The use of infrared radiation as a medium for high-speed short-range wireless digital communication is discussed. Available infrared links and local-area networks are described. Advantages and drawbacks of the infrared medium are compared to those of radio and microwave media. The physical characteristics of infrared channels using intensity modulation with direct detection (IM/DD) are presented including path losses and multipath responses. Natural and artificial ambient infrared noise sources are characterized. Strategies for designs of transmitter and receivers that maximize link signal-to-noise ratio (SNR) are described. Several modification formats are discussed in detail, including on-off keying (OOK) pulse-position modulation (PPM), and subcarrier modulation. The performance of these techniques in the presence of multipath distortion is quantified. Techniques for multiplexing the transmissions of different users are reviewed. The performance of an experimental 50-Mb/s on-off-keyed diffuse infrared link is described.

Wireless Infrared Communications

Wireless infrared communications can be used to establish short range high-data-rate wireless links. The primary commercial applications are short-term cable-less connectivity for information exchange and wireless local area networking. In the article, the major optical design issues and communication design issues are discussed. An overview of current systems and standards is provided, along with a comparison of infrared systems to radio systems.


Keywords:

wireless infrared communications;
channel modeling;
digital modulation;
IRDA

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We examine multiple-subcarrier modulation (MSM) schemes for wireless infrared digital communication in the indoor environment. Intensity modulation with direct detection (IM/DD) is employed, which results in equivalent baseband channels with a nonnegativity constraint on the input. The power efficiencies of modulation schemes are compared at 30 Mb/s and 100 Mb/s over an ensemble of experimentally determined multipath channels. Carrier selection and power shaping are examined as methods for improving MSM performance. It is found that MSM schemes can allow operation at higher data rates than single-carrier modulation schemes without equalization. Moreover, MSM schemes can be more bandwidth-efficient and also can provide a simple and flexible method for multiple access to the channel. However, they are not as power efficient as single-carrier schemes, and this will limit their use to applications which are not power limited.

Multiple-subcarrier modulation for non-directed wireless infrared communication

We propose an optical wireless indoor localization using light emitting diodes (LEDs) and demonstrate it via simulation. Unique frequency addresses are assigned to each LED lamp, and transmitted through the light radiated by the LED. Using the phase difference, time difference of arrival (TDOA) localization algorithm is employed. Because the proposed localization method used pre-installed LED ceiling lamps, no additional infrastructure for localization is required to install and therefore, inexpensive system can be realized. The performance of the proposed localization method is evaluated by computer simulation, and the indoor location accuracy is less than 1 cm in the space of 5m x 5 m x 3 m.

TDOA-based optical wireless indoor localization using LED ceiling lamps

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and RMS delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion.

Modeling of nondirected wireless infrared channels

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

Non-directed infrared links for high-capacity wireless LANs

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.

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

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 highpass electrical filtering, but such filtering induces intersymbol interference (ISI). The authors 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. They have measured the interference waveforms from lamps driven by 22 and 45 kHz ballasts, and have used the measured waveforms to evaluate the performance of IR links using OOK and 2, 4, 8 and 16 PPM at bit rates of 1, 10, and 100 Mbit/s. When the fluorescent interference is normalised to the signal power required in the absence of this interference, the penalties incurred by OOK are found to be essentially independent of bit rate. At 1 Mbit/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 Mbit/s.

Effect of electronic-ballast fluorescent lighting on wireless infrared links

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.

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

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. >

M.D. Audeh

Papers

Non-directed infrared links for high-capacity wireless LANs

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

Effect of electronic-ballast fluorescent lighting on wireless infrared links

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

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