802.11ax简介

简介"/>

802.11ax简介

参考文章：[802.11ax for Dummies Aerohive Special Edition.pdf]
这篇文章写得非常好，可以直接去看英文的，这里我也就翻译了一部分，后面直接粘贴的英文原文。
原版链接找不到了，找到了第2版书籍链接

在现实生活中，除了追求Wi-Fi的传输速度外，另一大挑战就是Wi-Fi的载体容量问题。一个好的WiFi网络需要能满足日益增长的客户端接入数量，并且能提供较好的用户体验。

Wi-Fi简史

IEEE 802家族是由一系列局域网络(local area network, LAN) 技术规格组成，802.11属于其中之一。

802.11协议发展历程

年份	标准	特性
1999年	802.11a	5G频段，最大54Mbps
1999年	802.11b	2.4G频段，5.5Mbps、11Mbps
2003年	802.11g	2.4G频段，最大54Mbps
2009年	802.11n	2.4G频段、5G频段，100Mbps
2013年	802.11ac	5G频段，433~2167Mbps，
2016年	802.11ax	2.4G频段、5G频段，简称WiFi6，最大10Gbps，侧重于efficiency

802.11ax

802.11ax相比于前几代标准，并不只是提升速率，更着重于效益，正如它的修订中描述，high efficiency，而前几代标准发布时修订中说的是high throughput，可以看出802.11ax的侧重点明显不同。

The changes in the 802.11ax standard will improve the way Wi-Fi networks work by leveraging technology that substantially improves capacity, provides better coverage, and even reduces congestion, resulting in a far better user experience overall. It’s Wi-Fi for the real world.

IEEE 802.11ax的目标有：

• Enhancing operation in the 2.4 GHz and 5 GHz band
• Increasing average throughput per station by at least four times in dense deployment scenarios
• Enhancing in both indoor and outdoor environments
• Maintaining or improving power efficiency in stations
• Improving the efficiency of traffic management in a variety of environments

802.11ax, 802.11ac, and 802.11n comparison

802.11ax关键技术

802.11ax通过一个新的channel-sharing capability来更高效的处理客户端稠密请求，通过在APs与Clients之间进行协商唤醒时间调度来节省能源延长电池使用寿命，吞吐量至少达到802.11ac的4倍之上。

MU

Multi-user(MU) 在802.11ax中泛指很多技术点，并不是指某一项具体的技术实现。MU表示AP与多个STA之间可以同时传输数据，不过这也要依赖于它们的具体实现技术。

MU-OFDMA

orthogonal frequency division multiple access(OFDMA)，正交频分复用多址接入技术是802.11ax中新题的技术。允许多用户同时以不同的带宽需求接入。OFDMA是OFDM(orthogonal frequency division multiplexing)的多用户版本，OFDM在802.11a/g/n/ac中使用，只能单用户接入。

不过802.11ax向下兼容，也能支持OFDM。

OFDMA将Wi-Fi信道细分为更小的频率分配，称为resource units(RUs)，AP就将特定的RU分配给不同的client，这样就可以同时允许多个客户端上行和下行数据通信。

OFDMA subdivides a Wi-Fi channel into smaller frequency allocations, called resource units (RUs), thereby enabling an access
point (AP) to synchronize communication (uplink and downlink) with multiple individual clients assigned to specific RUs. By subdividing the channel, small frames (such as streaming video) can be simultaneously transmitted to multiple users in parallel. The simultaneous transmission cuts down on excessive overhead at the medium access control (MAC) sublayer, as well as medium contention overhead. The AP can allocate the whole channel to a single user or partition it to serve multiple users simultaneously, based on client traffic needs.

OFDMA是大多数网络应用的理想选择，可以很好地频率复用、减少延迟、提升效率。

Subcarrier

OFDMA通过快速傅里叶逆变换(Inverse Fast Fourier Transform, IFFT)将一个channel细分更多的子载波(subcarrier)出来。subcarrier空间是正交的，所以尽管他们之间没有保护频带，都不会互相干扰。

802.11ac中OFDM的20MHz有64个312.5kHz subcarriers：

802.11ax引入了时间更长的OFDM symbol time – 12.8毫秒，是传统的symbol time – 3.2毫秒的4倍，所以802.11ax可以细分出更多的子载波，从312.5kHz缩至78.125kHz，在20MHz信道上，OFDMA可以包含256个子载波：

802.11ax中有三种subcarriers

types
data subcarrier	采用和802.11ac相同的调制和编码方案，且新增了两种调制和编码方案，1024正交幅度调制(1024-QAM)
pilot subcarrier	用户发射器和接收器之间同步，不载有调制数据
unused subcarrier	保护性质的子载波或者空子载波

Resource units(RUs)

802.11n/ac的AP与802.11n/ac的STA通信时，OFDM的信道整个frequence都是给每个独立的downlink transmission的。当使用20MHz OFDM信道时，所有的subcarriers都是给每个独立的传输使用的，换句话说，整个20MHz信道只能用于AP与一个STA之间的通信。同样的，对于uplink也是同样的场景，整个20MHz OFDM信道都只能给一个STA与AP之间的uplink通信。

OFDMA的一个channel中包含256个子载波，这些subcarriers可以被分组到不同的subchannels（也叫resource units, RUs）中，这样在OFDMA中，就可以将20MHz信道细分成26、52、106、242这样的子载波单元，分别对应2MHz、4MHz、8MHz、20MHz的信道，如下图所示。

802.11ax的AP可以同时与多个STA进行uplink和downlink通信，比如20MHz信道中，一个802.11ax的AP可以与一个802.11ax的STA在8MHz子信道通信，同时可以在4MHz子信道与另外3个802.1ax的STA通信。

此外，除了20MHz频率信道外，40MHz、80MHz甚至160MHz信道，都可以被细分为更多的RUs。如下图所示。比如如果一个80MHz信道被分为26个子载波RUs单元，那么可以同时与37个802.11ax的STA客户端通过OFDMA技术通信。

8021.11ax的APs与802.11ax的STAs之间可以同时uplink与downlink双向通信，这是之前的Wi-Fi技术不支持的。

MU-MIMO

在802.11ac中，多用户，多输入、多输出技术被引入。multi-user，multiple-input multiple-outpu（MU-MIMO)技术是在同一个channel上允许同时给多个接受者发送数据帧。

802.11ax与802.11ac的MU-MIMO技术的不同点之处在于：

同时能与多少个MU-MIMO客户端通信

802.11ac中限制了一个MU-MIMO group最多有4个clients（只能downlink），而802.11ax在设计的时候就设计支持8 x 8 x 8个同时uplink和downlink的MU-MIMO，也就是同时能支持8个客户端，且有更高的吞吐量。

The minimum RU size for MU-MIMO (downlink or uplink) is 106 subcarriers or greater.

802.11ax可以支持MU-OFDMA和MU-MIMO技术的同时使用，不过这不是很推荐，因为其主要侧重点不一样：

Spatial reuse(BSS coloring)

Wi-Fi是通过无线电频率(radio frequency)来通信，无线电频率是一种半双工介质 ---- 在一个frequency domain或channel上同时只能有一个radio在发射，所以每个人必须轮流交流。

Carrier sense with multiple access collision avoidance (CSMA/CA) is the method used in Wi-Fi networks to ensure that only one radio can transmit on the same channel at any given time. An 802.11 radio will defer transmissions if it hears the physical (PHY) preamble transmissions of any other 802.11 radio at a signal detect (SD) threshold of four decibels (dB) or greater. CSMA/CA consumes a lot of the available bandwidth. This problem is referred to as contention overhead. Unnecessary medium contention overhead that occurs when too many APs and clients hear each other on the same channel is called an overlapping basic service set (OBSS), also commonly referred to as co-channel interference (CCI).

For example, if AP-1 on channel 6 hears the preamble transmission of a nearby AP (AP-2), also transmitting on channel 6, AP-1
will defer and can’t transmit at the same time. Likewise, all the clients associated to AP-1 must also defer transmission if they hear the preamble transmission of AP-2. All these deferrals create medium contention overhead and consume valuable airtime because you have two basic service sets on the same channel that can hear each other, in the manner of OBSS.

In reality, Wi-Fi clients are the primary cause of OBSS interference. if a client associated to AP-2 is transmitting on channel 36, it is possible that AP-1 (and any clients associated to AP-1) will hear the PHY preamble of the client and must defer any transmissions.

Due to the mobile nature of Wi-Fi client devices, OBSS interference isn’t static: It changes as client devices move.

All this congestion and medium contention overhead means that efficiency at the MAC sublayer drops. This is further exacerbated by the fact that the bulk of data frames in a network are small (less than 256 bytes) and OBSS in dense deployments often unnecessarily blocks transmissions. Thus, the average transmission control protocol (TCP) throughput under ideal conditions in legacy a/b/g networks is roughly 40 to 50 percent of advertised data rates, and 60 to 70 percent of advertised data rates in 802.11n/ac networks. To increase capacity in dense environments, frequency reuse between basic service sets needs to be increased.

802.11ax was also tasked with addressing the OBSS challenge by improving spatial reuse, which is often referred to as BSS coloring. BSS coloring is a mechanism, originally introduced in 802.11ah, to address medium contention overhead due to OBSS by assigning a different “color” — a number between 0 and 7 that is added to the PHY header of the 802.11ax frame — to each BSS in an environment.

BSS coloring detects a color bit in the PHY header of an 802.11ax frame transmission. This means that legacy 802.11a/b/g/n clients will not be able to interpret the color bits because they use a different PHY header format.

When an 802.11ax radio is listening to the medium and hears the PHY header of an 802.11ax frame sent by another 802.11ax radio, the listening radio will check the BSS color bit of the transmitting radio. Channel access is dependent on the color detected:

• If the color bit is the same, then the frame is considered an intra-BSS transmission and the listening radio will defer.
• If the color bit is different, then the frame is considered an inter-BSS transmission from an OBSS and the listening radio treats the medium as busy only for the time it took to determine the color bit was different.

802.11ax radios can adjust the carrier sense operation based on the color of the BSS to improve spatial reuse efficiency and performance. Depending on the BSS from which the traffic is generated, the station can use different sensitivity thresholds to transmit or defer. This results in higher overall performance. Using adaptive clear channel assessment (CCA), an 802.11ax radio can raise the SD threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS frames. BSS coloring can thus potentially decrease the channel contention problem that is symptomatic of existing low SD thresholds.