Malik D. Audeh

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 of pulse-position modulation on measured non-directed indoor infrared channels

We examine the performance of pulse-position modulation (PPM) on measured channels with intersymbol interference (ISI). We summarize the bit-error-rate performance of unequalized systems and review the performance of maximum-likelihood sequence detection (MLSD) for PPM over ISI channels with additive white Gaussian noise. We evaluate the performance of PPM links over 46 experimentally measured indoor infrared channels. Detailed results are presented for 2, 4, 8, and 16-PPM at bit rates of 10 Mb/s and 30 Mb/s, and these techniques are compared to on-off keying. Our results show that when MLSD is employed, 16-PPM provides the best average-power efficiency among the modulation techniques considered in this study.

Decision-feedback equalization of pulse-position modulation on measured nondirected indoor infrared channels

We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured nondirected indoor infrared channels with intersymbol interference. PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However, the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD.

Performance evaluation of baseband OOK for wireless indoor infrared LAN's operating at 100 Mb/s

We investigate 100 Mb/s wireless nondirectional infrared communication in the indoor environment using baseband nonreturn-to-zero (NRZ) on-off keying (OOK) modulation. We show that intersymbol interference induced by multipath propagation impairs detection efficiency. Analytical and simulation results on specific channels demonstrate that an adaptive decision-feedback equalizer adapted according to the least-mean-squares algorithm recovers most of the performance degradation. We also evaluate the performance of a timing-recovery phase-locked loop operating independent of the adaptive equalizer; showing that it quickly and accurately determines the sampling phase with negligible performance degradation. We discuss effective methods of mitigating low-frequency noise induced by fluorescent lighting. We present a packet-based communication method and describe its features and performance. Our results support the feasibility of communication at 100 Mb/s over the infrared channel. >

Performance evaluation of L-pulse-position modulation on non-directed indoor infrared channels

We examine the performance of high-speed non-directed infrared links using L-pulse-position modulation (L-PPM). When used with intensity modulation and direct detection, L-PPM leads to high average-power efficiency, because of its high peak-to-average power ratio. We derive an expression for the BER of an L-PPM link on an ISI channel. Using the measured characteristics of multipath indoor infrared channels, we calculate BER curves and ISI power penalties for 2-, 4-, 8- and 16-PPM links operating at 10 and 30 Mb/s. Our results indicate that L-PPM is a very promising modulation technique for high-speed non-directed infrared communication.<<ETX>>

Effect of electronic-ballast fluorescent lighting on wireless infrared links

Fluorescent lamps driven by electronic ballasts emit an infrared (IR) signal that is periodically modulated at rates of tens of kilohertz, and which can severely impair the performance of IR wireless links. The impact of fluorescent interference can potentially be reduced by high-pass electrical filtering, but such filtering induces intersymbol interference (ISI). We present expressions for the bit-error rate (BER) of systems using on-off keying (OOK) and pulse-position modulation (PPM) in the presence of both a deterministic interfering signal and ISI. We evaluate the performance of IR links using OOK and 2-, 4-, 8-, and 16-PPM at bit rates of 1, 10, and 100 Mb/s. The penalties incurred by OOK are found to be essentially independent of bit rate. At 1 Mb/s, PPM suffers approximately the same penalties as OOK, but as the bit rate is increased, the degradation of PPM becomes progressively much smaller In the absence of measures to prevent ISI, first-order highpass filtering is not effective in improving the performance of OOK, but can substantially improve PPM link performance at bit rates of 10 and 100 Mb/s.

Decision-feedback equalization of pulse-position modulation on measured non-directed indoor infrared channels

We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured non-directed indoor infrared channels with intersymbol interference (ISI). PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Detailed performance results for 2-, 4-, 8-, and 16-PPM links at bit rates of 10 Mb/s and 30 Mb/s over 46 actual measured indoor infrared channels are presented for these two DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD.

Performance of PPM with maximum-likelihood sequence detection on measured non-directed indoor infrared channels

Examines the performance of pulse-position modulation (PPM) on indoor infrared channels with multipath-induced intersymbol interference (ISI). The authors review maximum-likelihood sequence detection (MLSD) for PPM over ISI channels in additive white Gaussian noise. They evaluate the performance of PPM links other 46 actual measured indoor infrared channels. Detailed performance results are presented for 2-, 4-, 8-, and 16-PPM at bit rates of 10 Mb/s and 30 Mb/s, and these techniques are compared to on-off keying. They results show that when MLSD is employed, 16-PPM provides the best average-power efficiency among the modulation techniques considered.

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>>

Performance simulation of baseband OOK modulation for wireless infrared LANs at 100 Mb/s

The authors present simulation results of a wireless indoor infrared communication link operating at 100 Mb/s using non-return-to-zero baseband on-off keying. They show that intersymbol interference induced by multipath dispersion significantly impairs detection efficiency. They demonstrate that a decision feedback equalizer adapted according to the least-mean square algorithm can recover most of this performance degradation. They evaluate the performance of a timing-recovery phase-locked loop operating independent of the adaptive equalizer, showing that it can accurately determine the sampling phase with negligible performance degradation. They also discuss strategies to counter low-frequency noise induced by fluorescent lighting.<<ETX>>