The overall approach to laying out a cellular system is founded on ensuring a capability for systematic growth. When a new system is deployed, user demand is low, and users are assumed to be uniformly distributed over the area to be served. Initial system layout is designed to provide uniformly reliable coverage and uniform capacity over the entire service area. As new users subscribe to the cellular service, the demand for channels may begin to exceed the capacity of some base stations. As we mentioned earlier, this increased demand often first shows up in the downtown areas of cities, where the population is dense during the working day. In the previous section we showed how the number of channels available to customers, or equivalently, the density of channels per square kilometer, can be increased by decreasing the cluster size. Once a system has been initially deployed, however, a system wide reduction in the cluster size may not be war- ranted, since user density does not grow at the same rate in all parts of the system.
Cell splitting is a technique that provides the capability to add new smaller cells in specific areas of the system to support increased demand in those areas, while minimizing the need to modify the existing cell parameters. Cell splitting is based on cell radius reduction.
There are two challenges to increasing the system capacity by reducing the cell radius.
Clearly, if cells are smaller there will have to be more of them, so additional base stations will be needed in the system. The challenge in this case is to reconfigure the system in such a way that existing base station towers do not have to be moved. The second challenge involves meeting an evolving and generally increasing demand that may vary dramatically between different geographic areas of a system. For example, a city center may have the highest density of users and therefore should be supported by cells with the smallest radius. The radii of cells in a typical system generally increase as one moves from urban to suburban to rural areas, since user density typically decreases as one moves away from a city center. The key challenge, then, is to add the minimum number of smaller cells wherever increased demand dictates the need for increased capacity. A gradual addition of new base stations and smaller cells implies that, at least for a
time, the cellular system may have to operate with cells of more than one size.
Figure 4.19(a) shows a cellular layout with seven-cell clusters. Let us suppose that the cells in the center of the diagram are becoming congested, and cell A in the center is approaching user capacity. Figure 4.19(b) shows an overlay of smaller cells superimposed on the original layout. The new smaller cells have half the cell radius of the original cells. At half the radius, the new cells will have one-fourth of the area and will consequently need to support one-fourth the number of subscribers. Notice that one of the new smaller cells lies in the center of each of the larger cells. If we assume that base stations are located in the cell centers, this allows the original base stations to be maintained as the new cell pattern spreads outward from the center. Of course new
base stations will have to be added for new cells that do not lie in the center of the larger cells.
Recall that the organization of cells into clusters is independent of the cell radius, so that the cluster size can be the same in the small-cell layout as it was in the large-cell layout. Recall that signal-to-interference ratio is determined by cluster size and not by cell radius. Consequently, if the cluster size is maintained, the signal-to-interference ratio will be the same after cell splitting as it was beforehand. If the entire system is replaced with new half-radius cells, and the cluster size is maintained, the number of channels per cell will be exactly as it was before, and the number of subscribers per cell will have been reduced. In large cities it is not uncommon for cellular systems to be configured into “microcells” whose radii are measured in hundreds of meters, rather than in kilometers.
When the cell radius is reduced by a factor, it is also possible, and desirable, to reduce the power in transmitted signals. The minimum required power level is determined by the need to maintain an adequate signal-to-noise ratio over a significant fraction of the cell area, which in turn requires a minimum signal-to-noise ratio at the cell radius. The following example illustrates the idea.
If a cellular layout is replaced in its entirety by a new layout with a smaller cell radius, the signal-to-interference ratio will not change, provided the cluster size does not change. Some special care must be taken, however, to avoid cochannel interference when both large and small cell radii coexist, as in the system of Figure 4.18. It turns out that the only way to avoid interference between the large-cell and small-cell systems is to assign entirely different sets of channels to the two systems. If a large-cell system becomes congested in the downtown, for example, channels can be taken away from the large-cell system to make up channel sets for the small-cell system. The capacity of the large-cell system will be reduced, but the large-cell system will now be used primarily in the suburbs, where the user density is low. The small-cell system also does not have a full complement of channels, but as the cell area is small, there may not be enough users per cell to demand a full channel set. As the small-cell system continues to spread, more and more channels can be reassigned from the large-cell to the small-cell system, until ultimately, the large-cell system is completely replaced.
Next: Operational Considerations
About the Authors
Bruce A. Black completed his B.S. at Columbia University, his S.M. at Massachusetts Institute of Technology, and his Ph.D. at the University of California at Berkeley, all in electrical engineering. Since 1983 he has been on the faculty of the Department of Electrical and Computer Engineering at Rose-Hulman Institute of Technology in Terre Haute, Indiana, where he has been advisor to Tau Beta Pi and is advisor to the Amateur Radio club (W9NAA). In 2004 he was named Wireless Educator of the Year by the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Philip S. DiPiazza received a B.E.E from Manhattan College in 1964, an M.E. in electrical engineering from New York University in 1965, and a Ph.D. (electrical engineering) from the Polytechnic Institute of New York in 1976. Dr. DiPiazza was responsible for the system integration and test of the first North American deployment of AMPS.. He is currently an Adjunct Professor at the Rose-Hulman Institute of Technology and a Senior Consultant with Award Solutions, Inc. Dr. DiPiazza is an advisor and member of the Global Wireless Educational Consortium and a member of the IEEE.
Bruce A. Ferguson received the B.S., M.S., and the Ph.D. degree in electrical engineering from Purdue University, West Lafayette, Indiana in 1987, 1988, and 1992 respectively. He is currently a Communication System Engineer with Northrop Grumman Space Technology. He has worked with space and ground communication systems and photonics at TRW Space and Electronics (now NGST), and taught at Rose-Hulman Institute of Technology and The University of Portland in Oregon. Dr. Ferguson is a member Eta Kappa Nu and IEEE.
David R. Voltmer received degrees from Iowa State University (B.S.), University of Southern California (M.S.), and The Ohio State University (Ph.D.), all in electrical engineering. During nearly four decades of teaching, Dr. Voltmer has maintained a technical focus in electromagnetics, microwaves, and antennas. His more recent efforts are directed toward the design process and project courses. He has served in many offices of the ERM division of ASEE and in FIE. Dr. Voltmer is an ASEE Fellow and a Life Senior member of IEEE.
Frederick C. Berry received the B.S., M.S., and D.E. degrees from Louisiana Tech University in 1981, 1983, and 1988 respectively. He taught in the Electrical Engineering Department at Louisiana Tech University from 1982 to 1995. Currently Dr. Berry is Professor and Head of the Electrical and Computer Engineering Department at Rose-Hulman Institute of Technology. In 2007 he became Executive Director of the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Title: Introduction to Wireless Systems ISBN: 0132447894 Chapter 4: Radio Frequency Coverage: Systems Engineering and Design
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