802.11n与802.11ac无线局域网标准详解

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"Cambridge.Next Generation Wireless LANs.802.11n and 802.11ac.2Ed.2013.pdf" 本书是针对IEEE 802.11n和802.11ac无线局域网（WLAN）标准的全面指南，旨在帮助读者快速理解这两个标准，而无需深入阅读整个802.11规范。作者深入浅出地介绍了802.11n和802.11ac的基本原理、实现细节以及关键增强功能，并解释了这些功能被引入标准背后的原因和历史背景。 802.11n和802.11ac是Wi-Fi技术的两个重要里程碑，它们显著提高了无线网络的速度和性能。802.11n是第一个引入多输入多输出（MIMO）技术的标准，该技术通过利用多个天线来提高数据传输速率和可靠性。书中详细讨论了MIMO技术，以及如何通过使用40MHz、80MHz甚至160MHz的更宽信道来增加带宽，从而提升吞吐量。此外，传输波束成形技术也被涵盖，这是一种优化无线信号传播的方法，可以提高信号强度和定向性。 802.11ac作为802.11n的后续标准，进一步提升了速度和效率，尤其是在5GHz频段的操作。在本书的第二版中，增加了对802.11ac物理层（PHY）和媒体访问控制（MAC）层的新内容，特别是关于多用户MIMO的章节。多用户MIMO允许路由器同时向多个设备发送数据，极大地提高了网络的并发处理能力。对于兼容性和共存问题，书中有清晰的总结，这些问题在部署新的高速标准时尤其重要，因为它们必须与现有的802.11a/b/g/n设备无缝协作。书中还涵盖了如何处理这些挑战，以确保不同标准设备之间的互操作性。这本书是无线通信领域设计师、网络管理员和研究人员的理想参考，它不仅提供了802.11n和802.11ac的深度解析，还关注了这些技术的实际应用和未来发展趋势。通过对这些关键技术的深入探讨，读者将能够更好地理解和设计高效的无线网络解决方案。

Foreword

The ﬁrst version of the 802.11 standard was ratiﬁed in 1997 after seven long years of

development. However, initial adoption of this new technology was slow, partly because

of the low penetration of devices that needed the “freedom of wireless.”

The real opportunity for 802.11 came with the increased popularity of laptop computers

just a few years later. This popularity brought a rapidly growing user base wanting network

connectivity not only while connected to an Ethernet cable at home or at work, but also in

between: in hotels, airports, conference centers, restaurants, parks, etc. 802.11 provided a

cheap and easy way to make laptop mobility a reality for anyone who wanted it.

However, technology by itself is rarely sufﬁcient, particularly in the networking space,

where interoperability of devices from multiple vendors is almost always the key to

market success. Having been formed as WECA in 1999, the Wi-Fi Allianc e was ready to

provide certiﬁcation of multi-vendor interoperability.

With the right technology from the IEEE 802.11 Working Group, certiﬁed interoper-

ability from the Wi-Fi Alliance, and a real market need based on a growing installed base

of laptops, the conditions were ripe for the Wi-Fi mark et to take off, and indeed it did. By

2007 virtually every new laptop contains Wi-Fi as standard equipment. More impor-

tantly, and unlike some other “successful” wireless technologies, many of these devices

are used regularly. With this wide use came a growing understanding of the power of

cheap, easy-to-deploy, and easy-to-manage interoperable Wi-Fi networks.

The natural next step was for people to ask, “What else can we use Wi-Fi for?” The answer

is increasingly becoming “everything, everywhere!” Not just laptops, but now almost any-

thing mobile and even many ﬁxed devices contain Wi-Fi, and they are used in a phenomenal

range of applications, including data, voice, games, music, video, location, public safety,

vehicular , etc. In 2007, more than 300 million W i-Fi devices were shipped. By 2012, some

analysts are forecasting that more than one billion Wi-Fi devices will be shipped every year .

The 2.4 GHz 802.11b 11 Mb/s DSSS/CCK PHY and the basic 802.11 contention-

based MAC provided the basis for a great industry. However, the rapid growth of the

Wi-Fi market challenged the capabilities of the technology. It was not long before better

security (802.11i certiﬁed by the Wi-Fi Alliance as WPA/WPA2TM) and better Quality

of Service (802.11e certiﬁed by the Wi-Fi Alliance as WMMTM and WMM Power Save)

were deﬁned, certiﬁed, and deployed.

It was also not long before higher data rates were demanded for greater d ata density

and to support the many new and exciting devices and applications. 802.11a, providing

54 Mbps based on OFDM in the 5 GHz band, failed to garner signiﬁcant support because

two radios were required to maintain backward compatibility with 2.4 GHz 802.11b

devices; the cost of two radios was often too high. The real success story was 802.11g,

which provided 54 Mbps based on OFDM in the 2.4 GHz band in a way that was

backward-compatible with 802.11b.

The success of 802.11g drove the use of Wi-Fi to new heights and expanded the

demands on the technology yet again; everyon e wanted more. Fortunately, the technol-

ogy continued to develop and in 2002 the IEEE 802.11 Working Group started deﬁning

the next generation of PHY and MAC features as part of 802.11n. 802.11n will deﬁne

mechanisms to provide users some combination of greater throughput, longer range and

increased reliability, using mandatory and optional features in the PHY (including

MIMO technology and 40 MHz channels) and the MAC (including more efﬁcient data

aggregation and acknowledgements).

Interestingly, 802.11n operates in both the 2.4 GHz and 5 GHz bands. It is expected

that 5 GHz operation will be more popula r than when 802.11a was introduced, because

2.4 GHz is now more congested, the number of available channels in the 5 GHz band has

been expanded with the introduction of DFS and TPC technology, there is more need for

high throughput 40 MHz channels, and the cost of dual-band radios has decreas ed.

The 802.11n standard is not yet complete, and is unlikely to be ratiﬁed by the IEEE

until at least mid 2009. Until August 2006, the Wi-Fi Allia nce had a policy to not certify

802.11n products until the standard was ratiﬁed. However, some vendors decided the

market could not wait for ratiﬁcation of the 802.11n standard and started releasing pre-

standard products. These products were often not interoperable at the expected perform-

ance levels because they were not based on a common interpretation of the draft 802.11n

speciﬁcation. The problem for the Wi-Fi Alliance was that these products were adversely

affecting the reputation of Wi-Fi. The Wi-Fi Alliance decided the only way forward was

to certify the basic features of 802.11n from a pre-standard draft. Such a decis ion is not

without precedent. In 2003, certiﬁcation of WPA started before the 802.11i standard was

ratiﬁed and in 2004 certiﬁcation of WMM started before 802.11e was ratiﬁed. The Wi-Fi

Alliance commenced certiﬁcation of 802.11n draft 2.0 on 26 June 2007.

The decision has turned out to be the right one for the industry and for users. The Wi-Fi

CERTIFIED 802.11n draft 2.0 programme has been remarkably successful, with more

than 150 products certiﬁed in less than ﬁve months. This represents a signiﬁcantly higher

number of certiﬁed products than for the 802.11g progra mme during a similar period

after launch. The Wi-Fi Alliance’s certiﬁcation progra m has helped ensure interoper-

ability for the many products that will be released before the ratiﬁcation of the 802.11n

standard. This is particularly important given that the likely ratiﬁcation date of the

802.11n standard has been extended by more than a year since the decision to start a

certiﬁcation program was announced by the Wi-Fi Alliance. The next challenge for the

Wi-Fi Alliance is to ensure a backw ard-compatible transition path from the 802.11n draft

2.0 as certiﬁed by the Wi-Fi Alliance to the ﬁnal rati ﬁed standard.

Standards are never the most accessible of documents. The 802.11 standard is partic-

ularly difﬁcult to understand because it has been amended so many times by different

groups and editors over a long period. A draft amendment to the standard, such as

xvi Foreword

Preface to the ﬁrst edition

Having worked on the development of the 802.11n stand ard for some time, we presented

a full day tutorial on the 802.11n physical layer (PHY) and medium access control

(MAC) layer at the IEEE Globecom conference held in San Francisco in December 2006.

Our objective was to provide a high level overview of the draft standard since, at the time,

there was very little information on the details of the 802.11n standard available to those

not intimately involved in its development. After the tutorial, we were approached by

Phil Meyler of Cambridge University Press and asked to consider expanding the tutorial

into a book.

Writing a book describing the standard was an intriguing prospect. We felt that a book

would provide more opportunity to present the technical details in the standard than was

possible with the tutorial. It would ﬁll the gap we saw in the market for a detailed

description of what is destined to be one of the most widely implemented wireless

technologies. While the standard itself conveys details on what is needed for interoper-

ability, it lacks the background on why particular options should be implemented, where

particular aspects came from, the constraints under which they were designed, or the

beneﬁt they provide. All this we hoped to capture in the book. The bene ﬁts various

features provide, particularly in the physical layer, are quantiﬁed by simulation results.

We wanted to provide enough information to enable the reader to model the physical

layer and benchmark their simulation against our results. Finally, with the amended

standard now approaching 2500 pages, we hoped to provide an accessible window into

the most important aspects, focusing on the throughput and robustness enhancements and

the foundations on which these are built.

The book we came up with is divided into three parts. The ﬁrst part covers the physi cal

layer (PHY), and begins with background information on the 802.11a/g OFDM PHY on

which the 802.11n PHY is based and interoperates, and proceeds with an overview of

spatial multiplexing, the key throughput enhancing technology in 802.11n. This is

followed by details on exactly how high throughput is achieved in 802.11n using spatial

multiplexing and wider, 40 MHz channels. This in turn is followed by details on robust-

ness enhancing features such as receive diversity, spatial expansion, space-time block

codes, and low density parity check codes.

The second part covers the medium access control (MAC) layer. This part provides

background on the original 802.11 MAC as well as the 802.11e quality of service (QoS)

enhancements. It gives an overview of why changes were needed in the MAC to achieve

higher throughput, followed by details on each of the new features introduced. Given the

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802.11n与802.11ac无线局域网标准详解

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Next Generation Wireless LANs - 802.11n and 802.11ac, 2ndEd 2013.zip

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