The demand for wireless mobile communications services is growing at an explosive rate, with the anticipation that communication to a mobile device anywhere on the globe at all times will be available in the near future. An array of antennas mounted on vehicles, ships, aircraft, satellites, and base stations is expected to play an important role in fulfilling the increased demand of channel requirement for these services, as well as for the realization of the dream that a portable communications device the size of a wristwatch be available at an affordable cost for such services. This paper is the first of a two-part study. It provides a comprehensive treatment, at a level appropriate to nonspecialists, of the use of an antenna array to enhance the efficiency of mobile communications systems. It presents an overview of mobile communications as well as details of how an array may be used in various mobile communications systems, including land-mobile, indoor-radio, and satellite-based systems. It discusses advantages of an array of antennas in a mobile communications system, highlights improvements that are possible by using multiple antennas compared to a single antenna in a system, and provides details on the feasibility of antenna arrays for mobile communications applications.

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Applications of antenna arrays to mobile communications. I. Performance improvement, feasibility, and system considerations

A mathematical formulation is developed for systematic tracking of the random movement of a mobile station in a cellular environment. It incorporates mobility parameters under the most generalized conditions, so that the model can be tailored to be applicable in most cellular environments. This mobility model is used to characterize different mobility-related traffic parameters in cellular systems. These include the distribution of the cell residence time of both new and handover calls, channel holding time, and the average number of handovers. It is shown that the cell residence time can be described by the generalized gamma distribution. It is also shown that the negative exponential distribution is a good approximation for describing the channel holding time.

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User mobility modeling and characterization of mobility patterns

Handover is the mechanism that transfers an ongoing call from one cell to another as a user moves through the coverage area of a cellular system. As smaller cells are deployed to meet the demands for increased capacity, the number of cell boundary crossings increases. The author presents an overview of published work on handover performance and control and discusses current trends in handover research. He discusses investigations that are applicable to a single tier of cells. He focuses on macrocells, but includes a brief discussion on how things change as cell sizes shrink. By assuming an overlay of macrocells and microcells he summarizes issues and approaches unique to such systems.

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Trends in handover design

The major focus of this paper is distributed call admission control in mobile/wireless networks, the purpose of which is to limit the call handoff dropping probability in loss systems or the cell overload probability in lossless systems. Handoff dropping or cell overload are consequences of congestion in wireless networks. Our call admission control algorithm takes into consideration the number of calls in adjacent cells, in addition to the number of calls in the cell where a new call request is made, in order to make a call admission decision. This is done by every base station in a distributed manner without the involvement of the network call processor. The admission condition is simple enough that the admission decision can be made in real time. Furthermore, we show that our distributed call admission control scheme limits the handoff dropping or the cell overload probability to a predefined level almost independent of load conditions. This is an important requirement of future wireless/mobile networks with quality-of-service (QoS) provisioning.

Distributed call admission control in mobile/wireless networks

A distributed recommendation system and method are disclosed that provides greater privacy for the user's private data. The method distributes the tasks of a recommendation system between wireless devices (100) and network servers (140), so as to protect the privacy of end users. The user's wireless device (100) sends (326) a current context-activity pair (515) to a network server (140) in response to either the user's selection (324) of an activity or automatically (322). The user's wireless device (100) includes a service history log (110). The activities stored in the service history log (110) include past recommendations (1) made by the network server (140), past services used (2), prestored service preferences (3), and special requested service requirements (4). Context-activity pair information (515) sent to the server (140) can include any combination of these activities. The server (140) then responds with an appropriate recommendation (515').

Method and business process to maintain privacy in distributed recommendation systems

The author presents a preliminary analytical model that characterizes the central issues of the hand-off problem when vehicles can support multiple calls simultaneously. In such cases a cell boundary crossing by a single vehicle can generate multiple hand-off attempts. A suitable vector state representation is identified which casts the problem as a multidimensional birth-death process. An iterative method is used to find implicit hand-off parameters for systems in statistical equilibrium. Theoretical performance characteristics that show blocking, hand-off failure, and forced termination probabilities as functions of communication traffic are determined. >

The multiple-call hand-off problem in high-capacity cellular communications systems

Channel holding times and user mobility are important topics in the study of wireless cellular communications. We present an approach to modeling user mobility and session time which enables both the calculation of teletraffic performance characteristics and a characterization of holding time which agrees with published reports. The model allows both the dwell time and unencumbered session time to have general distributions. A derivation of the channel holding time distribution is given. We then show how the model's parameters can be chosen to fit empirical data including observations of channel holding time.

A model for teletraffic performance and channel holding time characterization in wireless cellular communication with general session and dwell time distributions

A hierarchical overlaid scheme suitable for high-capacity microcellular communications systems is considered as a strategy to achieve high system performance and broad coverage. High-teletraffic areas are covered by microcells while overlaying macrocells cover low-teletraffic areas and provide overflow groups of channels for clusters of microcells. New calls and handoff calls enter at both the microcell and macrocell levels. Handoff calls are given priority access to channels at each level. The layout has inherent load-balancing capability, so spatial teletraffic variations are accommodated without the need for elaborate coordination of base stations (wireless gateways). An analytical model for teletraffic performance (including handoff) is developed. Theoretical performance characteristics that show carried traffic as well as blocking, handoff failure, and forced termination probabilities are derived. Effects of nonuniform teletraffic demand and channel allocation strategies on system performance are discussed. >

Microcellular communication systems with hierarchical macrocell overlays: traffic performance models and analysis

A personal communication system with multiple hierarchical cellular overlays is considered. The system can include a terrestrial segment and a space segment. The terrestrial trail segment, consisting of microcells and macrocells, provides high channel capacity by covering service areas with microcells. Overlaying macrocells cover spots that are difficult in radio propagation for microcells and provide overflow groups of channels for clusters of microcells. At the highest hierarchical level, communications satellites comprise a space segment. The satellite beams overlay clusters of terrestrial macrocells and provide primary access for satellite-only subscribers. Call attempts from cellular/satellite dual subscribers are first directed to the terrestrial cellular network, with satellites providing necessary overlay. At each level of the hierarchy, hand-off calls are given priority access to the channels. The mathematical structure is that of a multilayer hierarchical overflow system. An analytical model for teletraffic performance (including hand-off) is developed. Theoretical performance measures are calculated for users having different mobility characteristics. These show the carried traffic, traffic distribution, blocking, and forced termination probabilities. >

Personal communication systems using multiple hierarchical cellular overlays

Cellular communication systems that support a mixture of platform types distinguished by different mobility characteristics are considered. A tractable analytical model for traffic performance analysis is developed using multidimensional birth–death processes and the method of phases. The framework allows consideration of homogeneous and nonhomogeneous systems, a broad class of dwell time distributions, and ‘missed’ hand-off initiations. Cut-off priority for hand-offs and several platform types are considered to demonstrate the approach. The effects of different mobility parameters and of imperfect detection of hand-off needs are examined. Theoretical performance characteristics are obtained. These exhibit carried traffic, hand-off activity, blocking probability and forced termination probability for each platform type. The realisable exchange of blocking for hand-off performance is shown. Computational issues are discussed and a method for extrapolating performance characteristics for systems with many channels is presented.

Stephen S. Rappaport

Papers

The multiple-call hand-off problem in high-capacity cellular communications systems

A model for teletraffic performance and channel holding time characterization in wireless cellular communication with general session and dwell time distributions

Microcellular communication systems with hierarchical macrocell overlays: traffic performance models and analysis

Personal communication systems using multiple hierarchical cellular overlays

Blocking, hand-off and traffic performance for cellular communication systems with mixed platforms