MIMO antenna - what is it and what are its advantages? MIMO technology (Multiple Input Multiple Output) is a method of spatial signal encoding

MIMO (Multiple Input Multiple Output) is a method of coordinated use of multiple radio antennas in wireless network communications, common in modern home broadband routers and in LTE and WiMAX cellular networks.

How it works?

Wi-Fi routers with MIMO technology use the same network protocols as regular single-link routers. They provide higher performance by improving the efficiency of transmitting and receiving data over a wireless link. In particular, network traffic between clients and the router is organized into separate streams transmitted in parallel, with their subsequent restoration by the receiving device.

MIMO technology can increase transmission capacity, range, and reliability when there is a high risk of interference from other wireless equipment.

Application in Wi-Fi networks

MIMO technology has been included in the standard since 802.11n. Its use improves the performance and availability of network connections compared to conventional routers.

The number of antennas may vary. For example, MIMO 2x2 provides two antennas and two transmitters capable of receiving and transmitting on two channels.

To take advantage of this technology and realize its benefits, the client device and the router must establish a MIMO connection between themselves. The documentation for the equipment used should indicate whether it supports this feature. There is no other easy way to check whether a network connection uses this technology.

SU-MIMO and MU-MIMO

The first generation of technology, introduced in the 802.11n standard, supported the single-user (SU) method. Compared to traditional solutions, where all antennas on a router must be coordinated to communicate with a single client device, SU-MIMO allows each antenna to be distributed across different equipment.

Multi-user (MU) MIMO technology was created for use in 802.11ac Wi-Fi networks at 5 GHz. Whereas the previous standard required routers to manage their client connections one at a time (one at a time), MU-MIMO antennas can communicate with multiple clients in parallel. improves connection performance. However, even if an 802.11ac router has the necessary hardware support for MIMO technology, there are other limitations:

• supports a limited number of simultaneous client connections (2-4) depending on the antenna configuration;
• antenna coordination is provided in only one direction - from the router to the client.

MIMO and Cellular

The technology is used in different types of wireless networks. It is increasingly finding application in cellular communications (4G and 5G) in several forms:

• Network MIMO - coordinated signal transmission between base stations;
• Massive MIMO - use of a large number (hundreds) of antennas;
• millimeter waves - the use of ultra-high frequency bands in which the capacity is greater than in the bands licensed for 3G and 4G.

Multi-user technology

To understand how MU-MIMO works, we need to look at how a traditional wireless router processes data packets. It does a good job of sending and receiving data, but only in one direction. In other words, it can only communicate with one device at a time. For example, if a video is downloading, you cannot stream an online video game to the console at the same time.

A user can run multiple devices on a Wi-Fi network, and the router very quickly takes turns sending bits of data to them. However, it can only access one device at a time, which is the main reason for poor connection quality if Wi-Fi bandwidth is too low.

Since it works, it draws little attention to itself. However, the efficiency of a router that transmits data to multiple devices simultaneously can be improved. At the same time, it will work faster and provide more interesting network configurations. This is why developments like MU-MIMO emerged and were eventually incorporated into modern wireless standards. These developments allow advanced routers to communicate with multiple devices at once.

Brief History: SU vs MU

Single-user and multi-user MIMO are different ways for routers to communicate with multiple devices. The first one is older. The SU standard allowed sending and receiving data via several streams at once, depending on the available number of antennas, each of which could work with different devices. SU was included in the 2007 802.11n update and has begun to be gradually introduced into new product lines.

However, SU-MIMO had limitations in addition to antenna requirements. While there may be multiple devices connected, they are still dealing with a router that can only handle one at a time. Data rates have increased and interference has become less of an issue, but there is still plenty of room for improvement.

MU-MIMO is a standard that evolved from SU-MIMO and SDMA (Space Division Multiple Access). The technology allows a base station to communicate with multiple devices using a separate stream for each one, as if they all had their own router.

MU support was eventually added in an update to the 802.11ac standard in 2013. After several years of development, manufacturers began to include this feature in their products.

Benefits of MU-MIMO

This is an exciting technology because it has a noticeable impact on everyday Wi-Fi usage without directly changing bandwidth or other key wireless parameters. Networks are becoming much more efficient.

To ensure a stable connection with a laptop, phone, tablet or computer, the standard does not require the router to have multiple antennas. Each such device may not share its MIMO channel with others. This is especially noticeable when streaming videos or performing other complex tasks. Internet speeds are subjectively faster and the connection is more reliable, although in reality the networking becomes smarter. The number of simultaneously serviced devices also increases.

Limitations of MU-MIMO

Multi-user multiple access technology also has a number of limitations that are worth mentioning. Current standards support 4 devices, but allow you to add more and they will have to share the stream, which brings back the problems of SU-MIMO. The technology is mainly used in downlinks and is limited when it comes to uplinks. In addition, the MU-MIMO router must have more device and link state information than previous standards required. This makes wireless networks more difficult to manage and troubleshoot.

MU-MIMO is also a directional technology. This means that 2 devices located next to each other cannot use different channels at the same time. For example, if a husband is watching an online stream on TV and his wife is nearby streaming a PS4 game to her Vita via Remote Play, they will still have to share bandwidth. A router can only provide discrete streams to devices that are located in different directions.

Massive MIMO

As we move towards fifth generation (5G) wireless networks, the growth of smartphones and new applications has resulted in a 100-fold increase in their required bandwidth compared to LTE. New Massive MIMO technology, which has received a lot of attention in recent years, is designed to significantly increase the efficiency of telecommunications networks to unprecedented levels. Given the scarcity and high cost of available resources, operators are attracted by the opportunity to increase capacity in frequency bands below 6 GHz.

Despite significant progress, Massive MIMO is far from perfect. The technology continues to be actively researched in both academia and industry, where engineers strive to achieve theoretical results with commercially acceptable solutions.

Massive MIMO can help solve two key problems - throughput and coverage. For mobile operators, frequency range remains a scarce and relatively expensive resource, but is a key condition for increasing signal transmission speeds. In cities, base station spacing is driven by capacity rather than coverage, which requires large numbers of base stations to be deployed and incurs additional costs. Massive MIMO allows you to increase the capacity of an existing network. In areas where base station deployment is coverage-driven, technology can extend the range of base stations.

Concept

Massive MIMO fundamentally changes current practice by using a very large number of coherently and adaptively operating 4G service antennas (hundreds or thousands). This helps focus the transmission and reception of signal energy into smaller areas of space, greatly improving performance and energy efficiency, especially when combined with simultaneous scheduling of large numbers of user terminals (tens or hundreds). The method was originally intended for time division duplex (TDD) transmission, but could potentially also be used in frequency division duplex (PDD) mode.

MIMO technology: advantages and disadvantages

The advantages of the method are the widespread use of inexpensive low-power components, reduced latency, simplified access control (MAC) layer, and resistance to random and intentional interference. The expected throughput depends on the propagation medium providing asymptotically orthogonal links to the terminals, and experiments have so far revealed no limitations in this regard.

However, along with the elimination of many problems, new ones appear that require urgent solutions. For example, MIMO systems need to enable multiple low-cost, low-fidelity components to work together efficiently, collect channel state data, and allocate resources to newly connected terminals. There is also a need to exploit the additional degrees of freedom provided by redundant service antennas, reduce internal power consumption to achieve overall energy efficiency, and find new deployment scenarios.

The growing number of 4G antennas involved in MIMO implementations typically requires visits to each base station for configuration and wiring changes. The initial deployment of LTE networks required the installation of new equipment. This made it possible to produce a 2x2 MIMO configuration of the original LTE standard. Further changes to base stations are made only in extreme cases, and higher order implementations depend on the operating environment. Another problem is that MIMO operation results in completely different network behavior than previous systems, which creates some scheduling uncertainty. Therefore, operators tend to use other developments first, especially if they can be deployed through a software update.

Technology based on the IEEE 802.11n WiFi standard.

Wi-Life provides a brief overview of WiFi technology IEEE 802.11n .
Extended information to our video publications.

First generation of devices supporting the WiFi 802.11n standard appeared on the market several years ago. MIMO technology ( MIMO - multiple input / multiple output -multiple input/multiple output) is the core of 802.11n. It is a radio system with multiple separate transmission and reception paths. MIMO systems are described using the number of transmitters and receivers. The WiFi 802.11n standard defines a set of possible combinations from 1x1 to 4x4.

In a typical case of deploying a Wi-Fi network indoors, for example in an office, workshop, hangar, hospital, the radio signal rarely travels along the shortest path between the transmitter and the receiver due to walls, doors and other obstacles. Most such environments have many different surfaces that reflect the radio signal (electromagnetic wave) like a mirror reflects light. After reflection, multiple copies of the original WiFi signal are formed. When multiple copies of a WiFi signal travel along different paths from the transmitter to the receiver, the signal taking the shortest path will be the first, and the next copies (or the reflected echo of the signal) will arrive a little later due to longer paths. This is called multipath signal propagation (multipath). The conditions for multiple propagation are constantly changing because... Wi-Fi devices often move (a smartphone with Wi-Fi in the user’s hands), various objects move around creating interference (people, cars, etc.). If signals arrive at different times and at different angles, this can cause distortion and possible signal attenuation.

It is important to remember that WiFi 802.11 n with MIMO support and a large number of receivers can reduce multipath effects and destructive interference, but in any case it is better to reduce multipath conditions wherever and whenever possible. One of the most important points is to keep the antennas as far as possible from metal objects (primarily WiFi omni antennas that have a circular or omnidirectional radiation pattern).

Necessary clearly understand that not all Wi-Fi clients and WiFi access points are the same from a MIMO point of view.
There are 1x1, 2x1, 3x3, etc. clients. For example, mobile devices such as smartphones most often support MIMO 1x 1, sometimes 1x 2. This is due to two key problems:
1. the need to ensure low energy consumption and long battery life,
2. difficulty in arranging several antennas with adequate spacing in a small package.
The same applies to other mobile devices: tablet computers, PDAs, etc.

High-end laptops quite often already support MIMO up to 3x3 (MacBook Pro, etc.).

Let's Let's look at the main types MIMO in WiFi networks.
For now we will omit the details of the number of transmitters and receivers. It is important to understand the principle.

First type: Diversity when receiving a signal on a WiFi device

If there are at least two coupled receivers with antenna diversity at the receiving point,
then it is quite possible to analyze all copies on each receiver to select the best signals.
Further, various manipulations can be carried out with these signals, but we are interested, first of all, in
the possibility of combining them using MRC (Maximum Ratio Combined) technology. MRC technology will be discussed in more detail below.

Second type: Diversity when sending a signal to a WiFi device

If at the sending point there are at least two connected WiFi transmitters with spaced antennas, then it becomes possible to send a group of identical signals to increase the number of copies of information, increase reliability in transmission and reduce the need to resend data in the radio channel in case of loss.

Third type: Spatial multiplexing of signals on a WiFi device
(signal combining)

If at the sending point and at the receiving point there are at least two connected WiFi transmitters with separated antennas, then it becomes possible to send a set of different information over different signals in order to create the possibility of virtually combining such information flows into one data transmission channel, the total throughput of which tends to the sum of the individual streams of which it consists. This is called Spatial Multiplexing. But here it is extremely important to ensure the possibility of high-quality separation of all source signals, which requires a large SNR - signal/noise ratio.

MRC technology (maximum ratio combined ) is used in many modern Access Points Wi-Fi corporate class.
M.R.C. aimed at increasing the signal level in the direction from Wi-Fi client to the WiFi 802.11 Access Point.
Work algorithm M.R.C. involves the collection on several antennas and receivers of all direct and reflected signals during multipath propagation. Next is a special processor ( DSP ) selects the best signal from each receiver and performs the combination. In fact, mathematical processing implements a virtual phase shift to create positive interference with the signals being added. Thus, the resulting total signal has significantly better characteristics than all the original ones.

M.R.C. allows you to provide significantly better operating conditions for low-power mobile devices in the standard network Wi-Fi .

In 802.11n WiFi systems The advantages of multipath propagation are used to transmit multiple radio signals simultaneously. Each of these signals, called " spatial flows", is sent from a separate antenna using a separate transmitter. Because there is some distance between the antennas, each signal follows a slightly different path to the receiver. This effect is called " spatial diversity" The receiver is also equipped with several antennas with their own separate radio modules, which independently decode incoming signals, and each signal is combined with signals from other receiving radio modules. As a result, several data streams are received simultaneously. This provides significantly higher throughput than previous 802.11 WiFi systems, but also requires an 802.11n-capable client.

Now let's delve a little deeper into this topic:
In WiFi devices with MIMO It is possible to divide the entire incoming information flow into several different data streams using spatial multiplexing for their subsequent sending. Multiple transmitters and antennas are used to send different streams on the same frequency channel. One way to visualize this is that some text phrase can be transmitted so that the first word is sent through one transmitter, the second through another transmitter, etc.
Naturally, the receiving side must support the same functionality (MIMO) to fully isolate various signals, reassemble them and combine them using, again, spatial multiplexing. This way we get the opportunity to restore the original information flow. The presented technology allows you to divide a large data stream into a set of smaller streams and transmit them separately from one another. In general, this makes it possible to more efficiently utilize the radio environment and specifically the frequencies allocated for Wi-Fi.

WiFi 802.11n technology also defines how MIMO can be used to improve SNR at the receiver using transmit beamforming. With this technique, it is possible to control the process of sending signals from each antenna so that the parameters of the received signal at the receiver are improved. In other words, in addition to sending multiple data streams, multiple transmitters can be used to achieve a higher SNR at the receiving point and, as a result, a higher data rate at the client.
The following things need to be noted:
1. The transmit beamforming procedure defined in the Wi-Fi 802.11n standard requires collaboration with the receiver (in fact, with the client device) to obtain feedback about the state of the signal at the receiver. Here it is necessary to have support for this functionality on both sides of the channel - both on the transmitter and on the receiver.
2. Due to the complexity of this procedure, transmit beamforming was not supported in the first generation of 802.11n chips on both the terminal side and the Access Point side. Currently, most existing chips for client devices do not support this functionality either.
3. There are solutions for building networks Wi-Fi , which allow you to fully control the radiation pattern on Access Points without the need to receive feedback from client devices.

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MIMO(Multiple Input Multiple Output - multiple input multiple output) is a technology used in wireless communication systems (WIFI, cellular communication networks), which can significantly improve the spectral efficiency of the system, the maximum data transfer rate and network capacity. The main way to achieve the above benefits is to transmit data from source to destination through multiple radio connections, which is where the technology gets its name. Let's consider the background of this issue and determine the main reasons that led to the widespread use of MIMO technology.

The need for high-speed connections that provide high quality of service (QoS) with high fault tolerance is growing from year to year. This is greatly facilitated by the emergence of such services as VoIP (), VoD (), etc. However, most wireless technologies do not allow providing subscribers with high quality service at the edge of the coverage area. In cellular and other wireless communication systems, the quality of the connection, as well as the available data transfer speed, rapidly decreases with distance from the (BTS). At the same time, the quality of services also decreases, which ultimately leads to the impossibility of providing real-time services with high quality throughout the entire radio coverage area of ​​the network. To solve this problem, you can try to install base stations as densely as possible and organize internal coverage in all places with low signal levels. However, this will require significant financial costs, which will ultimately lead to an increase in the cost of the service and a decrease in competitiveness. Thus, to solve this problem, an original innovation is required that, if possible, uses the current frequency range and does not require the construction of new network facilities.

Features of radio wave propagation

In order to understand the operating principles of MIMO technology, it is necessary to consider the general ones in space. The waves emitted by various wireless radio systems in the range above 100 MHz behave in many ways like light rays. When radio waves encounter any surface during propagation, depending on the material and size of the obstacle, part of the energy is absorbed, part passes through, and the rest is reflected. The ratio of the shares of absorbed, reflected and transmitted energy is influenced by many external factors, including the frequency of the signal. Moreover, the signal energy reflected and transmitted through can change the direction of its further propagation, and the signal itself is divided into several waves.

The signal propagating according to the above laws from the source to the recipient, after encountering numerous obstacles, is divided into many waves, only part of which reaches the receiver. Each of the waves reaching the receiver forms the so-called signal propagation path. Moreover, due to the fact that different waves are reflected from different numbers of obstacles and travel different distances, different paths have different paths.

In dense urban environments, due to a large number of obstacles such as buildings, trees, cars, etc., a situation very often arises when there is no direct visibility between the MS and the base station antennas (BTS). In this case, the only option for the signal to reach the receiver is through reflected waves. However, as noted above, a repeatedly reflected signal no longer has the original energy and may arrive late. Particular difficulty is also created by the fact that objects do not always remain stationary and the situation can change significantly over time. This raises a problem - one of the most significant problems in wireless communication systems.

Multipath propagation - a problem or an advantage?

Several different solutions are used to combat multipath propagation of signals. One of the most common technologies is Receive Diversity - . Its essence lies in the fact that to receive a signal, not one, but several antennas are used (usually two, less often four), located at a distance from each other. Thus, the recipient has not one, but two copies of the transmitted signal, which arrived in different ways. This makes it possible to collect more energy from the original signal, because waves received by one antenna may not be received by another and vice versa. Also, signals arriving out of phase to one antenna may arrive in phase to another. This radio interface design can be called Single Input Multiple Output (SIMO), as opposed to the standard Single Input Single Output (SISO) design. The reverse approach can also be used: when several antennas are used for transmission and one for reception. This also increases the total energy of the original signal received by the receiver. This circuit is called Multiple Input Single Output (MISO). In both schemes (SIMO and MISO), several antennas are installed on the base station side, because It is difficult to implement antenna diversity in a mobile device over a sufficiently large distance without increasing the size of the terminal equipment itself.

As a result of further considerations, we come to the Multiple Input Multiple Output (MIMO) scheme. In this case, several antennas are installed for transmission and reception. However, unlike the above schemes, this diversity scheme not only combats multipath signal propagation, but also provides some additional advantages. By using multiple antennas for transmission and reception, each transmitting/receiving antenna pair can be assigned a separate path for transmitting information. In this case, diversity reception will be performed by the remaining antennas, and this antenna will also serve as an additional antenna for other transmission paths. As a result, theoretically, it is possible to increase the data transfer rate as many times as additional antennas are used. However, a significant limitation is imposed by the quality of each radio path.

How MIMO works

As noted above, to organize MIMO technology it is necessary to install several antennas on the transmitting and receiving sides. Typically, an equal number of antennas are installed at the input and output of the system, because in this case, the maximum data transfer rate is achieved. To show the number of antennas on reception and transmission, along with the name of the MIMO technology, the designation “AxB” is usually mentioned, where A is the number of antennas at the system input, and B is at the output. In this case, the system means a radio connection.

MIMO technology requires some changes in the transmitter structure compared to conventional systems. Let's consider just one of the possible, simplest ways to organize MIMO technology. First of all, a stream divider is needed on the transmitting side, which will divide the data intended for transmission into several low-speed substreams, the number of which depends on the number of antennas. For example, for MIMO 4x4 and an input data rate of 200 Mbit/s, the divider will create 4 streams of 50 Mbit/s each. Next, each of these streams must be transmitted through its own antenna. Typically, transmission antennas are installed with some spatial separation in order to provide as many spurious signals as possible that arise as a result of reflections. In one of the possible ways of organizing MIMO technology, the signal is transmitted from each antenna with a different polarization, which allows it to be identified when received. However, in the simplest case, each of the transmitted signals turns out to be marked by the transmission medium itself (time delay and other distortions).

On the receiving side, several antennas receive the signal from the radio air. Moreover, the antennas on the receiving side are also installed with some spatial diversity, thereby ensuring diversity reception, discussed earlier. The received signals arrive at receivers, the number of which corresponds to the number of antennas and transmission paths. Moreover, each of the receivers receives signals from all antennas of the system. Each of these adders extracts from the total flow the signal energy of only the path for which it is responsible. He does this either according to some predetermined attribute that was supplied to each of the signals, or through the analysis of delay, attenuation, phase shift, i.e. set of distortions or “fingerprint” of the propagation medium. Depending on the operating principle of the system (Bell Laboratories Layered Space-Time - BLAST, Selective Per Antenna Rate Control (SPARC), etc.), the transmitted signal may be repeated after a certain time, or transmitted with a slight delay through other antennas.

An unusual phenomenon that may occur in a MIMO system is that the data rate of the MIMO system may be reduced when there is a line of sight between the signal source and receiver. This is primarily due to a decrease in the severity of distortions in the surrounding space, which marks each of the signals. As a result, it becomes difficult to separate the signals at the receiving end and they begin to influence each other. Thus, the higher the quality of the radio connection, the less benefit can be obtained from MIMO.

Multi-user MIMO (MU-MIMO)

The principle of organizing radio communications discussed above refers to the so-called Single user MIMO (SU-MIMO), where there is only one transmitter and receiver of information. In this case, both the transmitter and the receiver can clearly coordinate their actions, and at the same time there is no surprise factor when new users may appear on the air. This scheme is quite suitable for small systems, for example, for organizing communication in a home office between two devices. In turn, most systems, such as WI-FI, WIMAX, cellular communication systems are multi-user, i.e. in them there is a single center and several remote objects, with each of which it is necessary to organize a radio connection. Thus, two problems arise: on the one hand, the base station must transmit a signal to many subscribers through the same antenna system (MIMO broadcast), and at the same time receive a signal through the same antennas from several subscribers (MIMO MAC - Multiple Access Channels).

In the uplink direction - from MS to BTS, users transmit their information simultaneously on the same frequency. In this case, a difficulty arises for the base station: it is necessary to separate signals from different subscribers. One of the possible ways to combat this problem is also a linear processing method, which involves preliminary transmission of the transmitted signal. The original signal, according to this method, is multiplied with a matrix, which is composed of coefficients reflecting the interference effect from other subscribers. The matrix is ​​compiled based on the current situation on the radio: the number of subscribers, transmission speeds, etc. Thus, before transmission, the signal is subject to distortion inverse to that which it will encounter during radio transmission.

In downlink - the direction from BTS to MS, the base station transmits signals simultaneously on the same channel to several subscribers at once. This leads to the fact that the signal transmitted for one subscriber affects the reception of all other signals, i.e. interference occurs. Possible options to combat this problem are to use or use dirty paper coding technology. Let's take a closer look at dirty paper technology. The principle of its operation is based on an analysis of the current state of the radio airwaves and the number of active subscribers. The only (first) subscriber transmits his data to the base station without encoding or changing his data, because there is no interference from other subscribers. The second subscriber will encode, i.e. change the energy of your signal so as not to interfere with the first one and not expose your signal to influence from the first one. Subsequent subscribers added to the system will also follow this principle, and will be based on the number of active subscribers and the effect of the signals they transmit.

Application of MIMO

In the last decade, MIMO technology has been one of the most relevant ways to increase the throughput and capacity of wireless communication systems. Let's look at some examples of using MIMO in various communication systems.

The WiFi 802.11n standard is one of the most striking examples of the use of MIMO technology. According to it, it allows you to maintain speeds of up to 300 Mbit/s. Moreover, the previous 802.11g standard allowed only 50 Mbit/s. In addition to increasing data transfer rates, the new standard, thanks to MIMO, also allows for better quality of service in areas with low signal strength. 802.11n is used not only in point/multipoint systems (Point/Multipoint) - the most common niche for using WiFi technology to organize a LAN (Local Area Network), but also for organizing point/point connections that are used to organize backbone communication channels at several speeds hundreds of Mbit/s and allowing data transmission over tens of kilometers (up to 50 km).

The WiMAX standard also has two releases that introduce new capabilities to users using MIMO technology. The first, 802.16e, provides mobile broadband services. It allows you to transmit information at speeds of up to 40 Mbit/s in the direction from the base station to the subscriber equipment. However, MIMO in 802.16e is considered an option and is used in the simplest configuration - 2x2. In the next release, 802.16m MIMO is considered a mandatory technology, with a 4x4 configuration possible. In this case, WiMAX can already be classified as cellular communication systems, namely their fourth generation (due to the high data transfer speed), because has a number of characteristics inherent to cellular networks: voice connections. In case of mobile use, theoretically, speeds of 100 Mbit/s can be achieved. In a fixed version, the speed can reach 1 Gbit/s.

Of greatest interest is the use of MIMO technology in cellular communication systems. This technology has been used since the third generation of cellular communication systems. For example, in the standard, in Rel. 6 it is used in conjunction with HSPA technology supporting speeds up to 20 Mbit/s, and in Rel. 7 – with HSPA+, where data transfer rates reach 40 Mbit/s. However, MIMO has not yet found widespread use in 3G systems.

Systems, namely LTE, also provide for the use of MIMO in up to 8x8 configurations. This, in theory, can make it possible to transmit data from the base station to the subscriber over 300 Mbit/s. Another important positive point is the stable connection quality even at the edge. In this case, even at a considerable distance from the base station, or when located in a remote room, only a slight decrease in the data transfer rate will be observed.

Thus, MIMO technology finds application in almost all wireless data transmission systems. Moreover, its potential has not been exhausted. New antenna configuration options are already being developed, up to 64x64 MIMO. This will allow us to achieve even higher data rates, network capacity and spectral efficiency in the future.

April 9th, 2014

At one time, the IR connection quietly and imperceptibly disappeared, then they stopped using Bluetooth for data exchange. And now it’s Wi-Fi’s turn...

A multi-user system with multiple inputs and outputs has been developed, allowing the network to communicate with more than one computer at the same time. The creators claim that when using the same radio wave range allocated for Wi-Fi, the exchange speed can be tripled.

Qualcomm Atheros has developed a multi-user, multiple-input/multiple-output (MU-MIMO) system that allows the network to communicate with more than one computer at the same time. The company plans to begin demonstrating the technology over the next few months before beginning deliveries to customers early next year.

However, in order to get this high transfer speed, users will have to upgrade both their computers and network routers.

Under the Wi-Fi protocol, clients are served sequentially - only one transmitting and receiving device is used during a certain time interval - so that only a small part of the network bandwidth is used.

The accumulation of these sequential events creates a drop in communication speed as more and more devices connect to the network.

The MU-MIMO (multi-user, multiple input, multiple output) protocol ensures simultaneous transmission of information to a group of clients, which makes more efficient use of the available Wi-Fi network bandwidth and thereby speeds up transmission.

Qualcomm believes such capabilities will be especially useful in conference centers and Internet cafes where multiple users connect to the same network.

The company also believes that it's not just about increasing absolute speed, but also using network and airtime more efficiently to support the growing number of connected devices, services and applications.

Qualcomm plans to sell MU-Mimo chips to manufacturers of routers, access points, smartphones, tablets and other Wi-Fi-enabled devices. The first chips will be able to handle four data streams simultaneously; technology support will be included in Atheros 802.11ac chips and Snapdragon 805 and 801 mobile processors. A demonstration of the technology will take place this year, and the first deliveries of chips are planned for the 1st quarter of next year.

Well, now if anyone wants to delve into this technology in more detail, let’s continue...

MIMO(Multiple Input Multiple Output - multiple input multiple output) is a technology used in wireless communication systems (WIFI, WI-MAX, cellular communication networks), which can significantly improve the spectral efficiency of the system, the maximum data transfer rate and network capacity. The main way to achieve the above benefits is to transmit data from source to destination through multiple radio connections, which is where the technology gets its name. Let's consider the background of this issue and determine the main reasons that led to the widespread use of MIMO technology.

The need for high-speed connections that provide high quality of service (QoS) with high fault tolerance is growing from year to year. This is greatly facilitated by the emergence of such services as VoIP (Voice over Internet Protocol), video conferencing, VoD (Video on Demand), etc. However, most wireless technologies do not allow providing subscribers with high quality service at the edge of the coverage area. In cellular and other wireless communication systems, the quality of the connection, as well as the available data transfer speed, rapidly decreases with distance from the base station (BTS). At the same time, the quality of services also decreases, which ultimately leads to the impossibility of providing real-time services with high quality throughout the entire radio coverage area of ​​the network. To solve this problem, you can try to install base stations as densely as possible and organize internal coverage in all places with low signal levels. However, this will require significant financial costs, which will ultimately lead to an increase in the cost of the service and a decrease in competitiveness. Thus, to solve this problem, an original innovation is required that, if possible, uses the current frequency range and does not require the construction of new network facilities.

Features of radio wave propagation

In order to understand the principles of operation of MIMO technology, it is necessary to consider the general principles of radio wave propagation in space. The waves emitted by various wireless radio systems in the range above 100 MHz behave in many ways like light rays. When radio waves encounter any surface during propagation, depending on the material and size of the obstacle, part of the energy is absorbed, part passes through, and the rest is reflected. The ratio of the shares of absorbed, reflected and transmitted energy is influenced by many external factors, including the frequency of the signal. Moreover, the signal energy reflected and transmitted through can change the direction of its further propagation, and the signal itself is divided into several waves.

The signal propagating according to the above laws from the source to the recipient, after encountering numerous obstacles, is divided into many waves, only part of which reaches the receiver. Each of the waves reaching the receiver forms the so-called signal propagation path. Moreover, due to the fact that different waves are reflected from different numbers of obstacles and travel different distances, different paths have different time delays.

In dense urban environments, due to a large number of obstacles such as buildings, trees, cars, etc., a situation very often arises when there is no direct visibility between the subscriber equipment (MS) and the base station (BTS) antennas. In this case, the only option for the signal to reach the receiver is through reflected waves. However, as noted above, a repeatedly reflected signal no longer has the original energy and may arrive late. Particular difficulty is also created by the fact that objects do not always remain stationary and the situation can change significantly over time. In this regard, the problem of multipath signal propagation arises - one of the most significant problems in wireless communication systems.

Multipath propagation - a problem or an advantage?

Several different solutions are used to combat multipath propagation of signals. One of the most common technologies is Receive Diversity. Its essence lies in the fact that to receive a signal, not one, but several antennas are used (usually two, less often four), located at a distance from each other. Thus, the recipient has not one, but two copies of the transmitted signal, which arrived in different ways. This makes it possible to collect more energy from the original signal, because waves received by one antenna may not be received by another and vice versa. Also, signals arriving out of phase to one antenna may arrive in phase to another. This radio interface design can be called Single Input Multiple Output (SIMO), as opposed to the standard Single Input Single Output (SISO) design. The reverse approach can also be used: when several antennas are used for transmission and one for reception. This also increases the total energy of the original signal received by the receiver. This circuit is called Multiple Input Single Output (MISO). In both schemes (SIMO and MISO), several antennas are installed on the base station side, because It is difficult to implement antenna diversity in a mobile device over a sufficiently large distance without increasing the size of the terminal equipment itself.

As a result of further considerations, we come to the Multiple Input Multiple Output (MIMO) scheme. In this case, several antennas are installed for transmission and reception. However, unlike the above schemes, this diversity scheme not only combats multipath signal propagation, but also provides some additional advantages. By using multiple antennas for transmission and reception, each transmitting/receiving antenna pair can be assigned a separate path for transmitting information. In this case, diversity reception will be performed by the remaining antennas, and this antenna will also serve as an additional antenna for other transmission paths. As a result, theoretically, it is possible to increase the data transfer rate as many times as additional antennas are used. However, a significant limitation is imposed by the quality of each radio path.

How MIMO works

As noted above, to organize MIMO technology it is necessary to install several antennas on the transmitting and receiving sides. Typically, an equal number of antennas are installed at the input and output of the system, because in this case, the maximum data transfer rate is achieved. To show the number of antennas on reception and transmission, along with the name of the MIMO technology, the designation “AxB” is usually mentioned, where A is the number of antennas at the system input, and B is at the output. In this case, the system means a radio connection.

MIMO technology requires some changes in the transmitter structure compared to conventional systems. Let's consider just one of the possible, simplest ways to organize MIMO technology. First of all, a stream divider is needed on the transmitting side, which will divide the data intended for transmission into several low-speed substreams, the number of which depends on the number of antennas. For example, for MIMO 4x4 and an input data rate of 200 Mbit/s, the divider will create 4 streams of 50 Mbit/s each. Next, each of these streams must be transmitted through its own antenna. Typically, transmission antennas are installed with some spatial separation in order to provide as many spurious signals as possible that arise as a result of reflections. In one of the possible ways of organizing MIMO technology, the signal is transmitted from each antenna with a different polarization, which allows it to be identified when received. However, in the simplest case, each of the transmitted signals turns out to be marked by the transmission medium itself (time delay, attenuation and other distortions).

On the receiving side, several antennas receive the signal from the radio air. Moreover, the antennas on the receiving side are also installed with some spatial diversity, thereby ensuring diversity reception, discussed earlier. The received signals arrive at receivers, the number of which corresponds to the number of antennas and transmission paths. Moreover, each of the receivers receives signals from all antennas of the system. Each of these adders extracts from the total flow the signal energy of only the path for which it is responsible. He does this either according to some predetermined attribute that was supplied to each of the signals, or through the analysis of delay, attenuation, phase shift, i.e. set of distortions or “fingerprint” of the propagation medium. Depending on the operating principle of the system (Bell Laboratories Layered Space-Time - BLAST, Selective Per Antenna Rate Control (SPARC), etc.), the transmitted signal may be repeated after a certain time, or transmitted with a slight delay through other antennas.

An unusual phenomenon that may occur in a MIMO system is that the data rate of the MIMO system may be reduced when there is a line of sight between the signal source and receiver. This is primarily due to a decrease in the severity of distortions in the surrounding space, which marks each of the signals. As a result, it becomes difficult to separate the signals at the receiving end and they begin to influence each other. Thus, the higher the quality of the radio connection, the less benefit can be obtained from MIMO.

Multi-user MIMO (MU-MIMO)

The principle of organizing radio communications discussed above refers to the so-called Single user MIMO (SU-MIMO), where there is only one transmitter and receiver of information. In this case, both the transmitter and the receiver can clearly coordinate their actions, and at the same time there is no surprise factor when new users may appear on the air. This scheme is quite suitable for small systems, for example, for organizing communication in a home office between two devices. In turn, most systems, such as WI-FI, WIMAX, cellular communication systems are multi-user, i.e. in them there is a single center and several remote objects, with each of which it is necessary to organize a radio connection. Thus, two problems arise: on the one hand, the base station must transmit a signal to many subscribers through the same antenna system (MIMO broadcast), and at the same time receive a signal through the same antennas from several subscribers (MIMO MAC - Multiple Access Channels).

In the uplink direction - from MS to BTS, users transmit their information simultaneously on the same frequency. In this case, a difficulty arises for the base station: it is necessary to separate signals from different subscribers. One of the possible ways to combat this problem is also the method of linear processing, which involves preliminary encoding of the transmitted signal. The original signal, according to this method, is multiplied with a matrix, which is composed of coefficients reflecting the interference effect from other subscribers. The matrix is ​​compiled based on the current situation on the radio: the number of subscribers, transmission speeds, etc. Thus, before transmission, the signal is subject to distortion inverse to that which it will encounter during radio transmission.

In downlink - the direction from BTS to MS, the base station transmits signals simultaneously on the same channel to several subscribers at once. This leads to the fact that the signal transmitted for one subscriber affects the reception of all other signals, i.e. interference occurs. Possible options to combat this problem are to use Smart Antena, or use dirty paper coding technology. Let's take a closer look at dirty paper technology. The principle of its operation is based on an analysis of the current state of the radio airwaves and the number of active subscribers. The only (first) subscriber transmits his data to the base station without encoding or changing his data, because there is no interference from other subscribers. The second subscriber will encode, i.e. change the energy of your signal so as not to interfere with the first one and not expose your signal to influence from the first one. Subsequent subscribers added to the system will also follow this principle, and will be based on the number of active subscribers and the effect of the signals they transmit.

Application of MIMO

In the last decade, MIMO technology has been one of the most relevant ways to increase the throughput and capacity of wireless communication systems. Let's look at some examples of using MIMO in various communication systems.

The WiFi 802.11n standard is one of the most striking examples of the use of MIMO technology. According to it, it allows you to maintain speeds of up to 300 Mbit/s. Moreover, the previous 802.11g standard allowed only 50 Mbit/s. In addition to increasing data transfer rates, the new standard, thanks to MIMO, also allows for better quality of service in areas with low signal strength. 802.11n is used not only in point/multipoint systems (Point/Multipoint) - the most common niche for using WiFi technology to organize a LAN (Local Area Network), but also for organizing point/point connections that are used to organize backbone communication channels at several speeds hundreds of Mbit/s and allowing data transmission over tens of kilometers (up to 50 km).

The WiMAX standard also has two releases that introduce new capabilities to users using MIMO technology. The first, 802.16e, provides mobile broadband services. It allows you to transmit information at speeds of up to 40 Mbit/s in the direction from the base station to the subscriber equipment. However, MIMO in 802.16e is considered an option and is used in the simplest configuration - 2x2. In the next release, 802.16m MIMO is considered a mandatory technology, with a 4x4 configuration possible. In this case, WiMAX can already be classified as cellular communication systems, namely their fourth generation (due to the high data transfer speed), because has a number of characteristics inherent to cellular networks: roaming, handover, voice connections. In case of mobile use, theoretically, speeds of 100 Mbit/s can be achieved. In a fixed version, the speed can reach 1 Gbit/s.

Of greatest interest is the use of MIMO technology in cellular communication systems. This technology has been used since the third generation of cellular communication systems. For example, in the UMTS standard, in Rel. 6 it is used in conjunction with HSPA technology supporting speeds up to 20 Mbit/s, and in Rel. 7 – with HSPA+, where data transfer rates reach 40 Mbit/s. However, MIMO has not yet found widespread use in 3G systems.

Systems, namely LTE, also provide for the use of MIMO in up to 8x8 configurations. This, in theory, can make it possible to transmit data from the base station to the subscriber over 300 Mbit/s. Another important positive point is the stable connection quality even at the cell edge. In this case, even at a considerable distance from the base station, or when located in a remote room, only a slight decrease in the data transfer rate will be observed.

Thus, MIMO technology finds application in almost all wireless data transmission systems. Moreover, its potential has not been exhausted. New antenna configuration options are already being developed, up to 64x64 MIMO. This will allow us to achieve even higher data rates, network capacity and spectral efficiency in the future.