Gene Wesley Marsh

Non-directed infrared links for high-capacity wireless LANs

he emergence of portable information terminals in future work and living environments is expected to accelerate t he introduction of wireless LANs. Such portable terminals should have access to all of the services that will be available on wired networks. Unlike their wired counterparts, portable devices are subject t o severe limitations on power consumption, size, and weight. The desire for inexpensive, high-speed links satisfying these requirements has motivated recent interest in infrared wireless communication [ 1-51. As a medium for short-range, indoor communication, infrared offers several significant advantages over radio, including a virtually unlimited spectral region that is unregulated worldwide. Near-infrared and visible light are close together in wavelength, and they exhibit qualitatively similar behavior. Both are absorbed by dark objects, diffusely reflected by light-colored objects, and directionally reflected from shiny surfaces. Both types of light penetrate through glass, but not walls or other opaque barriers. As a result, infrared communications can readily be secured against eavesdropping. Moreover, it is possible to operate at least one infrared link in every room of a building without interference, so that the potential capacity of an infrared-based network is extremely high. When an infrared link employs intensity modulation with direct detection (IMDD), the short carrier wavelength and large, square-law detector lead to efficient spatial diversity that prevents multipath fading. By contrast, radio links are typically subject to large fluctuations in received signal magnitude and phase. The infrared medium is not without drawbacks, however. In many indoor environments there exists an intense infrared ambient, arising from sunlight, incandescent lighting, and fluorescent lighting, which induces noise in an infrared receiver. Invirtually all short-range, indoor applications, IM/DD is the only practical transmission technique. The signal-to-noise ratio of a D D receiver is proportional to the square of the received optical power, implying that IM/DD links can tolerate only a comparatively limited path loss. Often, infrared links must employ relatively high transmit power levels and operate over a relatively limited range. While the transmitter power level can usually be increased without fear of interfering with other users, transmitter power maybe limited by concerns of power consumption and eye safety, particularly in portable transmitters. Some of the characteristics of infrared and radio indoor wireless links are compared in Table 1. Using directional infrared transmitters and receivers, it is possible to achieve high bit rates and long link ranges using relatively modest transmitter power [6]. In most applications of wireless LANs, however, i t is desirable to form links using omnidirectional transmitters and receivers, alleviating the need for careful alignment between them. This article will focus on such non-directed links. As illustrated in Fig. 1, non-directed infrared links may be classified into two categories: line-ofsight (LOS) and diffuse. LOS links depend upon the existence of an unobstructed path between transmitter and receiver. Diffuse links alleviate the need for a direct LOS path by relying on light scattered from a large diffuse reflector, such as a ceiling. Because it is difficult to block all of the light reflected from such a large surface, diffuse links are more robust than LOS links, and may be preferable for many applications. Fig. 2 illustrates two different paradigms for creating wireless infrared LANs serving portable information terminals. When two or more portables are located in the same room, they may communicate directly with each other on a peer-topeer basis, forming an ad hoc network. Portable transceivers designed for such ad hoc interconnection should consume little power and be relatively inexpensive. Alternatively, infrared links may also be used to connect portables to base stations that are interconnected by a wired backbone network. Such an installed network would permit portables to communicate with multimedia and compute servers, or with portables located in other rooms. In this scenario, the portable terminals should be inexpensive and low-power, but it might be permissible for the base stations to b e more complex and t o consume greater power. In some future high-performance multimedia wireless computing environments, the portable terminals may serve mainly as a human interface, accepting pen and keyboard input, but displaying full-motion video. The very high-capacity downlinks ( tens of Mb/s per base s ta t ion) and moderatecapacity uplinks (several Mb/s per base station) required of such a system would be particularly well-matched to the capabilities of infrared communication. Smaller rooms could be served by a single base station, while rooms larger than about 10 m x 10 m may require more than o n e base s ta t ion. Techniques for accommodation of multiple base stations in one room will be touched upon below. Despite a relative scarcity of research publications on wireless infrared communications, the technology has found wide commercial application. Directed infrared beams are commonly used in remote-control devices, as well as in serial links for

Performance evaluation of experimental 50-Mb/s diffuse infrared wireless link using on-off keying with decision-feedback equalization

We report an experimental nondirected optical link for short-range, indoor data transmission at 50 Mb/s. The system uses on-off keying (OOK) and achieves low bit-error rates (BERs) in the presence of intersymbol interference, background light noise, and shadowing, with a range of 2.9 m in a skylit room. The transmitter produces an eye-safe Lambertian pattern at 806 nm with an average power of 474 mW. The receiver utilizes a hemispherical concentrator with a hemispherical bandpass optical filter, a 1-cm/sup 2/ silicon p-i-n photodiode, and a high-impedance hybrid preamplifier to achieve a high signal-to-noise ratio (SNR). A high-pass filter is used to mitigate fluorescent light noise, with quantized feedback removing the resulting baseline wander. A decision-feedback equalizer provides resistance to intersymbol interference due to multipath. The system and its components are characterized, and compared to theory. We observe that decision-feedback equalization yields a reduction of multipath power penalties that is in good agreement with theory.

50-Mb/s diffuse infrared free-space link using on-off keying with decision-feedback equalization

We report an experimental 50-Mb/s free-space infrared link for indoor wireless communication, which achieves low bit-error rates (BERs) in the presence of multipath distortion, shadowing, and ambient natural and artificial lighting. The eye-safe 806 nm transmitter, directed at the ceiling, emits an on-off-keyed signal having 475-mW average power. The receiver employs a 1-cm/sup 2/ Si p-i-n photodiode coupled to a high-index hemispherical concentrator, which is covered by an optical bandpass filter of 30-nm bandwidth. A decision-feedback equalizer mitigates the effect of multipath ISI. In the presence of bright skylight, the link achieves a range of 2.9 m at 10/sup -7/ BER. >

High-speed non-directional infrared communication for wireless local-area networks

The use of wide-area infrared beams to establish high-speed digital links between portable terminals and a base station, allowing construction of in-building wireless local-area networks, is discussed. Key impairments to high-speed communication using nondirectional infrared links, namely, small received power in the face of potentially intense ambient infrared radiation and intersymbol interference caused by multipath optical propagation, are examined. Transmitter and receiver design strategies to achieve a high signal-to-noise ratio are discussed. Experimental measurements of multipath dispersion are presented, and the performance of two communication techniques that attempt to counter multipath distortion are evaluated. They are baseband on-off keying with adaptive decision-feedback equalization and multiple-subcarrier modulation.<<ETX>>

A closed-loop system for artifact mitigation in ambulatory electrocardiogram monitoring

Motion artifacts interfere with electrocardiogram (ECG) detection and information processing. In this paper, we present an independent component analysis based technique to mitigate these signal artifacts. We propose a new statistical measure to enable an automatic identification and removal of independent components, which correspond to the sources of noise. For the first time, we also present a signal-dependent closed-loop system for the quality assessment of the denoised ECG. In one experiment, noisy data is obtained by the addition of calibrated amounts of noise from the MIT-BIH NST database to the AHA ECG database. Arrhythmia classification based on a state-of-the-art algorithm with the direct use of noisy data thus obtained shows sensitivity and positive predictivity values of 87.7% and 90.0%, respectively, at an input signal SNR of -9 dB. Detection with the use of ECG data denoised by the proposed approach exhibits significant improvement in the performance of the classifier with the corresponding results being 96.5% and 99.1%, respectively. In a related lab trial, we demonstrate a reduction in RMS error of instantaneous heart rate estimates from 47.2% to 7.0% with the use of 56 minutes of denoised ECG from four physically active subjects. To validate our experiments, we develop a closed-loop, ambulatory ECG monitoring platform, which consumes 2.17 mW of power and delivers a data rate of 33 kbps over a dedicated UWB link.

50-Mbs diffuse infrared free-space link using on-off keying with decision-feedback equalization

We report an experimental 50-Mb/s free-space infrared link for indoor wireless communication, which achieves low bit-error rates (BERs) in the presence of multipath distortion, shadowing, and ambient natural and artificial lighting. The eye-safe 806 nm transmitter, directed at the ceiling, emits an on-off-keyed signal having 475-mW average power. The receiver employs a 1-cm/sup 2/ Si p-i-n photodiode coupled to a high-index hemispherical concentrator, which is covered by an optical bandpass filter of 30-nm bandwidth. A decision-feedback equalizer mitigates the effect of multipath ISI. In the presence of bright skylight, the link achieves a range of 2.9 m at 10/sup -7/ BER.<<ETX>>We report an experimental 50-Mb/s free-space infrared link for indoor wireless communication, which achieves low bit-error rates (BERs) in the presence of multipath distortion, shadowing, and ambient natural and artificial lighting. The eye-safe 805-nm transmitter, directed at the ceiling, emits an on-off-keyed signal having 475-mW average power. The receiver employs a 1-cm/sup 2/ silicon p-i-n photodiode coupled to a high-index hemispherical concentrator; which is covered by an optical bandpass filter of 30-nm bandwidth. A decision-feedback equalizer mitigates the effect of multipath ISI. In the presence of bright skylight, the link achieves a range of 2.9 mn at 10/sup -7/ BER.<<ETX>>