Huawei is heavily invested in Wi-Fi 6 for its developments. Questions from colleagues and clients about the new generation of the standard prompted us to write a post on the theoretical foundations and physical principles inherent in it. We'll transition from history to physics, thoroughly examining why OFDMA and MU-MIMO technologies are essential. We'll also discuss how the fundamentally reworked physical medium for data transmission has enabled guaranteed channel throughput and a reduction in overall latency, making it comparable to 'carrier-grade' solutions. This is noteworthy given that modern 5G networks are on average 20–30 times more expensive than similar indoor networks based on Wi-Fi 6.
For Huawei, this topic is far from trivial: Wi-Fi 6-supported solutions rank among our most groundbreaking products in 2020, into which enormous resources have been invested. Here's just one example: our research in materials science allowed us to select an alloy, whose use in the radio components of access points increased the signal-to-noise ratio by 2–3 dB—kudos to Doron Ezri for this achievement.
A Bit of History
The history of Wi-Fi can be traced back to 1971, when Professor Norman Abramson and a group of colleagues at the University of Hawaii developed, built, and launched the ALOHAnet wireless packet network.
In 1980, a set of IEEE 802 standards and protocols was ratified, describing the organization of the two lower layers of the seven-layer OSI network model. It took a long wait of 17 years until the first version of 802.11 was released.
With the adoption of the 802.11 standard in 1997, two years before the formation of the Wi-Fi Alliance, the first generation of the now-popular wireless data transmission technology stepped into the wider world.
IEEE 802 Standard. Wi-Fi Generations
The first standard that was widely supported by equipment manufacturers was 802.11b. As you can see, the frequency of innovations since the late 20th century has been quite stable: substantial changes require time. In recent years, significant efforts have focused on improving the physical medium for signal transmission. To better understand the current issues surrounding Wi-Fi, let's revisit its physical foundations.
Let's recall the basics!
Radio waves are a specific case of electromagnetic waves, propagating from sources of disturbances in the electric and magnetic fields. They are characterized by three main parameters: the wave vector, as well as the vectors of electric and magnetic field intensity. All three are mutually perpendicular to each other. The frequency of the wave is defined as the number of repeating oscillations that occur in a unit of time.
These are all well-known facts. However, to reach a conclusion, we must start from the very beginning.
On a conditional scale of frequency ranges of electromagnetic radiation, the radio frequency range occupies the lowest (low-frequency) part. It includes electromagnetic waves with oscillation frequencies from 3 Hz to 3000 GHz. All other ranges, including visible light, have much higher frequencies.
The higher the frequency, the more energy can be transmitted to the radio wave; however, it also has more difficulty bending around obstacles and dissipates more quickly. The reverse is also true. Considering these characteristics, 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 to ensure that messages do not interfere with each other due to the effect of interference, the frequency band is typically divided into separate narrow segments — channels with specific . In the diagram above, it can be seen that adjacent channels 1 and 2 with a bandwidth of 20 MHz will interfere with each other, while channels 1 and 6 will not.
The signal within a channel is transmitted via a radio wave at a specific carrier frequency. For information transmission, the wave parameters can be in frequency, amplitude, or phase.
Separation of channels in Wi-Fi frequency ranges
The 2.4 GHz frequency range is divided into 14 partially overlapping channels of optimal width — 20 MHz. It was once believed that this was sufficient for organizing a complex wireless network. It soon became apparent that the capacity of the range was being rapidly exhausted, leading to the addition of the 5 GHz range, which has a much higher spectral capacity. In it, in addition to 20 MHz channels, channels of 40 and 80 MHz widths can also be allocated.
To further enhance the efficiency of radio frequency spectrum utilization, the technology of Orthogonal Frequency Division Multiplexing (OFDM) is widely used today.).
It involves the use of multiple sub-carrier frequencies alongside the main carrier frequency within the same channel, allowing for parallel data transmission. OFDM enables fairly flexible traffic distribution in a 'granular' manner, but due to its long-standing existence, it retains several significant drawbacks. Among them are the operational principles of the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) network protocol, which dictate that only one user can operate on the same carrier and sub-carrier at any given time.
Spatial Streams
An important way to increase the capacity of a wireless network is through the use of spatial streams.
An access point carries several radio modules (one, two, or more), which are connected to a certain number of antennas. These antennas emit signals according to a specific pattern and modulation, allowing us to receive information transmitted over the wireless medium. A spatial stream can be formed between a specific physical antenna (radio module) of the access point and a user device. As a result, the total volume of information transmitted from the access point increases proportionally to the number of streams (antennas).
According to current standards, it is possible to implement up to four spatial streams in the 2.4 GHz band and up to eight in the 5 GHz band.
Previously, when operating in the 2.4 and 5 GHz ranges, we focused solely on the number of radio modules. The presence of a second radio module provided additional flexibility, allowing older subscriber devices to operate at 2.4 GHz and newer ones at 5 GHz. With the introduction of a third and subsequent radio modules, some issues arose. The radiating elements tend to create interference with each other, increasing the device's cost due to the need for more sophisticated design and equipping the access point with compensatory filters. Thus, only recently has it been possible to simultaneously support 16 spatial streams on a single access point.
Practical and theoretical speed
Due to the mechanisms of OFDM operation, we could not achieve maximum network throughput. Theoretical calculations for the practical implementation of OFDM were conducted a long time ago and only in relation to ideal environments, where a sufficiently high signal-to-noise ratio (SNR) and bit error rate (BER) were predictably expected. In modern conditions of strong noise across all the relevant radio frequency spectra, the throughput figures for OFDM-based networks are disappointingly low. The protocol continued to bear these shortcomings until the OFDMA (orthogonal frequency-division multiple access) technology came to the rescue. More on that later.
Let's talk about antennas
As you know, each antenna has a gain factor, based on which the spatial signal propagation pattern (beamforming) is formed with a specific coverage area (not taking into account signal reflections, etc.). This has always been the basis for designers' reasoning regarding the optimal placement of access points. For a long time, the pattern shape remained unchanged and only increased or decreased proportionally to the antenna's characteristics.
Modern antenna elements are becoming increasingly controllable and allow for dynamic changes to the spatial signal propagation pattern in real time.
The diagram on the left at the top illustrates the principle of radio wave propagation when using a standard omnidirectional antenna. By increasing the signal power, we could only change the coverage radius without the possibility of significantly affecting the channel usage quality — KQI (Key Quality Indicators). This indicator is extremely important when organizing communication in conditions where the subscriber device frequently moves 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 regulated in real time, forming propagation patterns depending on the user's spatial position.
Thus, we were able to closely approach the application of MU-MIMO technology (Multi-User Multiple Input, Multiple Output). With its help, the access point forms radiation streams directed specifically toward the subscriber devices at any given moment.
From Physics to 802.11 Standards
As Wi-Fi standards evolved, the principles of working with the network's physical layer changed. The use of different modulation mechanisms allowed — starting with versions 802.11g/n — for a much larger amount of information to fit within a time slot, thereby enabling work with a larger number of users. Among other things, this was also achieved through the use of spatial streams. The newly acquired flexibility regarding channel width allowed for the formation of more resources for MIMO.
The approval of the Wi-Fi 7 standard is scheduled for 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 become possible to work with wide aggregated channels such as 320 MHz. This is particularly interesting in the context of industrial applications.
Theoretical Throughput of Wi-Fi 6
The theoretical formula for calculating the nominal speed of Wi-Fi 6 is quite complex and depends on many factors, starting with the number of spatial streams and ending with the information that can be embedded in the subcarrier (or subcarriers if there are several) in a given time frame.
As you can see, much depends on spatial streams. Previously, increasing their number combined with the use of STC (Space-Time Coding) and MRC (Maximum Ratio Combining) worsened the overall performance of wireless solutions.
New Key Physical Layer Technologies
Let's move on to the key physical layer technologies, starting with the first layer of the OSI network model.
Recall that OFDM uses a certain number of subcarriers that can transmit a volume of information without interfering with each other.
In this example, we use a frequency range of 5.220 GHz, which accommodates 48 subchannels. By aggregating this channel, we can achieve a greater number of subcarriers, each utilizing its own modulation scheme.
Wi-Fi 5 uses 256 QAM (Quadrature Amplitude Modulation) quadrature modulation, which allows forming a 16 by 16 grid of points varying in amplitude and phase within a carrier frequency in a single time slot. The downside is that at any given moment, only one station can transmit on the carrier frequency.
Orthogonal Frequency Division Multiple Access (OFDMA) came from the world of mobile operators, became widespread alongside LTE, and is used for organizing downlink (the communication channel to the subscriber). It allows working with the channel at the level of so-called resource units. These units help break the block into a certain number of components. Within a block, we can at any moment not strictly work with one radiating element (user or access point) but combine dozens of elements. This allows achieving remarkable results.
Simple Channel Bonding in Wi-Fi 6
Channel Bonding in Wi-Fi 6 allows for combined channels ranging from 20 to 160 MHz. Moreover, the channels do not necessarily need to be in adjacent ranges. For instance, one block can be taken from the 5.17 GHz range, while the second can be from the 5.135 GHz range. This allows for flexible construction of the radio environment even in the presence of strong interference factors or when in proximity to other continuously transmitting stations.
From SIMO to MIMO
The MIMO method has not always been with us. There was a time when mobile communications had to rely on SIMO mode, which involved multiple antennas at the subscriber station working simultaneously to receive information.
MU-MIMO is designed to transmit information to users by utilizing the entire current antenna resource. This eliminates the constraints previously imposed by the CSMA/CA protocol concerning sending tokens to subscriber devices for transmission. Now, users are grouped together, and each group member receives their portion of the access point's antenna resources rather than waiting their turn.
Radiobeam formation
An important rule for the operation of MU-MIMO is to maintain an antenna resource operation mode that does not cause mutual interference of radio waves and loss of information due to phase summation.
This requires complex mathematical calculations on the access point's side. If the terminal supports this function, MU-MIMO allows it to notify the access point of the delay at which it receives a signal at each specific antenna. The access point, in turn, adjusts its antennas to form an optimally directed beam.
What does this give us overall?
The current use scenarios of previous generations of Wi-Fi are marked by white circles with numbers in the table. The blue circles (see the illustration above) describe the capabilities of Wi-Fi 6, while the gray ones represent the not-so-distant future.
The main benefits brought by new solutions supporting OFDMA are related to resource units implemented at a level analogous to TDM (Time Division Multiplexing). There was nothing like this in previous Wi-Fi implementations. This allows for precise control over the allocated bandwidth, ensuring minimal signal transit time through the medium and the required level of reliability. Fortunately, there is no doubt that the reliability metrics of Wi-Fi need improvement.
History moves in spirals, and the current situation resembles what once happened around Ethernet. At that time, it was established that the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) transmission medium provides no guaranteed throughput. This continued until the transition to IEEE 802.3z.
Regarding general application models, as you can see, with each generation of Wi-Fi, the scenarios for its use are multiplying, becoming increasingly sensitive to delays, overall and reliability.
Once again about the physical environment
Now, let's discuss what forms the new physical environment. When using CSMA/CA and OFDM, the increase in the number of active points (Active STA) significantly reduced the capacity of the 20 MHz channel. This was related to what has already been mentioned: less advanced STC (Space-Time Coding) and MRC (Maximum Ratio Combining) technologies.
OFDMA, through the use of resource units, can effectively interact with distant and low-power stations. We gain the ability to operate in the same carrier range with users consuming different amounts of resources. One user can occupy one unit, while another can occupy all the rest.
Why wasn't OFDMA used earlier?
And finally, the main question: why wasn't OFDMA used earlier? Strangely enough, it all came down to money.
For a long time, it was believed that the price of the Wi-Fi module had to be minimal. When the protocol was launched for 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 the technology took an inefficient path. Here we do not take into account LTE operations, where OFDMA has been utilized for quite some time.
Eventually, the working group on Wi-Fi decided to take these developments from the world of telecom operators and transfer them to the corporate network realm. The main task became the transition to the use of higher quality components, such as filters and oscillators.
Why was it so difficult for us to work with old MRC encodings with or without interference? Because the MVDR (Minimum Variance Distortionless Response) beamforming mechanism sharply increased the number of errors as soon as we tried to combine a large number of transmitting points. OFDMA proved that the problem is solvable.
The fight against interference is now based on mathematics. If the information transfer window is long enough, the resulting dynamic interference leads to problems. New operating algorithms allow us to bypass these issues, eliminating the influence of not only Wi-Fi transmission-related interference, but also any other interference occurring in that range.
Thanks to adaptive interference management, we can achieve gains of up to 11 dB even in a complex heterogeneous environment. The use of Huawei's proprietary algorithmic solutions has led to significant optimization precisely where it is needed—in indoor solutions. What works well in 5G is not necessarily effective in a Wi-Fi 6 environment. The approaches of Massive MIMO and MU-MIMO differ when it comes to indoor and outdoor solutions. Where needed, it is appropriate to use costly solutions, as in 5G. But other options, such as Wi-Fi 6, are also necessary to provide delays and other metrics that we have come to expect from telecom operators.
We draw from their tools that will be useful for us as corporate consumers, all to provide a reliable physical environment.
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By the way, don't forget about our various webinars on the latest Huawei developments of 2020, held not only in the Russian-speaking segment but also on a global level. The list of webinars for the coming weeks is available at .
Source: habr.com
