IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 49, NO. 3, MAY 2000 1303 Effect of Anisotropic Propagation Modeling on Microcellular System Design Dongsoo Har and Henry L. Bertoni, Fellow, IEEE Abstract—Microcells for wireless communications can be real- ized with low base station antennas operating at low power. The low base station antennas expected for microcells make the propaga- tion characteristics dependent on the direction relative to the street grid. Due to this anisotropic propagation, the shape of microcells is no longer circular, as is typically assumed so for macrocellular system planning. Therefore the infrastructure of microcells should be implemented with a different approach from that for macro- cells. This paper aims at finding the effect of microcellular wave propagation on the development of cellular design for channel- ized systems in residential/commercial environments by examining key aspects of cell layout. Using a measurement-based anisotropic propagation model, cell shape and frequency reuse patterns are in- vestigated for the downlink, and a methodology for frequency plan- ning is presented. Index Terms—Cell plannings, cell shape, propagation modelings, time-division multiple-access (TDMA) microcells, traffic capacity. I. INTRODUCTION F OR the macrocells of Cellular Mobile Radio (CMR), which make use of antennas located well above the rooftops, the variation of the radio signal over the coverage area can be rep- resented by a single isotropic path loss model [1], [2], together with the log-normal fading. For isotropic propagation, the equi- pathloss contours are circles, which are approximated by the familiar hexagonal cell shape that has been used to determine reuse patterns and study subsequent system performances. The growth of mobile radio networks requires efficient fre- quency planning. While various approaches such as frequency hopping [3]–[5], adaptive antenna arrays [6], [7], and fractional loading [8]–[10] give increased frequency reuse, reducing cell size is the most common approach for increasing capacity. In case of microcells using antennas located at a lamppost height, propagation is far from being isotropic [11], because of the prop- agation dependence on the direction relative to the street grid, thereby the use of symmetric hexagonal cells to tessellate the plane is no longer valid. To date, the issue of cell planning based on anisotropic propagation appears to have been addressed only for the case of high-rise building environments [12]–[14] and for linear microcells [15], [16]. In this paper, we examine design issues for a microcellular system that can accommodate the heavy traffic load forecast for mature systems, and study performance in typical residen- Manuscript received December 3, 1998; revised May 18, 1999. D. Har is with AirTouch Cellular, Walnut Creek, CA 94598 USA (e-mail: dhar@nit.airtouch.com). H. L. Bertoni is with Center for Advanced Technology in Telecom- munications, Polytechnic University, Brooklyn, NY 11201 USA (e-mail: hbertoni@poly.edu). Publisher Item Identifier S 0018-9545(00)03681-1. tial/commercial environments using appropriate anisotropic propagation models. A procedure suggested for finding loca- tions of significant cochannel cells in different reuse patterns, for each reuse factor, based on diamond cell shape is described in detail. The use of regular hexagons for cells provides a conceptual basis for analysis, as well as the zero order design, for macrocellular systems. However, in actual systems the cells are far from being hexagons and the cell design employs much more sophisticated methods [17]. In the same way, this study is intended to provide a conceptual basis for understanding the effects of direction dependent path loss, recognizing that actual system design will be more complex. Since the path loss values on some routes were shown to be smaller than on any other non-LOS route [11] a cochannel cell located at a large distance on those routes gives the same level of interference as does a cochannel cell at the consider- ably shorter distance on the other non-LOS routes. This dif- ference of wave propagation will be considered for the evalu- ation of microcellular system performance. This paper focuses on a methodology for downlink microcellular system design employing a diamond pattern for locating base stations for fre- quency reuse systems. The methodology determines system pa- rameters: frequency reuse factor ; reuse pattern; and base station separation in two dimensions based on achieving a de- sired QOS, which is specified in terms of call blocking proba- bility and the bumping probability e.g., a probability that falls below the predetermined protection ratio. To study system performance, compact fit formulas for the mean and standard deviation of effective slow fading from interfering sig- nals are developed in the Appendix for numerical analysis. II. CELL SHAPE IN RESIDENTIAL/COMMERCIAL ENVIRONMENTS Empirical path loss formulas for typical residential/commer- cial environments consisting of 2–5 story buildings were ob- tained in [11] for the base station located on the street in the middle of a block, as shown in Fig. 1(a), when the receivers were located on particular streets within the radial distance range km. In [11] use of those formulas to cover all streets for mid-block base stations was extended to treat the case of a backyard base station, as shown in Fig. 1(b) for which the path loss dependence on the distance is approximated by the for- mulas obtained from the measurements for a mid-block base station. It was shown in [11] that equi-pathloss contours at PCS fre- quency corresponding to 110–130 dB range give a diamond like cell shape. Table I lists the path loss formulas and parameters which are based on measurements made in the two low-rise 0018–9545/00$10.00 © 2000 IEEE