Jeffrey B. Carruthers

Experimental characterization of non-directed indoor infrared channels

We have experimentally characterized nondirected indoor channels that use intensity modulation and direct detection of an infrared carrier at a wavelength of 832 nm. At several locations in each of five different rooms, we have studied line-of-sight and diffuse link configurations, with and without shadowing, amounting to a total of approximately 100 different channels. We have measured channel frequency responses over the 2-300 MHz range by using a swept-modulation frequency technique, and from these data, we have computed channel impulse responses, path losses and r.m.s. delay spreads. Using channel impulse responses, we have calculated power penalties induced by multipath intersymbol interference in baseband on-off-keyed links operating at bit rates of 10, 30 and 100 Mb/s, considering unequalized links and those employing zero-forcing decision-feedback equalization. Unshadowed line-of-sight configurations generally have smaller path losses, r.m.s. delay spreads and power penalties than their unshadowed diffuse counterparts. Shadowed line-of-sight configurations, however, generally exhibit larger values of all three parameters than the corresponding shadowed diffuse configurations. We show that among the channels measured here, there is a strong correspondence between channel r.m.s. delay spread and multipath power penalty. Finally, we provide an analysis indicating why non-directed infrared channels using intensity modulation and direct detection do not exhibit multipath fading, and justifying their representation as linear, time-invariant systems. >

Multiple-subcarrier modulation for non-directed wireless infrared communication

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.

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

RTS/CTS-induced congestion in ad hoc wireless LANs

The RTS/CTS mechanism is widely used in wireless networks in order to avoid packet collisions and, thus, achieve high throughput. In ad hoc networks, however the current implementation of the RTS/CTS mechanism may lead to interdependencies so that nodes become unable to transmit any packets during long periods of time. This effect manifests itself in the form of congestion where, after a certain point, the network throughput decreases with increasing load instead of maintaining its peak value. In this paper, we describe and analyze this problem in detail and provide a backward-compatible solution, called RTS validation. Our simulations show that this solution leads to a 60% gain in the peak throughput in addition to stabilizing the throughput at high load.

Iterative site-based modeling for wireless infrared channels

We describe an iterative site-based method for estimating the impulse response of wireless infrared channels. The method can efficiently account for multiple reflections of any order. A simple geometrical model of indoor environments is presented which includes interior features such as partitions, people, and furniture, thus permitting accurate evaluation of shadowing effects. For a reflection order of three, the iterative method is over 90 times faster than the existing recursive technique. A computer implementation is described and used to demonstrate the efficiency and accuracy of the method.

Performance of wireless networks with hidden nodes: a queuing-theoretic analysis

Hidden nodes are a fundamental problem that can potentially affect any wireless network where nodes cannot hear each other. Although the hidden node problem is well known, so far only few papers have quantified its effects in a comprehensive manner. This paper represents a first step towards getting a quantitative insight into the impact of hidden nodes on the performance of wireless networks. We first carry out an exact queuing-theoretic analysis for a 4-node segment and derive analytical expressions for the probability of packet collision, the mean packet delay, and the maximum throughput, based on a model that closely follows the IEEE 802.11 standard. We then extend the analysis and provide an approximation for a general linear topology that is asymptotically exact at low load. Finally, we perform detailed simulations to validate our analytical results and show their applicability to predict the performance of IEEE 802.11 networks with hidden nodes. The simulation and analysis closely match. Moreover, they reveal that the impact of hidden nodes propagates through the network causing some nodes to saturate at load as low as 15% of the capacity.

Modeling of nondirected wireless infrared channels

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and r.m.s. 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.

Evaluation of the masked node problem in ad hoc wireless LANs

IEEE 802.11 wireless networks employ the so-called RTS/CTS mechanism in order to avoid DATA packet collisions. The main design assumption is that all the nodes in the vicinity of a sender and a rec...

Angle diversity for nondirected wireless infrared communication

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared communications systems. Multiple transmitter beams and multiple narrow-field-of-view receivers reduce the path loss, multipath distortion and background noise of the channel, which leads to improved range. The design and performance of a prototype angle-diversity infrared communication system are discussed. The prototype can maintain 70 Mb/s at a BER of 10/sup -9/ over a 4 m range.

Spotlighting for visible light communications and illumination

The trend toward solid state lighting with white LEDs has motivated much research for using these devices to provide wireless broadband data communications. Much work in this area has attempted to fit VLC into currently dominant indoor lighting modes, which broadcast the light in a wide field to achieve uniform coverage throughout a room. In this paper we explore spotlighting, which is appropriate lighting for many scenarios, as an alternative for implementing high datarate VLC. We find that spotlighting VLC has several benefits over uniform lighting implementations, including enabling higher datarate densities within a room and less channel distortion. We also introduce a hybrid scheme that combines spotlighting with uniform lighting to provide wide area data coverage as well as high-datarate “white hot spots” where needed.