602 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 20, NO. 3, APRIL 2002 Use of Cell-Site Diversity in Millimeter-Wave Fixed Cellular Systems to Combat the Effects of Rain Attenuation Gamantyo Hendrantoro, Member, IEEE, Robert J. C. Bultitude, Senior Member, IEEE, and David D. Falconer, Fellow, IEEE Abstract—An examination of potential advantages of cell-site diversity with selection combining in the context of mm-wave fixed cellular systems is reported. The study involved simulation of converging radio links over weather radar images of the radar reflectivity factor, from which the specific attenuation of rain at 30 GHz was derived. The average correlation of attenuation on two converging links as a function of their angular separation is shown to indicate the potential benefits in the use of cell-site diversity, especially in heavy rain. Results show that diversity gain exhibits a dependence on angular separation in the general form of . For links of identical lengths the model reduces to a root-sinusoidal shape , whereas links of unequal lengths lead to the ITU-R recommended model . Based on the model and observation of the length ratio of the links, a set of criteria for determining the benefit of cell-site diversity for a given subscriber location is proposed. Index Terms—Diversity methods, fixed wireless cellular systems, millimeter wave radio propagation meteorological factors, rain. I. INTRODUCTION M OTIVATED by recent interest in North America and Eu- rope in the use of millimeter-wave radio frequencies to provide wireless access to broadband services, embodied in a system generally termed Local Multipoint Distribution System (LMDS), research activities have been initiated worldwide to study the propagation characteristics of millimetric radio waves. One of the natural phenomena that significantly influence the performance of communication systems operating in the cited band, but the study of which has not yet been sufficiently thor- ough, is rain attenuation. This is particularly true when the influ- ence of rain is put into the context of point-to-multipoint, fixed cellular radio networks, in which spatial statistics of rain atten- uation are as important as single-point temporal statistics. Manuscript received March 12, 2001; revised September 11, 2001. This work was supported by the Canadian Institute for Telecommunications Research in cooperation with the Communications Research Centre. The work of G. Hen- drantoro was supported by a Higher Education Project scholarship award from the Indonesian government. G. Hendrantoro is with the Department of Systems and Computer Engi- neering, Carleton University, Ottawa, ON K1S 5B6, Canada on leave from the Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia (e-mail: gamantyo@sce.carleton.ca). R. J. C. Bultitude is with the Communications Research Centre, Ottawa, ON K2H 8S2, Canada (e-mail: robert.bultitude@crc.ca). D. D. Falconer is with Carleton University, Ottawa, ON K1S 5B6, Canada (e-mail: ddf@sce.carleton.ca). Publisher Item Identifier S 0733-8716(02)03381-4. Fig. 1. Typical scenario of cell-site diversity. At high rainfall intensities, the occurrence of which is of spe- cial interest in systems requiring high-reliability links, the hor- izontal structure of rain is highly variable [1]. It is frequently observed that during a shower, high intensity rain is localized in a very small area surrounded by a region of more uniform, low intensity rain. Hence, in a cellular system under rain with very localized storms, there is a potential that a subscriber ter- minal receiving a heavily attenuated signal from one hub can ob- tain less attenuated, acceptable reception from another hub. This makes cell-site diversity appear as a very promising method for improving link reliability and area coverage. Such a scenario can be roughly sketched as in Fig. 1 for cells with overlapping areas. When the link between a subscriber ter- minal (ST) and Hub 1, which is the closest and hence the default hub for ST, experiences a performance drop due to a high-inten- sity shower, there remains a possibility that Hub 2 or 3 can take over the service delivery. Which of the two hubs is selected to receive the handover depends on which hub location results in better signal reception at ST. This is all assuming that the re- ceiver and antenna technology used at the subscriber site allows these links to be monitored simultaneously and continually, and the antenna main beam to be rotated, either electrically or physi- cally, to execute the handover. In principle, it is akin to a macro- diversity scheme employed in mobile cellular systems to combat shadowing effects, taking the advantage of the fact that the ef- fects of obstruction losses around links that connect a mobile terminal to different base stations are only partially correlated [2], [3]. In order to take the spatial variation of rain attenuation into account, field data in the form of radar reflectivity factor 0733-8716/02$17.00 © 2002 IEEE