In light of the release of new wireless devices with support for MU-MIMO technology, in particular with the release of UniFi AC HD (UAP-AC-HD), there is a need to clarify what it is and why old hardware does not support this technology.

What is 802.11ac?

The 802.11ac standard is a wireless technology transformation that replaces the previous generation in the form of the 802.11n standard.

The advent of 802.11n was supposed to allow businesses to use this technology everywhere as an alternative to a conventional wired connection for working inside local network(LAN).

802.11ac is the next step in the evolution of wireless technology. Theoretically, the new standard can provide data transfer rates up to 6.9 Gbps in the 5 GHz band. This is 11.5 times the scope of 802.11n data transmission.

The new standard is available in two releases: Wave 1 and Wave 2. Below you can find a comparison table for current standards.

What is the difference between Wave 1 and Wave 2?

802.11ac Wave 1 products have been on the market since around mid-2013. The new revision of the standard is based on previous version standard, but with some very significant changes, namely:

• Improved performance from 1.3 Gbps to 2.34 Gbps;
• Added support for Multi User MIMO (MU-MIMO);
• Use of wide channels in 160 MHz is allowed;
• Fourth spatial stream (Spatial Stream) for greater performance and stability;
• More channels in the 5GHz band;

What exactly are the Wave 2 enhancements for the real user?

Bandwidth growth has a positive effect on applications that are sensitive to bandwidth and delays within the network. This is primarily the transmission of streaming voice and video content, as well as an increase in network density and an increase in the number of clients.

MU-MIMO provides great opportunities for the development of the "Internet of Things" (Internet of Things, IoT), when one user can connect several devices at the same time.

MU-MIMO technology allows multiple simultaneous downstreams, providing simultaneous service to multiple devices at once, which improves network performance as a whole. MU-MIMO also has a positive effect on latency, providing faster connection and overall client experience. In addition, the features of the technology allow you to connect to the network an even greater number of simultaneous clients than in the previous version of the standard.

Using a channel width of 160 MHz requires certain conditions (low power, low noise figure, etc.), and the channel can provide a huge performance boost when transmitting large amounts of data. In comparison, 802.11n can provide up to 450Mbps of channel speed, the newer 802.11ac Wave 1 up to 1.3Gbps, while 802.11ac Wave 2 with a 160MHz channel can provide up to 2.3Gbps of channel speed.

In the previous generation of the standard, the use of 3 transceiver antennas was allowed, the new revision adds the 4th stream. This change improves the range and stability of the connection.

There are 37 channels in the 5 GHz band used worldwide. Some countries have a limited number of channels, some do not. 802.11ac Wave 2 allows for more channels, allowing more devices to work simultaneously in one location. In addition, more channels are needed for wide channels of 160 MHz.

Are there any new channel rates in 802.11ac Wave 2?

The new standard inherits the standards introduced since the first release. As before, the speed depends on the number of streams and the channel width. The maximum modulation remained unchanged - 256 QAM.

If earlier a channel rate of 866.6 Mbps required 2 streams and a channel width of 80 MHz, now this channel rate can be achieved using only one stream, while increasing the channel rate by two - from 80 to 160 MHz.

As you can see, no major changes have taken place. In connection with the support of 160 MHz channels, the maximum channel speeds have also increased - up to 2600 Mbps.

In practice, the real speed is approximately 65% ​​of the channel (PHY Rate).

Using 1 stream, 256 QAM modulation and a 160 MHz channel, you can achieve a real speed of about 560 Mbps. Accordingly, 2 streams will provide an exchange rate of ~1100 Mbps, 3 streams - 1.1-1.6 Gbps.

What bands and channels does 802.11ac Wave2 use?

In practice, Waves 1 and Waves 2 operate exclusively on the 5 GHz band. The frequency range is subject to regional restrictions, typically the 5.15-5.35 GHz and 5.47-5.85 GHz bands are used.

In the US, a 580 MHz band is allocated for 5 GHz wireless networks.

802.11ac, as before, can use channels at 20 and 40 MHz, at the same time, good performance can be achieved using only 80 MHz or 160 MHz.

Since in practice it is far from always possible to use a continuous 160 MHz band, the standard provides for an 80 + 80 MHz mode, which will divide the 160 MHz band into 2 different bands. All this adds more flexibility.

Please note that the standard channels for 802.11ac are 20/40/80 MHz.

Why are there two waves of 802.11ac?

The IEEE implements standards in waves as technology advances. This approach allows the industry to immediately release new products, without waiting for this or that feature to be finalized.

The first wave of 802.11ac provided a significant step forward from 802.11n and laid the groundwork for future developments.

When should we expect 802.11ac Wave 2 products?

According to analysts' initial forecasts, the first consumer-level products should have gone on sale as early as mid-2015. Higher-level enterprise and carrier solutions usually come out with a delay of 3-6 months, just like it was with the first wave of the standard.

Both consumer and commercial grades are usually released before the WFA (Wi-Fi Alliance) starts certifying (second half of 2016).

As of February 2017, the number of devices supporting 802.11ac W2 is not as high as we would like. Especially from Mikrotik and Ubiquit.

Will Wave 2 devices be significantly different from Wave 1?

In the case of the new standard, the general trend of previous years is preserved - smartphones and laptops are produced with 1-2 streams, 3 streams are designed for more demanding tasks. It makes no practical sense to implement the full functionality of the standard on all devices.

Is Wave 1 compatible with Wave 2?

The first wave allows 3 streams and channels up to 80 MHz, in this part, client devices and access points are fully compatible.

To implement the second generation features (160 MHz, MU-MIMO, 4 streams), both the client device and the access point must support the new standard.

The next generation access points are compatible with 802.11ac Wave 1, 802.11n and 802.11a client devices.

So use additional features a second generation adapter will not work with a first generation point, and vice versa.

What is MU-MIMO and what does it do?

MU-MIMO is short for "multiuser multiple input, multiple output". In fact, this is one of the key innovations of the second wave.

For MU-MIMO to work, both the client and the AP must support it.

In short, an access point can simultaneously send data to multiple devices at once, while previous standards only allow data to be sent to one client at a particular time.

In fact, conventional MIMO is SU-MIMO, i.e. SingleUser, single user MIMO.

Consider an example. There is a point with 3 streams (3 Spatial Streams / 3SS) and 4 clients are connected to it: 1 client with 3SS support, 3 clients with 1SS support.

The access point distributes time equally among all clients. While working with the first client, the point uses 100% of its capabilities, because the client also supports 3SS (MIMO 3x3).

The remaining 75% of the time, the point works with three clients, each of which uses only 1 stream (1SS) out of 3 available. At the same time, the access point uses only 33% of its capabilities. The more such clients, the less efficiency.

IN specific example, the average channel speed will be 650 Mbps:

(1300 + 433,3 + 433,3 + 433,3)/4 = 650

In practice, it will mean an average speed of about 420 Mbps, out of a possible 845 Mbps.

Now let's look at an example using MU-MIMO. We have a second generation point using 3x3 MIMO, the channel speed will remain unchanged - 1300 Mbps for a channel width of 80 MHz. Those. At the same time, clients, as before, can use no more than 3 channels.

The total number of clients is now 7, while the access point has divided them into 3 groups:

1. one 3SS client;
2. three 1SS clients;
3. one 2SS client + one 1SS;
4. one 3SS client;

As a result, we get a 100% implementation of AP capabilities. A client from the first group uses all 3 streams, clients from another group use one channel, and so on. The average channel speed will be 1300 Mbps. As you can see, at the output it gave a twofold increase.

Is the MU-MIMO point compatible with older clients?

Unfortunately no! MU-MIMO is not compatible with the first version of the protocol, i.e. for this technology to work, your client devices must support the second version.

Differences between MU-MIMO and SU-MIMO

In SU-MIMO, the access point transmits data to only one client at a time. With MU-MIMO, an access point can transmit data to multiple clients at once.

How many clients are supported in MU-MIMO at the same time?

The standard provides for simultaneous maintenance of up to 4 devices. General maximum amount streams can be up to 8.

Depending on the configuration of the equipment, a wide variety of options are possible, for example:

• 1+1: two clients, each with one stream;
• 4+4: two clients, each using 4 streams;
• 2+2+2+2: four clients, 2 streams for each;
• 1+1+1: three clients in one thread;
• 2+1, 1+1+1+1, 1+2+3, 2+3+3 and other combinations.

It all depends on the hardware configuration, usually devices use 3 streams, therefore, the point can serve up to 3 clients at the same time.

It is also possible to use 4 antennas in a MIMO 3x3 configuration. The fourth antenna in this case is additional, it does not implement an additional stream. In this case, it will be possible to simultaneously serve 1 + 1 + 1, 2 + 1 or 3SS, but not 4.

Is MU-MIMO only supported for Downlink?

Yes, the standard only supports Downlink MU-MIMO, i.e. point can simultaneously transmit data to multiple clients. But the dot cannot “listen” at the same time.

The implementation of Uplink MU-MIMO was deemed impossible in the short term, so this functionality will only be added in the 802.11ax standard, which is scheduled for release in 2019-2020.

How many streams are supported in MU-MIMO?

As mentioned above, MU-MIMO can work with any number of streams, but no more than 4 per client.

For high-quality operation of multi-user transmission, the standard recommends the presence of a number of antennas, more streams. Ideally, for MIMO 4x4 there should be 4 antennas for receiving and the same number for sending.

Is it necessary to use special antennas for the new standard?

The design of the antennas remained the same. As before, you can use any compatible antennas designed for use in the 5 GHz band for 802.11a/n/ac.

The second release also added Beamforming, what is it?

Beamforming technology allows you to change the radiation pattern, adapting it to a specific client. During operation, the point analyzes the signal from the client and optimizes its radiation. An additional antenna may be used during the beamforming process.

Can an 802.11ac Wave 2 access point handle 1 Gb of traffic?

Potentially, new generation access points are able to handle such traffic flow. Real throughput depends on a number of factors, starting with the number of supported streams, communication range, the presence of obstacles and ending with the presence of interference, the quality of the access point and client module.

What frequency bands are used in 802.11ac Wave?

The choice of operating frequency depends solely on local legislation. The list of channels and frequencies is constantly changing, below are the data for the US (FCC) and Europe, as of January 2015.

In Europe, the use of a channel width of more than 40 MHz is allowed, so there are no changes in terms of the new standard, all the same rules apply to it as for the previous standard.

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One of the most significant and important innovations Wi-Fi over the past 20 years - Multi User - Multiple Input Multiple Output (MU-MIMO) technology. MU-MIMO extends the functionality of a recent update wireless standard 802.11ac "Wave 2". Undoubtedly, this is a huge breakthrough for wireless communication. This technology helps to increase the maximum theoretical speed wireless connection from 3.47 Gbps in the original 802.11ac specification to 6.93 Gbps in the upgrade to 802.11ac Wave 2. This is one of the most complex Wi-Fi features to date.

Let's see how it works!

MU-MIMO technology raises the bar by allowing multiple devices to receive multiple data streams. It is based on Single User MIMO (SU-MIMO), which was introduced nearly 10 years ago with the 802.11n standard.

SU-MIMO increases the speed of a Wi-Fi connection by allowing a pair of wireless devices to receive or send multiple streams of data at the same time.

Figure 1. SU-MIMO technology provides multi-channel input and output streams to the same device at the same time. MU-MIMO technology enables simultaneous communication with multiple devices.

Essentially, there are two technologies that are revolutionizing Wi-Fi. The first of these technologies, called beamforming, allows Wi-Fi routers and access points to use radio channels more efficiently. Before the advent of this technology, Wi-Fi routers and access points worked like light bulbs, sending out a signal in all directions. The problem was that It is difficult for an unfocused signal of limited power to reach Wi-Fi client devices.

Using beamforming technology, a Wi-Fi router or access point exchanges information about its location with a client device. The router then changes its phase and power to form a better signal. As a result: radio signals are used more efficiently, data transfer is faster and possibly the maximum connection distance is increased.

The possibilities of beamforming are expanding. Until now, Wi-Fi routers or access points have been inherently single-tasking, sending or receiving data from only one client device at a time. In earlier versions of the family of standards wireless transmission 802.11 data, including the 802.11n standard and the first version of the 802.11ac standard, it was possible to receive or transmit multiple data streams at the same time, but until now there was no method that allowed a Wi-Fi router or access point to “talk” at the same time with multiple clients at once. From now on, with the help of MU-MIMO, such an opportunity has appeared.

This is indeed a big breakthrough, as the ability to transmit data to multiple client devices simultaneously greatly expands the available bandwidth for wireless clients. MU-MIMO technology advances wireless networks from the old way CSMA-SD, when only one device was served at the same time, to a system where several devices can “talk” at the same time. To make this example clearer, imagine moving from a single-lane country road to a wide highway.

Today, second-generation 802.11ac Wave 2 wireless routers and access points are taking over the market. Everyone who deploys Wi-Fi understands the specifics of how MU-MIMO technology works. We bring to your attention 13 facts that will accelerate your learning in this direction.

1. MU-MIMO only uses"Downstream" stream (from the access point to the mobile device).

Unlike SU-MIMO, MU-MIMO currently only works for transferring data from the access point to the mobile device. Only wireless routers or access points can transmit data to multiple users at the same time, whether it be one or more streams for each of them. The wireless devices themselves (such as smartphones, tablets, or laptops) still have to take turns sending data to the wireless router or access point, although they can individually use SU-MIMO technology to transmit multiple streams when it is their turn.

MU-MIMO technology will be especially useful in networks where users download more data than they upload.

Perhaps in the future a version of Wi-Fi technology will be implemented: 802.11ax, where the MU-MIMO method will be applicable for "Upstream" traffic.

2. MU-MIMO only works in the 5GHz Wi-Fi band

SU-MIMO technology operates in both the 2.4GHz and 5GHz frequency bands. 802.11ac Wave 2 2nd generation wireless routers and access points can serve multiple users at the same time on the same frequency band 5 GHz. On the one hand, of course, it is a pity that we will not be able to use the new technology on the narrower and more congested 2.4 GHz frequency band. But, on the other hand, there are more and more dual-band wireless devices on the market that support MU-MIMO technology, which we can use to deploy high-performance corporate Wi-Fi networks.

3. Beamforming technology helps guide signals

In the literature of the USSR, one can come across the concept of a Phased Antenna Array, which was developed for military radars in the late 80s. A similar technology has been applied to modern Wi-Fi. MU-MIMO uses directional signal shaping (known as "beamforming" in the English technical literature). Beamfiorming allows signals to be directed towards the intended location of a wireless device (or devices) rather than being sent out randomly in all directions. Thus, it turns out to focus the signal and significantly increase the range and speed of the Wi-Fi connection.

Although beamforming technology became optionally available with the 802.11n standard, however, most manufacturers implemented their own proprietary versions of this technology. These vendors still offer proprietary implementations of the technology in their devices, but now they will have to include at least a simplified and standardized version of the directional signaling technology if they want to support MU-MIMO technology in their 802.11ac product line.

4. MU-MIMO supports a limited number of simultaneous streams and devices

Unfortunately, routers or access points with implemented MU-MIMO technology cannot simultaneously serve an unlimited number of streams and devices. The router or access point has its own limit on the number of streams it serves (often 2, 3, or 4 streams), and this number of spatial streams also limits the number of devices that the access point can serve at the same time. For example, an access point with support for four streams can simultaneously serve four various devices, or, for example, send one stream to one device, and aggregate three other streams to another device (increasing the speed from combining channels).​

5. User devices are not required to have multiple antennas

As with SU-MIMO technology, only wireless devices with built-in MU-MIMO support can aggregate streams (rate). But, unlike the situation with SU-MIMO technology, wireless devices do not necessarily need to have multiple antennas in order to receive MU-MIMO streams from wireless routers and access points. If wireless device equipped with only one antenna, it can receive only one MU-MIMO data stream from the access point, using beamforming to improve reception.

More antennas will allow the wireless user device to receive more data streams at the same time (typically one stream per antenna), which will certainly have a positive effect on the performance of this device. However, the presence of multiple antennas in a user device negatively affects the power consumption and size of this product, which is critical for smartphones.

However, MU-MIMO technology imposes fewer hardware requirements on client devices than the cumbersome technical terms SU-MIMO technology, it is safe to assume that manufacturers will be much more willing to equip their laptops and tablets supporting MU-MIMO technology.​

6. Access points do the heavy lifting

In an effort to simplify the requirements for end-user devices, the developers of MU-MIMO technology have tried to shift most of the signal processing work to access points. This is another step forward from SU-MIMO technology, where the burden of signal processing was mostly on the user devices. And again, this will help client device manufacturers save on power, size and other costs in the production of their product solutions with support for MU-MIMO, which should have a very positive effect on the popularization of this technology.

7. Even budget devices benefit from simultaneous transmission through multiple spatial streams

Similar to link aggregation in Ethernet networks(802.3ad and LACP), 802.1ac stream aggregation does not increase the speed of a point-to-point connection. Those. if you are the only user and you have only one application running, you will use only 1 spatial stream.

However, it is possible to increase the overall network bandwidth by providing the ability to service the access point of several user devices at the same time.

But if all used in your network user devices support only one stream, MU-MIMO will allow your access point to serve up to three devices at the same time, instead of one at a time, while others(more advanced) user devices will have to wait in line.

8. Some user devices have hidden support for MU-MIMO technology

Although there are still not many routers, access points or mobile devices support MU-MIMO, the Wi-Fi chip company claims that some manufacturers considered hardware requirements in their manufacturing process to support the new technology for some of their end-user devices a few years ago. Relatively easy upgrade for such devices software will add support for MU-MIMO technology, which should also accelerate the popularization and diffusion of the technology, as well as encourage companies and organizations to upgrade their corporate wireless networks with equipment that supports the 802.11ac standard.

9. Devices without MU-MIMO support also benefit

Although Wi-Fi devices must have MU-MIMO support in order to use this technology, even those client devices that do not have such support can indirectly benefit from operating on a wireless network where a router or access points support MU-MIMO technology. It should be remembered that the data transfer rate over the network directly depends on the total time during which subscriber devices are connected to the radio channel. And if MU-MIMO technology allows you to serve some devices faster, then this means that access points in such a network will have more time to serve other client devices.

10. MU-MIMO Helps Increase Wireless Bandwidth

When you increase your Wi-Fi connection speed, you also increase your wireless network bandwidth. As devices are served more quickly, the network has more airtime to serve more client devices. Thus, MU-MIMO technology can greatly optimize the performance of wireless networks with heavy traffic or a large number of connected devices, such as public Wi-Fi networks. This is great news as the number of smartphones and other mobile devices with Wi-Fi connectivity is likely to continue to increase.

11. Any channel width is supported

One way to expand the bandwidth of a Wi-Fi channel is channel bonding, when two adjacent channel into one channel that is twice as wide, effectively doubling the speed of the Wi-Fi connection between the device and the access point. The 802.11n standard provided support for channels up to 40 MHz wide, in the original specification of the 802.11ac standard, the supported channel width was increased to 80 MHz. The updated 802.11ac Wave 2 standard supports 160 MHz channels.

Figure 3. 802.11ac currently supports channels up to 160 MHz wide in the 5 GHz band

However, it should not be forgotten that the use of wider channels in a wireless network increases the likelihood of interference in co-channels. Therefore, this approach will not always be the right choice to deploy all Wi-Fi networks without exception. However, MU-MIMO technology, as we can see, can be used for channels of any width.

However, even if your wireless network uses narrower 20MHz or 40MHz channels, MU-MIMO can still help it run faster. But how much faster will depend on how many client devices need to be served and how many streams each of these devices supports. Thus, the use of MU-MIMO technology, even without wide associated channels, can more than double the throughput of the outgoing wireless connection for each device.

12. Signal processing improves safety

An interesting side effect of MU-MIMO technology is that the router or access point encrypts the data before sending it over the air. It is rather difficult to decode data transmitted using MU-MIMO technology, since it is not clear which part of the code is in which spatial stream. Although special tools may later be developed to allow other devices to intercept transmitted traffic, today MU-MIMO technology effectively masks data from nearby listening devices. Thus, new technology helps to improve Wi-Fi security, which is especially true for open wireless networks such as public Wi-Fi networks, as well as access points operating in personal mode or using a simplified user authentication mode (Pre-Shared Key, PSK) based on Wi-Fi security technologies WPA or WPA2.

13. MU-MIMO is best for fixed Wi-Fi devices

There is also one caveat about MU-MIMO technology: it does not work well with fast moving devices, as the beamforming process becomes more complex and less efficient. Therefore, MU-MIMO will not provide you with a meaningful benefit for devices that frequently roam on your corporate network. However, it should be understood that these "problem" devices should in no way affect either MU-MIMO data transmission to other client devices that are less mobile, or their performance.

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mimo-m multiple antenna technologies in LTE

MIMO functions (M multiple input–multiple output)

The use of MIMO technologies (multiple input - multiple output) solves two problems:

Increasing the quality of communication due to spatial time / frequency coding and (or) beamforming (beamforming),

Increasing the transmission rate when using spatial multiplexing.

MIMO structure

Various implementations of MIMO mean the simultaneous transmission of several independent messages in one physical channel. In order to implement the MIMO action, multi-antenna systems are used: on the transmitting side there is N t transmitting antennas, and on the receiving side N r reception rooms. This structure is shown in fig. 1.

Rice. 1. MIMO structure

What is MIMO?

MIMO (English) Multiple Input Multiple Output) -a method of spatial signal coding that allows you to increase the channel bandwidth, in which data is transmitted using N antennas and their reception M antennas. The transmitting and receiving antennas are separated enough to achieve a weak correlation between adjacent antennas.

History of MIMO

The history of MIMO systems as an object of wireless communication is not yet very long. The first patent for the use of the MIMO principle in radio communications was filed in 1984 on behalf of Bell Laboratories employee Jack Winters. Based on his research, Jack Salz of the same company published the first paper on MIMO solutions in 1985. The development of this direction continued by Bell Laboratories and other researchers until 1995. In 1996, Greg Raleigh and Gerald J. Foschini proposed new version implementation of the MIMO system, thereby increasing its efficiency. Subsequently, Greg Raleigh, who is credited with OFDM ( Orthogonal Frequency Division Multiplexing– multiplexing via orthogonal carriers) for MIMO, founded Airgo Networks, which developed the first MIMO chipset called True MIMO.

However, despite the rather short period of time since its inception, the MIMO direction has been developing in a very multifaceted way and includes a heterogeneous family of methods that can be classified according to the principle of signal separation in the receiver. At the same time, MIMO systems use both approaches to signal separation that have already entered into practice, as well as new ones. These include, for example, space-time, space-frequency, spatial-polarization coding, as well as super-resolution in the direction of signal arrival at the receiver. Thanks to the abundance of signal separation approaches, it was possible to ensure such a long development of standards for the use of MIMO systems in communications. However, all varieties of MIMO are aimed at achieving the same goal - increasing the peak data rate in communication networks by improving noise immunity.

The simplest MIMO antenna is a system of two asymmetric vibrators (monopoles) oriented at an angle of ±45° relative to the vertical axis (Fig. 2).

Rice. 2 The simplest MIMO antenna

Such a polarization angle allows the channels to be in equal conditions, since with a horizontal-vertical orientation of the emitters, one of the polarization components would inevitably receive greater attenuation when propagating along the earth's surface. The signals emitted independently by each monopole are mutually orthogonal polarized with a sufficiently high mutual decoupling in the cross-polarization component (not less than 20 dB). A similar antenna is used on the receiving side as well. This approach allows simultaneous transmission of signals with the same carriers modulated in different ways. The principle of polarization separation provides a doubling of the bandwidth of the radio link compared to the case of a single monopole (in ideal line-of-sight conditions with identical orientation of the receiving and transmitting antennas). Thus, essentially any system with dual polarization can be considered a MIMO system.

Further evolution of MIMO

By the time MIMO technology was specified in Release 7, the standard was actively spreading around the world. There were attempts to combine the networks of the third generations with MIMO technology, but not widely used. According to the Global Association of Mobile Equipment Suppliers ( global mobile Suppliers Association, GSA) dated 11/04/2010 at that time out of 2776 types of devices with support HSPA on the market, only 28 models support MIMO. In addition, the introduction of a MIMO network with low penetration of MIMO terminals leads to a decrease in network throughput. Nokia developed the technology to minimize bandwidth losses, but it would only be effective if MIMO terminal penetration was at least 40% of subscriber devices. Adding to the above, it is worth recalling that on December 14, 2009, the launch of the world's first mobile network based on technology LTE, which made it possible to achieve much higher speeds. Based on this, it can be seen that the operators were aimed at the speedy deployment of LTE networks, rather than on the modernization of third generation networks.

Today, we can note the rapid growth in the volume of traffic in 4th generation mobile networks, and in order to provide the necessary speed to all their subscribers, operators have to look for various methods to increase the data transfer rate or to increase the efficiency of using the frequency resource. MIMO, on the other hand, allows transmitting almost 2 times more data in the available frequency band for the same time period with the 2x2 option. If we use the 4x4 antenna implementation, then, unfortunately, maximum speed downloading information will be 326 Mbps, not 400 Mbps, as the theoretical calculation suggests. This is due to the peculiarity of transmission through 4 antennas. Each antenna is allocated certain resource elements (RE) for transmitting reference symbols. They are necessary for organizing coherent demodulation and channel estimation. The location of these REs is shown in Fig. 3. The transmit antennas are assigned logical antenna port numbers. Characters marked R0 are on port 0, R1 on port 1, and so on. As a result, 14.3% of all REs are allocated for the transmission of reference symbols, which is the reason for the difference between theoretical and practical speeds.

One approach to increase data rates for 802.11 WiFi and 802.16 WiMAX is to use wireless systems with multiple antennas for both transmitter and receiver. This approach is called MIMO (literal translation - “multiple input multiple output”), or “smart antenna systems” (smart antenna systems). MIMO technology plays an important role in the implementation of the 802.11n WiFi standard.

MIMO technology uses several different kinds of antennas tuned to the same channel. Each antenna transmits a signal with different spatial characteristics. Thus, MIMO technology uses the radio spectrum more efficiently and without sacrificing operational reliability. Each wi-fi receiver "listens" for 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 the amplification of desired signals in wireless networks.

Another advantage of MIMO technology is that this technology provides spatial division multiplexing (Spatial Division Multiplexing (SDM)). SDM spatially multiplexes several independent data streams simultaneously (virtual channels, mostly) within a single channel spectral bandwidth. In essence, multiple antennas transmit different individually coded data streams (spatial streams). These streams, moving in parallel through the air, “push” more data through a given channel. At the receiver, each antenna sees different combinations signal streams and the receiver "demultiplexes" these streams for their use. MIMO SDM can significantly increase the throughput for data transmission if the number of spatial data streams is increased. Each spatial stream needs its own transmit/receive (TX/RX) antenna pairs at each end of the transmission. The operation of the system is shown in Fig. 1

It should also be understood 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 circuit must be carefully designed to keep costs low while maintaining an adequate level of efficiency.

An important tool for increasing the physical speed of data transmission in wireless networks is the expansion of the bandwidth of spectral channels. By using a wider channel bandwidth with orthogonal frequency division multiplexing (OFDM) data transmission is carried out with maximum performance. OFDM is a digital modulation that has proven itself as a tool for implementing bi-directional high-speed wireless data transmission in WiMAX / WiFi networks. The channel capacity expansion method is cost effective and fairly easy to implement with moderate digital signal processing (DSP) growth. When applied correctly, it is possible to double the bandwidth of 802.11 Wi-Fi from a 20 MHz channel to a 40 MHz channel, and more than double the bandwidth of channels currently in use. By combining a MIMO architecture with a higher channel bandwidth, a very powerful and cost-effective approach is obtained to increase the physical transmission rate.

The use of MIMO technology with 20 MHz channels is expensive to meet the IEEE 802.11n WiFi standard (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 at 20 MHz provides reliable performance for applications that require high bandwidth in a real user environment.

The combined use of MIMO technologies and channel expansion meets all the requirements of the user and is a fairly reliable tandem. This is also true when using several resource-intensive network applications at the same time. The combination of MIMO and 40 MHz channel extension will also meet more complex requirements such as Moore's Law and the implementation of CMOS technology to improve DSP technology.

When using an extended channel of 40 MHz in the 2.4 GHz band, initially there were 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 non-high throughput (non-HT) dual mode. Before Using the Transfer Protocol WiFi data 802.11n standard, this mechanism sends one packet to each of the halves of the 40 MHz channel to advertise a network distribution vector (NAV). By following the non-HT duplicate mode NAV message, the 802.11n data transfer protocol can be used for the time specified in the message, without violating the legacy (integrity) of the network.

Another mechanism is a kind of signaling and does not wireless networks expand the channel more than 40 MHz. For example, a laptop has 802.11n and Bluetooth modules installed, this mechanism is aware of the potential for interference when these two modules operate simultaneously and disables the 40 MHz transmission of one of the modules.

These mechanisms ensure that 802.11n WiFi will work with networks of earlier 802.11 standards 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 form for sending messages in the section