Introduction to 802.11ax High-Efficiency Wireless

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Abstract

802.11ax, also called High-Efficiency Wireless (HEW), has the challenging goal of improving the average throughput per user by a factor of at least 4X in dense user environments. This new standard focuses on implementing mechanisms to serve more users a consistent and reliable stream of data (average throughput) in the presence of many other users. This paper will explore the new mechanisms that will give the popular 802.11ax standard the title of High-Efficiency Wireless.

802.11ax也称为高效无线技术（High-Efficiency Wireless, HEW），在用户密集环境下,802.11ax能至少将每个用户的平均吞吐量提高至以前的4倍。此项标准的重点是实现在许多其他用户在场的情况下为更多用户提供一致和可靠的数据流（平均吞吐量）的机制。本文将探讨802.11ax为啥能被称为HEW中的新技术/机制。

Introduction

In 2015 the iconic car manufacturer, Ferrari, released a new version of its entry-level model: the Ferrari California T. This sleek sports car has a 3.9 liter turbocharged V8 engine capable of generating more than 412 KW (553 Horsepower), good for smashing zero to 100 Km/h (0 to 62 mph) in 3.6 seconds. Keeping the foot on the accelerator will propel this engineering marvel to its top speed of 315 Km/h (196 mph). [1]

2015年，标志性汽车制造商法拉利（Ferrari）发布了其入门级车型的新版本：Ferrari California T。这款时尚跑车配备了一台3.9升涡轮增压V8发动机，能够产生412千瓦（553马力）以上的功率，使得其百公里加速时间适仅需3.6秒。一脚踩在油门上将推动这一工程奇迹达到315公里/小时（196英里/小时）的最高速度。

The Ferrari designers considered many details of the engine, body and interior to make this vehicle a daily driver, while delivering the most precise handling, fluid motion, and performance at breakneck speeds. That kind of design would certainly make for an exhilarating – though significantly shorter – daily commute to the office. However, what good would that red Ferrari convertible be on the heavily congested streets of a large metropolitan area, with mostly stop-and-go traffic?

法拉利设计师考虑了发动机，车身和内饰的许多细节，以使司机能以极快的速度做到最精确的操控、流畅的动作和性能。这么令人兴奋的设计，是不是会显然地降低我们日常去办公室上班的时间呢:)？然而，在一个大都市区拥挤的街道上，对大多是走走停停的交通，红色法拉利敞篷车会有什么好处呢？

Today many people find themselves in that kind of situation. Perhaps not as privileged to be driving an Italian sports car, but able to enjoy blazing fast wireless connectivity links. Consider that the first 802.11b Wi-Fi standard (1999), had a top link speed of 11 Mbps. A good first step, but significantly slower than a wired connection. Then a few years later the 802.11a/g revision (2003) increased the speed to 54 Mbps with the introduction of Orthogonal Frequency Division Multiplexing (OFDM) technology.

今天，许多人发现自己处于这种情况。也许没有驾驶意大利跑车那么幸运，但却能够享受高速无线连接。考虑到第一版 Wi-Fi标准（802.11b, 1999），其最高链路速度为11 Mbps。良好的第一步，但比有线连接慢得多。几年后，802.11a/g修订版（2003年）引入了正交频分复用（OFDM）技术后，无线链路速度提高到54 Mbps。

The next link speed improvement came with 802.11n (2009) presenting users with single stream links up to 150 Mbps. The 802.11ac revision of the standard (2013) brought with it the possibility of link speeds around 866 Mbps on a single spatial stream with wider channels (160MHz) and higher modulation orders (256-QAM). Using the specified maximum number of 8 spatial streams, this engineering marvel would, in theory, reach its top speed of 6.97 Gbps. In theory, using 802.11ac is the equivalent of replacing your bicycle or even your family sedan with a souped-up Ferrari.

Wi-Fi 4（802.11n, 2009）带来了下一个链接速度改进，单用户的最高链路速度提高到了150 Mbps。Wi-Fi 5标准（802.11ac, 2013）引入了160Mhz 的更大带宽信道和更高阶调制方式（256-QAM）, 使其单个空间流的链路速度最高能达到866 Mbps。在使用指定的最大8个空间流(小何:这里指MIMO的阶数为8,即单个AP所能达到的链路速度上限)，理论上将达到6.97 Gbps的顶级速度。从理论上讲，使用802.11ac相当于用加大马力的法拉利取代自行车甚至家庭轿车。

However, speeds approaching 7 Gbps might only be achievable in the controlled race-track environment of the RF lab. In reality, users commonly experience frustratingly slow data traffic when trying to check their email on a public Wi-Fi at a busy airport terminal. A new revision of the IEEE 802.11 wireless LAN standard – 802.11ax – seeks to remedy exactly this precise situation.

然而，接近7 Gbps的速度可能只有在射频实验室的受控(赛道)环境中才能实现。事实上，用户在繁忙的机场航站楼尝试在公共Wi-Fi上查看电子邮件时，通常会遇到令人沮丧的缓慢数据速度。IEEE 802.11无线局域网标准的新修订版802.11ax-旨在确切的纠正这一情况。

802.11ax, also called High-Efficiency Wireless (HEW), has the challenging goal of improving the average throughput per user by a factor of at least 4X in dense user environments. Looking beyond the raw link speeds of 802.11ac, this new standard implements several mechanisms to serve more users consistent and reliable data throughput in crowded wireless environments.

Wi-Fi 6(802.11ax)中最具挑战性的目标是在密集用户环境中将每个用户的平均吞吐量提高至少4倍。除了802.11ac的原始链路速度之外，Wi-Fi 6还实现了多种机制，以便在拥挤的无线环境中为更多用户提供一致和可靠的数据吞吐量。

Key Features and Applications

High-Efficiency Wireless includes the following key features:

Wi-Fi 6(802.11ax)包括以下主要特色改进：

• Backwards compatible with 802.11a/b/g/n/ac 向后兼容802.11a/b/g/n/ac

• Increase 4X the average throughput per user in high-density scenarios, such as train stations, airports and stadiums.在高密度场景中，如火车站、机场和体育场馆，将每个用户的平均吞吐量提高4倍。

• Data rates and channel widths similar to 802.11ac, with the exception of new Modulation and Coding Sets (MCS 10 and 11) with 1024-QAM.数据速率和信道宽度类似于802.11ac，带来了更高阶调制方式(1024-QAM)和编码集（MCS 10和11）。

• Specified for downlink and uplink multi-user operation by means of MU-MIMO and Orthogonal Frequency Division Multiple Access (OFDMA) technology.通过MU-MIMO和正交频分多访问（OFDMA）技术指定了下行链路和上行链路多用户操作。

• Larger OFDM FFT sizes (4x larger), narrower subcarrier spacing (4X closer), and longer symbol time (4X) for improved robustness and performance in multipath fading environments and outdoors.更大的OFDM FFT大小（大4倍）、更窄的子载波间隔（近4倍）和更长的符号时间（4倍），可在多径衰落环境和户外提高鲁棒性和性能。

• Improved traffic flow and channel access 改进了流控和信道访问

• Better power management for longer battery life带来了更好的电源管理和更长的电池寿命

High-Efficiency Wireless also serves the following target applications:

Wi-Fi 6(802.11ax)还可用于以下目标应用：

• Cellular data offloading: By 2020, 38.1 exabytes Wi-Fi offload traffic will be generated each month, continuing to exceed projected monthly mobile/cellular traffic (30.6 exabytes). [2] That’s equivalent to moving more than 6000 Blue-ray movies per minute on these networks.

  蜂窝数据卸载：到 2020 年，每月将产生 38.1 EB的 Wi-Fi 卸载流量，继续超过预计的每月移动/蜂窝流量（30.6 EB）。 这相当于在这些网络上每分钟移动 6000 多部蓝光电影。

• Environments with many access points and a high-concentration of users with heterogeneous devices (Airport Wi-Fi ≠ Home Wi-Fi)

  具有多个接入点且用户高度集中的异构设备的环境(机场Wi-Fi 与家庭Wi-Fi 不同)

• Outoors/outdoors mixed environments.户内/户外混合环境。

Figure 1. Example scenario of a stadium with high user density and mixed environments targeted for 802.11ax deployment

图1。用于802.11ax部署的具有高用户密度和混合环境的体育场的示例场景

Current Challenges to Wi-Fi Throughput in Dense Environments

The 802.11 protocol uses a carrier sense multiple access (CSMA) method in which the wireless stations (STA) first sense the channel and attempt to avoid collisions by transmitting only when they sense the channel to be idle. That is, when they don’t detect any 802.11 signals. When an STA hears another one, it waits for a random amount of time for that STA to stop transmitting before listening again for the channel to be free. When they’re able to transmit, STAs transmit their whole packet data.

802.11 协议使用载波侦听多路访问 (CSMA) 方法，其中无线节点 (STA) 首先侦听信道 并 仅在所侦听信道空闲时尝试进行传输来避免设备间冲突。 也就是说，当他们没有检测到任何 802.11 信号时。 当一个 STA 听到另一个STA时，在再次侦听该信道是否空闲之前，它会等待一个随机的时间，先让另一个STA 停止传输。 当它们能够传输时，STA 会传输它们的整个数据包数据。

Wi-Fi STAs may use Request to Send/Clear to Send (RTS/CTS) to mediate access to the shared medium. The Access Point (AP) only issues a CTS packet to one STA at a time, which in turn sends its entire frame to the AP. The STA then waits for an acknowledgement packet (ACK) from the AP indicating that it received the packet correctly. If the STA doesn’t get the ACK in time, it assumes the packet collided with some other transmission, moving the STA into a period of binary exponential backoff. It will try to access the medium and re-transmit its packet after the backoff counter expires.

Wi-Fi STA可使用请求发送（Request to Send， RTS）/清除发送（Clear to Send， CTS）来调解对共享介质的访问。接入点（AP）一次仅向一个STA发送CTS分组，该STA反过来向AP发送其整个帧。STA然后等待来自AP的确认数据包（acknowledgement packet， ACK），ACK指示AP已正确接收该数据包。如果STA没有及时获得ACK，它将假定数据包与其他传输相撞，从而将STA移动到二进制指数退避（BEB， Binary Exponential Back off）的时段中。在退避计数器过期后，它将再次尝试访问介质并重新传输其数据包。（小何：注意，这里指的介质有点拗口，其实指的就是信道）

Figure 2. Clear Channel Assessment Protocol

图2.CCA(Clear Channel Assessment)协议

Although this Clear Channel Assessment and Collision Avoidance protocol serves well to divide the channel somewhat equally among all participants within the collision domain, its efficiency decreases when the number of participants grows very large. Another factor that contributes to network inefficiency is having many APs with overlapping areas of service. Figure 3 depicts a user (User 1) that belongs to the Basic Service Set (BSS, a set of wireless clients associated to an AP) on the left. User 1 would contend for access to the medium with other users in its own BSS and then exchange data with its AP. However, this user would still be able to hear traffic from the overlapping BSS on the right.

尽管CCA和冲突避免协议能够在冲突域内的所有参与者之间公平地划分信道，但当参与者数量增加时，其效率会降低。导致网络效率低下的另一个因素是有许多AP具有重叠的服务区域。图3在左侧描述了属于基本服务集（Basic Service Set ,BSS，与AP关联的一组无线客户端）的用户（用户1）。用户1将与其自己的BSS中的其他用户争用对介质的访问权，然后与其AP交换数据。但是，该用户仍然能够听到来自右侧重叠BSS（overlapping BSS, OBSS）的流量。

Figure 3. Medium access inefficiency from overlapping BSS

图3。重叠BSS导致的介质访问效率低下

In this case, traffic from the OBSS would trigger User 1’s backoff procedure. This kind of situation results in users having to wait longer for their turn to transmit, effectively lowering their average data throughput.

在这种情况下，来自OBSS 的通信过程将触发用户1的退避过程。这种情况导致用户不得不等待更长的时间等待轮到他们传输，从而事实上降低了他们的平均数据吞吐量。

A third factor to consider is the shared use of wider channels. For example, for 802.11ac operation in North America there is only one 160 MHz channel available, and in Europe only two.

要考虑的第三个因素是共享频带更宽的信道。例如，对于北美的802.11ac中，只有一个160 MHz信道可用，而在欧洲只有两个。

Figure 4. Example 802.11ax channel allocation on the 5GHz band

图4. 5GHz频段上的示例802.11ax通道分配

Planning dense coverage with a reduced number of channels becomes very difficult, forcing network managers to reuse channels in nearby cells. Without careful and deliberate power management, users will experience co-channel interference, which degrades performance and negates much of the expected gain from the wider channels. This is especially true for the top data rates of MCS 8, 9, 10, and 11, which are much more susceptible to low signal to noise ratio. Also, on the current implementation of 802.11 networks a 20 MHz channel overlapping an 80 MHz channel will basically render the 80 MHz channel useless, while a user transmits on the narrower channel. Implementing 802.11ac’s Channel Sharing in a high density network compromises the gains of the 80 MHz channel for transmissions on a 20 MHz channel.

规划密集覆盖和减少信道数量变得非常困难，这迫使网络管理员在附近小区中重用信道。如果不进行仔细和深思熟虑的电源管理，用户将遇到同信道干扰，这会降低性能，并抵消更宽信道带来的大部分预期收益。对于MCS 8,9,10和11的顶部数据速率尤其如此，它们更容易受到低信噪比的影响。此外，在802.11网络的当前实现中，20 MHz信道与80 MHz信道重叠基本上会使80 MHz信道无效，使得用户不得不在较窄的信道上传输。在高密度网络中实现802.11ac的信道共享会损害在20 MHz信道上传输的80 MHz信道的增益。

PHY Mechanisms for High Efficiency

PHY Changes

The 802.11ax specification introduces significant changes to the physical layer of the standard. However, it maintains backward compatibility with 802.11a/b/g/n and /ac devices, such that an 802.11ax STA can send and receive data to legacy STAs. These legacy clients will also be able to demodulate and decode 802.11ax packet headers – though not whole 802.11ax packets – and backoff when an 802.11ax STA is transmitting.

802.11ax规范对标准的物理层进行了重大更改。但是，它保持与802.11a/b/g/n和/ac设备的向后兼容性，以便802.11ax STA可以向传统STA发送和接收数据。这些传统客户端还能够解调和解码802.11ax数据包头（尽管不是整个802.11ax数据包），并在802.11ax STA传输时退避。

The following table highlights the most important changes to this revision of the standard, in contrast to the current 802.11ac implementation:

下表突出显示了与当前802.11ac实施相比，本标准修订版的最重要更改：

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Table 1. 802.11ac vs. 802.11ax

Notice that the 802.11ax standard will operate in both the 2.4 GHz and 5 GHz bands. The specification defines a four times larger FFT, multiplying the number of subcarriers. However, one critical change with 802.11ax is that the subcarrier spacing has been reduced to one fourth the subcarriers spacing of previous 802.11 revisions, preserving the existing channel bandwidths.

请注意，802.11ax标准将在2.4 GHz和5 GHz频段上运行。该规范定义了四倍于子载波数的FFT。然而，802.11ax的一个关键变化是，子载波间隔已减少到先前802.11版本的子载波间隔的四分之一，从而保留了现有的信道带宽(小何:兼容性设计)。

Figure 5. Narrower sub-carrier spacing

图5。较窄的子载波间隔

The OFDM symbol duration and cyclic prefix also increased 4X, keeping the raw link data rate the same as 802.11ac, but improving efficiency and robustness in indoor/outdoor and mixed environments. Nevertheless, the standard does specify 1024-QAM and smaller cyclic prefix ratios for indoor environment, which will increase the maximum data rate.

OFDM符号持续时间和循环前缀(cyclic prefix, CP)也增加了4倍，使原始链路数据速率保持与802.11ac相同，但在室内/室外和混合环境中提高了效率和鲁棒性。然而，该标准确实为室内环境规定了1024-QAM和更小的循环前缀比率(小何:注意在计算CP对OFDM影响时算的是CP/duration 的比例,而不是纯粹CP时间)，这将增加最大数据速率。

Beamforming

802.11ax will employ an explicit beamforming procedure, similar to that of 802.11ac. Under this procedure, the beamformer initiates a channel sounding procedure with a Null Data Packet. The beamformee measures the channel and responds with a beamforming feedback frame, containing a compressed feedback matrix. The beamformer uses this information to compute the channel matrix, H. The beamformer can then use this channel matrix to focus the RF energy toward each user.

802.11ax将采用与802.11ac类似的显式波束形成过程。在此过程下，波束形成器启动具有空数据分组的信道探测过程。波束形成器测量信道并使用包含压缩反馈矩阵的波束形成反馈帧进行响应。波束形成器使用该信息计算信道矩阵H。然后，波束形成器可以使用该信道矩阵将射频能量聚焦到每个用户。

Multi-User Operation: MU-MIMO and OFDMA

The 802.11ax standard has two modes of operation:

802.11ax标准有两种操作模式：

Single User: in this sequential mode the wireless STAs send and receive data one at a time once they secure access to the medium, as this paper has described above.

单用户：正如上文所述, 在这种顺序模式下，一旦无线STA安全地访问介质，就会将数据发送和接收数据。

Multi-User: this mode allows for simultaneous operation of multiple non-AP STAs. The standard divides this mode further into Downlink and Uplink Multi-user.

多用户：此模式允许同时操作多个非AP STA。标准将此模式划分为下行链路和上行链路多用户。

• Downlink multi-user refers to data that the AP serves to multiple associated wireless STAs at the same time. The existing 802.11ac standard already specifies this feature.下行链路多用户是指AP同时为多个关联的无线STA服务的数据。现有的802.11ac标准已有此功能。
• Uplink multi-user involves simultaneous transmission of data from multiple STAs to the AP. This is new functionality of the 802.11ax standard, which did not exist in any of the previous versions of the Wi-Fi standard.上行链路多用户涉及从多个STA到AP的同时传输数据。这是802.11ax标准的新功能，该标准不存在于Wi-Fi标准的任何版本中。

Under the Multi-User mode of operation, the standard also specifies two different ways of multiplexing more users within a certain area: Multi-User MIMO and Orthogonal Frequency Division Multiple Access (OFDMA). For both of these methods, the AP acts as the central controller of all aspects of multi-user operation, similar to how an LTE cellular base station controls the multiplexing of many users. An 802.11ax AP can also combine MU-MIMO with OFDMA operation.

在多用户操作模式下，标准还规定了两种不同的方式多路复用更多用户在某个区域内：多用户MIMO和正交频分多址（OFDMA）。对于这两种方法，AP充当多用户操作的所有方面的中央控制器，类似于LTE蜂窝基站如何控制多个用户的复用。 802.11ax AP还可以将MU-MIMO与OFDMA操作相结合。

Multi-User MIMO

Borrowing from the 802.11ac implementation, 802.11ax devices will use beamforming techniques to direct packets simultaneously to spatially diverse users. That is, the AP will calculate a channel matrix for each user and steer simultaneous beams to different users, each beam containing specific packets for its target user. 802.11ax supports sending up to eight multi-user MIMO transmissions at a time, up from four for 802.11ac. Also, each MU-MIMO transmission may have its own Modulation and Coding Set (MCS) and a different number of spatial streams. By way of analogy, when using MU-MIMO spatial multiplexing, the AP could be compared to an Ethernet switch that reduces the collision domain from a large computer network to a single port.

借鉴于802.11ac，802.11ax设备将使用波束成形技术将数据包同时定向到空间不同的用户。也就是说，AP将计算每个用户的信道矩阵，并将同时的波束转向不同的用户，每个波束包含其目标用户的特定分组。802.11ax支持一次最多发送八个多用户MIMO传输，而802.11ac支持四个。此外，每个MU-MIMO传输可以具有其自己的调制和编码集（MCS）和不同数量的空间流。作为类比，当使用MU-MIMO空间复用时，AP可以被比作以太网交换机，它将冲突域从大型计算机网络减少到单个端口。

As a new feature in the MU-MIMO Uplink direction, the AP will initiate a simultaneous uplink transmission from each of the STAs by means of a trigger frame. When the multiple users respond in unison with their own packets, the AP applies the channel matrix to the received beams and separates the information that each uplink beam contains. The AP may also initiate Uplink multi-user transmissions to receive beamforming feedback information from all participating STAs as shown in Figure 7.

作为MU-MIMO上行链路方向中的新特征，AP将通过触发帧从每个STA发起同时上行链路传输。当多个用户与自己的数据包有一致响应时，AP将信道矩阵应用于接收的波束并分离每个上行链路波束包含的信息。AP还可以发起上行链路多用户传输，以从所有参与的STA的波束成形反馈信息，如图7所示。

Figure 6. AP using MU-MIMO beamforming to serve multiple users located in spatially diverse positions

图6。AP使用MU-MIMO波束形成服务于位于空间不同位置的多个用户

Figure 7. A beamformer (AP) requesting channel information for MU-MIMO operation

图7。为MU-MIMO操作请求信道信息的波束形成器（AP）

Multi-User OFDMA

The 802.11ax standard borrows a technological improvement from 4G cellular technology to multiplex more users in the same channel bandwidth: Orthogonal Frequency-Division Multiple Access (OFDMA). Building on the existing orthogonal frequency-division multiplexing (OFDM) digital modulation scheme that 802.11ac already uses, the 802.11ax standard further assigns specific sets of subcarriers to individual users. That is, it divides the existing 802.11 channels (20, 40, 80 and 160 MHz wide) into smaller sub-channels with a predefined number of subcarriers. Also borrowing from modern LTE terminology, the 802.11ax standard calls the smallest subchannel a Resource Unit (RU), with a minimum size of 26 subcarriers.

802.11ax标准借鉴了4G蜂窝技术的一项技术改进(小何:这里应当是4G+或指5G)，在同一信道带宽内复用更多用户：正交频分多址（OFDMA）。基于802.11ac已经使用的现有正交频分复用（OFDM）数字调制方案，802.11ax标准进一步将特定的子载波集分配给各个用户。也就是说，它将现有的802.11信道（20、40、80和160 MHz宽）划分为具有预定义子载波数量的较小子信道。同样借用现代LTE术语，802.11ax标准调用最小的子信道资源单元（Resource Unit, RU），最小大小为26个子载波。

Based on multi-user traffic needs, the AP decides how to allocate the channel, always assigning all available RUs on the downlink. It may allocate the whole channel to only one user at a time – just as 802.11ac currently does – or it may partition it to serve multiple users simultaneously (see Figure 8).

基于多用户业务需求，AP决定如何分配信道，始终分配下行链路上的所有可用RU。它可以一次仅将整个信道分配到一个用户—就像802.11ac目前所做的那样-或者将其分区以同时服务多个用户（见图8）。

Figure 8. A single user using the channel Vs. multiplexing various users in the same channel using OFDMA

图8。使用信道的单个用户与使用OFDMA在同一信道中复用不同用户

In dense user environments where many users would normally contend inefficiently for their turn to use the channel, this OFDMA mechanism now serves them simultaneously with a smaller – but dedicated – subchannel, thus improving the average throughput per user. Figure 9illustrates how an 802.11ax system may multiplex the channel using different RU sizes. Note that the smallest division of the channel accommodates up to 9 users for every 20MHz of bandwidth. [4]

在密集的用户环境中，许多用户通常会低效地争夺使用该信道的机会，这种OFDMA机制现在可以同时为他们提供一个更小但专用的子信道，从而提高每个用户的平均吞吐量。图9说明了802.11ax系统如何使用不同的RU大小多路复用信道。请注意，信道的最小划分每20MHz带宽最多可容纳9个用户。

Figure 9. Subdividing Wi-Fi channels using various Resource Unit sizes

图9。使用各种资源单元大小细分Wi-Fi通道

The following table shows the number of users that can now get frequency-multiplexed access when the 802.11ax AP and STAs coordinate for MU-OFDMA operation.

下表显示了当802.11ax AP和STA协调MU-OFDMA操作时，现在可以获得频率复用访问的用户数。

Table 2. Total number of RUs by channel bandwidth

表2。按通道带宽列出的RU总数

Multi-User Uplink Operation

To coordinate uplink MU-MIMO or uplink OFDMA transmissions the AP sends a trigger frame to all users. This frame indicates the number of spatial streams and/or the OFDMA allocations (frequency and RU sizes) of each user. It also contains power control information, such that individual users can increase or reduce their transmitted power, in an effort to equalize the power that the AP receives from all uplink users and improve reception of frames from nodes farther away. The AP also instructs all users when to start and stop transmitting. As Figure 10depicts, the AP sends a multi-user uplink trigger frame that indicates to all users the exact moment at which they all start transmitting, and the exact duration of their frame, to ensure that they all finish transmitting simultaneously as well. Once the AP receives the frames from all users, it sends them back a block ACK to finish the operation.

为了协调上行链路MU-MIMO或上行链路OFDMA传输，AP向所有用户发送触发帧。该帧指示每个用户的空间流的数量和/或OFDMA分配（频率和RU大小）。它还包含功率控制信息，使得各个用户可以增加或减少其发射功率，以努力均衡AP从所有上行链路用户接收的功率，并改善来自更远节点的帧的接收。AP还指示所有用户何时开始和停止传输。如图10所示，AP发送一个多用户上行链路触发帧，该帧向所有用户指示他们开始传输的确切时刻，以及他们帧的确切持续时间，以确保他们同时完成传输。一旦AP从所有用户接收到帧，它会将其发送回块ACK以完成操作。

Figure 10. Coordinating uplink multi-user operation

One of the main goals of the 802.11ax is to support 4X higher average per-user throughput in dense user environments. With that goal in mind, the standard designers have specified that 802.11ax devices support Downlink and Uplink MU-MIMO operation, MU-OFDMA operation, or both for an even larger number of simultaneous users.

802.11ax的主要目标之一是在密集用户环境中支持4倍的平均每用户吞吐量。考虑到这一目标，标准设计者已经指定802.11ax设备支持下行和上行MU-MIMO操作、MU-OFDMA操作，或者对于更多的同时用户同时支持这两种操作。

MAC Mechanisms for High Efficiency

Spatial Reuse with Color Codes

To improve the system level performance and the efficient use of spectrum resources in dense deployment scenarios, the 802.11ax standard implements a spatial reuse technique. STAs can identify signals from overlapping Basic Service Sets (BSS) and make decisions on medium contention and interference management based on this information.

为了在密集部署场景中提高系统级性能和频谱资源的有效利用，802.11ax标准实施了空间重用技术。STA可以识别来自重叠基本服务集（BSS）的信号，并基于此信息做出媒体竞争和干扰管理决策。

When an STA that is actively listening to the medium detects an 802.11ax frame, it checks the BSS color bit or MAC address in the MAC header. If the BSS color in the detected PPDU is the same color as the one that its associated AP has already announced, then the STA considers that frame as an intra-BSS frame.

当主动监听信道的STA检测到802.11ax帧时，它会检查MAC报头中的BSS颜色位或MAC地址。如果检测到的PPDU中的BSS颜色与其相关AP已经宣布的颜色相同，则STA将该帧视为BSS帧。

However, if the detected frame has a different BSS color than its own, then the STA considers that frame as an inter-BSS frame from an overlapping BSS. The STA then treats the medium as BUSY only during the time it takes the STA to validate that the frame is from an inter-BSS, but not longer than the time indicated as the length of the frame’s payload.

然而，如果检测到的帧具有与其自身不同的BSS颜色，则STA将该帧视为来自重叠BSS的BSS间帧。然后，STA仅在STA验证帧来自BSS间的时间内将媒体视为忙，但不超过指示为帧有效载荷长度的时间。

The standard still has to define some of the mechanisms for ignoring traffic from overlapping BSSs, but the implementation could include raising the clear channel assessment signal detection (SD) threshold for inter-BSS frames, while maintaining a lower threshold for intra-BSS traffic (see Figure 11). That way, traffic from neighboring BSS wouldn’t create unnecessary channel access contention.

该标准仍然必须定义一些忽略来自重叠BSS的流量的机制，但实现可能包括提高BSS帧间的清晰信道评估信号检测（SD）阈值，同时保持BSS内流量的较低阈值（见图11）。这样，来自相邻BSS的流量就不会产生不必要的信道访问争用。

Figure 11. Using Color Codes for Clear Channel Assessment

When 802.11ax STAs use the color code based CCA rule, they are also allowed to adjust the OBSS signal detection threshold together with transmit power control. This adjustment improves system level performance and the use of spectrum resources. Furthermore, 802.11ax STAs can adjust CCA parameters, such as the energy detection level and the signal detection level.

当802.11ax sta使用基于颜色代码的CCA规则时，还允许它们与发射功率控制一起调整OBSS信号检测阈值。这种调整提高了系统级性能和频谱资源的使用。此外，802.11ax sta可以调整CCA参数，例如能量检测电平和信号检测电平。

In addition to using CCA to determine if the medium is idle or busy for the current frame, the 802.11 standard employs a Network Allocation Vector (NAV) – a timer mechanism that maintains a prediction of future traffic – for STAs to indicate the time required for the frames immediately following the current frame. The NAV acts as a virtual carrier sense that ensures medium reservation for frames critical to operation of the 802.11 protocol, such as control frames, and data and ACKs following an RTS/CTS exchange.

除了使用CCA来确定媒体在当前帧中是空闲还是忙外，802.11标准还为STA使用网络分配向量（NAV）——一种维持未来流量预测的计时器机制——来指示紧跟当前帧之后的帧所需的时间。NAV充当虚拟载波感知，确保对802.11协议的操作至关重要的帧（如控制帧）以及RTS/CTS交换后的数据和ACK的介质保留。

Figure 12. Example of MU PPDU exchange and NAV setting

The 802.11 Task Group working on High-Efficiency Wireless will possibly include not just one NAV field, but two different NAVs to the 802.11ax standard. Having an intra-BSS NAV and an inter-BSS NAV could help STAs to predict traffic within their own BSS and feel free to transmit when they know the state of overlapping traffic.

致力于高效无线的802.11任务组可能不仅包括一个导航字段，还包括802.11ax标准的两个不同导航字段。拥有BSS内部导航和BSS内部导航有助于STA预测其自身BSS内的流量，并在了解重叠流量的状态时自由传输。

Power-saving with Target Wake Time

An 802.11ax AP can negotiate with the participating STAs the use of the Target Wake Time (TWT) function to define a specific time or set of times for individual stations to access the medium. The STAs and the AP exchange information that includes an expected activity duration. This way the AP controls the level of contention and overlap among STAs needing access to the medium. 802.11ax STAs may use TWT to reduce energy consumption, entering a sleep state until their TWT arrives. Furthermore, an AP can additionally devise schedules and deliver TWT values to STAs without individual TWT agreements between them. The standard calls this procedure Broadcast TWT operation (see Figure 13).

802.11ax AP可以与参与的sta协商使用目标唤醒时间（Target Wake Time， TWT）功能来定义单个站点访问介质的特定时间或时间集。STA和AP交换包括预期活动持续时间的信息。通过这种方式，AP控制需要访问介质的STA之间的争用和重叠级别。在802.11ax中，STA可使用TWT来降低能耗，进入睡眠状态，直到TWT到达。此外，AP还可以另外设计计划并将TWT值传递给STA，而不会与它们之间的单个TWT协议。标准将此过程称为广播TWT操作（见图13）。

Figure 13. Example of Target Wake Time Broadcast operation

Conclusion

802.11ax promises to improve the average data throughput per user in dense environments by 4X. One of the biggest enablers of this efficiency is multi-user technology, both in the form of MU-MIMO and MU-OFDMA. This improvement in spectrum use in crowded environments will likely drive 802.11ax market adoption at faster rates than ever before. However, implementing this functionality will present a whole new set of challenges for the scientists, engineers, and technologists in charge of making these engineering marvels a reality.

802.11ax承诺将密集环境中每个用户的平均数据吞吐量提高4倍。这种效率的最大促成因素之一是多用户技术，包括MU-MIMO和MU-OFDMA。在拥挤环境中频谱使用的这种改进可能会以前所未有的速度推动802.11ax的市场采用。 However之后就是广告的接入了，这里不翻咯。