to transmit or receive a data stream, avoiding the impact of individual channel fading on the whole link, with the aim of improving the quality of the link, i.e. improving the quality of the communication.
A representative technique for transmission diversity is Space-Time Coding (STC), which is a joint coding technique combining transmit diversity on the spatial domain and channel coding on the temporal domain to increase the redundancy of the signal by joint coding at the transmitter side so that the signal gains diversity at the receiver side.
Space-time coding (STC) divides data into multiple data sub-streams transmitted simultaneously at multiple antennas, establishing a relationship between spatial separation and temporal separation, and obtains diversity gain by introducing coding redundancy in the time domain between transmitting antennas.
The essence is to establish the relationship between spatial separation and temporal separation, to achieve the purpose of mutual protection between each antenna (that is, each antenna transmits signals independently or with little correlation), to reduce the chance of deep fading of the same symbol on all antennas, and to reduce the average BER.
Space-time coding (STC) is mainly divided into space-time lattice code (STTC) and space-time grouping code (STBC).
STTC (Space-Time Trellis Code) is a joint coding method that combines transmit diversity with grid-coded modulation. The resulting coding scheme achieves full diversity gain and high coding gain without sacrificing system bandwidth, thereby improving transmission quality.
The decoding of the space-time lattice code is performed using a maximum likelihood decoder, usually a Viterbi decoder for maximum likelihood decoding. Using STTC can get both coding gain and diversity gain, and although it can provide 3-4 times higher spectral efficiency than the current system, its decoding complexity increases exponentially with the increase of the number of states.
Space-Time Block Code (STBC: Space-Time Block Code) is the use of the principle of orthogonal design to allocate the transmit signal format on each transmitting antenna, which is actually a joint space-domain and time-domain orthogonal grouping coding method.
Space-time block code can make the receiver decode to obtain full diversity gain, and ensure that the decoding operation is only a simple linear merger so that the decoding complexity is greatly reduced.
Spatial multiplexing of MIMO technology
Spatial multiplexing of MIMO technology is to transmit multiple data streams in parallel to a terminal/base station to increase the link capacity by using the uncorrelation between antenna elements with larger spacing or between fugitive beams.
The MIMO technology spatial multiplexing technique is to transmit multiple independent data streams at different antennas, at the same frequency point, and the receiver must use not less than the number of receiving antennas to decode the data streams correctly, which can improve the throughput of the whole system under the condition of frequency point resources.
Comparison of transmission diversity and spatial multiplexing in MIMO technology
MIMO technology transmission diversity
– is the transmission of the same data on multiple independent paths
– The receiver side uses diversity-merging techniques to
– Resists channel fading and reduces BER
– Improves system reliability, does not increase the data rate
MIMO technology spatial multiplexing
– Transmits different data on multiple independent paths
– Multi-user detection and separation are performed at the receiving end
– Maximize system resources and increase system capacity
– Improve the effectiveness of the system, which can improve the data rate
The development history of MIMO technology
First：
3G MIMO technology: WCDMA HSPA standard
Only SISO can be used
Peak downlink rate 7.2Mb/s
Then:
3G MIMO technology: WWDCMA HSPA+ standard
Supports 2×2MIMO
Peak downlink rate of 42Mb/s
After：
4G MIMO technology: 3GPP LTE standard
Supports SISO/2×2MIMO/4×4MIMO
Downlink peak rate 10OMb/s
Later：
4G MIMO technology: 3GPP LTE-A standard
Supports up to 8×8MIMO downlink peak rate of 1Gb/s
The application of MIMO technology in LTE technology
There are 8 main modes as follows
| Transmission Modes | PDSCH Transmission Solution | Advantages | Typical Application Scenarios |
| TM1 | Single antenna transmission mode | Low CRS overhead generated | Various scenarios |
| TM2 | Transmit Diversity | Improves link transmission quality and cell coverage radius | As a return to other MIMO modes Retreat mode |
| TM3 | Open-loop spatial multiplexing | Improves cell average buffer efficiency and peak rate | High-speed mobile scenarios |
| TM4 | Closed-loop spatial multiplexing | Improves cell-averaged buffer spectrum efficiency and peak rate | Low-speed mobile scenarios |
| TM5 | Multi-user MIMO technology | Improves cell average spectrum efficiency and peak rate | Dense urban areas |
| TM6 | Pre-coding with Rank=1 | Improves cell coverage | Only support transmission with rank=1 |
| TM7 | Single-stream beamforming | Improves link transmission quality and cell coverage | Suburban area, large coverage scenario |
| TM8 | Dual-stream beamforming | Improve cell coverage and cell center user throughput | High cell center throughput demand scenarios |
Note:
Closed-Loop MIMO technology: channel a priori information is obtained through feedback or channel reciprocity
Open-Loop MIMO technology: no channel a priori information
Rank=1 means that the transmitter uses a single layer of precoding to adapt to the current channel, which is a special scenario of closed-loop space division multiplexing
5G Massive MIMO technology
What is 5G Massive MIMO technology?
The massive antenna is also called Massive MIMO antenna.
Massive MIMO antenna relative to the traditional base station antenna or traditional integrated active antenna, its form difference for the array number is very large, the unit has independent transceiver capability.
It is equivalent to more antenna units to achieve simultaneous transmission and reception of data. High-frequency Massive MIMO antennas are used for hotspot areas, indoor capacity, and wireless backhaul. Mixed high and low-frequency networking for optimal spectrum utilization.
Advantages of Massive MIMO technology
Multi-beam capability to increase network capacity through multi-user space division multiplexing gain (MU-MIMO);
Large array beamforming, through the algorithm to suppress inter-user interference, significantly improve the single-user SINR;
3D-beamforming feature to achieve the coverage requirements of various scenarios;
Multi-channel uplink reception maximizes uplink reception gain.
Massive MIMO technology and traditional MIMO technology performance comparison
Massive MIMO technology vs. Traditional MIMO technology | ||
Technology Content | Traditional MIMO technology | Massive MIMO technology |
Number of antennas | ≤8 | ≥100 |
Channel angle domain value | Uncertain | Forms a deterministic function as the matrix domain grows |
Channel matrix | Low requirement | High requirement |
Channel capacity | Low | High |
Diversity gain | Low | High |
Link stability | Low | High |
Noise immunity | Low | High |
Array Resolution | Low | High |
Antenna Correlation | Low | High |
Coupling | Low | High |
SER | High | Low |
Conductive frequency contamination | None | Yes |
The research content of Massive MIMO Antenna Technology
Application scenario and channel modeling
The potential application scenarios of Massive antenna technology mainly include macro coverage, high-rise buildings, heterogeneous networks, indoor and outdoor hotspots, and wireless backhaul links.
In addition, the construction of large-scale antenna systems in the form of distributed antennas may also become one of the application scenarios of this technology.
In the scenario where wide area coverage is required, the large-scale antenna technology can utilize the existing frequency band;
In scenarios such as hotspot coverage or backhaul links, higher frequency bands can be considered.
Transmission and detection technology
The large-scale antenna performance gain is mainly guaranteed by the quasi-orthogonal characteristics between multi-user channels formed by a large number of antenna array elements.
In the actual channel conditions, due to the equipment. And a propagation environment there are many non-ideal factors, in order to obtain stable multi-user transmission gain, still need to rely on the design of the downlink transmit and uplink receive algorithm to effectively suppress the inter-user and even inter-small channel interference, and the computational complexity of the transmission and detection algorithm is directly related to the size of the antenna array and the number of users.
Channel state information measurement and feedback technology
Channel state information measurement, feedback, and reference signal design for the application of MIMO technology is of great importance.
In order to better balance the channel state information measurement overhead and accuracy, in addition to the traditional codebook-based implicit feedback and channel reciprocity-based feedback mechanisms, such as hierarchical CSI measurement and feedback.
In addition to the traditional codebook-based implicit feedback and channel reciprocity-based feedback mechanisms, new feedback mechanisms such as hierarchical CSI measurement and feedback, Kronecker-based CSI measurement and feedback, compression-awareness, and pre-experiential feedback are also worth considering.
Coverage enhancement technology and high-speed mobile solutions
Antenna scale expansion for the service channel coverage will bring great gains, but for the broadcast channel that needs to effectively cover all terminals in the whole area, it will bring many adverse effects.
In this case, access technology similar to internal and external double-ring beam scanning can solve the problem of wide coverage with a narrow beam.
In addition, large-scale antennas also need to consider how to achieve reliable and high-speed transmission of signals in high-speed mobile scenarios.
The beam tracking and beam widening techniques, which are less dependent on the channel state information acquisition, can effectively use the array gain of large-scale antennas to improve the data transmission reliability and transmission rate.
Multi-user scheduling and resource management technology
The large-scale antennas provide finer spatial granularity and more spatial freedom for wireless access networks, so the multi-user scheduling technology, service load balancing technology, and resource management technology based on large-scale antennas will obtain considerable performance gain.
Large-scale active array antenna technology
The large-scale antenna front-end system from the structure can be divided into a digital array and digital-mode hybrid array two categories.
Out of complexity, power consumption and cost considerations, digital-mode hybrid array architecture in the high-frequency band will have great potential for application.
The key technologies of large-scale active array antenna architecture, high-efficiency/high-reliability/miniaturization/low-cost/modular transceiver components, high-precision monitoring, and calibration scheme will directly affect the performance and efficiency of large-scale antenna technology in the actual application environment and will become the key link directly related to whether large-scale antenna technology can finally enter the practical stage.
Pre-coding technology
Pre-coding technology is in the downlink of the transmitter using CSI to pre-process the transmitted signal technology. Assuming that the complete channel state information is available at the transmitter side, the signal can be pre-processed at the transmitter side to eliminate the interference caused by the transmit signal through the wireless channel.
This includes inter-antenna interference between data streams transmitted on multiple transmitting antennas, as well as multi-user interference caused by signals from multiple users transmitting at the same time and frequency resources, thus achieving the purpose of ensuring communication reliability and improving system performance.
The coding methods of precoding technology mainly include ZF precoding, MF precoding, MRC precoding, and MMSE precoding. Large-scale antenna technology provides an important guarantee for the improvement of system spectrum efficiency, user experience, and transmission reliability, and also provides a flexible means of interference control and coordination for heterogeneous and dense network deployment environments.
With the breakthrough of a series of key technologies and the further development of devices, antennas, and other technologies, large-scale antenna technology will play a major role in the 5G system.
The future development of MIMO technology
Regarding the future direction of large-scale antenna development, since 5G NR is in the process of commercialization, different aspects are to be further enhanced according to the actual deployment scenarios, which include the following parts.
Considering high-speed vehicle scenarios (e.g., UE moving at high power) further reduction of delay and overhead, as well as reduction of the probability of beam failure events.
Rel 16, while examining enhancements to uplink beam selection across multiple panels, provides some options potentially used to enhance uplink coverage, but there has not been sufficient time to complete the standardization effort.
(b) In addition to the benefit of having multiple transmit points for downlink data channels, uplink-dense deployments could include multipoint transmissions between small cells in macrocells or heterogeneous network deployment scenarios.
Further enhancements to uplink pilot SRS to improve capacity and coverage.
Further enhancements to the Rel 16 enhanced Type II CSI for multi-sender point/multi-panel CSI design in low-frequency FDD deployments and better use of the channel statistics perspective and partial reciprocity of delay for joint transmission.
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