Wi-Fi 6详解：802.11ax高效率WLAN技术教程

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"这篇文档是关于IEEE 802.11ax标准的教程，重点关注在高密度场景下提升无线局域网(WLAN)的每区域吞吐量。该标准，也被称为WiFi 6，旨在实现10Gbps的速度，并在2016年和2017年发布了D1.0和D2.0草案。本文档将基于更成熟的D3.0版本，介绍802.11ax的关键创新，如新的正交频分多址(OFDMA)随机访问方法和空间频率复用技术，以及物理层增强、多用户多输入多输出(MU-MIMO)扩展和电源节省方面的改进。" 正文： WiFi 6，正式名称为IEEE 802.11ax，是无线网络技术的最新迭代，它旨在显著提高无线局域网的性能，特别是在高密度环境（如体育馆、购物中心或办公室）中。自1999年的第一个IEEE 802.11标准（即最初的2Mbps无线局域网标准）以来，WiFi技术经历了多次升级，而802.11ax则是在2014年开始研发，目标是提升每区域的吞吐量。 802.11ax标准的两个早期草案，D1.0和D2.0，分别于2016年底和2017年发布，为最终版本奠定了基础。D3.0版本进一步成熟，提供了更多细节和改进，使得该标准更加完善。本教程文档特别关注了802.11ax的几个关键技术创新。首先，引入了一种基于OFDMA的新随机访问方法，这是一种允许多个设备同时在不同子载波上通信的技术，提高了频谱效率，减少了延迟，尤其适合物联网(IoT)设备和大规模并发连接。其次，802.11ax采用了先进的空间频率复用技术，允许在同一时隙内为多个用户分配不同的频率资源，这极大地提高了频谱利用率和系统容量。结合MU-MIMO扩展，802.11ax不仅支持下行链路的MU-MIMO（与802.11ac相比），还支持上行链路，进一步优化了多用户环境中的数据传输。物理层的增强也是802.11ax的一个重要方面，包括更高的调制阶数（如1024-QAM），这使得每个信号符号可以携带更多的信息，从而提升了数据速率。此外，目标唤醒时间(TWT)功能允许设备更有效地管理其睡眠和唤醒模式，减少了能源消耗，这对于电池驱动的设备尤其有利。 802.11ax还引入了其他优化措施，如窄带传输、更高密度的编码方案和增强的调度算法，这些都旨在提高网络的效率和用户体验。802.11ax通过一系列创新技术，为现代高需求的无线环境提供了解决方案，确保了更高效、更可靠且节能的无线连接。总结来说，这篇教程文档深入浅出地介绍了802.11ax标准的主要特性，包括OFDMA、MU-MIMO、物理层增强和电源管理，为读者提供了理解这一先进无线技术的全面视角。对于希望深入了解WiFi 6及其在高密度环境中的应用的专业人士，这是一个宝贵的资源。

200 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019

the decoding of at least some packets in case of short noise

bursts, at the expense of slightly increased overhead.

Since contention-based channel access inevitably leads to

collisions, from the very beginning IEEE 802.11 tried to add

various contention-free channel access mechanisms to the stan-

dard. Both the “historical” Point coordinated function (PCF,

obsolete now) and the subsequent Hybrid Controlled Channel

Access (HCCA) allow an AP to access the channel without

contention. Channel access coordination is accomplished by

introducing an Interframe Space called PIFS (PCF InterFrame

Space) which, being shorter than the DIFS (Distributed coor-

dination function InterFrame Space) used by the remaining

STAs, permits the AP to acquire the channel access with-

out any contention, so as to transmit data or poll the STAs

and grant them channel access. In practice, contention-free

access techniques have seen a very marginal deployment,

especially because of their inefﬁciency in scenarios when sev-

eral APs work in the same area. Indeed, if several APs use

PIFS, their transmissions will start simultaneously and col-

lide. This problem is partially addressed in the HCCA TXOP

Negotiation mechanism introduced in 802.11aa. The mecha-

nism allows various APs to use different time intervals for

transmission. Unfortunately, HCCA TXOP Negotiation can

only avoid collisions between APs which can communicate

with each other. Moreover, it does not reduce the collision

probabilities between an AP and the alien STAs, which still

can use random access.

The IEEE 802.11 Working Group has historically put a sig-

niﬁcant effort to improve the Quality of Service (QoS) in

Wi-Fi networks. Speciﬁcally, the 802.11e amendment intro-

duces Enhanced Distributed Channel Access (EDCA) and

HCCA which distinguish voice, video, best effort and back-

ground trafﬁc and serve them differently. While EDCA just

assigns different priorities to these types of trafﬁc, the sophis-

ticated HCCA allows an AP to schedule transmissions taking

into account speciﬁc QoS requirements, like the delay bound,

the packet loss ratio, or the required bandwidth. However,

determining exact requirements is a non trivial task, and

arguably another key reason behind the scarce deployment of

the contention-free HCCA.

For many devices which use Wi-Fi (e.g., laptops and smart-

phones) power consumption is an important issue. In 802.11

networks, power management is based on alternating between

two states: awake and doze. In the awake state, a STA can

transmit and receive frames, while in the doze state, its radio

is switched off. An active STA is always awake, while a

power-saving (PS) STA alternates between these states. The

AP buffers data destined for PS STAs until the STA wakes

up and retrieves it. Many amendments introduce new power-

saving features, but most of them are related to switching off

the radio for a rather long time, i.e., for hundreds of mil-

liseconds or even for seconds. Some of them require a PS

STA to contend for the channel if it wants to retrieve data

from the AP. Such methods are inefﬁcient in dense environ-

ments because of collisions, huge overhead and large delays.

Some other methods allow an AP and a PS STA to schedule

a series of times when the STA retrieves data from the AP.

The period of the series depends on the QoS requirements.

The tight dependence of these methods with HCCA function-

ality — speciﬁcally with the Trafﬁc Speciﬁcation (TSPEC)

information element which parametrizes QoS requirements —

prevents their usage in consumer electronics.

Finally, the 802.11ac amendment [17]–[19] was introduced

mainly with the purpose of signiﬁcantly increasing the data

rate of a 10x factor with respect to 802.11n. Besides increas-

ing the number of spatial streams up to 8, 802.11ac addresses

the problem of how to cope with terminals that, for obvious

manufacturing reasons, could not deploy more than 1 or 2

antennas. To this purpose, the 802.11ac ﬁrst introduces the

DL MU-MIMO, which allows an AP to assign various DL

spatial streams to different STAs — the UL MU transmis-

sion was postponed to subsequent standards owing to the tight

synchronization requirements which would have required a

signiﬁcant re-design. Additionally, 802.11ac widens the trans-

mission bands up to 160 MHz (also exploting non-contiguous

80+80Mhz channels) and increases the constellation order to

256-QAM, which raises data rates up to 7 Gbps. To reduce

the header-induced overhead at such high data rates, the

amendment increases the maximal length of a frame from

65 535 (802.11n) to 4 692 480 octets. Nevertheless, for

short packets, such as instant messages, Web requests, TCP

acknowledgments, etc. the channel is still used inefﬁciently.

B. Main Features of 802.11ax

Similarly to the previous amendments that improve the

nominal bit rates, 802.11ax contains a new PHY protocol

with higher modulation and coding schemes. In contrast to

802.11ac, 802.11ax does not increase the number of the

MIMO spatial streams and does not widen the channel. Thus

the nominal data rates are increased up to 9.6 Gbps, which is

just 37% higher than that of 802.11ac (rather small compared

to the 10x growth of 802.11n or 802.11ac!) [20]. The desired

increase of the user throughput is achieved by more efﬁcient

spectrum usage.

The key feature of 802.11ax is the adoption of an OFDMA

approach, an approach widely used in cellular networks, but

brand new in Wi-Fi. The rationale is that the very wide chan-

nels (80 MHz, 80+80 MHz and 160 MHz) introduced by

802.11ac suffer from frequency selective interference, which

signiﬁcantly impairs the practically achievable rates. With

OFDMA, adjacent subcarriers (tones) are grouped together

into a resource unit (RU) and a sender can choose the best RU

for each particular receiver, which actually results in higher

Signal-to-Interference-plus-Noise Ratio (SINR), Modulation

and Coding Scheme (MCS) and throughput. Moreover, since

the efﬁciency of high data rates degrades when a STA has

only few data to transmit, advanced aggregation techniques

aimed to reduce channel access, acknowledgment (ACK) and

preamble-induced overhead become useless. Allocating nar-

row RUs for such STAs is an efﬁcient remedy. According to

the latest TGax investigations, OFDMA provides a 6 times

higher throughput than legacy DCF [21], see Fig. 1.

OFDMA makes Wi-Fi radio access closer to the LTE one.

However in contrast to LTE, OFDMA works on top of the

legacy DCF and is coordinated by the AP. It means that having

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