Wi-Fi 7 Scenarios
As bandwidth-hungry, latency-sensitive services like 4K/8K video, virtual reality (VR), augmented reality (AR), real-time gaming, remote working and cloud computing gain widespread adoption, users are demanding higher throughput and lower latency from Wi-Fi technology. Wi-Fi 7 features efficient physical layer (PHY) and medium access control (MAC) technologies to deliver increased throughput and decreased latency needed to support the services. The IEEE 802.11be Task Group (TGbe) is discussing drafts of the Wi-Fi 7 standard and is expected to release the finalized version at the end of 2023.
Key Innovative Technologies of Wi-Fi 7
To meet the requirements of high-bandwidth and low-latency application scenarios, TGbe discussed a number of key innovative technologies to be incorporated into the 802.11be, or Wi-Fi 7 standard. For example, to increase throughput, TGbe adopted the 320 MHz, 16-spatial-stream (SS) multi-user multiple-input multiple-output (MU-MIMO), 4096-quadrature amplitude modulation (QAM), and multi-link architecture in the extremely high throughput (EHT) PHY specification. To decrease latency, the discussions focused on enhancing spectrum utilization, improving anti-interference performance and making technological adjustments for real-time applications (RTAs). For spectrum utilization, TGbe discussed technologies including the EHT preamble, multi-resource unit (RU) support, implicit sounding, explicit feedback, virtual basis service set (BSS), hybrid automatic repeat request (HARQ), distributed MU-MIMO, and multi-access point (AP) sounding. For anti-interference performance, it deliberated over techniques like preamble puncturing, synchronous channel access, null steering, coordinated optical orthogonal frequency division multiplexing access (Co-OFDMA), and coordinated spatial reuse (CSR). For technological adjustments for RTAs, TGbe considered using applicable features of the IEEE 802.1 time-sensitive networking (TSN) specification and discussed technologies like faster backoff, new access categories (ACs), transmission opportunity (TXOP) capturing, multi-AP joint reception, and asynchronous channel access.
PHY Improvements of Wi-Fi 7 Are Decisive Factor in Increasing Throughput and Reducing Latency
—320 MHz bandwidth: Because the unlicensed spectrum of 2.4 GHz and 5 GHz bands is limited and congested, Wi-Fi 7 adds new bandwidth modes, including contiguous 320 MHz, 160+160 MHz, 240 MHz, and 160+80 MHz, and can operate in the 6 GHz band. Wi-Fi 7 also has effective mechanisms to improve the spectrum utilization of non-contiguous bandwidth. Non-contiguous bandwidth facilitates the coexistence of adjacent networks and can provide high speeds in the absence of contiguous spectrum. TGbe also considered band aggregation, which enables the establishment of multiple links across different frequencies.
—4096-QAM: In scenarios where the AP and the only station (STA) have the same number of antennas, MU-MIMO is not applicable. In this case, the only way of increasing bandwidth is to improve the QAM consternation. However, the bandwidth gain decreases with every improvement of the QAM consternation. For example, while 1024-QAM increases the nominal data rate by 25% compared with 256-QAM, 4096-QAM manages only a 20% rise over 1024-QAM.
—More-effective preamble puncturing formats and mechanisms: In Wi-Fi 7, preamble puncturing is extended to 320 MHz bandwidth, improved for the multi-user (MU) PHY protocol data unit (PPDU), and added for the SU PPDU. These enhancements improve channel utilization.
—Multi-RU allocation: An AP can allocate RUs to different STAs for downlink (DL) or uplink (UL) transmission. Allocating only one RU for each STA will reduce RU diversity, and RU diversity greatly improves the experience of RTAs. Wi-Fi 7 supports allocating multiple RUs to each STA. Because the main drawback of multi-RU allocation is that implementing the technology and scheduling the RUs are complex processes, TGbe limits the number of RU combinations. To improve spectrum utilization, it is better to combine a large RU(s) with a large RU(s) and a small RU(s) with a small RU(s).
—Advanced PHY: TGbe discussed multiple advanced PHY technologies including HARQ, full-duplex (FD) operation, and non-orthogonal multiple access (NOMA). Although these technologies significantly boost spectral efficiency in transmission retries and bidirectional simultaneous transmissions, they incur a high cost. Whether to include them in the final Wi-Fi 7 standard will require more discussions.
MAC Enhancements Consolidate PHY Improvements of Wi-Fi 7
One of the revolutionary features distinguishing Wi-Fi 7 from the previous Wi-Fi standards is its local support for multi-link operation (MLO). This helps Wi-Fi 7 deliver very-high data rates and very-low latencies, and is essential to meet the 802.11be project authorization requirements (PARs) for high bandwidth and low latency. In addition, this feature allows for efficient use of channel resources and prevents interference in dense deployments.
—Multi-link device (MLD): Wi-Fi 7 introduces the MLD concept, which consists of several affiliated devices. While each device has a PHY interface toward the MAC layer, the MLD has a single interface to the logical link control (LLC) layer. This concept simplifies packet fragmentation and reassembly, duplicate detection, and dynamic link switching. Wi-Fi 7 includes two multi-band MAC architectures: independent MAC and distributed MAC. Each MAC architecture is divided into the upper and lower MAC layers. The upper MAC layer supports most MAC operations such as aggregate MAC service data unit (A-MSDU) aggregation/de-aggregation, and sequence/packet number allocation, while the lower MAC layer supports only a small amount of MAC operations like MAC protocol data unit (MPDU) header creation, cyclic redundancy check (CRC) verification, and MPDU aggregation/de-aggregation. This mechanism implements the switchover between a single traffic ID and multiple traffic IDs without causing a big MAC overhead.
—Multi-link channel access: The MLD can access and transmit data asynchronously through multiple links, and transmit and receive data simultaneously in the 2.4 GHz, 5 GHz and 6 GHz bands. The closer the channels of the affiliated device to each other, the more power leakage from the affiliated device to the other devices. This leakage or interference complicates simultaneous transmission and reception. To address the problem, Wi-Fi 7 introduces a synchronous MLO scheme at the cost of decreasing the channel access and lowering the throughput. Another solution for cutting inter-device interference is to disable transmission while the receiver is receiving data.
—MLO for RTAs: Due to the diversity of channels, MLO is regarded as an effective method of improving transmission reliability and reducing latency. Wi-Fi 7 provides two modes of MLO: duplicate mode and joint mode. In duplicate mode, the transmitter sends copies of a data frame through multiple links. After the receiver obtains a copy, it discards all the copies delivered thereafter. This mode significantly enhances transmission robustness. In joint mode, the transmitter sends a data frame over multiple links without generating any copies. This mode reduces transmission latency.
—Multi-AP operation: One goal of TGbe is to improve network performance by using the MAC layer protocol to strictly coordinate channel access, transmission scheduling, and the joint transmission of the same data. TGbe considered two types of multi-AP systems. The coordinated multi-AP system uses a single AP to send and receive the entire data, while the joint multi-AP system employs multiple APs to send and receive the data. TGbe discussed CSR, Co-OFDMA, coordinated beamforming (CBF), and joint transmission and reception (JTR) as the multi-AP techniques. These techniques require different levels of synchronousness, with coarse frame-level synchronization for CSR, symbol-level synchronization for CBF and Co-OFDMA, and accurate time and phase synchronization, which is the most difficult, for JTR.
—MAC EDCA QoS improvement: In order to use IEEE 802.1 TSN techniques to improve enhanced distributed channel access (EDCA), TGbe analyzed the backoff process, ACs, and packet policies. However, many techniques used in the IEEE 802.1 TSN, which is a wired Ethernet network, are not directly applicable to Wi-Fi networks. The techniques must be applied selectively or modified before application. Consider a typical scenario where video traffic and online gaming traffic are concurrently transmitted. In this case, EDCA needs to be upgraded by placing the gaming traffic in the voice (A-VO) AC queue or introducing a new AC. If an RTA frame is about to expire, the backoff count can also be sped up. In the worst-case scenario, the channel can be permanently allocated to the RTA frame. Moreover, Wi-Fi 7 also allows changing the TXOP rule.
The core functions of the Wi-Fi 7 standard are providing extremely high throughput and supporting RTAs. Wi-Fi 7 improves EHT PHY technologies to implement very-high rates and ultra-low latency. However, EHT PHY alone cannot reliably provide end users with high throughput and low latency in complex real-world environments. In light of this limitation, Wi-Fi 7 also introduces enhanced MAC technologies. Because some of the EHT PHY and enhanced MAC technologies incur a high cost, their implementation may be pushed back to Wi-Fi 8.
Currently, 6 GHz spectrum has been authorized for Wi-Fi use in the US, Europe and South America. In countries and regions where 6 GHz spectrum has not been authorized for Wi-Fi use, bandwidth gain can also be produced by employing MLO technology that utilizes both 2.4 GHz and 5 GHz spectrum. For example, in China, MLO is used to combine 40 MHz in the 2.4 GHz band and 160 MHz in the 5 GHz band to generate 200 MHz bandwidth.
ZTE has long been actively involved in the development of the IEEE 802.11, or Wi-Fi standards. With its experts serving as the chair of the 802.11 TGbe and AMP TIG, ZTE is participating in the entire process of Wi-Fi 7 development and has made important contributions to the definition of key interfaces and parameters of the PHY and MAC layers of the standard.