Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-11
2002-08-20
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000
Reexamination Certificate
active
06438376
ABSTRACT:
TECHNICAL FIELD
The present invention relates to management and control of mobile telephone communications and, in particular, to use of information representing one or more of the location, speed and direction of travel of mobile stations to reduce the instances of call disruption and enhance the quality of service.
BACKGROUND
Mobile telephone systems rely on a variety of management and control techniques to provide simultaneous services to large numbers of mobile stations, even though the spectrum of radio frequencies available for this purpose is quite limited in a relative sense. Each of the multitudes of mobile telephones in use at any given moment in a mobile system must be allowed to communicate with the stationary, ground-based network separately from others. However, insufficient room exists across the spectrum of radio frequencies available for all users to utilize a separate radio frequency to conduct a conversation or data transmission.
The most basic management and control technique used universally by mobile systems is to provide an array of adjacent “cells” that separate the use of any given radio frequency by distance. While the cells cover the service area like patches in a quilt, each cell employs a set of radio frequencies or channels to communicate with mobile stations within the cell that are different from the set of frequencies used by immediately neighboring or adjacent cells. In this way, each cell contributes to separation by distance of frequency sets of other cells around it. This separation therefore allows frequencies to be reused throughout the array of cells, generally without overlap or interference. Separate calls can be made by different users over the same frequency in different cells spaced sufficiently apart. Each cell may also be further subdivided into sectors, each using separate sets of frequencies or channels. Three sectors are typically used in a cell.
The number of mobile users serviced and the capacity of the system are increased further by dividing each frequency used within a cell or sector into a number of separate time slots. Each mobile phone operating on that frequency or frequency band is assigned a separate time slot in which to broadcast transmissions to the base transceiver station (BTS) within its cell. The BTS broadcasts to the mobile station in a similar manner, sometimes on the same and sometimes on a different frequency. Thus, a number of mobile stations can use the same frequency to communicate with the same BTS within the cell, separately and without interference. This sort of division of frequencies into time slots or intervals is widely known as Time Division Multiple Access (TDMA).
Other techniques for providing access to the mobile system by multiple users within the same cell are Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). Each technique is well-known by persons who are skilled in the field of mobile telecommunication systems and therefore need not be discussed in more detail.
Maintaining substantially continuous communication with a mobile station as it moves from one cell or sector to another and a high quality of service (QOS) are objectives to which existing systems aspire. Achieving these objectives would result in service similar to that provided by land-line communication networks that do not rely on radio or other electromagnetic transmission. Meeting these objectives, however, in FDMA and TDMA systems is hampered by the need to change the frequency or channel through which a mobile station communicates as it travels into an adjacent cell or sector within a cell. In general, the terms “cell” and “sector” are often used interchangeably; however, their meanings differ in the sense that cells are formed by different BTSs, while sectors share the same BTS and together form a larger cell. Because the adjacent cell or sector operates using different sets of frequencies to avoid interference, the mobile station must adjust to the frequency environment of the cell or sector into which it travels. This sort of adjustment is known as a “hard hand-off.”
In TDMA systems, this inter-cell (adjacent cells) or intra-cell (adjacent sectors) “hand-off” of the traveling mobile station to the adjacent cell or sector is made more difficult by the need to synchronize the time slot in which the mobile station transmits and receives “bursts” of signals to and from the associated BTS. This time slot adjustment is required to avoid interfering with the transmission bursts of other mobile stations operating over the same frequency. In addition to resetting the time slot assignment, when a mobile station travels from a servicing cell to a target cell having radii appreciably different, the transmission of the mobile station must be retarded or advanced to compensate for a different transmission or “propagation” delay in transmissions to the BTS of the target cell. This burst transmission adjustment compensates for any appreciable difference in the transmission propagation delay resulting from different lengths of the transmission paths to the BTS of the prior serving cell and the new target cell. Under existing standards, such burst transmission adjustments are necessary for approximately every 500 meter difference in transmission distance; however, circumstances may make adjustments based on other increments desirable.
Because existing TDMA systems currently do not determine this distance in advance of the hand-off, transmission to and from the mobile station must be temporarily interrupted or “muted” while the mobile station transmits small bursts of synchronization codes to the BTS of the target cell, in accordance with IS-54B and IS-13 standards. The BTS determines the correct burst advance or retard to be implemented and transmits the information to the mobile station, which completes the adjustment. This muting avoids undesirable burst collisions with other mobile stations operating on the same channel or frequency while the adjustment is made. Unfortunately, an undesirable consequence is that the mobile station user is temporarily disconnected from any communication and typically hears a “click” noise. Therefore, it would be desirable to provide a management and control system that adjusts the burst transmission of the mobile station in connection with the hand-off with less disruption of communication.
Other difficulties experienced with current TDMA management and control systems during hand-off stem from system determinations of what new frequency of the target cell or sector to assign a mobile station. Mobile stations operating with systems employing the Mobile Assisted Hand-off (MAHO)procedure of the IS-136 standard frequently monitor the strength of the Digital Control Channels (DCCHs) of immediately adjacent cells (as distinguished from the traffic or voice channel or frequency) and provide this information to the BTS of the currently serving cell. Using this information, the BTS communicates to the mobile station its preferred DCCH assignment and adjustment in the target cell.
However, mobile stations sometimes incorrectly monitor a stronger DCCH signal of a cell that is not immediately adjacent, but that is of the same frequency as a DCCH of an adjacent cell. This often occurs due to shielding of the adjacent cell DCCH by buildings in urban areas, for example. Such a “false” readings will result in a dropped call as the hand-off is made to the incorrect, unavailable or unacceptably weak DCCH of the actual target cell. It is therefore desirable to provide an ability to distinguish or eliminate such “false” readings.
When a target cell or cell sector does not have available a frequency or channel for one or more mobile stations moving into that cell or sector, each station is placed by the management and control system in a “queue,” while awaiting availability of a channel. Placement of a mobile station in the “queue” by the system for such inter- or intra-cell hand-off and channel assignment, is triggered primarily by a reduction of signal strength of the BTS to a predetermi
Baumann Jill R.
Carter Daniel
Elliott Stephen B.
Eswara Srinivas
Carr Law Firm, L.L.P.
Nortel Networks Limited
Vo Nguyen T.
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