Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-07-30
2001-05-22
Hunter, Daniel (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S456500, C455S434000, C455S435100
Reexamination Certificate
active
06236861
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to cellular communications and, more particularly, to a method for tracking mobile users in a cellular network.
The increasing demand for personal communication services (PCS) requires wireless networks to gracefully accommodate mobility of both users and services. Contrary to wired networks, in which user location is fixed, in wireless networks a user can potentially be located anywhere within the system service area. As the number of mobile users keeps increasing, the amount of signaling traffic required for location management keeps growing. The cost associated with the need to locate a mobile user is composed of two parts: 1) The cost of accessing data bases, such as Home Location Register (HLR) and Visitor Location Register (VLR), and 2) The cost of radio signaling over the control channel.
The problem of tracking mobile users has been addressed by several studies, many of which attempt to reduce the wireless cost of user tracking. In Rose C., “Minimizing the Average Cost of Paging and Registration: A Timer-Based Method”,
ACM Journal of Wireless Networks
, Vol. 2, No. 2, pp. 109-116 (1996), a timer based method was suggested in which the user updates its location every T time units, where T is a time threshold. Each time the user makes no contact with the network for T units of time, the user initiates a registration message. Another strategy, suggested in Bar-Noy A., I. Kessler and M. Sidi, “Mobile Users: to Update or not to Update?”,
Wireless Networks
, Vol. 1, No. 2, pp. 175-185 (1995), and in Madhow U., L. Honig and K. Steiglitz, “Optimization of Wireless Resources for Personal Communications Mobility Tracking”,
IEEE Trans. on Networking
, Vol. 3, No. 6, pp. 698-707 (1995), is to use a distance-based method in which the user tracks the distance it moved from its last known location, in terms of cells. Whenever this distance exceeds a parameter D the user transmits a registration message. The distance-based strategy is considered as the most efficient tracking strategy, however its implementation is the most difficult. In Bar-Noy et al. (1995), and in Akyildiz, I. F., J. S. M. Ho and Y. B. Lin, “Movement Based Location Update and Selective Paging Schemes”,
IEEE/ACM Trans. on Networking
, Vol. 4, No. 4, pp. 629-638 (1996), a movement based method was suggested, in which the user counts the number of cell transitions, and transmits a registration message whenever this number exceeds a pre-defined threshold. A load-sensitive approach, in which the tracking activity adapts to both user and system activity was suggested in Levy H. and Naor Z., “Initiated Queries and their Application for Tracking Users in Wireless Networks”,
Sixth WINLAB Workshop
, 1997, pp. 381-399, and in Naor Z. and Levy H., “Minimizing the Wireless Cost of Tracking Mobile Users: An Adaptive Threshold Scheme”,
IEEE INFOCOM
'98 pp. 720-727. The basic assumption underlying all these methods is that the user location, as well as other derived parameters (such as the distance traveled from its last known location), are always known to the user. However, in reality, this information is not available to the user.
Existing cellular systems use the following tracking strategy, known as the geographic-based (GB) strategy: The geographic area is partitioned into location areas, based on the commercial licenses granted to the operating companies. A location area (LA) is a group of cells, referred to as a home-system. The term location area is used by GSM systems, while IS-41 refers to the LA as registration area. Users register whenever they change LA, while within the LA they never register. The implementation of the geographic-based (GB) strategy is very simple: All base stations within the same LA periodically broadcast a location area code wherein is encoded the ID of the LA. Each user receives the location area code of the LA wherein it is located, compares its last LA ID with the current ID, and transmits a registration message whenever the ID changes. Hence, the user is not aware of its exact location within the LA. When there is an incoming call directed to a user, all the cells within its current LA are paged. Because the number of cells within a typical LA is very large, the tracking cost associated with the GB strategy is very high.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for tracking users of a cellular communications network that is based on user location measured at the cell level.
SUMMARY OF THE INVENTION
According to the present invention there is provided, in a system, including at least one location area, each of the at least one location area including a plurality of cells, each cell having a certain size, wherein at least one user moves among the cells of the at least one location area, and wherein, for each cell, a location area code, representative of the location area wherein the each cell is included, is transmitted to each of the at least one user located within the each cell, a method for tracking the at least one user, including the steps of: (a) assigning a cell identity to each cell; (b) for each cell, broadcasting a cell code representative of the cell identity of the each cell to each of the at least one user located within the each cell; (c) for each at least one user, receiving both the location area code and the cell code transmitted for the each cell wherein the each at least one user is located; and (d) for each at least one user, transmitting a registration message, the transmitting being based both on the location area code received by the at least one user and on the cell code received by the each at least one user.
The present invention is based on a new approach for providing the users with location information, or other related information, necessary to reduce the tracking cost. The basic idea is to assign an identity to each cell that identifies the cell and its orientation relative to the other cells, and to encode that identity in a Cell Identification Code (CIC). It is important to note that this cell identity is not necessarily unique. In fact, in two of the three preferred embodiments of the present invention that are presented below, the cell identity is not unique. Each cell periodically broadcasts its identification code through the down link control channel (DCCH in GSM systems). The goal of the CIC is to provide the location information required to the users, in order to perform the registration strategy. The users then base the frequency with which the registration messages are broadcast on the CICs that they have received.
Three preferred CIC encoding schemes are presented below: The first, achieving the best performance, is the basis of an embodiment of the present invention that implements a distance-based tracking strategy. For a realistic two dimensional topology, a four bit message is sufficient to provide a reliable distance-based tracking. The second preferred embodiment of the present invention is based on a combined timer and movement tracking strategy, based on either a one-bit or a two-bit CIC, depending on system topology and user mobility. A movement based tracking strategy is a special case of the combined timer and movement tracking strategy. The third preferred embodiment of the present invention is a conditional timer strategy, in which the user examines its current location every T time units, using a CIC, and if the current location differs from the last known location, the user transmits a registration message.
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Naor Z. et al., “Minimizing the Wireless Cost of Tracking Mobile Users: An Adaptive Threshold Scheme”,IEEE Infocom'98pp. 720-727.
Levy H. and Naor Z., “Initiated Queries and their Application for Tracking Users in Wireless
Levy Hanoch
Naor Zohar
Friedman Mark M.
Gantt Alan T.
Hunter Daniel
Ramot University Authority for Applied Research and Development
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