🥇What awaits us in Wi-Fi 7, IEEE 802.11be?

Recently, devices supporting Wi-Fi 6 technology (IEEE 802.11ax) have entered the market, generating much discussion. However, few are aware that development of the next generation of Wi-Fi technology—Wi-Fi 7 (IEEE 802.11be)—is already underway. This article outlines what Wi-Fi 7 will encompass.

Background

In September 2020, we will celebrate the 30th anniversary of the IEEE 802.11 project, which has significantly impacted our lives. Currently, Wi-Fi technology, defined by the IEEE 802.11 standards family, is the most popular wireless technology used for internet connectivity: Wi-Fi accounts for more than half of user traffic. While cellular technologies rebrand every decade, such as changing from 4G to 5G, Wi-Fi consumers experience speed increases and new service implementations almost unnoticed. Only a few customers pay attention to the letters 'n', 'ac', or 'ax' that follow '802.11' on equipment boxes. However, this does not mean that Wi-Fi is not evolving.

One indicator of Wi-Fi's evolution is the dramatic increase in nominal data transfer speeds: from 2 Mbps in the 1997 version to nearly 10 Gbps in the latest 802.11ax standard, also known as Wi-Fi 6. Modern Wi-Fi achieves this performance boost through faster signal coding schemes, wider channels, and the use of technologies. MIMO.

In addition to the main focus on high-speed wireless local area networks, the evolution of Wi-Fi includes several niche projects. For example, Wi-Fi HaLow (802.11ah) aims to bring Wi-Fi to the wireless Internet of Things market. Millimeter wave Wi-Fi (802.11ad/ay) supports nominal data transfer speeds of up to 275 Gbps, albeit over very short distances.

New applications and services involving high-definition video streaming, virtual and augmented reality, gaming, remote work, and cloud computing, along with the need to support a large number of users with high traffic in wireless networks, demand high performance.

Goals of Wi-Fi 7

In May 2019, a subgroup of the BE (TGbe) working group of the 802.11 committee for local and metropolitan area network standards began work on a new amendment to the Wi-Fi standard that will increase the nominal throughput to over 40 Gbit/s in a frequency channel typical for the Wi-Fi range <= 7 GHz. While many documents mention a 'maximum throughput of at least 30 Gbit/s', the new physical layer protocol will be providing a nominal speed exceeding 40 Gbit/s.

Another significant area of development for Wi-Fi 7 is support for real-time applications (gaming, virtual and augmented reality, robotics control). Notably, although Wi-Fi uniquely serves audio and video traffic, it has long been believed that ensuring guarantees for minimal delays (on the order of milliseconds), also known as Time-Sensitive Networking, in Wi-Fi networks is fundamentally impossible. In November 2017, our team from IPI RAN and HSE (not to be taken as publicity) presented a relevant proposal to the IEEE 802.11 group. The proposal garnered significant interest, and in July 2018, a special subgroup was launched for further exploration of this issue. Since real-time applications require both high nominal data transfer speeds and expanded channel layer functionality, the 802.11 working group decided to develop methods for supporting real-time applications within Wi-Fi 7.

An important issue related to Wi-Fi 7 is its coexistence with cellular network technologies (4G/5G), developed by 3GPP and operating in the same unlicensed frequency bands. This concerns LTE-LAA/NR-U. To study the issues related to the coexistence of Wi-Fi and cellular networks, IEEE 802.11 launched the Coexisting Standing Committee (Coex SC). Despite numerous meetings and even a joint seminar of 3GPP and IEEE 802.11 participants in July 2019 in Vienna, technical solutions have not yet been approved. One possible explanation for this unproductive activity is that both IEEE 802 and 3GPP do not want to modify their respective technologies to align with the other. Thus, it remains unclear whether discussions within the Coex SC will influence the Wi-Fi 7 standard.

The development process

Although the development process of Wi-Fi 7 is still in its early stages, around 500 proposals for new features for the future Wi-Fi 7, also known as IEEE 802.11be, have been submitted to date. Most of the ideas are still under discussion within the be subgroup, and no decisions have been made on them yet. Other ideas have been recently approved. Below will clearly indicate which proposals are approved and which are still being discussed.

Initially, it was planned that the development of the main new mechanisms would be completed by March 2021. The final version of the standard is expected by early 2024. In January 2020, there were concerns expressed within the 11be subgroup regarding whether the development would stay on schedule given the current pace of work. To expedite the standard's development, the subgroup agreed to select a small set of high-priority features that could be released by 2021 (Release 1), leaving the others for Release 2. The high-priority features are expected to provide a significant performance boost and include support for 320 MHz, 4K-QAM, obvious enhancements of OFDMA from Wi-Fi 6, and MU-MIMO with 16 streams.

Due to the coronavirus, the group is not meeting in person at the moment but is regularly holding teleconferences. Thus, the development has slowed somewhat but has not stopped.

Technology Details

Let's consider the main innovations of Wi-Fi 7.

The new physical layer protocol is an advancement of the Wi-Fi 6 protocol with a doubling of the bandwidth up to 320 MHz and a doubling of the number of MU-MIMO spatial streams,which increases the nominal throughput by 2×2 = 4 times. Wi-Fi 7 also begins to utilize modulation 4K-QAM,which adds another 20% to the nominal throughput. Thus, Wi-Fi 7 will provide a nominal data transfer rate of 2x2x1.2 = 4.8 times higher compared to Wi-Fi 6: the maximum nominal capacity of Wi-Fi 7 is 9.6 Gbps x 4.8 = 46 Gbps. Additionally, there will be a revolutionary change in the physical layer protocol related to ensuring compatibility with future versions of Wi-Fi, but it will remain unnoticed by users.
Changes in the channel access method for supporting real-time applications. will be conducted taking into account the IEEE 802 TSN experience for wired networks. Ongoing discussions in the standards committee revolve around the random delay procedure for channel access, traffic service categories, and, accordingly, separate queues for real-time traffic along with packet servicing policies.
Introduced in Wi-Fi 6 (802.11ax) OFDMA – a method of channel access with time and frequency division (similar to that used in 4G and 5G networks) – provides new opportunities for optimal resource allocation. However, in 11ax, OFDMA is not flexible enough. First, it allows the access point to allocate to a client device only one resource block of a predetermined size. Second, it does not support direct transmission between client stations. Both shortcomings reduce spectral efficiency. Furthermore, the inherited lack of flexibility from Wi-Fi 6 OFDMA deteriorates performance in dense networks and increases latency, which is critical for real-time applications. 11be will address these OFDMA issues.
One of the approved revolutionary changes in Wi-Fi 7 is the built-in support for simultaneous use of multiple parallel connections across different frequencies, which is highly beneficial for both enormous data transfer rates and extremely low latency. While current chipsets can already utilize multiple connections simultaneously, for instance, in the 2.4 and 5 GHz bands, these connections are independent, limiting the effectiveness of such operations. In 11be, there will be a synchronization level between channels that allows for efficient channel resource utilization and will lead to significant changes in channel access protocol rules.
The use of very wide channels and a large number of spatial streams leads to the problem of high overhead associated with the channel state estimation procedure necessary for MIMO and OFDMA. This overhead negates the benefits obtained from increased nominal data transfer rates. It is expected that the channel state estimation procedure will be revised.
In the context of Wi-Fi 7, the standardization committee is discussing the use of several 'advanced' data transmission methods. In theory, these methods improve spectral efficiency in the case of retransmissions, as well as during simultaneous transmissions in the same or opposite directions. This includes Hybrid Automatic Repeat reQuest (HARQ), which is currently used in cellular networks, full-duplex mode, and Non-Orthogonal Multiple Access (NOMA). These methods have been well studied in the literature theoretically, but it remains unclear whether the performance gains they provide will justify the efforts required for their implementation.
- Using HARQ is complicated by the following issue. In Wi-Fi, packets are concatenated to reduce overhead. In the current versions of Wi-Fi, the delivery of each packet within the concatenation is confirmed, and if the acknowledgment does not arrive, the packet is retransmitted using methods from the channel access protocol. HARQ moves the retransmission attempts from the channel to the physical layer, where there are no longer packets, but code words, and the boundaries of the code words do not align with the packet boundaries. This desynchronization complicates the implementation of HARQ in Wi-Fi.
- As for Full-Duplex, currently, neither in cellular networks nor in Wi-Fi networks can data be transmitted simultaneously to and from an access point (base station) within the same frequency channel. From a technical perspective, this is related to the significant difference in power between the transmitted and received signals. Although there are prototypes that combine digital and analog subtraction of the transmitted signal from the received one, capable of detecting Wi-Fi signals during their transmission, the practical gains they may offer could be minimal since, at any given moment, the downstream traffic is not equal to the upstream traffic (on average, the downstream is significantly greater). At the same time, such two-way transmission would substantially complicate the protocol.
- If multiple streams using MIMO require several antennas for the sender and receiver, in the case of non-orthogonal access, the access point can simultaneously transmit data to two receivers using a single antenna. Various non-orthogonal access options are included in the latest 5G specifications. Prototype NOMA Wi-Fi was first created in 2018 at IPPI RAS (please don’t consider this as advertising). It demonstrated a performance improvement of 30-40%. The advantages of the developed technology include its backward compatibility: one of the two receivers can be an outdated device that does not support Wi-Fi 7. Overall, the issue of backward compatibility is very important, as devices of various generations can operate simultaneously on the Wi-Fi network. Currently, several teams around the world are analyzing the effectiveness of combining NOMA and MU-MIMO, the results of which will determine the future of this approach. We are also continuing work on the prototype: its next version will be presented at the IEEE INFOCOM conference in July 2020.
Finally, another important innovation, though with an unclear future, is coordinated operation of access points. While many providers have their own centralized controllers for enterprise Wi-Fi networks, the capabilities of such controllers have generally been limited to configuring long-term parameters and channel selection. The standards committee is discussing closer cooperation between neighboring access points, which includes coordinated transmission scheduling, beamforming, and even distributed MIMO systems. Some of the considered approaches utilize sequential interference suppression (similar to NOMA). Although the coordination approaches for 11be are not yet fully developed, there is no doubt that the standard will allow access points from different manufacturers to coordinate transmission schedules to reduce mutual interference. As for other, more complex approaches (such as distributed MU-MIMO), integrating them into the standard will be more challenging, although some members of the group are determined to achieve this in Release 2. Regardless of the outcome, the fate of access point coordination methods is uncertain. Even if included in the standard, they may not reach the market. A similar situation has occurred before when trying to regulate Wi-Fi transmissions using solutions like HCCA (11e) and HCCA TXOP Negotiation (11be).

In summary, it seems that most proposals related to the first five groups will become part of Wi-Fi 7, while the proposals concerning the last two groups require significant further research to prove their effectiveness.