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1994 | Buch

Wireless Infrared Communications

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The demand for wireless access to network services is growing in virtually all communications and computing applications. Once accustomed to unteathered opera­ tion, users resent being tied to a desk or a fixed location, but will endure it when there is some substantial benefit, such as higher resolution or bandwidth. Recent technolog­ ical advances, however, such as the scaling of VLSI, the development of low-power circuit design techniques and architectures, increasing battery energy capacity, and advanced displays, are rapidly improving the capabilities of wireless devices. Many of the technological advances contributing to this revolution pertain to the wireless medium itself. There are two viable media: radio and optical. In radio, spread-spectrum techniques allow different users and services to coexist in the same bandwidth, and new microwave frequencies with plentiful bandwidth become viable as the speed of the supporting low-cost electronics increases. Radio has the advantage of being available ubiquitously indoors and outdoors, with the possibility of a seam­ less system infrastructure that allows users to move between the two. There are unan­ swered (but likely to be benign) biological effects of microwave radiation at higher power densities. Optical communications is enhanced by advances in photonic devices, such as semiconductor lasers and detectors. Optical is primarily an indoor technology - where it need not compete with sunlight - and offers advantages such as the immediate availability of a broad bandwidth without the need for regulatory approval.

Inhaltsverzeichnis

Frontmatter
1. Introduction
Abstract
Trends in the telecommunications and computer industries suggest that the network of the future will consist of a fiber-optic backbone with short-range wireless communication links providing network access to portable communicators and portable computers. In this vision of the future, users moving from room to room will have access to the same high-speed network services available to wired terminals. The technology base for implementing this concept does not yet exist, however. Radio technology, although well-suited for moderate-speed applications such as voice, may not be sufficient to support many high-speed applications.
John R. Barry
2. Link Analysis and Optics Design
Abstract
Before we can address such issues as electrical receiver design and modulation design, we need a solid understanding of the optical components that underlie a wireless infrared communication system. The purpose of this chapter is to present a link budget analysis for non-directed line-of-sight optical communications, and to present design strategies for optimizing the transmitter and receiver optical components. We examine the signal-to-noise ratio (SNR) improvement of a hemispherical concentrator as an optical antenna. We propose a hemispherical thin-film optical filter, and compare its performance to that of a traditional planar thin-film filter. For both filter types, we jointly optimize the transmitter radiation pattern, the filter orientation, and the filter bandwidth. The results of this chapter indicate that a 269-mW transmitter and a 1-cm2photodetector are sufficient to achieve a 100-Mb/s non-directed link over a range of 4 m, even in the presence of intense background light. These numerical results are based on a sphere radius of 2 cm; in principle, an arbitrary performance improvement can be achieved by an arbitrarily large sphere radius, implying a direct trade-off between performance and size.
John R. Barry
3. Receiver Design
Abstract
In this chapter we examine the problem of designing the electrical front end of the receiver, which consists of one or more photodetectors followed by a low-noise, wide-band preamplifier. The design of the preamplifier is challenging due to the high capacitance of large-area photodiodes and a potentially intense background light. The error performance of the entire system depends to a large degree on the ability of the preamplifier to amplify the detected signal over the desired bandwidth without adding undue noise. We show that it is always possible to make the preamplifier noise negligible in comparison to the background light shot noise. We show that, in practical cases, a PIN photodiode is preferable to an avalanche photodiode. We propose a transimpedance amplifier using a current-feedback pair, and present a detailed bandwidth and noise analysis. We present design procedures for two typical design scenarios. Numerical examples show that, using wide-gate FETs, low-noise operation over bandwidths near 100 MHz can be achieved for moderate detector areas near 1 cm2.
John R. Barry
4. Modeling Multipath Dispersion
Abstract
The focus of the previous two chapters was design; chapter 2 examined optics design, and chapter 3 examined electronics design. In contrast, this chapter investigates the properties of the wireless infrared channel itself, over which the designer has no control. Specifically, we examine the severity of multipath dispersion in typical indoor environments, and assess its effect on system performance. We present a recursive method for calculating the impulse response of a room with Lambertian reflectors [57][58]. The method, which accounts for multiple reflections of any order, enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication. We present a simple algorithm for computer implementation of the technique. We present computer simulation results for both line-of-sight and diffuse transmitter configurations.Inboth cases we find that reflections of multiple order are a significant source of intersymbol interference. We also describe experimental measurements of optical multipath, which help verify the accuracy of our simulations.
John R. Barry
5. Modulation and Equalization
Abstract
In this chapter we examine the performance of a number of modulation schemes for wireless infrared communication systems. At first, we neglect multipath dispersion. We review the unique characteristics of the intensity-modulation channel, which differs from a conventional linear Gaussian-noise channel in two ways: the input cannot be negative, and the average amplitude — not power — of the input is limited. We then compare the power efficiency and bandwidth efficiency of a number of modulation schemes for the intensity-modulation channel. We find that the intensity-modulation channel favors baseband modulation schemes over subcarrier and multiplesubcarrier modulation schemes, particularly those baseband modulation schemes with low duty cycles like pulse-position modulation (PPM). Unfortunately, at high data rates, the performance of PPM is severely degraded by multipath dispersion. Therefore, we also investigate several detection and equalization strategies for PPM in the presence of multipath dispersion.
John R. Barry
6. System-Level Issues
Abstract
Chapters 2 through 5 have concentrated on the physical-layer problem of point-to-point communication using non-directed infrared radiation. Although the results presented there stand alone, the purpose of this chapter is to address some of the issues that arise when one tries to use non-directed infrared links as building blocks for a wireless LAN. Because a complete design of a wireless LAN is beyond the scope of this book, the discussion presented here will not be in depth; nevertheless, it should convince the reader that there are no fatal obstacles to an infrared LAN lurking in the higher network layers.
John R. Barry
7. Conclusions and Future Work
Abstract
This book has addressed the general problem of designing a high-speed wireless link using non-directed infrared radiation. The results suggest that speeds near 100 Mb/s are practical. The only true test of this hypothesis, of course, is to build such a link and verify it experimentally. Researchers at the University of California at Berkeley are currently developing a 50-Mb/s prototype that incorporates many of the ideas in this book. Although final results are forthcoming, preliminary reports have been encouraging [5] [37] [38].
John R. Barry
Backmatter
Metadaten
Titel
Wireless Infrared Communications
verfasst von
John R. Barry
Copyright-Jahr
1994
Verlag
Springer US
Electronic ISBN
978-1-4615-2700-8
Print ISBN
978-1-4613-6162-6
DOI
https://doi.org/10.1007/978-1-4615-2700-8