A wide variety of wireless indoor communication systems are enjoying increased popularity in industrial, commercial, and residential environments. These communication systems include, for example, cordless, wireless, and cellular telephones, paging systems, two-way radio services, video conferencing systems, and others. Quite frequently, the overall effectiveness of a given system is determined by the performance and reliability of one or more RF communications links. Whenever possible, it is important to position transmitting and receiving antennas such that solid, reliable communications links are provided throughout the desired operational area of a wireless communications system. For example, wireless systems often employ a number of portable units used in conjunction with one or more base stations. Although portable units may be used throughout the building, the base stations generally remain at fixed locations, thus affording the system designer an opportunity to optimize the position of the base station antenna.
The task of selecting suitable antenna locations is especially challenging in indoor environments. Buildings fabricated in conformance with modern construction practices typically utilize numerous RF-absorbing and/or reflecting materials such as steel-reinforced concrete, metallic partitions, aluminum air ducts, electrical conduit, plumbing, aluminum-clad fiberglass insulation, steel structural members, and others. These materials significantly degrade the performance of indoor RF communications systems. RF signals may be severely attenuated over relatively short distances. Multiple reflections and/or multiple signal pathways set up standing wave patterns which may cause signals to fade in and out as receivers, transmitters, people, and/or objects move or are moved about the building.
One approach to resolving the difficulties inherent in indoor RF propagation is to employ a multiplicity of receivers, transmitters, and/or antennas. A "brute force" approach may also be used, where transmitters are equipped to operate at relatively high RF power levels. However, these approaches serve to increase the cost and complexity of a communications system. Furthermore, the use of high RF power levels is undesirable due to recent public concern over the health effects of electromagnetic radiation.
A better approach to resolving indoor propagation difficulties is to optimize antenna placement for antennas which are to remain at a fixed location within a building. Several techniques may be employed to determine suitable locations for transmitting and/or receiving antennas. The simplest technique is intuitive, and involves placing the antennas (a) in centralized locations, (b) at relatively high elevations, (c) away from nearby conductive objects, and/or (d) such that a relatively obstacle-free path is provided between the antenna and the desired coverage area.
The process of intuitively determining proper antenna placement can be tedious and time-consuming. In practice, these intuitive techniques must often be combined with trial-and-error, cut-and-try methods to arrive at suitable antenna locations. The resonant frequency, standing wave ratio, and efficiency of the antenna are significantly influenced by proximity to nearby objects. Unpredictable nulls and standing-wave patterns can occur throughout the desired operational area. A multiplicity of on-site field strength measurements must generally be taken to assess and to optimize system performance. This procedure is labor-intensive, and may not result in the best practical solution for a given communications system.
It may be possible to reduce the number of antennas and/or transceivers required under the aforementioned "intuitive" approach by using one or more mathematical propagation models. Thus, the intuitive approach may result in a relatively expensive system which includes unnecessary redundancies in terms of antennas and/or transceivers. Likewise, the intuitive approach may not adequately account for all propagation obstacles, resulting in a system having poor coverage in certain areas of the building.
It is theoretically possible to calculate exact RF field strength values throughout the operational area of a communications system. Such a task could be accomplished by solving Maxwell's equations, using the building geometry to provide boundary conditions. Unfortunately, this approach is beyond the computing power of present-day personal computing devices, and is better suited for large mainframe computers. Moreover, the factors affecting RF propagation cannot always be represented with mathematical precision. For example, the electrical and magnetic properties of building materials are subject to field variations. Objects within the building, such as metal desks, bookcases, and filing cabinets, may be relocated from time to time. Propagation is also influenced by the movements of people throughout the structure. Although these building materials and objects can be characterized through actual measurements, such a task would prove very time-consuming.
Due to the significant complexities involved in performing exact RF propagation calculations, various techniques for estimating or predicting RF field strength have been developed. Illustrative examples of propagation models are disclosed in "Ray tracing as a design tool for radio networks", by J. W. McKown and R. L. Hamilton, IEEE Network Magazine, Vol. 5, No. 6, pp. 27-30, November 1991, and "Mechanisms governing UHF propagation on single floors in modern office buildings", by W. Honcharenko, H. L. Bertoni, J. Dailing, J. Qian, and H. D. Yee, IEEE Transactions on Vehicular Technology, Vol. 41. No. 4, pp. 496-504, November 1992. These RF propagation models facilitate the calculation of RF field strength values by implementing one or more simplifying assumptions.
Existing mathematical prediction techniques do not provide a practical, efficient approach for designing indoor RF communications systems. These techniques were often developed in academic environments for use with large mainframe computers. Although the prediction of indoor field strength can be implemented with a high degree of accuracy, the required computational time is typically measured in terms of hours or days. Furthermore, the relatively complex mathematical computations are not expeditiously performed by state-of-the-art personal, laptop, or notebook computers. Unreasonable computational time and/or impractically large amounts of computing power would be required to implement these mathematical propagation prediction techniques in the field.
The simplifying assumptions implemented by prior an propagation prediction techniques do not sufficiently enhance computational efficiency for many practical, real-world applications. What is needed is an improved approximation technique for analytically characterizing and optimizing indoor RF communications systems. For these applications, it is not generally necessary to have exact agreement between predicted and measured field strength values. Rather, for the purpose of comparatively evaluating several possible indoor communication system design alternatives, it is more important to characterize average field strength values at locations throughout the desired service area.