(1) Multi-link operation: 802.11be targets efficient operation in all available frequency bands (i.e., 2.4, 5, and 6 GHz) for load balancing, multi-band aggregation, and simultaneous downlink/uplink transmissions. In 802.11be, a multi-link device (MLD) is defined as a device with multiple subordinate APs or STAs and a single MAC service access point (SAP) to the above-mentioned logical link control (LLC) layer. A MAC address that uniquely identifies the MLD management entity is also introduced. The relevant functions of multi-link control and operation are summarized as follows:

• Multi-link discovery and setup: MLD has the ability to dynamically update its frame exchange on each pair of links simultaneously. In addition, each individual AP/STA can also provide information about the operating parameters of other affiliated AP/STAs within the same MLD.

• Traffic Link Mapping: In a multi-link setup, all traffic identifiers (TIDs) used to classify frames according to their Quality of Service (QoS) are mapped to all setup links. Updates to this mapping may be made subsequently by any associated MLD. In addition, the receiving MLD will utilize a single reordering buffer for QoS data frames of the same TID transmitted over multiple links.

• Channel access and power saving: Each AP/STA of MLD performs independent channel access through its link and maintains its own power state. To facilitate efficient STA power management, the AP can also use the enabled link to carry indications of buffered data for transmission on other links.

(3) Direct enhancements to 802.11ax: The 802.11be TG will also specify some upgrades to the current 802.11ax standard. These include:

• Support for 320 MHz transmissions, doubling the 160 MHz transmissions of 802.11ax.

• Using higher modulation orders, 4096 QAM can be supported, while only 1024-QAM is supported in 802.11ax, and there is a strict -38 dB requirement for the error vector magnitude (EVM) of the transmitter.

• Each STA is allocated multiple resource units, namely OFMDA. Its flexibility can improve spectrum utilization.

4. Advanced AP coordination : In order to fully tap the potential of multi-AP coordination, 802.11be TG agreed to support the following three solutions:

• Coordinated OFDMA: In 802.11be, an AP that obtains a TXOP will be able to share its frequency resources in multiples of a 20 MHz channel with a set of neighboring APs. For efficiency, a shared AP can request neighboring APs to report their resource requirements.

• Single-user and multi-user joint transmission: Sending data to its connected STAs requires the AP to bind its phase synchronization error and timing offset. When considering the reasonable range of these deviations, it is found that joint transmission can bring gains under the premise of sufficient backhaul. Since the cooperative AP needs CSI from related and non-related STAs, 802.11be will define a joint multi-AP detection scheme. In this way, the AP will send its detection frame at the same time, and the addressed STA will transmit the CSI feedback of all APs.

• Coordinated Beamforming: This technique exploits the ability of modern multi-antenna APs to spatially multiplex their STAs while jointly placing radiation nulls to or from neighboring non-associated STAs. While the CSI required to control the radiation nulls can be obtained through the joint multi-AP sounding scheme described above, CBF can also take advantage of the simpler sequential sounding procedure that will be part of 802.11be. In addition, CBF does not require joint data processing because each STA sends or receives data to or from a single AP, thus significantly reducing the number of joint transmissions required for the backhaul. This is because CBF can provide significant throughput and latency enhancements while maintaining complexity, which we will explore further in the next section.

To provide a more detailed description of the PSR framework, let us look at Figure 2(a) with an example of two BSSs, where:

• BSS1 consists of AP1, STA11, and STA12; BSS2 includes AP2, STA21, STA22, and STA23.

• Figure 2(b) shows how AP1 initiates PSR processing by sending a trigger frame after gaining channel access. This trigger frame has a dual function:

• Transmitting synchronization and scheduling information required for uplink transmission of its associated STA11 and STA12; and

• The spatial reuse opportunity is advertised to the OBSS device, which spans the subsequent uplink data received by AP1.

• Increases network throughput because it allows more concurrent transfers;

• Increased STA file throughput because STA spends less time in contention; importantly,

• Reduced latency, because STAs with time-sensitive short file traffic may not need to wait until the broadband STA terminates its long transmission. This is the case for STA21 and STA22 in Figure 2.

2. PSR Challenges: Although the PSR framework allows for greater spatial reuse, two challenges were discovered during the 802.11be research:

• Devices that take advantage of spatial reuse opportunities must reduce their transmit power to limit the interference generated. In some cases, for STA21, STA22 in Figure 2, this translates into reduced throughput. In other cases, for STA23, the device cannot even access the spatial reuse opportunity because its maximum allowed transmit power is insufficient to reach its receiver.

• Devices that exploit spatial multiplexing opportunities are unaware of and cannot control the interference perceived by their respective receivers. In Figure 2, this means that if STA21, STA22 is close to AP2, the uplink transmission from STA21, STA22 to AP2 may fail because AP2 will receive a non-negligible amount of interference from STA21, STA22.

• Establishment and maintenance of coordination sets: In order for CBF to be effective, APs need to communicate with OBSS STAs, such as obtaining the necessary CSI to place radiation nulls at specific spatial locations. To this end, an inter-BSS coordination set is defined between the cooperating APs, which must contain the IDs of all APs and STAs participating in the CBF transmission. These IDs can be kept in memory by all relevant devices without discarding the relevant frames generated by the OBSS devices included in their coordination set as in the traditional way. Once defined, the inter-BSS coordination set can be updated in a semi-static manner (i.e., after tens or hundreds of TXOPs).
• Dynamic coordination of subsequent spatial reuse opportunities: Once AP1 obtains a TXOP, it needs to announce the incoming uplink triggered transmission and, together with the devices in its coordination set, determine which STAs will participate in the subsequent CSI acquisition and data communication phases. In the example of Figure 3(b), AP2 replies to the dynamic coordination frame sent by AP1, indicating which of its STAs will benefit most from being granted a secure spatial reuse opportunity, e.g., STA21 and STA22.

3. Data communication: The implementation of the first two phases addresses the two fundamental challenges of the 802.11ax PSR framework highlighted in the previous section, making spatially multiplexed transmissions from STA21, STA22, and STA23 more likely to succeed under adverse conditions. This is because:

• STA21, STA22 and even STA23 are more likely to find spatial reuse opportunities and use their maximum transmission power. This is thanks to the spatial interference mitigation performed by AP1, which helps to publish relaxed messages about the channel access conditions of the associated OBSS devices.

• AP2 is now able to suppress incoming interference generated by STA11 and STA12 while receiving uplink transmissions from STA21, STA22, and STA2.

• IEEE 802.11ax device setup without spatial multiplexing: The results in Figure 4 show that IEEE 802.11 based systems may be able to provide low latency but struggle to maintain consistent performance in the worst case. In fact, we can observe that latency remains below 3ms for about 50% of the considered scenarios, but increases significantly to over 200ms in the 0.01% worst case scenario. This is mainly due to the combined effects of the random channel access mechanism and collisions leading to retransmissions.

• Setup with IEEE 802.11ax devices supporting PSR: Figure 4 illustrates that the implementation of PSR does not help to significantly reduce the worst-case latency. This is because, similar to STA23 in Figure 2, the AR STAs are not far enough from their neighboring APs in the considered dense scenario. This prevents these latency-sensitive STAs from discovering spatial reuse opportunities because the channel access conditions they need to comply with to prevent harmful interference are too stringent, as detailed in Section 4.2.

• Equipment setup implementing the IEEE 802.11be CBF scheme described in the previous section: The results in Figure 4 illustrate how the proposed scheme significantly reduces the worst-case latency compared to other IEEE 802.11ax systems. In fact, we can observe that the system with multi-AP coordination capability reduces the worst-case latency of 0.01% by a factor of 9 relative to the system with PSR capability. This significant performance enhancement is a direct result of i) the large number of spatial multiplexing opportunities discovered by the AR STAs due to their relaxed channel access conditions, and ii) the OBSS interference mitigation provided in the spatial domain, which maximizes the chances of performing a successful data transmission.