MIMO technology: what is it and what is it eaten with? MIMO data transmission technology in WIFI wireless networks

Multi-user MIMO is an integral part of the 802.11 ac standard. But so far, there have not yet been devices that support a new kind of multi-antenna technology. Previous generation 802.11ac WLAN routers were referred to as Wave 1 equipment. Only Wave 2 introduces multi-user MIMO technology(MU-MIMO), and at the head of this second wave devices coming.

■ yes □ no

Since multi-user MIMO transmits a signal simultaneously to multiple devices, the transmission protocol is extended accordingly in terms of the formation of data block headers: instead of transmitting several spatially separated streams for one client, multi-user MIMO distributes the transmission for each user separately, as well as encoding . The bandwidth allocation and coding remain the same.

Single User If four devices share the same WLAN, then a 4×4:4 MIMO router transmits four spatial data streams, but always only to the same device. Devices and gadgets are serviced alternately. Multi User With Multi User MIMO (Multi User MIMO) supported, there is no queue of devices waiting to access the resources of the WLAN router. Laptop, tablet, phone and TV are provided with data at the same time.

A WLAN network is like a busy highway: depending on the time of day, in addition to PCs and laptops, tablets, smartphones, TVs and game consoles. The average household has more than five devices connected to the Internet via WLAN, and this number is constantly growing. With the speed of 11 Mbps, which is provided under the main IEEE 802.11b standard, surfing the web and downloading data requires a lot of patience, because the router can only be connected to one device at a time. If radio communication is used by three devices at once, then each client receives only a third of the duration of the communication session, and two-thirds of the time is spent waiting. Although the latest IEEE 802.11ac WLANs provide data rates up to 1 Gbps, they also have the problem of speed drops due to queuing. But already the next generation of devices (802.11ac Wave 2) promises higher performance for radio networks with multiple active devices.

To better understand the essence of innovation, you should first recall what changes have occurred with WLAN networks in the recent past. One of the most effective techniques increasing the data rate, starting with the IEEE 802.1In standard, is the MIMO technology (Multiple Input Multiple Output: multi-channel input - multi-channel output). It involves the use of several radio antennas for parallel transmission of data streams. If, for example, one video file is transmitted over a WLAN and a MIMO router with three antennas is used, each transmitter will ideally (if the receiver has three antennas) send a third of the file.

Rising costs with each antenna

In the IEEE 802.11n standard, the maximum data rate for each individual stream, together with the overhead, reaches 150 Mbps. Devices with four antennas are thus capable of transmitting data at up to 600 Mbps. The current IEEE 802.11ac standard theoretically comes out at about 6900 Mbps. In addition to wide radio channels and improved modulation, the new standard provides for the use of up to eight MIMO streams.

But just increasing the number of antennas does not guarantee multiple acceleration of data transmission. Conversely, with four antennas, the amount of overhead increases greatly, and the process of detecting radio collisions also becomes more costly. To justify the use of more antennas, MIMO technology continues to improve. For the sake of distinction, it is more correct to call the former MIMO single-user MIMO (Single User MIMO). Although it provides simultaneous transmission of several spatial streams, as mentioned earlier, but always only at one address. Such a disadvantage is now eliminated with the help of multi-user MIMO. With this technology, WLAN routers can simultaneously transmit a signal to four clients. A device with eight antennas can, for example, use four to provide a laptop and in parallel with the help of two others - a tablet and a smartphone.

MIMO - accurate directional signal

In order for a router to forward WLAN packets to different clients at the same time, it needs to know where the clients are located. To do this, first of all, test packets are sent in all directions. Clients respond to these packets and the base station stores signal strength data. Beamforming technology is one of the most important helpers of MU MIMO. Although it is already supported by the IEEE 802.11n standard, it has been improved in IEEE 802.11ac. Its essence boils down to establishing the optimal direction for sending a radio signal to clients. The base station specifically sets for each radio signal the optimal directivity of the transmitting antenna. For multi-user mode, finding the optimal signal path is especially important, because changing the location of only one client can change all transmission paths and disrupt the throughput of the entire WLAN network. Therefore, every 10 ms, a channel analysis is performed.

In comparison, single-user MIMO only analyzes every 100ms. Multi-user MIMO can serve four clients simultaneously, with each client receiving up to four data streams in parallel, for a total of 16 streams. This multi-user MIMO requires new WLAN routers as the need for processing power grows.

One of the biggest problems in multi-user MIMO is client-to-client interference. Although channel congestion is often measured, this is not enough. If necessary, some frames are given priority, while others, on the contrary, are adhered to. To do this, 802.11ac uses various queues that different speed perform processing depending on the type of data packet, giving preference, for example, to video packets.

MIMO(Multiple Input Multiple Output - multiple input multiple output) is a technology used in wireless communication systems (WIFI, cellular 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 advantages is to transmit data from the source to the destination via multiple radio links, which is where the technology gets its name from. Consider the backstory this issue, and determine the main reasons for the widespread use of MIMO technology.

Need for high speed connections providing high quality of service (QoS) with high fault tolerance is growing year by year. This is largely facilitated by the emergence of such services as VoIP (), VoD (), etc. However, most wireless technologies do not allow to provide subscribers with high quality of 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 rate, drops rapidly with distance from (BTS). Along with this, the quality of services also decreases, which ultimately leads to the impossibility of providing real-time services with high quality throughout the network's radio coverage. To solve this problem, you can try to install base stations as tightly as possible and organize internal coverage in all places with low level signal. 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, using, if possible, the current frequency range and does not require the construction of new network facilities.

Features of the propagation of radio waves

In order to understand the principles of operation of MIMO technology, it is necessary to consider the general ones in space. Waves emitted by various wireless radio systems in the range above 100 MHz behave in many ways like light beams. When radio waves propagate on a surface, depending on the material and size of the obstacle, some of the energy is absorbed, some passes through, and the rest is reflected. The ratio of the shares of the absorbed, reflected and passed through parts of the energies is influenced by many external factors, including the frequency of the signal. Moreover, the reflected and transmitted through signal energies can change the direction of their further dissemination, and the signal itself is divided into several waves.

The signal propagating according to the above laws from the source to the receiver, after meeting with numerous obstacles, is divided into many waves, only a part of which will reach the receiver. Each of the waves reaching the receiver forms a so-called signal propagation path. Moreover, due to the fact that different waves are reflected from a different number of obstacles and pass different distance, different paths have different .

In a dense urban environment, due to a large number of obstacles such as buildings, trees, cars, etc., it is very common for a situation where there is no line of sight between (MS) and base station (BTS) antennas. In this case, the only way to reach the receiver's signal is through reflected waves. However, as noted above, the repeatedly reflected signal no longer has the initial energy and may arrive with a delay. A 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, a problem arises - one of the most significant problems in wireless communication systems.

Multipath propagation - a problem or an advantage?

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

As a result of further reasoning, we arrive at the Multiple Input Multiple Output (MIMO) scheme. In this case, several antennas are installed for transmitting and receiving. However, unlike the above schemes, this diversity scheme allows not only to deal with multipath signal propagation, but also to obtain some additional advantages. By using multiple transmit and receive antennas, each transmit/receive 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 will be used. However, a significant limitation is imposed by the quality of each radio path.

How MIMO Works

As noted above, the organization of MIMO technology requires the installation of several antennas on the transmitting and receiving sides. Usually, an equal number of antennas is installed at the input and output of the system, since in this case, the maximum data transfer rate is reached. To show the number of antennas at the 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 input of the system, and B is at the output. Under the system this case refers to a radio connection.

For the MIMO technology to work, some changes in the structure of the transmitter are required compared to conventional systems. Let us consider only one of the possible, most simple, ways of organizing MIMO technology. First of all, on the transmitting side, a stream divider is needed, 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 Mbps, the divider will create 4 streams of 50 Mbps each. Further, each of these streams must be transmitted through its own antenna. Typically, transmitting antennas are set up with some spatial separation to allow for as many spurious signals as possible that result from multipaths. In one of possible ways organization of MIMO technology, the signal is transmitted from each antenna with a different polarization, which makes it possible to identify it upon reception. However, in the simplest case, each of the transmitted signals is marked by the transmission medium itself (time delay, and other distortions).

On the receiving side, several antennas receive a signal from the radio. Moreover, the antennas on the receiving side are also installed with some spatial diversity, due to which the diversity reception discussed earlier is provided. The received signals are fed to 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 sign that each of the signals was equipped with, or due to the analysis of delay, attenuation, phase shift, i.e. a set of distortions or "fingerprint" of the distribution medium. Depending on how the system works (Bell Laboratories Layered Space-Time - BLAST, Selective Per Antenna Rate Control (SPARC), etc.), the transmitted signal may be repeated every certain time, or transmitted with a slight delay through other antennas.

In a system with MIMO technology, an unusual phenomenon may occur in that the data rate in the MIMO system may decrease if there is a line-of-sight between the signal source and the receiver. This is due primarily to a decrease in the severity of distortions of the surrounding space, which marks each of the signals. As a result, it becomes problematic on the receiving side to separate the signals, and they begin to influence each other. Thus, the higher the quality of the radio connection, the less benefit can be gained from MIMO.

Multi-user MIMO (MU-MIMO)

The above principle of organizing radio communication 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 can appear on the air. Such a 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. they exist single center and several remote objects, with each of which it is necessary to establish 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 the signals from different subscribers. One possible way to deal with this problem is also the linear processing method, which provides for a preliminary transmitted signal. The original signal, according to this method, is multiplied with a matrix, which is composed of coefficients reflecting interference from other subscribers. The matrix is ​​compiled based on the current situation on the air: the number of subscribers, transmission speeds, etc. Thus, prior to transmission, the signal is subjected to distortion inverse to that which it encounters 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. Options To combat this problem is the use or application of dirty paper coding technology (“dirty paper”). Let's take a closer look at the dirty paper technology. The principle of its operation is based on the analysis of the current state of the radio and the number of active subscribers. The only (first) subscriber transmits his data to the base station without encoding, changing his data, because. there is no interference from other subscribers. The second subscriber will encode, i.e. change the energy of his signal so as not to interfere with the first and not to subject his signal to influence from the first. Subsequent subscribers added to the system will also follow this principle, based on the number of active subscribers and the effect of the signals they transmit.

Application of MIMO

MIMO technology in the last decade is one of the most relevant ways to increase the throughput and capacity of wireless communication systems. Consider some examples of using MIMO in various systems connections.

The WiFi 802.11n standard is one of the most prominent examples of the use of MIMO technology. According to him, it allows you to maintain speeds up to 300 Mbps. Moreover, the previous standard 802.11g allowed to provide only 50 Mbps. In addition to increasing the data rate, the new standard, thanks to MIMO, also allows you to provide best performance quality of service in places with low signal strength. 802.11n is used not only in point/multipoint systems (Point/Multipoint) - the most common niche for using WiFi technology for organizing LAN (Local Area Network), but also for organizing point-to-point connections that are used to organize trunk communication channels at a speed of several hundred Mbps and allow data to be transmitted over tens of kilometers (up to 50 km).

The WiMAX standard also has two releases that bring new possibilities to users with the help of MIMO technology. The first, 802.16e, provides mobile broadband services. It allows you to transfer information at a speed of up to 40 Mbps 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 possible 4x4 configuration. In this case, WiMAX can already be attributed to cellular systems communication, namely their fourth generation (due to high speed data transmission), because has a number of inherent cellular networks signs: , voice connections. When mobile use theoretically 100 Mbps can be achieved. In the fixed version, the speed can reach 1 Gbps.

Of greatest interest is the use of MIMO technology in cellular communication systems. This technology finds its application starting from the third generation of cellular communication systems. For example, in the standard, in Rel. 6, it is used in conjunction with HSPA technology with support for speeds up to 20 Mbps, and in Rel. 7 - with HSPA+, where data transfer rates reach 40 Mbps. However, MIMO has not found wide application in 3G systems.

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

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

April 9th, 2014

At one time, the IR connection somehow quietly and imperceptibly left, 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 range of radio waves allocated for Wi-Fi, the exchange rate can be tripled.

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

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

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

The accumulation of these consecutive events creates a drop in the exchange rate as more and more devices connect to the network.

The MU-MIMO protocol (multi-user, multiple input, multiple output) provides simultaneous transmission of information to a group of clients, which makes more efficient use of available bandwidth WiFi networks and thereby speed up the transmission.

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

The company also believes that it is not only about increasing the absolute speed, but also more efficient use of the network and airtime to support a 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 work simultaneously with four data streams; technology support will be included in Atheros 802.11ac chips and mobile Snapdragon processors 805 and 801. The demonstration of the technology will take place this year, and the first shipments of chips are scheduled for the 1st quarter of next year.

Well, now who wants to delve into this technology in more detail, we continue ...

MIMO(Multiple Input Multiple Output - multiple input multiple output) is a technology used in wireless communication systems (WIFI, WI-MAX, cellular 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 advantages is to transmit data from the source to the destination via multiple radio links, which is where the technology gets its name from. Consider the background of this issue, and determine the main reasons that served as a widespread use of MIMO technology.

The need for high-speed connections that provide high quality of service (QoS) with high fault tolerance is growing year by year. This is largely 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 of 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 rate, drops rapidly 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 radio coverage of the network. To solve this problem, you can try to install the base stations as tightly as possible and organize internal coverage in all places with a low signal level. 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, using, if possible, the current frequency range and not requiring the construction of new network facilities.

Features of the propagation of radio waves

In order to understand the principles of operation of MIMO technology, it is necessary to consider the general principles of the propagation of radio waves in space. Waves emitted by various wireless radio systems in the range above 100 MHz behave in many ways like light beams. When radio waves propagate on a surface, depending on the material and size of the obstacle, some of the energy is absorbed, some passes through, and the rest is reflected. The ratio of the shares of the absorbed, reflected and transmitted parts of the energy is affected by many external factors, including the frequency of the signal. Moreover, the reflected and passed through signal energies can change the direction of their further propagation, and the signal itself is divided into several waves.

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

In a dense urban environment, due to a large number of obstacles such as buildings, trees, cars, etc., a situation often arises when there is no line of sight between the subscriber equipment (MS) and the base station antennas (BTS). In this case, the only way to reach the receiver's signal is through reflected waves. However, as noted above, the repeatedly reflected signal no longer has the initial energy and may arrive with a delay. A 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?

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

As a result of further reasoning, we arrive at the Multiple Input Multiple Output (MIMO) scheme. In this case, several antennas are installed for transmitting and receiving. However, unlike the above schemes, this diversity scheme allows not only to deal with multipath signal propagation, but also to obtain some additional advantages. By using multiple transmit and receive antennas, each transmit/receive 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 rate by as many times as many additional antennas will be used. However, a significant limitation is imposed by the quality of each radio path.

How MIMO Works

As noted above, the organization of MIMO technology requires the installation of several antennas on the transmitting and receiving sides. Usually, an equal number of antennas is installed at the input and output of the system, since in this case, the maximum data transfer rate is reached. To show the number of antennas at the 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 input of the system, and B is at the output. The system in this case refers to the radio connection.

For the MIMO technology to work, some changes in the structure of the transmitter are required compared to conventional systems. Let us consider only one of the possible, most simple, ways of organizing MIMO technology. First of all, on the transmitting side, a stream divider is needed, 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 Mbps, the divider will create 4 streams of 50 Mbps each. Further, each of these streams must be transmitted through its own antenna. Typically, transmitting antennas are set up with some spatial separation to allow for as many spurious signals as possible that result from multipaths. In one of the possible ways of organizing MIMO technology, the signal is transmitted from each antenna with a different polarization, which makes it possible to identify it upon reception. 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 a signal from the radio. Moreover, the antennas on the receiving side are also installed with some spatial diversity, due to which the diversity reception discussed earlier is provided. The received signals are fed to 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 sign that each of the signals was equipped with, or due to the analysis of delay, attenuation, phase shift, i.e. a set of distortions or "fingerprint" of the distribution medium. Depending on how the system works (Bell Laboratories Layered Space-Time - BLAST, Selective Per Antenna Rate Control (SPARC), etc.), the transmitted signal can be repeated after a certain time, or transmitted with a slight delay through other antennas.

In a system with MIMO technology, an unusual phenomenon may occur in that the data rate in the MIMO system may decrease if there is a line-of-sight between the signal source and the receiver. This is due primarily to a decrease in the severity of distortions of the surrounding space, which marks each of the signals. As a result, it becomes problematic on the receiving side to separate the signals, and they begin to influence each other. Thus, the higher the quality of the radio connection, the less benefit can be gained from MIMO.

Multi-user MIMO (MU-MIMO)

The above principle of organizing radio communication 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 can appear on the air. Such a 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. they have 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 the signals from different subscribers. One possible way to deal with this problem is also the linear processing method, which involves pre-encoding the transmitted signal. The original signal, according to this method, is multiplied with a matrix, which is composed of coefficients reflecting interference from other subscribers. The matrix is ​​compiled based on the current situation on the air: the number of subscribers, transmission speeds, etc. Thus, prior to transmission, the signal is subjected to distortion inverse to that which it encounters 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 for dealing with this problem are the use of Smart Antena, or the use of dirty paper coding technology (“dirty paper”). Let's take a closer look at the dirty paper technology. The principle of its operation is based on the analysis of the current state of the radio and the number of active subscribers. The only (first) subscriber transmits his data to the base station without encoding, changing his data, because. there is no interference from other subscribers. The second subscriber will encode, i.e. change the energy of his signal so as not to interfere with the first and not to subject his signal to influence from the first. Subsequent subscribers added to the system will also follow this principle, based on the number of active subscribers and the effect of the signals they transmit.

Application of MIMO

MIMO technology in the last decade is one of the most relevant ways to increase the throughput and capacity of wireless communication systems. Let's consider some examples of using MIMO in various communication systems.

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

The WiMAX standard also has two releases that bring new possibilities to users with the help of MIMO technology. The first, 802.16e, provides mobile broadband services. It allows you to transfer information at a speed of up to 40 Mbps 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 possible 4x4 configuration. In this case, WiMAX can already be attributed to cellular communication systems, namely their fourth generation (due to the high data transfer rate), because has a number of features inherent in cellular networks: roaming, handover, voice connections. In the case of mobile use, theoretically 100 Mbps can be achieved. In the fixed version, the speed can reach 1 Gbps.

Of greatest interest is the use of MIMO technology in cellular communication systems. This technology has found its application 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 with support for speeds up to 20 Mbps, and in Rel. 7 - with HSPA+, where data transfer rates reach 40 Mbps. However, MIMO has not found wide application in 3G systems.

Systems, namely LTE, also provide for the use of MIMO in configurations up to 8x8. This, in theory, can make it possible to transmit data from the base station to the subscriber over 300 Mbps. Also an important positive point is the stable quality of the connection even at the edge of the honeycomb. In this case, even at a considerable distance from the base station, or when you are 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. And its potential has not been exhausted. New antenna configuration options are already being developed, up to 64x64 MIMO. This will make it possible to achieve even higher data rates, network capacity and spectral efficiency in the future.