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 split 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 reasoning, we arrive at 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 allows not only to combat multipath signal propagation, but also to obtain 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 the 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 also allows for better quality of service in areas with low signal strength thanks to MIMO. 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 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 access 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.
Existing mobile networks are used for more than just making calls and sending messages. Thanks to the digital transmission method, data transmission is also possible using existing networks. These technologies, depending on the level of development, are designated 3G and 4G. 4G technology is supported by the LTE standard. The data transfer speed depends on some network features (determined by the operator), theoretically reaching up to 2 Mb/s for a 3G network and up to 1 Gb/s for a 4G network. All of these technologies work more efficiently if there is a strong and stable signal. For these purposes, most modems provide for connecting external antennas.
Panel antenna
On sale you can find various antenna options to improve the quality of reception. 3G panel antenna is very popular. The gain of such an antenna is about 12 dB in the frequency range 1900-2200 MHz. This type of device can also improve the quality of the 2G signal - GPRS and EDGE.
Like the vast majority of other passive devices, it has a one-way directionality, which, together with an increase in the received signal, reduces the level of interference from the sides and rear. Thus, even in conditions of unstable reception, it is possible to raise the signal level to acceptable values, thereby increasing the speed of reception and transmission of information.
Application of panel antennas for operation in 4G networks
Since the operating range of 4G networks practically coincides with the range of the previous generation, there are no difficulties in using these antennas in 3G 4G LTE networks. For any of the technologies, the use of antennas allows data transmission rates to be brought closer to maximum values.
New technology using separate receivers and transmitters in the same frequency band has made it possible to further increase the speed of receiving and transmitting data. The design of the existing 4G modem involves the use of MIMO technology.
The undoubted advantage of panel antennas is their low cost and exceptional reliability. There is practically nothing in the design that can break even if dropped from a great height. The only weak point is the high-frequency cable, which can break where it enters the housing. To extend the life of the device, the cable must be securely fastened.
MIMO technology
To increase the capacity of the communication channel between the receiver and the data transmitter, a signal processing method has been developed when reception and transmission are carried out on different antennas.
Note! By using LTE MIMO antennas, you can increase throughput by 20-30% compared to working with a simple antenna.
The basic principle is to eliminate the coupling between antennas.
Electromagnetic waves can have different directions relative to the plane of the earth. This is called polarization. Mainly used are vertically and horizontally polarized antennas. To eliminate mutual influence, the antennas differ from each other in polarization by an angle of 90 degrees. To ensure that the influence of the earth's surface is the same for both antennas, the polarization planes of each are shifted by 45 degrees. relative to the ground. Thus, if one of the antennas has a polarization angle of 45 degrees, then the other, accordingly, has 45 degrees. Relative to each other, the displacement is the required 90 degrees.
The figure clearly shows how the antennas are deployed relative to each other and relative to the ground.
Important! The polarization of the antennas must be the same as at the base station.
If for 4G LTE technologies MIMO support is available by default at the base station, then for 3G due to the large number of devices without MIMO, operators are in no hurry to introduce new technologies. The fact is that devices will work much slower on a MIMO 3G network.
Installing antennas for a modem yourself
The rules for installing antennas do not differ from the usual ones. The main condition is the absence of obstacles between the client and base stations. A growing tree, the roof of a nearby building, or, worse, a power line, serve as reliable shields for electromagnetic waves. And the higher the frequency of the signal, the greater the attenuation will be caused by obstacles located in the path of radio waves.
Depending on the type of mounting, the antennas can be installed on the wall of a building or mounted on a mast. There are two types of antennasMIMO:
- monoblock;
- spaced.
Monoblock ones already contain two structures inside, installed with the necessary polarization, and spaced ones consist of two antennas that need to be mounted separately, each of them must be directed exactly at the base station.
All the nuances of installing a MIMO antenna with your own hands are clearly and in detail described in the accompanying documentation, but it is better to first consult with the provider or invite a representative for installation, paying a not very large amount, but receiving a certain guarantee for the work performed.
How to make an antenna yourself
There are no fundamental difficulties in making it yourself. You need skills in working with metal, the ability to hold a soldering iron, desire and accuracy.
An indispensable condition is strict adherence to the geometric dimensions of all, without exception, component parts. The geometric dimensions of high-frequency devices must be maintained to the nearest millimeter or more accurately. Any deviation leads to deterioration in performance. The gain will drop and the coupling between MIMO antennas will increase. Ultimately, instead of strengthening the signal, it will weaken.
Unfortunately, exact geometric dimensions are not widely available. As an exception, the materials available on the network are based on the repetition of some factory designs, which are not always copied with the required accuracy. Therefore, you should not place high hopes on diagrams, descriptions and methods published on the Internet.
On the other hand, if extremely strong gain is not required, then a MIMO antenna made independently, in compliance with the specified dimensions, will still give, although not a large, positive effect.
The cost of materials is low, and the time required if you have the skills is also not too high. In addition, no one bothers you to try several options and choose the acceptable one based on the test results.
In order to make a 4G LTE MIMO antenna with your own hands, you need two absolutely flat sheets of galvanized steel 0.2-0.5 mm thick, or better yet, one-sided foil fiberglass laminate. One of the sheets will be used for the manufacture of a reflector (reflector), and the other for the manufacture of active elements. The cable for connecting to the modem must have a resistance of 50 Ohms (this is the standard for modem equipment).
TV cable cannot be used for two reasons:
- A resistance of 75 Ohms will cause a mismatch with the modem inputs;
- large thickness.
It is also necessary to select connectors that must exactly match the connectors on the modem.
Important! The specified distance between the active elements and the reflector must be measured from the foil layer if foil material is used.
In addition, you will need a small piece of copper wire 1-1.2 mm thick.
The manufactured structure must be placed in a plastic case. Metal cannot be used, since in this way the antenna will be enclosed in an electromagnetic shield and will not work.
Note! Most of the drawings refer not to MIMO antennas, but to panel antennas. Externally, they differ in that one cable is supplied to a simple panel antenna, and two are needed for MIMO.
By making two panel antennas, you can get a diversity version of a DIY MIMO 4G antenna.
To summarize, we can say that making a MIMO antenna with your own hands is not a very difficult task. With proper care, it is quite possible to get a working device while saving some money. It is somewhat easier to make a 3G antenna yourself. In remote areas where there is no LTE coverage yet, this may be the only option to improve connection speeds.
Video
MIMO technology played a huge role in the development of WiFi. A few years ago it was impossible to imagine other devices with a throughput of 300 Mbit/s and higher. The emergence of new high-speed communication standards, for example, 802.11n, was largely due to MIMO.
In general, it’s worth mentioning here that when we talk about WiFi technology, we actually mean one of the communication standards, specifically IEEE 802.11. WiFi became a brand after tempting prospects for using wireless data transmission emerged. You can read a little more about Wi-Fi technology and the 802.11 standard in.
What is MIMO technology?
To give the simplest possible definition, then MIMO is Multi-Stream Data Transmission. The abbreviation can be translated from English as “several inputs, multiple outputs.” Unlike its predecessor (SingleInput/SingleOutput), in devices with MIMO support the signal is broadcast on one radio channel using not one, but several receivers and transmitters. When indicating the technical characteristics of WiFi devices, their number is indicated next to the abbreviation. For example, 3x2 means 3 signal transmitters and 2 receiving antennas.
Besides, MIMO uses spatial multiplexing. Behind the terrifying name lies the technology of simultaneous transmission of several data packets over one channel. Thanks to this “densification” of the channel, its throughput can be doubled or more.
MIMO and WiFi
With the growing popularity of wireless data transmission over WiFi connections, of course, the requirements for their speed have increased. And it was MIMO technology and other developments that took it as a basis that made it possible to increase throughput several times. The development of WiFi follows the path of development of 802.11 standards - a, b, g, n and so on. It’s not for nothing that we mentioned the emergence of the 802.11n standard. Multiple Input Multiple Output is its key component, which makes it possible to increase the channel speed of a wireless connection from 54 Mbit/s to more than 300 Mbit/s.
The 802.11n standard allows you to use either a standard 20 MHz channel width or a 40 MHz broadband line with higher throughput. As mentioned above, the signal is reflected many times, thereby using multiple streams on one communication channel.
Thanks to this, WiFi-based Internet access now allows not only surfing, checking email and communication in ICQ, but also online games, online video, communication on Skype and other “heavy” traffic.
The newer standard also uses MIMO technology.
Problems with using MIMO in WIFI
At the dawn of the technology, there was a difficulty in combining devices, working with and without MIMO support. However, now this is no longer so relevant - almost every self-respecting manufacturer of wireless equipment uses it in their devices.
Also, one of the problems with the advent of data transmission technology using multiple receivers and multiple transmitters was the price of the device. However, here the company made a real price revolution. She not only managed to establish the production of wireless equipment with MIMO support, but also did it at very affordable prices. Look, for example, at the cost of a typical company package - (base station), (client side). And in these devices it’s not just MIMO, but proprietary improved airMax technology based on it.
The only problem that remains is to increase the number of antennas and transmitters (currently a maximum of 3) for devices with PoE. It is difficult to provide power to a more energy-intensive design, but again, Ubiquiti is making constant progress in this direction.
AirMAX technology
Ubiquiti Networks is a recognized leader in the development and implementation of innovative WiFi technologies, including MIMO. It was on this basis that Ubiquiti developed and patented the technology AirMAX. Its essence is that the reception and transmission of a signal by several antennas on one channel is ordered and structured by the TDMA protocol with hardware acceleration: data packets are separated into separate time slots, transmission queues are coordinated.
This allows you to expand the channel capacity and increase the number of connected subscribers without loss of communication quality. This solution is effective, easy to use and, importantly, inexpensive. Unlike similar equipment used in WiMAX networks, equipment from Ubiquiti Networks with AirMAX technology is pleasantly priced.
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27.08.2015
Surely many have already heard about the technology MIMO, in recent years it has often been full of advertising brochures and posters, especially in computer stores and magazines. But what is MIMO (MIMO) and what is it eaten with? Let's take a closer look.
MIMO technology
MIMO (Multiple Input Multiple Output; multiple inputs, multiple outputs) is a method of spatial signal encoding that allows you to increase the channel bandwidth, in which two or more antennas are used for data transmission and the same number of antennas for reception. The transmitting and receiving antennas are spaced so far as to achieve minimal mutual influence on each other between adjacent antennas. MIMO technology is used in Wi-Fi, WiMAX, LTE wireless communications to increase capacity and more efficiently use frequency bandwidth. In fact, MIMO allows you to transmit more data in one frequency range and a given frequency corridor, i.e. increase a speed. This is achieved through the use of several transmitting and receiving antennas. 
History of MIMO
MIMO technology can be considered a fairly recent development. Its history begins in 1984, when the first patent for the use of this technology was registered. Initial development and research took place in the company Bell Laboratories, and in 1996 the company Airgo Networks The first MIMO chipset was released called True MIMO. MIMO technology received its greatest development at the beginning of the 21st century, when Wi-Fi wireless networks and 3G cellular networks began to develop at a rapid pace. And now MIMO technology is widely used in 4G LTE and Wi-Fi 802.11b/g/ac networks.
What does MIMO technology provide?
For the end user, MIMO provides a significant increase in data transfer speed. Depending on the configuration of the equipment and the number of antennas used, you can get a twofold, threefold or up to eightfold increase in speed. Typically, wireless networks use the same number of transmitting and receiving antennas, and this is written as, for example, 2x2 or 3x3. Those. if we see a MIMO 2x2 recording, it means two antennas are transmitting the signal and two are receiving. For example, in the Wi-Fi standard one 20 MHz wide channel gives a throughput of 866 Mbps, while an 8x8 MIMO configuration combines 8 channels, giving a maximum speed of about 7 Gbps. The same is true for LTE MIMO - a potential increase in speed by several times. To fully use MIMO in LTE networks, you need , because As a rule, built-in antennas are not sufficiently spaced and provide little effect. And of course, there must be MIMO support from the base station.
An LTE antenna with MIMO support transmits and receives signals in horizontal and vertical planes. This is called polarization. A distinctive feature of MIMO antennas is the presence of two antenna connectors, and accordingly the use of two wires to connect to the modem/router.
Despite the fact that many say, and not without reason, that a MIMO antenna for 4G LTE networks is actually two antennas in one, you should not think that using such an antenna will double the speed. This can only be the case in theory, but in practice the difference between a conventional and MIMO antenna in a 4G LTE network does not exceed 20-25%. However, more important in this case will be the stable signal that a MIMO antenna can provide.
One approach to increasing data rates for 802.11 WiFi and 802.16 WiMAX is to use wireless systems that use multiple antennas for both the transmitter and receiver. This approach is called MIMO (literal translation - “multiple input multiple output”), or “smart antenna systems”. MIMO technology plays an important role in the implementation of the 802.11n WiFi standard.
MIMO technology uses multiple antennas of different types tuned to the same channel. Each antenna transmits a signal with different spatial characteristics. Thus, MIMO technology uses the radio wave spectrum more efficiently and without compromising reliability. Each Wi-Fi receiver “listens” to all signals from each Wi-Fi transmitter, which allows you to make data transmission paths more diverse. In this way, multiple paths can be recombined, resulting in amplification of the desired signals in wireless networks.
Another advantage of MIMO technology is that this technology provides spatial division multiplexing (SDM). SDM spatially multiplexes multiple independent data streams simultaneously (mostly virtual channels) within a single channel spectral bandwidth. In essence, multiple antennas transmit different data streams with individual signal encoding (spatial streams). These streams, moving in parallel through the air, “push” more data along a given channel. At the receiver, each antenna sees a different combination of signal streams and the receiver “demultiplexes” these streams to use them. MIMO SDM can significantly increase data throughput if the number of spatial data streams is increased. Each spatial stream requires its own transmit/receive (TX/RX) antenna pairs at each transmission end. The operation of the system is shown in Fig. 1
It is also necessary to understand that MIMO technology requires a separate RF circuit and analog-to-digital converter (ADC) for each antenna. Implementations requiring more than two antennas in a chain must be carefully designed to avoid increasing costs while maintaining an appropriate level of efficiency.
An important tool for increasing the physical speed of data transmission in wireless networks is expanding the bandwidth of spectral channels. By using wider channel bandwidth with Orthogonal Frequency Division Multiplexing (OFDM), data transmission is carried out at maximum performance. OFDM is a digital modulation that has proven itself as a tool for implementing bidirectional high-speed wireless data transmission in WiMAX / WiFi networks. The channel capacity expansion method is cost-effective and fairly easy to implement with moderate increases in digital signal processing (DSP). When properly implemented, it is possible to double the bandwidth of the Wi-Fi 802.11 standard from a 20 MHz channel to a 40 MHz channel, and can provide more than twice the bandwidth of channels currently used. By combining MIMO architecture with higher channel bandwidth, the result is a very powerful and cost-effective approach for increasing physical transmission rates.
MIMO technology with 20 MHz channels is expensive to meet IEEE 802.11n WiFi requirements (100 Mbps throughput on MAC SAP). Also, to meet these requirements when using a 20 MHz channel, you will need at least three antennas, both on the transmitter and on the receiver. But at the same time, operation on a 20 MHz channel ensures reliable operation with applications that require high throughput in a real user environment.
The combined use of MIMO and channel expansion technologies meets all user requirements and is a fairly reliable tandem. This is also true when using several resource-intensive network applications simultaneously. The combination of MIMO and 40 MHz channel extension will allow it to meet more complex requirements, such as Moore's Law and CMOS implementation of advanced DSP technology.
When using an extended 40 MHz channel in the 2.4 GHz band, there were initially difficulties with compatibility with equipment based on WiFi standards 802.11a / b / g, as well as with equipment using Bluetooth technology for data transmission.
To solve this problem, the 802.11n Wi-Fi standard provides a number of solutions. One such mechanism, specifically designed to protect networks, is the so-called low-throughput (non-HT) redundant mode. Before using the 802.11n WiFi data protocol, this mechanism sends one packet to each half of the 40 MHz channel to advertise the Network Distribution Vector (NAV). Following the non-HT redundant mode NAV message, the 802.11n data transfer protocol can be used for the duration stated in the message, without violating the legacy (integrity) of the network.
Another mechanism is a kind of signaling that prevents wireless networks from extending the channel beyond 40 MHz. For example, a laptop has 802.11n and Bluetooth modules installed; this mechanism knows about the possibility of potential interference when these two modules operate simultaneously and disables transmission over the 40 MHz channel of one of the modules.
These mechanisms ensure that WiFi 802.11n will work with older 802.11 networks without the need to migrate the entire network to 802.11n equipment.
You can see an example of using the MIMO system in Fig. 2
If you have any questions after reading, you can ask them through the message sending form in the section
