Wi-Fi 2.4G中20/40MHz共存对WLAN性能的影响 - CSDN文库

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"这篇资源是关于无线局域网络（WLANs）中20/40MHz共存问题的研究文章。研究重点在于分析20MHz和40MHz基本服务集（BSSs）重叠时对性能的影响。作者通过模拟实验展示了802.11n标准下20/40MHz模式的BSS与传统20MHz模式BSS的相互作用。主要发现是，当不使用清晰信道评估（CCA）时，传统BSS的吞吐量几乎为零，而20/40MHz BSS的吞吐量显著下降。如果在扩展信道上使用CCA，两者吞吐量都会有显著提升。此外，支持20/40MHz模式的站点（STAs）/接入点（AP）能动态切换传输/接收模式，并使用CCA，将使两个BSS的吞吐量进一步增加。文章还探讨了当20/40MHz BSS采用802.11e增强功能时，对传统BSS的影响，以及40MHz传输的RTS/CTS保护机制对网络性能的影响。减小40MHz传输的传输机会（TXOP）间隔可以提高传统BSS的公平性和吞吐量。" 本文深入研究了WIFI 2.4G频段中20MHz和40MHz带宽共存的情况，特别是在Wi-Fi 802.11n标准下，这种共存可能带来的问题和解决方案。首先，研究揭示了当两个BSSs重叠，且扩展信道未使用CCA进行信道检测时，20MHz BSS的性能严重受损，几乎无法工作，而20/40MHz BSS的吞吐量也会大幅下降。这是因为没有CCA，20MHz设备无法正确感知40MHz设备的存在，导致冲突。接着，作者指出，启用扩展信道的CCA可以显著改善两个BSS的性能。通过让20/40MHz模式的STAs/AP能够动态切换到20MHz模式，可以进一步优化性能。这种方法允许设备根据信道条件调整其操作模式，从而减少干扰并提高整体效率。文章还探讨了802.11e增强功能对20MHz BSS性能的影响，特别是当20/40MHz BSS使用这些增强功能时，如何减轻对传统BSS的干扰。此外，作者讨论了40MHz传输的RTS/CTS（请求发送/清楚发送）保护机制的作用，这是一种防止冲突的技术，但也会带来额外开销。通过减少40MHz传输的TXOP间隔，可以提高传统BSS的公平性，使得它们有更多机会进行数据传输，从而提升吞吐量。这篇文章对于理解WLANs中的频谱共存策略和优化网络性能具有重要意义，为网络设计者提供了重要的理论依据和实践指导。

On the 20/40 MHz Coexistence of Overlapping

BSSs in WLANs

Ariton E. Xhafa, Anuj Batra and Artur Zaks

†

Texas Instruments Inc., 12500 TI Boulevard, Dallas, TX 75243, USA

Email: {axhafa, abatra}@ti.com

†

Texas Instruments, Inc., 26 Zarchin St., Raanana 43662, Israel

Email: {arturz}@ti.com

Abstract— We investigate the impact of 20/40 MHz coexis-

tence on the performance of wireless local area networks

(WLANs). To that end, we present simulation results of

overlapping basic service sets (BSSs), a 802.11n BSS op-

erating in 20/40 MHz mode and a legacy BSS operating

in 20 MHz mode, where the overlapping channel is the

extension channel of the 20/40 MHz BSS. Our results show

that if clear channel assessment (CCA) is not used in the

overlapping channel, the throughput in legacy BSS is almost

zero, while the throughput of 20/40 MHz BSS decreases

drastically. If CCA is used in the overlapping channel,

the throughput of legacy and 20/40 MHz BSSs increase

dramatically compared to the case when no CCA is used.

In addition, when stations (STAs)/AP that operate in 20/40

MHz BSS are able to dynamically switch between 20 MHz

and 40 MHz transmit/receive modes and CCA is used in

the extension channel, the throughput of both BSSs further

increases. We also report the impact of 40 MHz operation on

legacy BSS when the 20/40 MHz BSS employs IEEE 802.11e

enhancements. In addition, the impact on the network

performance of RTS/CTS protection mechanism for 40

MHz transmissions are also shown. Reducing transmission

opportunity (TXOP) interval for 40 MHz transmissions also

improves fairness and throughput for legacy BSSs.

Index Terms— medium access control, wireless lan, coexis-

tence.

I. INTRODUCTION

Tomorrow’s wireless LAN networks will provide

high data rates and multimedia services to end

users [1]-[9]. Currently, IEEE 802.11n is working to-

ward a multiple-input-multiple-output (MIMO) orthognal-

frequency-division-multiplexing (OFDM) solution for

next generation wireless local area network (WLAN)

standard, which is expected to be standardized by end

of 2008 / beginning of 2009. The presence of MIMO-

OFDM physical (PHY) layer in 802.11n will yield higher

data rate in the network.

To further increase the data rate, the working group has

proposed an optional 40 MHz operation, where, stations

(STAs) and/or the access point (AP) can transmit/receive

in two-20 MHz channels simultaneously. These two

channels consists of control and extension channels. To

distinguish between the control and the extension channel

This paper is based on “On the coexistence of overlapping BSSs in

WLANs,” by A. E. Xhafa, A. Batra, and A. Zaks, which appeared in the

Proceedings of the 24th IEEE Vehicular Technology Conference (VTC

Fall 2007), Baltimore, USA, October 2007.

c

° 2007 IEEE.

one should refer to the fact that a 20/40 MHz capable

basic service set (BSS) will use the control channel for

20 MHz operation. While the use of 40 MHz channels

in WLANs increases the data rate in the network, it will

also introduce fairness issues in the network, since 40

MHz channels will increase the number of overlapping

BSSs.

In IEEE 802.11n [2], an optional 20/40 MHz coex-

istence feature has been proposed. This feature is called

phased coexistence operation (PCO) where the AP divides

time into alternating 20 MHz and 40 MHz phases. Clear

to send to self (CTS2S) frames as well as contention free

end (CF-End) frames are used for starting/ending these

phases as deﬁned in the Beacon frame or set PCO phase

frame. Setting of a 40 MHz phase does not allow for a

STA that gains access of the control channel to transmit at

20 MHz control channel if the extension channel is busy.

Thus, the proposed approach suffers from inefﬁciency due

to transmission of many control frames (CTS2S and CF-

End) and the rules of starting 40 MHz phase are vague

or not deﬁned.

In [10], the authors propose a two-level carrier sensing

solutions for 20 MHz overlapping BSS. In this approach,

two additional network allocation vector (NAV) ﬁelds are

proposed, one for the self-BSS and the other for over-

lapping BSS. The problem with the proposed approach is

that it will require changes to IEEE 802.11 standard. In

addition, reading of the overlapping BSS NAV might be

quite a challenge.

In [11], the authors propose interferential packet detec-

tion scheme for legacy overlapping BSSs. Dynamic chan-

nel switching for the STAs that detect overlapping BSS

(based on the received BSSID) is proposed. However, this

approach does not consider 20/40 MHz BSS and it applies

to only STAs that are located in the overlapping area of

the two 20 MHz BSSs. Other work on coexistence [12]-

[14] is focused mostly on the coexistence between WLAN

networks and other technologies operating in Industrial,

Scientiﬁc and Medical (ISM) band, such as Bluetooth.

In this paper, we investigate the problems with 20/40

MHz coexistence and discuss the impact of various over-

lapping BSSs scenarios on the overall network perfor-

mance. We assume that the extension channel is the

overlapping channel; i.e., the channel used by both 20/40

MHz BSS and legacy BSS. Throughout the paper the

56 JOURNAL OF NETWORKS, VOL. 3, NO. 7, JULY 2008

© 2008 ACADEMY PUBLISHER

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