Standardization Trends in IEEE 802.11be/bn

3.1 Main technical features of IEEE 802.11be

New features for improving frequency-utilization efficiency, wider frequency bandwidth, and latency reduction will be defined in the 11be. Multi-link operation is a new feature that was never defined in the old standard before 11ax. Combining these features enables a 2.4 times improvement of the maximum data rate at the physical (PHY) layer from 9.6 Gbit/s (11ax) to 23 Gbit/s (11be). Applying the multi-link operation that enables simultaneous transmissions will enhance throughput at the MAC layer, and 11be aims to achieve a maximum throughput of at least 30 Gbit/s by using this feature. Table 2 lists the effect of throughput enhancement of 11be compared with 11ax.

Table 2. Effect of throughput enhancement of 11be.

(1) Features of frequency-utilization efficiency and wider frequency bandwidth

The 11be standard defines 4096 quadrature amplitude modulation (QAM) as a modulation scheme of each subcarrier of the orthogonal frequency division multiplexing (OFDM) symbol, which achieves a 1.2 times higher modulation rate than 11ax (1024 QAM). In 11ax, a resource unit (RU) that indicates grouped subcarriers in a channel bandwidth of orthogonal frequency division multiple access (OFDMA), is assigned to each station (STA). Multiple RU (MRU) defined in 11be, however, enables the assignment of multiple RUs to an STA. This feature establishes several patterns of RU combinations, allowing for more flexible resource allocation.

(2) Multi-link operation

The 11be standard specified a new feature defined as multi-link operation that was never defined in the old standard before 11ax. In multi-link operation, a wireless device defined as a multi-link device (MLD) can support more than one affiliated STA or access point (AP) and operate using one or more affiliated STAs or APs. This feature enables an AP/STA to bundle multiple transmission paths (links), thus achieves a higher transmission rate in accordance with the number of links. Figure 1 shows a conceptual image of multi-link operation.

Fig. 1. Concept of multi-link operation.

(3) Feature of latency reduction

In 11be, the latency-reduction feature defined as restricted target wake time (TWT) will be supported. This feature ensures a specific period within the scheduled service period (SP) for latency-sensitive traffic. Therefore, contention between latency-sensitive traffic and non-latency sensitive traffic is reduced, and latency-sensitive traffic generated periodically can be transmitted more reliably.

(4) Other new features

A new peer-to-peer (P2P) feature without setting up any connections between STAs will be defined. In this new P2P feature, called triggered transmission opportunity (TXOP) sharing mode 2, an AP obtains time for P2P transmission representatively, and the designated STA can transmit data frames to the other STA directly. Emergency preparedness communications service (EPCS) will also be defined in 11be. This feature can prioritize the transmission of specific STAs authorized by networks in advance.

3.2 Standardization activities of NTT for 11be

The predecessors of TGbe were EHT TIG and EHT SG, which were launched in 2018 ( TGbe was formed in 2019). The focus was initially only on improving maximum throughput, and reducing latency was not included in the scope of 11be. In the subgroup called Real Time Application (RTA) TIG, NTT and several companies discussed and considered latency and jitter reduction for industrial automation, cloud gaming, and so on around the same time as EHT TIG and SG. NTT collaborated with the RTA TIG’s leading contributors to create a report document [3] that summarizes RTA TIG’s activities. We input this document into EHT SG and emphasized the importance of latency-reduction features when EHT SG created the Project Authorization Request (PAR), which defines the scope and necessity of the standard, and Criteria for Standards Development (CSD), which describes the significance of the standard and market requirements. This document is referenced not only in 11be but also in the PAR and CSD of 11bn. After the establishment of TGbe, NTT continued to solicit that 11be should have some form of latency reduction and notification of latency related information features. Therefore, 11bn will have a latency-reduction feature, such as restricted TWT, and new signaling information, such as quality-of-service characteristic elements.

4.1 Features under consideration for 11bn

In UHR SG and TGbn, many companies have proposed various features for 11bn as follows. Note that these features have not been decided for technical specifications but are under discussion.

(1) Multi-AP coordination

Multi-link operation defined in 11be enables coordination using multiple wireless interfaces equipped with a single MLD. In contrast, multi-AP coordination improves frequency efficiency and reliability through cooperation between multiple independent APs. This feature had been discussed in early TGbe; however, TGbe decided not to include this feature in 11be specifications due to the timeline compliance and implementation complexity. Therefore, discussions of multi-AP coordination have been taken over by TGbn. Several types of coordination forms are under consideration. Coordinated spatial reuse (Co-SR) controls transmission power to decrease interference to STAs. Coordinated beamforming (Co-BF) enables simultaneous transmission to each target STA of each AP without generating interference by coordinated beamforming and nulling. In joint transmission (JT), multiple APs transmit identical signals simultaneously with synchronization, and a receiver STA can combine multiple identical signals. Figure 2 shows conceptual images of multi-AP coordination.

Fig. 2. Conceptual images of multi-AP coordination.

(2) Distributed RU

As described in the previous section, the MRU feature was defined in 11be. TGbn is discussing its enhancement to distributed-tone RUs (dRUs). This feature does not assign RUs by several patterns of combinations but freely, which offers more flexibility. For example, the same data block can be assigned to multiple RUs for redundancy, and the reliability of data transmission improves.

(3) Latency reduction

Restricted TWT defined in 11be can protect a specific period for latency-sensitive traffic generated periodically. In addition to the enhancement of restricted TWT, support for non-periodic (sporadic) latency-sensitive traffic is under consideration in 11bn. Preemption can interrupt latency-sensitive traffic into ongoing transmission by the frequency and/or time domain and transmit with priority. A function that ensures transmission of latency-sensitive traffic by using triggered TXOP sharing and enhancement of a buffer status report, which notifies an AP or an STA of the requirement of latency bound in real time, are also considered.

(4) Other features under consideration

Power-saving functions applying multi-link operation and/or multi-AP coordination, extension of area coverage of a wireless LAN network using relay architecture, and enhancement of security functions are under discussion in 11bn.

4.2 Discussion on utilizing millimeter wave

Whether to include the millimeter-wave frequency band (60 GHz) as the target frequency in addition to existing frequency bands such as 2.4/5/6 GHz was an important topic in UHR SG, which discusses the scope of 11bn. Although 11ad/ay are standards for wireless LAN systems that use millimeter waves, this discussion topic in UHR SG assumes using millimeter waves as additional links for multi-link operation features, which is unrelated to 11ad/11ay. As a result of the discussion, UHR SG decided that the millimeter-wave band should be discussed in a separate subgroup from TGbn, and the Integrated Millimeter Wave (IMMV) SG was launched in November 2023.

4.3 Standardization activities of NTT for 11bn

NTT has continued to promote the need to improve latency characteristics and reliability in wireless LAN since RTA TIG launched in 2018 and proposes to expand the Wi-Fi market not only for the consumer but also for business use cases requiring certain communication qualities such as low latency. This contribution changed the scope of mainstream wireless LAN standards from improving maximum throughput, such as 11be or before, to improving reliability in 11bn. NTT also proposed the PAR [4] and CSD [5] of 11bn with other companies and contributed to including important business use cases such as industrial automation, robotics, logistics, and smart agriculture in these prospective documents for establishing TGbn [6]. At the same time as the TGbn foundation, Yusuke Asai, a distinguished researcher of NTT, was elected 11bn leadership officer (Secretary).