1 Introduction Uvod In wireless technologies information is sent by electromagnetic waves. During propagation an interaction between waves and environment attenuates the signal level. It causes path loss and finally it limits coverage area. The accurate path loss prediction is a crucial element in the first step of network planning. The capability of determining optimum base-station locations, obtaining suitable data rates and estimating coverage without conducting a series of propagation measurements (what is very expensive and time consuming) can be achieved with empirical propagation models. Empirical propagation models are designed for a specific type of communication systems, specific system parameters and types of environment. Therefore, selection of a suitable propagation model is the first step in the wireless network design. Okumura-Hata [1- 2], COST 231-Hata [3] and COST 231 Walfisch-Ikegami [4] are widely used models for the path loss prediction in frequency bands below 2 GHz. However, new wireless systems are designed to operate on higher frequencies, i.e. 2,4 GHz, 3,5 GHz, 5 GHz. A new model (i.e. SUI model) for the band below 11 GHz has been developed by Stanford University, as an extension of the Hata model. In contrast to numerous publications that exist for path loss measurements for frequencies below 2 GHz, there are very few works that present experimental results for higher frequency bands. A comparison between a ray-tracing approach and empirical models for a frequency of 2,154 GHz is given in [5]. A modification of the ITU-R P.1411 model [6] to enhance prediction accuracy in urban environments is presented in [7], as well as measurement results at 2,17 GHz. In [8] experimental results for a system working at 3,5 GHz are compared against prediction made by different empirical propagation models. A simple 43 J. Milanović, S. Rimac-Drlje, I. Majerski ISSN 1330-3651 UDC/UDK 621.391.8 : 004.733 (497.13 Osijek) RADIO WAVE PROPAGATION MECHANISMS AND EMPIRICAL MODELS FOR FIXED WIRELESS ACCESS SYSTEMS Josip Milanović, Snježana Rimac-Drlje, Ivo Majerski This paper provides a survey of the basic mechanisms which influence the propagation of electromagnetic waves at most. It also deals with features of empirical models often used in a process of fixed wireless access network planning and implementation. Four empirical models, SUI, COST 231-Hata, Macro and Ericsson, which are most suitable for path loss prediction for such a system, are presented. By using these propagation models the receiving signal levels are predicted for different types of environment for a WiMAX (Worldwide Interoperability for Microwave Access) system installed in the city Osijek, Croatia. Measurement results of receiving WiMAX power at 3,5 GHz are also presented and compared with the results predicted by using the propagation models. Keywords: empirical models, radio propagation, WiMAX Original scientific paper Ovaj rad daje pregled osnovnih mehanizama koji najviše utječu na prostiranje elektromagnetskih valova. Također se bavi značajkama empirijskih modela koji se često koriste u procesu planiranja i implementacije fiksnih radijskih pristupnih mreža. Predstavljena su četiri empirijska modela koja najbolje odgovaraju za predviđanje gubitaka za ove sustave: SUI, COST 231- Hata, Macro i Ericsson model. Korištenjem ovih modela prostiranja napravljena je predikcija razine prijemnog signala za različite tipove okruženj uspoređeni s rezultatima predviđenim uporabom modela prost a za WiMAX (eng. Worldwide Interoperability for Microwave Access) sustav postavljen u gradu Osijeku, u Hrvatskoj. Predstavljeni su i rezultati mjerenja prijemne snage WiMAX sustava na 3,5 GHz te su iranja. Klju ne rije i: č č empirijski modeli, prostiranje radio vala, WiMAX Izvorni znanstveni članak Mehanizmi prostiranja radio vala i empirijski modeli za fiksne radijske pristupne sustave Mehanizmi prostiranja radio vala i empirijski modeli za fiksne radijske pristupne sustave Technical Gazette 17, (2010), 43 1 -52 empirical model based on measurements at 5,3 GHz is proposed in 9. Since accuracy of the path loss prediction significantly depends on the type of environment, as more experimental data for different environments are available, the better model fitting to real conditions can be done. The paper is organized as follows: in Section 2 basic mechanisms of radio wave propagation are discussed. Characteristics of the radio wave propagation in built-up areas are given in Section 3. In Section 4 empirical propagation models suitable for the path loss prediction in a fixed wireless access system are presented. Section 5 gives experimental results for a WiMAX system on 3,5 GHz as well as a comparison between measured results and predictions obtained by different empirical propagation models. Concluding remarks are given in Section 6. During propagation between the transmitting and the receiving antenna, radio waves interact with environment, causing path loss. Path loss ( ) is defined as the difference between the transmitted and the received power as shown in (1): 2 Basic mechanisms of electromagnetic wave propagation Temeljni mehanizam elektromagnetskog prostiranja vala PL where and are the transmitted and the received power, and are gain of the transmitting and the receiving antenna and and are feeder losses, all in a dB scale. Propagation in free space path loss (FSL) can be expressed as [10]: P P G G L L T R T R T R T T R R T R PL P G G P L L = + + - - - , (1) freespace 32,4 20 lg 20 lg PL d f = + × + × , dB (2)