In its developments, Huawei relies on Wi-Fi 6. And questions from colleagues and customers about the new generation of the standard prompted us to write a post about the theoretical foundations and physical principles embedded in it. Let's move on from history to physics, let's take a closer look at why OFDMA and MU-MIMO technologies are needed. Let's also talk about how the fundamentally redesigned physical data transmission medium made it possible to achieve guaranteed channel throughput and such a reduction in the overall level of delays that they became comparable to "operator's". And this despite the fact that modern networks based on 5G are more expensive (on average 20β30 times) than similar indoor networks on Wi-Fi 6.
For Huawei, the topic is by no means an idle one: Wi-Fi 6 solutions are among our most breakthrough products in 2020, in which huge resources have been invested. To give just one example, research in the field of materials science allowed us to find an alloy whose use in the radio elements of the access point increased the signal-to-noise ratio by 2-3 dB: we take off our hats in respect to Doron Ezri (Doron Ezri) for this achievement.
A bit of history
It makes sense to count the history of Wi-Fi since 1971, when at the University of Hawaii, Professor Norman Abramson and a group of colleagues developed, built and launched the ALOHAnet wireless packet data network.
In 1980, a group of standards and protocols IEEE 802 was approved, describing the organization of the two lower layers of the seven-layer OSI network model. Before the release of the first version of 802.11 had to wait a long 17 years.
With the adoption of the 1997 standard in 802.11, two years before the emergence of the Wi-Fi Alliance, the first generation of today's most popular wireless technology stepped into the big world.
IEEE 802 Wi-Fi Generations
802.11b became the first standard to be truly massively supported by equipment manufacturers. As you can see, the frequency of innovations since the end of the XNUMXth century has been fairly stable: qualitative changes take time. In recent years, the main work was carried out to improve the physical environment of signal transmission. In order to better understand the modern problems of Wi-Fi, let's turn to its physical foundations.
Let's remember the basics!
Radio waves are a special case of electromagnetic waves - propagating from a source of disturbances in the electric and magnetic fields. They are characterized by three main parameters: the wave vector, as well as the vectors of the electric and magnetic fields. All three are mutually perpendicular to each other. In this case, it is customary to call the frequency of a wave the number of repetitive oscillations that fit into a unit of time.
All these are well-known facts. However, in order to reach the end, we have to start from the very beginning.
On the conditional scale of the frequency ranges of electromagnetic radiation, the radio range occupies the lowest (low-frequency) part. It includes electromagnetic waves with an oscillation frequency of 3 Hz to 3000 GHz. All other bands, including visible light, have a much higher frequency.
The higher the frequency, the more energy can be imparted to the radio wave, however, at the same time, it bends around obstacles worse and decays faster. The reverse is also true. Taking into account these features, two main frequency ranges were chosen for Wi-Fi operation - 2,4 GHz (frequency band from 2,4000 to 2,4835 GHz) and 5 GHz (frequency bands 5,170-5,330, 5,490-5,730 and 5,735-5,835 GHz).
Radio waves propagate in all directions, and in order for messages not to affect each other due to the interference effect, it is customary to divide the frequency band into separate narrow segments - channels with one or another . The diagram above shows that adjacent channels 1 and 2 with a bandwidth of 20 MHz will interfere with each other, but 1 and 6 will not.
The signal inside the channel is transmitted using a radio wave at a certain carrier frequency. To transmit information, wave parameters can frequency, amplitude or phase.
Channel separation in Wi-Fi frequency bands
The 2,4 GHz frequency band is divided into 14 partially overlapping channels of optimal width - 20 MHz. It was once thought that this was enough to organize a complex wireless network. It soon became clear that the capacity of the band was rapidly depleted, so the 5 GHz band was added to it, the spectral capacity of which is much higher. In it, in addition to 20 MHz, it is possible to allocate channels with a width of 40 and 80 MHz.
To further increase the efficiency of using the radio frequency spectrum, orthogonal frequency division multiplexing technology is now widely used ().
It implies the use, along with the carrier frequency, of several more subcarrier frequencies in the same channel, which makes it possible to carry out parallel data transmission. OFDM allows you to distribute traffic in a fairly convenient "granular" way, but due to its venerable age, it retains a number of significant disadvantages. Among them are the principles of operation using the CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance) network protocol, according to which at certain times only one user can work on one carrier and subcarrier.
Spatial flows
An important way to increase the capacity of a wireless network is to use spatial streams.
The access point carries several radio modules (one, two or more) that are connected to a number of antennas. These antennas radiate according to a certain scheme and modulation, and you and I receive information transmitted over a wireless medium. A spatial stream may be formed between a specific physical antenna (radio module) of the access point and the user device. Due to this, the total amount of information transmitted from the access point increases by a multiple of the number of streams (antennas).
According to current standards, up to four spatial streams can be implemented in the 2,4 GHz band, up to eight in the 5 GHz band.
Previously, when working in the 2,4 and 5 GHz bands, we focused only on the number of radio modules. The presence of a second radio module gave additional flexibility, as it allowed old subscriber devices to operate at a frequency of 2,4 GHz, and new ones at a frequency of 5 GHz. With the advent of the third and subsequent radio modules, some problems arose. The radiating elements tend to create interference with each other, which increases the cost of the device due to the need for better design and equipping the access point with compensation filters. So it has only recently become possible to support 16 spatial streams simultaneously per access point.
Speed ββpractical and theoretical
Due to OFDM mechanisms, we could not get the maximum network bandwidth. Theoretical calculations for the practical implementation of OFDM were carried out a very long time ago and only in relation to ideal environments, where a fairly high signal-to-noise ratio (SNR) and bit error probability (BER) were predictably expected. In today's conditions of strong noisiness of all the radio frequency spectra of interest to us, the bandwidth indicators of networks based on OFDM are depressingly small. And the protocol until recently continued to carry these shortcomings, until OFDMA (orthogonal frequency-division multiple access) technology came to the rescue. About her - a little further.
Let's talk about antennas
As you know, each antenna has a gain, depending on the value of which a spatial pattern of signal propagation (beamforming) is formed with a certain coverage area (we do not take into account signal re-reflection, etc.). This is what designers have always relied on when it comes to exactly where access points should be placed. For a long time, the shape of the pattern remained unchanged and only increased or decreased in proportion to the characteristics of the antenna.
Modern antenna elements are becoming more controllable and allow you to dynamically change the spatial pattern of signal propagation in real time.
The left figure above shows the principle of radio wave propagation using a standard omnidirectional antenna. By increasing the signal strength, we could only change the coverage radius without the ability to significantly affect the quality of channel usage - KQI (Key Quality Indicators). And this indicator is extremely important when organizing communication in conditions of frequent movement of the subscriber device in a wireless environment.
The solution to the problem was the use of a large number of small antennas, the load on which can be adjusted in real time, forming propagation patterns depending on the spatial position of the user.
Thus, it was possible to come close to the use of MU-MIMO (Multi-User Multiple Input, Multiple Output) technology. With its help, the access point at any time generates radiation fluxes directed specifically towards subscriber devices.
From physics to 802.11 standards
As Wi-Fi standards have evolved, the principles of working with the physical layer of the network have changed. The use of other modulation mechanisms made it possible - starting with versions 802.11g / n - to fit much more information into the time slot and, accordingly, work with a large number of users. Among other things, this was achieved through the use of spatial streams. And the newfound flexibility in terms of channel width allowed more resources to be generated for MIMO.
Wi-Fi 7 is scheduled to be approved next year. What will change with its arrival? In addition to the usual increase in speed and the addition of the 6 GHz band, it will be possible to work with wide aggregated channels, such as 320 MHz. This is especially interesting in the context of industrial applications.
Wi-Fi 6 Theoretical Bandwidth
The theoretical formula for calculating the nominal speed of Wi-Fi 6 is quite complicated and depends on many parameters, starting with the number of spatial streams and ending with the information that we can put into a subcarrier (or subcarriers, if there are several) per unit time.
As you can see, a lot depends on spatial flows. But before, an increase in their number in combination with the use of STC (Space-Time Coding) and MRC (Maximum Ratio Combining) worsened the performance of the wireless solution as a whole.
New Key Physical Layer Technologies
Let's move on to the key technologies of the physical layer - and start with the first layer of the OSI network model.
Recall that OFDM uses a certain number of subcarriers that, without affecting each other, are capable of transmitting a certain amount of information.
In the example, we are using the 5,220 GHz band, which has 48 sub-channels. By aggregating this channel, we get a larger number of subcarriers, each of which uses its own modulation scheme.
Wi-Fi 5 uses quadrature modulation 256 QAM (Quadrature Amplitude Modulation), which allows you to form within the carrier frequency in one time slot a field of 16 x 16 points that differ in amplitude and phase. The disadvantage is that only one station can transmit on the carrier frequency at any one time.
Orthogonal frequency division multiplexing (OFDMA) came from the world of mobile operators, spread simultaneously with LTE and is used to organize a downlink (communication channel to the subscriber). It allows you to work with the channel at the level of so-called resource units. These units help to break the block into a certain number of components. Within the framework of the block, we can not work strictly with one radiating element (user or access point) at each moment, but combine dozens of elements. This allows you to achieve remarkable results.
Easy linking of channels in Wi-Fi 6
Channel bonding in Wi-Fi 6 allows you to get combined channels with a width of 20 to 160 MHz. Moreover, the connection does not have to be made in nearby ranges. For example, one block can be taken from the 5,17 GHz band, and the second from the 5,135 GHz band. This allows you to flexibly build a radio environment even in the presence of strong interference factors or in the vicinity of other constantly emitting stations.
From SIMO to MIMO
The MIMO method has not always been with us. Once upon a time, mobile communications had to be limited to the SIMO mode, which meant that the subscriber station had several antennas that simultaneously worked to receive information.
MU-MIMO is designed to transmit information to users using the entire current antenna fund. This removes the restrictions previously imposed by the CSMA / CA protocol associated with sending tokens to subscriber devices for transfer. Now users are united in a group and each member of the group receives their part of the resource of the antenna fund of the access point, and does not wait in line.
Beamforming
An important rule of operation of MU-MIMO is to maintain such a mode of operation of the antenna fund, which would not lead to mutual overlap of radio waves and loss of information due to phase addition.
This requires complex mathematical calculations on the side of the access point. If the terminal supports this feature, MU-MIMO allows it to tell the access point how much delay it receives a signal on each specific antenna. And the access point, in turn, adjusts its antennas to form an optimally directed beam.
What does it give us in general?
White circles with numbers in the table indicate current scenarios for using previous generations of Wi-Fi. The blue circles (see the illustration above) describe the capabilities of Wi-Fi 6, and the gray circles are a matter of the near future.
The main advantages that new solutions with OFDMA support bring are associated with resource units implemented at a level similar to TDM (Time Division Multiplexing). This was not the case with Wi-Fi before. This allows you to clearly control the allocated band, ensuring the minimum signal transit time through the medium and the required level of reliability. Fortunately, no one doubts that Wi-Fi reliability indicators need to be improved.
History moves in a spiral, and the current situation is similar to the one that developed at one time around Ethernet. Even then, the opinion was established that the CSMA / CD (Carrier Sense Multiple Access with Collision Detection) transmission medium does not provide any guaranteed throughput. And so it continued until the transition to IEEE 802.3z.
As for the general application models, as you can see, with each generation of Wi-Fi, scenarios for its use are multiplying, more and more sensitive to delays, general and reliability.
And again about the physical environment
Well, now about how a new physical environment is formed. When using CSMA / CA and OFDM, the increase in the number of active points (Active STA) led to the fact that the throughput of the 20 MHz channel fell seriously. This was due to what has already been mentioned: with not the newest technologies STC (Space-Time Coding) and MRC (Maximum Ratio Combining).
OFDMA due to the use of resource units can effectively interact with distant and low-power stations. We get the opportunity to work in the same carrier range with users consuming different amounts of resources. One user can occupy one unit, and another can occupy all the others.
Why was there no OFDMA before?
And finally, the main question: why was there no OFDMA before? Oddly enough, it all came down to money.
For a long time, it was believed that the price of a Wi-Fi module should be minimal. When the protocol was launched into commercial operation in 1997, it was decided that the production cost of such a module could not exceed $1. As a result, the development of technology has taken a suboptimal path. Here we do not take into account carrier LTE, where OFDMA has been used for quite a long time.
In the end, the Wi-Fi working group decided to take these developments from the world of telecom operators and transfer them to the world of enterprise networks. The main task was the transition to the use of higher quality elements, such as filters and oscillators.
Why was it so difficult for us to work in the old MRC encodings with or without interference? Because the MVDR (Minimum Variance Distortionless Response) beamforming mechanism dramatically increased the number of errors as soon as we tried to align a large number of transmit points. OFDMA has proven that the problem is solvable.
The fight against interference is now based on mathematics. If the communication window is long enough, the resulting dynamic interference leads to problems. New operation algorithms allow you to get away from them, excluding the influence of not only the interference associated with the transmission of Wi-Fi, but also any other that occurs in this range.
Thanks to adaptive interference suppression, we can gain up to 11 dB gain even in a complex heterogeneous environment. The use of Huawei's own algorithmic solutions made it possible to achieve serious optimization exactly where needed - in indoor solutions. What is good in 5G is not necessarily good in a Wi-Fi 6 environment. Massive MIMO and MU-MIMO approaches differ between indoor and outdoor solutions. Where required, it is appropriate to use expensive solutions, as in 5G. But other options are also needed, such as Wi-Fi 6, capable of delivering the latency and performance we've come to expect from carriers.
We borrow from them the tools that will be useful to us as corporate consumers, all in order to provide a physical environment that can be relied upon.
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By the way, donβt forget about our numerous webinars on Huawei 2020 novelties, held not only in the Russian-speaking segment, but also at the global level. A list of webinars for the coming weeks is available at .
Source: habr.com