深入理解IEEE 802.11n：无线LAN的新纪元

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"这篇文档详细介绍了802.11n标准的发展背景、技术特点以及其带来的性能提升，特别是利用MIMO技术实现的高速率和更远的传输范围。" 在无线局域网（Wireless LAN）领域，IEEE 802.11n标准是一个里程碑式的修订，它旨在显著提高无线网络的吞吐量，从而带来更快的下载速度和更广泛的覆盖范围。802.11n标准始于2003年，经过多轮草案修改，最终在2009年成为正式标准。虽然早期开发阶段的延迟以及市场对预标准产品的保守态度减缓了该技术的普及，但随着技术的成熟，人们对802.11n的兴趣逐渐升温。 802.11n标准的核心技术是多输入多输出（Multiple Input, Multiple Output，简称MIMO）系统。MIMO技术通过使用多个发射和接收天线，实现了数据传输速率的显著提升。在Draft 3.0版本中，802.11n能够提供超过200Mbps的理论吞吐量，物理层数据速率最高可达600Mbps。这种性能的提升主要得益于以下几个关键技术： 1. **空间分集多路复用（Spatial Division Multiplexing, SDM）**：MIMO系统可以同时发送和接收多个独立的数据流，每个流都在不同的空间路径上传输，这样可以在不增加频谱宽度的情况下增加总的传输容量。 2. **发射波束成形（Transmitter Beamforming）**：这是一种信号处理技术，允许发射端根据接收端的反馈信息调整信号的方向，以优化信号到达接收端时的能量集中，从而提高信号质量和传输效率。 3. **空间时间块编码（Space-Time Block Coding, STBC）**：STBC是一种编码策略，通过在时间和空间上编码数据，增强了信号的抗干扰能力，提高了无线链路的可靠性。 MIMO技术的应用不仅极大地提升了传输速率，还能够根据环境条件优化接收范围。在某些情况下，它可以将单天线系统的吞吐量提升两到四倍，或者在保持相同吞吐量的同时，增加接收范围。这使得802.11n标准在网络性能上取得了重大突破，对于家庭、办公室和公共场所的无线网络应用具有重要意义。 IEEE 802.11n标准通过引入MIMO技术，成功地解决了传统无线局域网的速率和覆盖问题，为用户带来了更高效、更可靠的无线网络体验。随着技术的不断发展，802.11n之后的标准，如802.11ac和802.11ax（Wi-Fi 6），继续扩展这些概念，将无线网络的速度和功能推向新的高度。

(d) =−10 log



(4πd)



(2)

where

are the transmitter and receiver antenna

gains, respectively,

is the distance between them, in

meters (known as T-R separation [11]), and

is the wave-

length of the transmitted carrier frequency. Assuming

unity antenna gains (

= G

= 1

), equation (2) can be

written as (with carrier frequency,

= 5.25

GHz):

(d ) = 10 log



(4πd )



= 2 · 10 log

(4π/λ) + 2 · 10 log

(d )

= 47 dB+ 2 · 10 log

(d ) (3)

Note that the slope is 2 for the path loss expression in

(3). For distances larger than

, the slope is 3.5, which

can be seen from (1). The parameters used to model path

loss for the different TGn channel models in [7] are shown

below. Note that channel models A to C represent small

environments (0 to 30 ns delay spread), and models D to F

represent larger environments (50 to 150 ns delay spread):

The shadow fading values in Table 1, above, represent

the fact that, for any given transmitter/receiver configu-

ration, the regions surrounding these stations can differ,

resulting in the received signal having an average

strength differing from (1). From [11], this configuration-

dependent variation in path loss (known as shadow fad-

ing, or log-normal shadowing) can be modeled with a

zero-mean, Gaussian random variable, with standard

deviation,

(in dB), added to the path loss.

With shadow fading added to (1), the equations

become:

L(d)= L

(d)+ X

, d ≤ d

L(d)= L

)+3.5·10 log

(d/d

)+ X

, d > d

(4)

where

is the random variable described above. Note

that the value for

differs before and after the breakpoint

distance (see Table 1), and that the breakpoint distance is

generally assumed to be the boundary for LOS (Line-Of-

Sight) conditions [7].

The table below shows the computed path loss for var-

ious distances with TGn Channel D, based on the above

described model, as well as the expected average SNR at

the receiver, assuming the transmit station uses

5 dBm

transmit power, and the receive station has a

−

90 dBm

receive sensitivity (or signal detection threshold), with

no external interference.

III.1.2 Small-Scale Signal Fading

Small-scale fading are fading effects caused by small

changes in distance (only a few wavelengths). These

fading effects are caused by multiple reflected

Shadow Fading Shadow Fading std.

std. dev. (dB) dev. (dB) After

New Model

(m) Slope Before

Slope After

Before

BP (LOS)

BP (NLOS)

A (Optional) 5 2 3.5 3 4

B523.534

C523.535

D1023.535

E2023.536

F3023.536

Table 1.

Path Loss Parameters for TGn Channel Models (reprinted from [7]).

Distance Path Loss SNR

1 m (3 ft) 47.0 dB 48.0 dB

3 m (10 ft) 56.5 dB 38.5 dB

6 m (20 ft) 62.5 dB 32.5 dB

9 m (30 ft) 66.0 dB 29.0 dB

12 m (40 ft) 70.0 dB 25.0 dB

15 m (50 ft) 73.0 dB 22.0 dB

18 m (60 ft) 76.0 dB 19.0 dB

··· ··· ···

30 m (100 ft) 84.0 dB 11.0 dB

··· ··· ···

46 m (150 ft) 90.0 dB 5.0 dB

··· ··· ···

55 m (180 ft) 93.0 dB 2.0 dB

Table 2.

Path Loss, SNR, at various distances for TGn Channel D.

FIRST QUARTER 2008 IEEE CIRCUITS AND SYSTEMS MAGAZINE

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深入理解IEEE 802.11n：无线LAN的新纪元

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