Propagation measurements and models for wireless communications channels

Abstract

To achieve ubiquitous PCS, new and novel ways of classifying wireless environments will be needed that are both widely encompassing and reasonably compact. ireless personal communicationscouldinprincipleusesevera1 physical media, ranging from sound to radio to light. Since we want to overcome the limitations of acoustical communications, we shall concentrate on propagation of electromagnetic wavcs in the frequency range from some hundreds of MHz to a few GHz. Although thereisconsiderable interest atthe moment in millimeter wave communications in indoor environments, they will be mentioned only brieflyin this survey of propagation of signals.It is interesting to observe that propagation results influence personal communications systems in several ways. First there is obviously the distribution ofmeanpoweroveracertainareaorvolumeofinterest, which is the basic requirement for reliable communications. The energy should be sufficient for the link in question, but not too strong, in order not to create cochannel interfcrcnce at a distance in another cell. Also, since the radio link is highly variable over short distances, not only the mean power is significant; the statistical distribution is also important. This is especially true when the fading distribution is dependent on thc bandwidth of the signal. Secondly. even if there is sufficient power available for communications, the quality of the signal may be such that large errors occur anyway. This results from rapid movement through thescatteringenvironment,or impairments due to long echoes leading to inter-symbol-interference. A basic understanding of the channel is important for finding modulation andcodingschemes that improve thc channel, for designing equalizers or, if this is not possible, for deploying basc station antcnnas in such a way that the detrimental effects are less likely to occur.In this article we will describe the type of signals that occur invarious cnvironments and the modeling of the propagation parameters. Models are essentially of two classes. The first class consists of parametric statistical models that on average describcthephenomenonwithinagivenerror.They are simple to use, but relativcly coarse. In the last few years a second class ofenvironment-specific mode1shasbeenintroduced.Thesemodelsareofamore deterministicnaturc,characterizingaspecificstreet, building, etc. They are necessarily more time consuming to use, but are also more revealing concerning physical details and hopefully morc accurate.Firstsome keyparametersandthemeasurement ofthemwill be discussed and then the differentwireless environments will be treated. The latter topic is divided here into outdoor environments, indoor environments, and radio penetration from outdoor to indoor environments.The Physics of Propagation T hemechanismswhich governradio propagation are complex and diverse, and they can generally be attributed to three basic propagation mechanisms: reflection, diliraction, and scattering.Reflection occurs when a propagating electromagnetic wave impinges upon an ...

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