Propagation Losses and Impulse Response of the Indoor Optical Channel: A Simulation Package

Dept. of Electronics and Telecommunications University of Aveiro 3800AVEIRO PORTUGAL Tel: Fax: email: +351 34381937 +351 34381941 [email protected] In this paper we present a simulation package developed to calculate both the channel propagation losses and the multipath dispersion in the indoor optical channel. The simulation package is based on a model of the indoor optical channel which includes the total optical power and radiation pattern of the emitter, the channel free-space losses and reflection properties and the active area and fiel-of-view of the receiver. We illustrate the use of the simulation package by means of two case studies corresponding to: i) a satellite based cell and ii) a passive reflection based cell. It is seen that optimization of the emitting pattern by tunning the number, orientation and radiation pattern of the LEDs can significantly reduce the maximum channel losses. This work is being camed out as part of the ESPRIT.6892 POWER (Portable Workstation for Education in Europe) project commisioned by the CEC. I Introduction The need and demand to communicate and share software applications. data bases and information in general while keeping mobility have increased very rapidly over the past few years. This growth required the development of practical and flexible communication networks. Wireless networks represent a very good alternative to cabled networks for indoor applications. Indoor wireless communication systems can be used in a wide range of applications to provide for portability. user mobility and easy. setup networking. Indoor wireless systems can use two kinds of carrier to convey the information: radio waves or infrared radiation. There are already systems commercially available using each of the technologies. The use of a particular technology depends mainly upon the envisaged application and the physical channel characteristics. In this work, we concentrate on indoor communication systems using infrared technology. Submission page 1 Cipriano Lomba / Univ. of Aveiro May 1993 doc: IEEE P802~11/93 78 Recently, infrared technology based systems have been receiving significant interest because they: . • are easier to implement than radio systems. • do not require any licensing. • are not affected by electromagnetic noise interference and • provide for privacy outside the room premises. Indoor infrared systems are mainly impaired by three factors: • Interference from ambient light (sun light and artificial illumination), • Mutipath dispersion (for data rates higher than a few Mbit/s, depending on the room dimensions) and • Technological limitations of the infrared devices available (LEDs and PIN photodetectors ). There are mainly three basic propagation modes on indoor infrared systems: • Point-to-pomt communication links, • Quasi-diffuse systems and • Diffuse systems. These modes have been extensively discussed in the literature [1] [2] [3]. The main interest of our study is the characterization of the indoor channel under the quasi-diffuse and diffuse propagation modes. In this paper we present a simulation package developed to calculate both the channel propagation losses and the multipath dispersion. A brief discussion of the characteristics of the quasi-diffuse propagation mode is given in section II. The model of the optical channel used in the simulation package is presented and discussed in section III. In section IV we illustrate the use of the simulation package by means of two case studies. Finally, in section V we present the main conclusions and some guidelines for future work. This work is being carried out as part of the ESPRIT.6892 POWER (Portable Workstation for Education in Europe) project commisioned by the CEC. II The Quasi-diffuse propagation mode In the quasi-diffuse mode the signal emitted by one station is broadcasted to all the other stations in the room. This requires emitters and receivers to have relatively large emitting beams and field-ofviews, respectively. A direct line-of-sight between emitter and receiver is not required. However all stations must have a direct path to the reflecting surface. Usually, the reflecting surface is the room ceiling which must present good reflection characteristics. If the room ceiling surface does not reflect properly the infrared signals or if one intends to extend the coverage area an active reflector can be used. This active refle·ctor also called "satellite" is mounted at the ceiling. The satellite receives the signal broadcasted by the emitting stations, amplifies the signals and then broadcasts them again. The satellite increases system complexity and cost and therefore its use should be avoided. Two variations of the quasi-diffuse mode will be considered: • Targeted orientation: assumes the user will have to point the optical interface of the transceiver to the satellite or to some specific area at the room ceiling if the system uses passive reflection. • Natural orientation: assumes that all terminals have a large emitting beam and receiving FOVand all will be oriented vertically. Submission page 2 Cipriano Lomba I Univ. of Aveiro May 1993 doc: IEEE P802.11193 78 III • Model of the Indoor Optical Channel and Simulation Package Before one invest in the design and implementation of a communication system it is fundamental to have an accurate model for the channel characteristics. In this section, we will present the model of the optical indoor channel used in the simulation package. The model includes the characteristics of the emitted optical beam, the effects of the channel over that beam and the receiver collecting characteristics. This model can be used for the evaluation of: • the channel propagation losses and • the channel impulse response. The simulation package allows for optimization of the power distribution over the cell area. The optimization process has in view the reduction of the worst-case propagation losses. The simulation package is being improved to obtain the impulse response of the indoor channel. For that purpose, multiple-order reflections from the room walls are being considered. The channel impulse response is important on the design of infrared indoor systems operating at data rates higher that a few Mbitls. The main characteristics of the quasi-diffuse indoor channel are now presented. The source emitted power is spread over the room space. The power spreading degree depends directly on the source position, orientation and radiation pattern. The emitted beam incides on the room walls and furniture and is partially reflected according to the reflection coefficients and patterns of the surfaces. The optical power collected at the receiver depends on the receiver position. orientation, flelf-of-view and active area. The received signal results from the direct signal (line-of-sight) and multiple-order reflected signals that incide into the detector active area. The signal collected from the multiple-order reflections will result on a signal spread or intersymbol interference. The components of the optical channel are the optical sources, the room space and configuration and the optical collectors. Usually, the optical sources and collectors are fonned by sets of LEOs and PINs, respectively. The models used in the simulation package will now be presented. 1) The source model Indoor infrared communication systems usually use short wavelength (820-900 nm) Light Emitting Diodes (LEOs). Following Gfeller [1], the radiant intensity of a LED can be modeled using an extension of the Lambertian law given by: E(tfJ) = ~:; Prcos"(tfJ) (1) where P t (usually supplied by the manufacturer data sheets) is the total emitted power, ¢ is the angle with the perpendicular to the LED lens and n is a value given by 0.693 n = In[cos(HPBW)] (2) where HPBW is the LED half power emission angle. For HPBW=60° n is unity and Eqn. (1) reduces to the case of the ideal Lambertian radiator. The higher the value of n the more directive is the radiation pattern of the LED. 2) The Channel Propagation Model There are mainly two independent factors influencing the channel propagation characteristics: the free-space propagation losses and the signal n~flections on the room surfaces. The free-space Submission page 3 Cipriano Lomba / Univ. of Aveiro May 1993 doc: IEEE P802.11193 78 losses under consideration are decribed by the 1Ir2 law. Other characteristics of atmospheric systems, like signal attenuation, dispersion and scattering, are not considered since the ranges of indoor systems are very small and those effects are negligible. The reflection characteristics of any surface depends upon the surface material and texture. The reflection pattern from a surface is usually composed of a diffuse and a specular component. There are several models describing the reflection properties of typical surfaces available from the literature [4}. The most appropriate model to use depends on the surface characteristics and also on the complexity of the model we are able to implement. We will consider the Lambertian reflection model which is characteristic of perfectly diffusing surfaces. It has been shown that this model is a good approximation for some surfaces [1]. Measurements of the reflection pattern of the most common room surfaces will be carried out and, if required, more complex models will be included in the simulation package. In the simulation package, the reflecting surfaces are divided into incremental areas characterised by a reflection coefficient and a pure Lambertian radiation pattern given by R(fJ) = }..cos(fJ) (3) 7r where fJ is the emission angle. The incremental areas should be relatively small for the point source approximation to be satisfactory. The simulation of the impulse response of the indoor optical channel requires high resolution and a very large number of incremental areas have to be considered. This factor will result in a simulation package requiring very demanding computational capabilities. 3) The Detector Model In indoor wireless infrared systems, the most used detectors are large area silicon PositiveIntrinsic-Negative, PIN, photodetectors. To achieve larger collecting areas, without increasing the PIN capacitance, a lens to concentrate the incident radiation on the detector active area can also be employed. In both cases, the detector is modeled as having an active area which collects the power incident for angles smaller than the FOV. In the case of a lens being in use the transmitance factor of the lens has to be considered.