一文让你全面了解802.11be最新规范

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1. 802.11 Standard Evolution

Standard Origin: 802.11-1997 defeats other standards to become the first widely used WLAN standard in the industry.

Standard Enhancement: 802.11b makes the large-scale commercial use of WLAN possible by delivering speeds of 11 Mbit/s. 802.11a further increases the WLAN speeds to 54 Mbit/s by applying OFDM technology to the 5 GHz frequency band.

Standard Extension and Compatibility: 802.11g extends the use of OFDM technology to the 2.4 GHz frequency band and is backward compatible with 802.11b.

HT Standard based on MIMO and OFDM: 802.11n supports single-user MIMO and OFDM, and delivers speeds of up to 600 Mbit/s.

VHT Standard: 802.11ac supports downlink multi-user MIMO, provides channel bandwidth of up to 160 MHz and delivers speeds of up to 6933.33 Mbit/s.

HE Standard: 802.11ax introduces technologies such as OFDMA, uplink MU-MIMO, BSS coloring, and TWT, further improving the throughput in high-density scenarios and increasing the speeds to 9607.8 Mbit/s.

EHT Standard: Based on the 6 GHz spectrum introduced in Wi-Fi 6E, 802.11be supports various technologies such as MRU and multi-link to further improve the throughput and deliver speeds of up to 46120 Mbit/s.

2. 802.11 Standard Comparison

lEEE802.11be aims to specify MAC and PHY features that will meet the high through-put and low latency requirements for these applications. The lEEE802.11be PHY is very similar to the PHY defined in the highly successful lEEE802.11ax standard. For example IEEE802.11be will support MU-MIMO, OFDMA and longer symbol duration(smaller subcarrier spacing). Some key lEEE802.11be PHY improvements and changes include the ability to assign multiple resource units to a single user in OFDMA transmissions as well as 320 MHz bandwidth and 4096QAM modulation.

3. Multilink ­Operation(MLO)

EHT introduces a new MAC feature called multilink operation (MLO). An MLO device (MLD) can establish multiple links on different channels with other MLDs as shown in the Figure. Two example use cases – higher throughput using link aggregation and load balancing – benefit from this feature. Aggregation uses two or more links for data transmission, achieving higher throughput than if only one link was used. In load balancing, MLO can be used to quickly switch to a channel link with fewer users. Fewer users on the medium means the latency due to channel access contention/retries is reduced. Although MLO uses multiple links, setup/association between the AP and non-AP is done via a single link, reducing the amount of overhead compared to legacy operation.

MLO devices with two or more radios can transmit and receive on different links at the same time, called simultaneous transmit and receive (STR). The Figure shows a link pair with one link at a channel in the 5 GHz band and another link at a channel in the 6 GHz band. In this example, AP1 in the 5 GHz band is transmitting data to STA1 in the 5 GHz band while STA2 simultaneously sends data to AP2.

Not all MLDs are STR-capable at any time. The MLD must be able to receive on one of the links in the presence of transmissions on the other link. If the frequency of the two channels is too close, the device may not be able to meet the IEEE802.11be receiver requirements in case of transmissions on the other link. With an established multilink, the STA indicates via an MLD capability field how much frequency separation it needs to be able to operate as an STR link.  

4. Restricted Target Wake Time

The target wake time (TWT) feature was introduced in lEEE802.11ah to support low power IoT applications by allowing STAs to go into sleep mode outside wake time periods after AP negotiation. lt also allows the AP to distribute STA wake periods over time to minimize contention. lEEE802.11be extends TWT capabilities and provides predictable latency to support time sensitive application requirements. STAs must ensure that any transmission ends before the start of a restricted TWT service period so that the channel can be exclusively used by STAs with established membership to the restricted TWT schedule as shown in Figure, with STA3 as member of a restricted TWT (rTWT3) and STA1 using TWT (TWT1) for power saving.

5. OFDM & OFDMA 

One of the key differences between Wi-Fi 6 and Wi-Fi 5 is that the former introduces the multi-user technology- OFDMA, which makes it possible for users to improve spectrum utilization by sharing channel resources. We can view OFDMA as a multi-user version of OFDM, which is a single-user transmission technology. This means that each time data is sent, one user occupies the entire channel regardless of the user data amount. Let's imagine Wi-Fi communication is express delivery, and information represents the goods to be transported to the receiver. In OFDM, the van delivers one package per trip, regardless of its size. As a consequence, some of the space in the van is usually wasted, as shown in Figure.

To make better use of the van's space, Wi-Fi 6 introduces OFDMA, which is essentially a multiple access technique. Put differently, it divides channel resources into multiple RUs. Different users are allocated these RUs, which carry their respective data. in this way, the data of multiple users can be sent on one channel simultaneously. Let's revisit the delivery van analogy. With OFDMA, the van is divided into several compartments to simultaneously carry different packages. As such, it can deliver several packages to different receivers on a single trip, as shown in Figure.

6. Minimum Transmission Unit: RU  

Before we discuss how RUs are divided, we first need to describe the concept of tone. Wireless signals are transmitted on fixed frequencies, which are also known as carriers, and the 802.11 standard further divides these frequencies into subcarriers (tones). For example, a 20 MHz channel in Wi-fi 6 is divided into 256 tones, with 78.125 kHz spacing, which represents only one quarter compared to Wi-Fi 5 (312.5 kHz). Among these tones, 234 data tones are used fortransmission,7 direct current (DC) tones (located at the spectrum center) are used for identification only, 4 pilot tones are used for functions such as channel estimation, and 11 guard tones are used to provide guard intervals (Gls).

To simplify OFDMA-based scheduling, Wi-Fi 6 defines seven types of RUs: 26-tone RU, 52-tone RU, 106-tone RU, 242-tone RU, 484-tone RU, 996-tone RUs and 2x996-tone RUs. Base on these, Wi-Fi 7 supports one more RU type thanks to the new 320 MHz channel. The table lists the number of XX-tone RUs supported at different channel bandwidth values. Assuming that a 320 MHz channel is only divided into 26-tone RUs, then theoretically, it allows an AP to communicate with a maximum of 148 terminals simultaneously.

7. Multiple­ Resource­ Units ­per ­user

While much of the lEEE802.11be PHY is the same or very similar to lEEE802.11ax, a key differentiator for lEEE802.11be is the capability to allocate more than one resource unit to a single user. Assigning multiple RUs per user provides scheduling flexibility to take advantage of frequency diversity and to efficiently allocate resources within the spectrum. The drawback of this additional flexibility is an increased overhead needed to describe all possible RU combinations. EHT avoids this by defining rules to limit which RUs may be combined by focusing on those combinations that provide the most benefit. The rules are based on the EHT RU classification as small or large size RU. Small size RUs are less than20 MHz, i.e.26,52,106-tone RUs. Large size RUs are 20 MHz or larger, i.e. 242-tone(20 MHz), 484-tone (40 MHz), 996-tone (80 MHz) RUs. Small and large size RUs are not used together in an MRU. So, a multiple resource unit (MRU) contains either two small RUs or two large RUs.

Small RUs in an MRU are contiguous and lie within a 20 MHz channel boundary. in addition, 26+26-tone MRU, 52+52-tone MRU and 106+106-tone MRU are not allowed since the AP should schedule a single 52-tone RU, 106-tone RU or 242-tone RU, respectively.

8. 802.11be MAX TPUT

The EHT physical layer supports wider bandwidth, more spatial streams and a higher modulation scheme to achieve extremely high throughput.
The maximum throughput of a single physical link can be calculated by the simple formula below.

A link using a 320 MHz channel, 16x16 MIMO, short guard interval of 0.8 µs, 4096QAM modulation with 5/6 coding can reach a theoretical data rate of 46 Gbps. A more typical example is a 160 MHz channel with 2x2 MIMO,1024QAM with 3/4 coding for example, achieving up to 2.1 Gbps.

9. WiFi Software Framwork Overview

10. Key benefits from new and enhanced IEEE 802.11be features

lEEE802.11be also has an eye towards future compatibility. lEEE802.11 PHY always considers backward compatibility with legacy lEEE802.11 generations, but lEEE802.11be introduces forward compatibility concepts. For example, lEEE802.11be includes a new preamble field called the universal SlG (U-SlG) that will be used in lEEE802.11be and all future lEEE802.11 generations as well as more precise terms and rules for reserved bits.

The lEEE802.11be MAC layer introduces significant changes and new features such as multilink operation, multi-AP support, restricted target wake time (TWT) and 1024-bit block acknowledgement (1K BA) to meet the targeted low latency applications.
The following table provides an overview of new and enhanced lEEE802.11be features and their benefits compared to lEEE802.11ax.

Feature	Support in IEEE 802.11ax	Support in IEEE 802.11be	Benefit
Bandwidth	20 MHz, 40 MHz, 80 MHz, 80+80 MHz and 160 MHz channels	adds 320 MHz and support noncontiguous bandwidth such as 80+80 MHz	increased data rate and lower complexity
Modulation	BPSK, QPSK, 16QAM, 64QAM, 256QAM, 1024QAM	adds 4096QAM option	increased throughput
Resource­units	single resource unit (RU) per user	adds support for multiple RU (MRU) assignments to a single user and modifies the RU allocation plan for bandwidth ≥ 80 MHz	efficient spectrum utilization and more scheduling flexibility
MU-MIMO	UL and DL supported with up to 8 spatial streams (SS), 4 users with 1 or 2 SS	up to 16 spatial streams and an enhanced sounding protocol (MAC)	increased throughput
Compatibility	supports backward compatibility in the 2.4 GHz and 5 GHz band	enhanced preamble that adds a universal SIG field to indicate the PHY version	easier/better coexistence with future Wi-Fi generations
6 GHz ­band support	yes	yes	availability of more spectrum and afew 320 MHz channels
Preamble puncturing	yes	slight modifications to some spectral emission masks when puncturing is used	efficient spectrum utilization, better coexistence with other users in the band
Multilink operation	no	new to IEEE802.11be	improved spectral efficiency, load balancing, higher data throughput, lower latency
Restricted target ­wake time ­(TWT)	no	new to IEEE802.11be	enables dedicated service period for low latency traffic
1K block ack (BA)	yes	IEEE802.11be extends the nego tiated block ack buffer size to 1024 bits	more efficient MAC data unit aggre gation and better support for MLO

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