红外无线通信技术与应用

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"Wireless Infrared Communications - 一篇关于无线红外通信的重要技术文档，主要讨论了红外辐射在高速、短距离无线数字通信中的应用。该文档由JOSEPH M. KAHN和JOHN R. BARRY撰写，是研究可见光通信(VLC)的参考资料。" 无线红外通信是一种利用红外辐射作为传输介质的高速、短距离无线通信技术。这种技术在目前的红外链接和局域网络中得到了应用。红外通信与无线电和微波通信相比，具有独特的优点和局限性。文档中比较了这些媒介的特点，为读者提供了全面的理解。红外通信的一个关键物理特性是使用强度调制与直接检测（IM/DD）的通道特性。这包括路径损耗和多径响应。文档还深入分析了自然和人为环境中的红外噪声源，这对于优化链路信噪比(SNR)至关重要。为了最大化链路SNR，文档详细介绍了发射器和接收器的设计策略。几种常见的调制格式如开/关键控（OOK）、脉冲位置调制（PPM）和副载波调制被详细讨论。这些调制技术在多径失真环境下的性能被量化评估。此外，文档还回顾了用于不同用户传输复用的技术，这对于提高无线红外通信的效率和容量至关重要。特别地，文档描述了一个实验性的50 Mbps开/关键控扩散红外链路的性能，这为实际应用提供了实例。文档"Wireless Infrared Communications"不仅涵盖了理论基础，还涉及到实际应用案例，对于理解红外通信系统的工作原理、设计挑战以及性能评估具有极大的价值，尤其对于从事可见光通信研究的人员来说，是一份极具参考价值的资料。

Table 2 Comparison Between LED’s and LD’s (Shading Denotes an Advantage)

Characteristic LED’s LD’s

Spectral width 25–100 nm

(10–50 THz)

to 5 nm

(< 1 MHz to 2 THz)

Modulation Bandwidth Tens of kiloherz to tens of megahertz Tens of kiloherz to tens of gigahertz

E/O Conversion Efﬁciency 10–20% 30–70%

Eye Safety Generally considered eye-safe

Must be rendered eye-safe, especially for

<

1400

Cost Low Moderate to high

some of the means to overcome them. Section II describes

how to achieve a high SNR, which is the single most

difﬁcult problem faced by the designer of an infrared link.

Multipath distortion on infrared channels is characterized

in Section III. Section IV provides a survey of various

modulation techniques for infrared systems, comparing

their power and bandwidth efﬁciencies, and characterizing

their performance on multipath channels. In Section V,

angle-diversity receivers and quasidiffuse transmitters are

discussed. Multiple-access techniques are the subject of

Section VI. Section VII describes an experimental 50-Mb/s

diffuse infrared link, and Section VIII provides some con-

cluding remarks.

II. D

ESIGN OF POWER-EFFICIENT LINKS

Achieving a high electrical SNR is the single biggest

problem facing the designer of an infrared link. The difﬁ-

culty arises for two reasons. Firstly, the SNR of an IM/DD

link depends upon the square of the received optical average

power. This implies that one should transmit at relatively

high power, but available transmitter power may be limited

by considerations of eye safety and power consumption.

It also implies that one should design the link so as to

minimize path loss and employ a receiver having a large

light-collection area. Second, in many environments there

exists intense ambient infrared noise, which introduces

white shot noise and low-frequency cyclostationary noise

into the receiver. This noise can be minimized through

optical ﬁltering and by employing a directional receiver,

which can separate the desired signal from the ambient

noise.

A. Infrared Transmitters and Eye Safety

The wavelength band between about 780 and 950 nm is

presently the best choice for most applications of infrared

wireless links, due to the availability of low-cost LED’s

and laser diodes (LD’s), and because it coincides with the

peak responsivity of inexpensive, low-capacitance silicon

photodiodes. The primary drawback of radiation in this

band relates to eye safety: it can pass through the human

cornea and be focused by the lens onto the retina, where it

can potentially induce thermal damage [49]. The cornea is

opaque to radiation at wavelengths beyond about 1400 nm,

considerably reducing potential ocular hazards, so that it has

been suggested that the 1550-nm band may be better suited

for infrared links. Unfortunately, the photodiodes presently

available for this band, which are made of germanium or

InGaAs, have much higher costs and capacitances per unit

area than their silicon counterparts. To our knowledge, at

present, all commercially available systems operate in the

shorter-wavelength band.

Table 2 presents a comparison between LED’s and LD’s.

LED’s are currently used in all commercial systems, due

to their extremely low cost and because most LED’s emit

light from a sufﬁciently large surface area that they are

generally considered eye-safe. Typical packaged LED’s

emit light into semiangles (at half power) ranging from

about 10

–30 , making them suitable for directed transmit-

ters. Nondirected transmitters frequently employ multiple

LED’s oriented in different directions. Potential drawbacks

of LED’s include: 1) typically poor electro-optic power con-

version efﬁciencies of 10–20% (though new devices have

efﬁciencies as high as 40%), 2) modulation bandwidths that

are limited to tens of MHz in typical low-cost devices, 3)

broad spectral widths (typically 25–100 nm), which require

the use of a wide receiver optical passband, leading to

poor rejection of ambient light, and 4) the fact that wide

modulation bandwidth is usually obtained at the expense of

reduced electro-optic conversion efﬁciency.

LD’s are much more expensive than LED’s, but offer

many nearly ideal characteristics: 1) electro-optic conver-

sion efﬁciencies of 30–70%, 2) wide modulation band-

widths, which range from hundreds of MHz to more than 10

GHz, and 3) very narrow spectral widths (spectral widths

ranging from several nm to well below 1 nm are available).

To achieve eye safety with an LD requires that one pass

the laser output through some element that destroys its

spatial coherence and spreads the radiation over a sufﬁ-

ciently extended emission aperture and emission angle. For

example, one can employ a transmissive diffuser, such as

a thin plate of translucent plastic. While such diffusers can

achieve efﬁciencies of about 70%, they typically yield a

Lambertian radiation pattern, offering the designer little

freedom to tailor the source radiation pattern. Computer-

generated holograms [29] offer a means to generate custom-

tailored radiation patterns with efﬁciencies approaching

100%, but must be fabricated with care to insure that any

residual image of the LD emission aperture is tolerably

weak.

The eye safety of infrared transmitters is governed by

International Electrotechnical Commission (IEC) standards

[49]. It is desirable for infrared transmitters to conform to

the IEC Class 1 allowable exposure limit (AEL), implying

that they are safe under all foreseen circumstances of use,

and require no warning labels. At pulse repetition rates

KAHN AND BARRY: WIRELESS INFRARED COMMUNICATIONS 271

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