802.11ay: Enhanced Directional Multi-Gigabit Wireless

Max Camp-Oberhauser (A paper written under the guidance of Prof. Raj Jain)

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Abstract

Enhanced directional multi-Gigabit (EDMG) wireless, as standardized by IEEE 802.11ay, is a promising technology that can provide up to 100 Gbps data speeds for local area networks and serve other applications such as wireless backhaul and virtual reality. By operating with millimeter waves in the 60 GHz spectrum, EDMG is able to have high bandwidth per channel and take advantage of beamforming for spatial reuse, although the protocol must counter the effects of increased attenuation. In this paper, we examine the key features of 802.11ay and its differences from its predecessor 802.11ad. These include multiple input multiple output (MIMO), channel bonding and aggregation, and an enhanced beamforming training process. We also survey relevant applications that serve to benefit the most from these recent advances.

Keywords

EDMG, IEEE 802.11ay, Millimeter Wave, Wi-Fi, Wireless Local Area Networks, Wi-Gig, Beamforming Antennas, IEEE 802.11ad, Wireless Backhaul, MIMO

Table of Contents

• 1. Introduction
• 2. Overview and Background
• • 2.1 Benefits
  • 2.2 Challenges
  • 2.3 802.11ad
• 3. Changes in 802.11ay
• • 3.1 General Protocol
  • 3.2 MIMO
  • 3.3 Channel Bonding
  • 3.4 Channel Aggregation
  • 3.5 Dual-Polarized Antennas
  • 3.6 Enhanced Beamforming Training
• 4. Applications
• • 4.1 High-Speed WLANs
  • 4.2 Wireless Backhaul
  • 4.3 Virtual Reality
• 5. Summary
• References
• List of Acronyms

1. Introduction

The history of wireless networking has seen an exponential increase in users' demand for speed and performance. These improvements have in turn led to new applications with evolved user experiences and yet even higher requirements. Modern use cases from gaming to high-definition video streaming require multi-Gigabit connections and have pushed networking to its limits whether through modulation, multiple input multiple output (MIMO), or the part of the spectrum being used. IEEE 802.11ay, the enhanced directional multi-Gigabit (EDMG) protocol, represents the current culmination of these advancements. Table 1 shows where EDMG fits in with the history of 802.11 protocols [McCann22].

This paper presents a survey of the current state of EDMG. Section 2 provides background including benefits, challenges, and a look at 802.11ad which laid the groundwork for 802.11ay. In section 3 we tackle the specific features of 802.11ay that distinguish it from prior protocols including MIMO with beamforming, support for dual-polarized antennas, and an enhanced beamforming training process. Section 4 examines EDMG use cases such as wireless backhaul and virtual reality before a summary of the survey is presented in section 5.

2. Overview and Background

Work on the IEEE 802.11ay protocol began in 2015 and continued, for longer than expected, until the approval of the final standard in March 2021 [McCann22]. However, this technology builds off of developments that have occurred since the release of the original 802.11 specification in 1997. Before discussing the 802.11ay standard itself, we must first cover necessary background information such as the motivation for the technology and the specific problems that it is trying to solve. In particular, it is important to note that 802.11ay is not the first directional multi-Gigabit (DMG) IEEE standard. That distinction goes to 802.11ad, first officially released in 2012. Understanding the design and flaws of this protocol is relevant for understanding the changes introduced in 802.11ay.

2.1 Benefits

Because the amount of available spectrum is limited, existing 802.11 protocols such as 802.11n and 802.11ac can only provide so much speed for up-and-coming applications. Millimeter-wave communications, defined as those using frequencies between 30 and 300 GHz, solve this problem with the immense amount of spectrum obtainable. For example, a single un-bonded channel in 802.11ay uses 2.16 GHz of bandwidth in the unlicensed band between 57 and 71 GHz [Chen19]. This is over 20 times the amount of bandwidth that would be accessible if every channel in the 2.4 GHz band were bonded together. It is this single factor that compels researchers to overcome the challenges posed by millimeter waves and ensure that the next generation of devices, including those that use 802.11ay, 5G, and 6G, are taking advantage of this opportunity.

However, it is not only high speeds that are a benefit of using millimeter waves. Because the wavelength is much shorter, smaller antennas suffice for both sending and receiving data. The ease of having an array of antennas enables both beamforming and MIMO as well as their concurrent use [Chen19]. As a result, beams in 802.11ay are highly directional, allowing the possibility of spatial reuse. While these advantages pale in comparison to the ultra-high data rates, they provide a glimpse of a future in which interference, being reduced to a primarily spatial concern, could be solved simply with increased infrastructure.

2.2 Challenges

Of course, these benefits come alongside plenty of practical challenges that make the implementation of such a system difficult. The largest hurdle for millimeter-wave technology is the huge amount of propagation loss inherent for the high frequency. Research has shown that attenuation is 21-22 dB worse at 60 GHz compared to 5 GHz, a scale factor of over 100x [Baza21]. This is partially due to the effects of water vapor and oxygen molecule absorption on this frequency, effects which are negligible below 10 GHz. Under this lens, beamforming in 802.11ay is a necessary evil that adds complexity in the form of sector-level sweeps and beam refinement. Specifically, if the beamforming training (BFT) process takes too long or if link alignment can not be maintained, users will experience delay no matter how fast their speeds are once the connection is established.

Other obstacles for 802.11ay include finding better methodology for power consumption as well as channel bonding and aggregation. Due to the simultaneous use of a large number of high-power antennas to create directional transmissions, power management is particularly relevant. While 802.11ad provided a solid starting ground, 802.11ay seeks to further minimize the time in which stations remain in an active mode, thereby maintaining or improving each node's efficiency [Zhou18]. The difficulties with channel bonding are not new to 802.11ay, but are amplified when the base channels are so wide to begin with. These wider channels have higher sensitivity which may result in reduced range, and of course are more likely to interfere with other networks operating in the same area. The challenges discussed in this section are not insurmountable, but rather provide an impetus for the design decisions presented later in this survey.

2.3 802.11ad

As the first millimeter-wave IEEE standard in the 60 GHz band, 802.11ad laid the groundwork for the follow-up 802.11ay. This was the first 802.11 protocol to support directional communication, therefore meaning it was the first protocol to tackle beamforming and the ensuing developments necessary to the method of channel access. These are introduced here but will be relevant to 802.11ay as well. 802.11ad breaks beamforming training within a personal basic service set (PBSS) into two parts: sector-level sweep (SLS) and beam refinement protocol (BRP). These exist within the superstructure of a beacon header interval (BHI) and data transfer interval (DTI) contained within the overall scheduled beacon interval. The BHI encompasses a beacon transmission interval (BTI), association beamforming training (A-BFT), and an announcement transmission interval (ATI), while the DTI consists of any number and order of contention-based access periods (CBAPs) and scheduled service periods (SPs) [Zhou18]. An example of this entire process is shown below in Figure 1.

While the 7 Gbps maximum speed represented the highest available for 802.11 at the time, several limitations plagued 802.11ad. The primary limit was the range, which was no more than 10 meters and thus could not cover many homes or even rooms. Others included the lack of support for multi-channel and MIMO operations. Because all of the functionality is limited to a single channel in this way, 802.11ad constitutes a more rigid protocol than what was built into 802.11ay. Ultimately, 802.11ad did not see much adoption. This was due to factors such as the restrictive range, the lack of support for the protocols necessary for fixed wireless access, and options such as 802.11ac providing a more proven alternative [Nitsche14].

3. Changes in 802.11ay

In order to take 802.11ad's DMG to 802.11ay's EDMG, several key additions have been made. Cumulatively, these result in a theoretical maximum speed of 40 Gbps as well as an increased potential range of several hundred meters. To begin, we introduce the general features of the protocol that, while often similar to 802.11ad, provide a foundation for understanding EDMG. Following that we examine MIMO support, which accounts for most of the speed improvement in 802.11ay on its own. Finally, implementation of channel bonding and aggregation, support for dual-polarized antennas, and an enhanced beamforming training sequence round out the major attributes of 802.11ay that are of relevance to this survey.

3.1 General Protocol

The 802.11ay specification ensures backwards compatibility with 802.11ad while additionally providing access to the most powerful tools available to high-end equipment and an ideal environment. For example, while 802.11ad supports quadrature amplitude modulation (QAM) up to a factor of 64 and a maximum error-correcting code of 13/16, 802.11ay goes one step further with 256-QAM and a coding of 7/8 [Baza21]. The same 60 GHz band and 6 non-overlapping 2.16 GHz channels are used, although as discussed later bonding and aggregation are possible.

While the majority of the packet structure is left unchanged from 802.11ad, several EMDG-related fields must be included in order to share information such as the number of bonded channels, the number of streams, the MIMO mode, and new training (TRN) information. This is shown below in Figure 2 where the different parts include the short training field (STF), the channel estimation field (CEF), and EDMG headers. Header-B is only included in multi-user MIMO (MU-MIMO) packets because Header-A contains all the other necessary options [Ghasempour17].

3.2 MIMO

802.11ay enables single-user MIMO (SU-MIMO) and MU-MIMO functionality for up to 8 simultaneous streams of data. Given the constant increase in the number of both stations and users on modern networks, multi-stream communication is necessary for scaling capacity. However, channels in the 60 GHz band tend to be more sparse than channels at lower frequencies, making the separation of streams at the receiver a more challenging task. Before accessing the channel, the transmitter must indicate to one (SU-MIMO) or more (MU-MIMO) stations its intention to transmit a MIMO packet [Ghasempour17]. This will occur most often with a ready-to-send (RTS) frame that will specify the antenna configuration and channel number to be used. The receiver can then respond with an ACK to confirm its availability.

What makes MIMO for 802.11ay unique is that unlike in other 802.11 protocols, it is done alongside beamforming. To accomplish this, 802.11ay breaks MIMO beamforming up into two phases, with the first being single in single out (SISO). In this phase, both participants collect the information needed for possible candidate sectors to be used in the next phase, and the receiver curates a ranked list of sectors from best to worst based on their estimated signal to noise ratios. In the MIMO phase, simultaneous training can occur for each individual antenna array. The process of selection for the best sector combinations can be somewhat implementation-dependent, but ultimately one transmit and one receive sector are chosen per antenna array. MU-MIMO faces the additional hurdle of avoiding inter-stream interference, but given one transmitter and a group of receivers, the transmitter is able to act as the decision maker and coordinate the necessary packets to obtain feedback from every receiver [Aggarwal22].

3.3 Channel Bonding

The other main source of 802.11ay's speedup, channel bonding allows for up to 4 of the 6 adjacent channels to be combined to achieve a higher bandwidth. In order to maintain backwards compatibility with 802.11ad while supporting larger channels, design choices such as the channel access scheme had to be modified. The standard specifies that a primary channel must be chosen on which the entire BHI is to still be sent. However, if one channel is allocated to legacy 802.11ad devices while another is only 802.11ay devices, the 802.11ay devices can use their channel regardless of the state of the other [Zhou18]. This means that multiple-channel operation is more efficient and channel utilization is improved.

3.4 Channel Aggregation

While broadly similar to channel bonding, channel aggregation is a unique feature of 802.11ay with its own costs and benefits. Because the channels do not have to be adjacent, a single waveform is not sent but rather split up over multiple spaced-out channels and a slightly different frame format is required. This makes the channel usage more flexible, but bonding generally results in a higher throughput and lower power. One problem that can be encountered is a channel allocation made by the PBSS central point that does not include the primary channel. This can increase the system's efficiency overall, but then requires that the secondary allocation perform carrier sensing in case of overlapping PBSS. By not being tuned to the primary channel, these secondary stations would otherwise be unable to set their network allocation vectors [daSilva18].

3.5 Dual-Polarized Antennas

While representing a smaller change in 802.11ay, the support for dual-polarized antennas is a notable facet of the protocol because it serves to further enhance existing strengths. Specifically, it is able to reduce interference when using MIMO, increase spatial reuse in line-of-sight environments, and decrease the time needed for BFT. In order to obtain these benefits, the antenna arrays must be independently steerable and able to send signals with different polarizations. If achieved, a 2x2 SU-MIMO setup can have two streams that both operate in the exact same space if one of them is vertically polarized and the other is horizontally polarized [Zhou18]. This can cut out a lot of time from the critical BFT period because before the optimal antenna configuration is found, BFT frames can still be transmitted in parallel using orthogonal polarizations.

3.6 Enhanced Beamforming Training

A final key addition to 802.11ay is upgraded BFT methods that are efficient and low-complexity. In comparison, the procedure in 802.11ad is fixed, which means it is time-consuming in situations where a flexible and reactive methodology could vastly improve the training process. A representation of the TRN field for the BRP is shown below in Figure 3. As seen, N TRN-Units are each made up of P + M TRN subfields. The first P are always transmitted in the same way as the prior fields in the packet, while the remaining M are used to test different configurations of the antenna parameters. P final repetitions are needed after the Nth TRN-Unit in order for the receiver to measure the frequency offset in unit N [daSilva18].

802.11ay beamforming supports a vast array of options for different scenarios and use cases. Of course, like all of 802.11ay, accommodations are made for both MIMO as well as channel bonding and aggregation. Next there is the idea of hybrid beamforming, which says that amplitude and phase variations imposed by the antennas can be done as either analog, digital, or a combination of both. Further, 802.11ay supports asymmetric links in which one station can transmit to the other but not vice-versa. This results from the use of quasi-omni antennas and requires a different procedure. Lastly, 802.11ay has several methods for tracking beams and ensuring their robustness. If an operating channel becomes blocked, beam tracking can help quickly find another set of beams to continue transmission or, if necessary, transfer the session to a fallback in the 2.4 or 5 GHz bands [Zhou18].

4. Applications

To fully understand EDMG, it is relevant to consider some applications for which it is intended to be used. These applications are selected to showcase the range of possibilities, because EDMG is able to sustain both high-speed short-range links as well as longer wireless backhaul connections. The first and most obvious use case is high-speed wireless local area networks (WLANs) which can benefit both the consumer and commercial sectors. The second is the aforementioned wireless backhaul, which is a means of replacing the wired backbone with a wireless link when connecting a subnetwork to the broader internet. The third and final is virtual reality, which neatly encompasses many of the benefits of using EDMG .

4.1 High-Speed WLANs

Due to the nature of millimeter-wave being highly directional while also unable to penetrate through walls, it does not represent an end all be all solution for the most common types of home routers and network setups. That said, direct, local, point-to-point connections within a room could obtain insanely fast transfer speeds using 802.11ay. This could be a single fixed device or an entire office setup, with wireless monitors and webcams still able to provide 8K resolution. EDMG could serve a dense work environment where some devices act as docks and others have limited mobility. Alternatively, it could augment a smart home where peripherals like smart TVs could wirelessly receive a source stream from a variety of other devices.

4.2 Wireless Backhaul

A more distinct use case that 802.11ay is uniquely suited to is the replacement of what would have once been internet cables with wireless links. Because EDMG can reach 300 or even 1000 meters with optimal unobstructed placement, cables would not have to get as far as every single home or building to still achieve connection and high speeds. One benefit is that this infrastructure could be quickly rolled out, with nodes placed on already-existing street poles or rooftops. Coverage could still be provided even if the connection went through multiple hops, with redundancy potentially coming from a mesh topology. Challenges for this use case are that line of sight is required and that by being outdoors and on such a high frequency, weather effects could impact the throughput of the channel [Aldubaikhy20]. A diagram showing an example of what this may look like can be seen in Figure 4, where the circles represent EDMG nodes.

4.3 Virtual Reality

Virtual reality requires fast interactivity, high data rates to support 360Es video, and some amount of mobility as the user navigates a room. To achieve the best user experience, virtual reality headsets must be made wireless, and 802.11ay offers a solution to all of these constraints. Whether 802.11ay will become more commonly commercially available and consistent enough to not cause the kind of micro-delays that can induce nausea for some users is still an open question. One study showed that inter-stream interference was the main limiting factor and that even changing the orientation of the headset affected the link quality [Zhang22]. However, by taking advantage of EDMG features, the required data rate of more that 20 Gbps for the user was achieved on average.

5. Summary

Given the benefits, increased adoption of millimeter-wave communication in the future is inevitable. Challenges such as attenuation, power usage, link stability, and cost can be overcome via beamforming and efficient protocols to deliver on previously-unseen wireless data rates. This survey explored IEEE 802.11ay, or EDMG, a recently-approved standard that greatly improves the DMG protocol introduced in 802.11ad. The highest-impact changes are the additions of MIMO and channel bonding support, while other modifications such as enhanced beamforming training support these new features and still ensure backwards compatibility. Taken together, EDMG is able to have an upgraded range and maximum speeds of 40 Gbps with limited additional overhead. Lastly, we looked at the applications of high-speed WLANs, wireless backhaul, and virtual reality to see how EDMG opens up brand new possibilities and use cases for wireless networking.

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List of Acronyms