Source: Edge Computing Community
Original Author: ZTE Corporation Zhang Zhigang
With the widespread application of 4K and 8K video, AR/VR, real-time gaming, remote offices, and cloud computing, users have higher demands for the high throughput and low latency of Wi-Fi technology. Wi-Fi 7 enhances throughput and reduces latency by designing efficient PHY (Physical Layer) and MAC (Medium Access Control Sub-layer Protocol), providing technical support for real-time services.
To meet the needs of high-bandwidth and low-latency application scenarios, the 802.11be Work Group explored various key innovative technologies; to improve throughput, the EHT (Extremely High Throughput) PHY adopted 320MHz, 16SS MU-MIMO, 4096QAM technology, and Multi-Link architecture; to reduce latency, it approached from different angles such as spectrum utilization, anti-interference, and targeted adjustment technologies for real-time services.
In terms of spectrum utilization, technologies such as Multi-RU and HARQ were explored; in terms of anti-interference, technologies such as Co-OFDMA and CSR were discussed; in terms of targeted adjustment technologies for real-time services, the applicability of IEEE 802 TSN (Time Sensitive Network) was also explored.
Enhancements in PHY Technology
The enhancements in PHY technology of Wi-Fi 7 are decisive factors for its increased throughput and reduced latency.
Due to the limited and crowded unlicensed spectrum of 2.4GHz and 5GHz, EHT added new bandwidth modes and can operate in the 6GHz band, including configurations such as continuous 320MHz, 160+160MHz, 240MHz, and 160+80MHz. EHT designed effective methods to improve the spectrum utilization of both contiguous and non-contiguous bandwidth, with non-contiguous bandwidth being beneficial for co-existence with neighboring networks, providing high bandwidth without contiguous spectrum. Task Group 11be (TGbe) also considered band aggregation, which refers to the joint use of multiple links at different frequencies.
In scenarios where the number of antennas for the AP and the unique STA is the same, since MU transmission cannot be used, the only effective way to increase bandwidth is to enhance the QAM modulation constellation. However, the gain decreases with each increment of the modulation constellation. 1024-QAM only increases the nominal data rate by 25% compared to 256-QAM, while 4096-QAM only improves by 20% compared to 1024-QAM.
More Efficient Preamble Puncturing Format and Mechanism
In EHT, improvements were made to the puncturing design for MU PPDU and an increase in the puncturing design for SU PPDU, enhancing channel utilization, and TGbe extended the preamble puncturing to the 320MHz band.
Assigning only one RU to each STA reduces diversity, while diversity is effective for real-time service RTA. 802.11be supports assigning multiple RUs to each STA. The main drawback of multi-RU allocation is the complexity of implementation and scheduling, so TGbe restricts the types of RU combinations. Frequency diversity provides minimal gain, so RU combinations should aim to combine large RUs and small RUs separately to improve utilization.
TGbe also discussed various Advanced PHY technologies such as Automatic Repeat reQuest (HARQ), Full Duplex operation (FD), and Non-Orthogonal Multiple Access (NOMA), which will significantly improve spectral efficiency in cases of transmission retries and simultaneous bi-directional transmission, but the corresponding cost is high, and whether they will be included in the standard needs further evaluation.
Improvements in MAC Technology
The improvements in MAC technology of Wi-Fi 7 further optimize the effects of the enhancements in Wi-Fi 7 PHY. One of the revolutionary changes in Wi-Fi 7 is the local support for Multi-Link Operation, which benefits massive data rates and extremely low latency, serving as an important means to meet the high bandwidth and low latency PAR (Project Authorization Request) requirements of EHT, effectively utilizing channel resources and avoiding interference in dense deployments.
802.11be introduces the concept of multi-link devices, composed of several subordinate Wi-Fi devices, each having a PHY interface to the wireless medium, but a single interface to the LLC (Logical Link Control) layer. This solution simplifies fragmentation and data packet reassembly, duplicate detection, and dynamic link switching. 802.11be recommends two types of Multi-band MAC architectures, namely Independent MAC and Distributed MAC. It divides MAC into Upper and Lower MAC layers, the former supporting most MAC operations (such as A-MSDU aggregation/de-aggregation, sequence/packet number allocation), while the latter supports a few MAC operations (such as MPDU header and cyclic redundancy check registration/verification and MPDU aggregation/de-aggregation), achieving the switching of single traffic ID and multiple traffic IDs without causing significant MAC overhead.
Multi-Link Channel Access
Multi-link channel access allows asynchronous access and transmission of data through multiple links, simultaneously transmitting and receiving across different bands of 2.4/5/6GHz. As the channels of subordinate devices become closer, the power leakage from the transmitting subordinate devices to other devices becomes stronger, complicating the simultaneous transmission and reception capability. To address this, a synchronous transmission scheme is proposed, at the cost of reduced channel access proportion and decreased throughput. Another solution for cross-subordinate device interference is to prohibit transmission during the anticipated receiver’s transmission process.
Multi-Link RTA (Real-Time Application) Business Transmission
Due to channel diversity, multi-link operation is considered an effective method to enhance reliability and reduce latency. There are two modes of multi-link operation: replication mode and joint mode. In replication mode, the transmitter sends copies of each frame through multiple links, and once the receiver obtains one frame, it discards all subsequent copies, significantly improving transmission robustness. In joint mode, the transmitter distributes data frames across multiple links without producing any copies, reducing transmission delay.
TGbe aims to enhance network performance through strictly coordinated channel access, transmission scheduling, and joint transmission of the same data at the MAC layer, considering two types of Multi-AP systems: Coordinated and Joint. The Coordinated system sends/receives each part of the data through a single AP, while the Joint system sends and receives data through multiple APs. The schemes discussed in Multi-AP include Coordinated Spatial Reuse (CSR), Coordinated OFDMA (Co-OFDMA), Coordinated Beamforming (CBF), Joint Transmission and Reception (JTR), among others.
The required level of synchronization between the above Multi-AP systems varies; CSR can operate under coarse frame-level synchronization, while CBF and Co-OFDMA require symbol-level synchronization, and JTR necessitates tight time and phase synchronization, which is the most difficult to achieve.
MAC EDCA QoS Improvements
TGbe drew on the results of IEEE 802 TSN in Ethernet networks to improve EDCA (Enhanced Distributed Channel Access), analyzing its backoff procedures, AC (Access Category), and packet policies. However, many wired 802 TSN solutions do not directly apply to wireless Wi-Fi and need to be selectively applied or improved. A more realistic scenario is the concurrent situation of video streaming and online gaming, which requires upgrading EDCA to place gaming streams in the voice (A-VO) AC queue or introducing a new AC. If RTA frames are about to time out, the backoff count can be accelerated. In the worst case, long-term channel allocation can be adopted. Wi-Fi 7 also allows changes to TXOP (Transmission Opportunity) rules.
The core function of the 802.11be amendment is to provide extremely high throughput and support real-time applications. 802.11be first improved EHT PHY technology, which can achieve ultra-high rates and lower latency, but relying solely on EHT PHY cannot provide high throughput and latency gains for end-users, which is why TGbe has introduced many other MAC improvement technologies; of course, the advanced PHY, MAC methods, and some methods in Multi-AP introduced may be costly and could potentially be pushed to Wi-Fi 8 implementation.
The reproduced content only represents the author’s views
It does not represent the position of the Institute of Semiconductors, Chinese Academy of Sciences
Editor: Muxin
