Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-10-08
2001-05-08
Trost, William G. (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S525000
Reexamination Certificate
active
06230014
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mobile communication systems and methods, and in particular, to handing off communications with a mobile station from a first radio transceiver to a second radio transceiver in a mobile communication system where the first and second radio transceivers service the same or different coverage areas of the mobile communication system.
BACKGROUND OF THE INVENTION
In traditional cellular type mobile communication systems, a mobile station is “handed off” from a servicing base station to a target base station when it appears that the signal quality with the target base station will be superior to the signal quality of the servicing base station. Typically “handoff” occurs when the mobile station is moving from the coverage area of the servicing base station into the coverage area of the target base station.
One type of handoff, commonly referred to as a “hard handoff”, requires the mobile station to change the frequency and/or time slot of its communication signal when it is handed off from the servicing base station to the target base station. Commonly, a hard handoff is an asynchronous function in which the mobile station and the base station asynchronously perform a break-before-make switching. For example, an initial voice path is established between a mobile station and the servicing base station when the mobile station is in the coverage area of the servicing base station. When it is determined that the mobile station should be handed off from the servicing base station to the target base station, a handoff order is transmitted to the mobile station with instructions to switch to the frequency and/or time slot allocated for the target base station, and in response, the mobile station retunes to the allocated frequency and/or time slot and synchronizes with the target base station. Next, via signalling between the base station and the communication system, the target base station verifies the presence of the mobile station, and the communication system establishes a voice path to the mobile station through the target base station. The period of time between the handoff order and the reestablishment of the voice path through the target base station is commonly referred to as the handoff period and often is in the range of 1-3 seconds. The handoff period results in a loss of voice communications and degradation in voice quality that may exceed what is acceptable, particularly for mobile communication systems that are intended to replicate the quality of traditional fixed communication systems.
Another type of hard handoff occurs when the communication signal with the mobile station at a particular frequency and time slot experiences some type of disturbance or degradation not necessarily associated with moving from one coverage area into another coverage area. In this type of hard handoff, the mobile station retunes from its current frequency and/or time slot to a new frequency and/or time slot, without switching from the servicing base station.
More detailed descriptions of various mobile communication systems and the handoff procedures used therein are contained in “Wireless and Personal Communication Systems”, Vijay K. Garg and Joseph E. Wilkes, published in 1996 by Prentice Hall PTR and “Cellular Radio Systems”, D. M. Balston and R. C. V. Macario published in 1993 by Artech House Publishers, the disclosures of which are incorporated herein by reference.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a method is provided for controlling handoff of communications with a mobile station from a first radio transceiver to a second radio transceiver in a communication system. The method includes the steps of maintaining a first voice path at a first frequency and a first time slot with the mobile station through the first radio transceiver, and while the mobile station is tuned to the first voice path, establishing a second voice path at a second frequency and a second time slot through the second radio transceiver, with at least one of the second frequency and the second time slot being different from the first frequency and the first time slot, respectively.
In accordance with one aspect of the invention, a method is provided for controlling handoff of communications with a mobile station from a first radio transceiver to a second radio transceiver in a communication system having a common baseband processing unit operably connected to each of the radio transceivers.
In one aspect, the method includes the steps of maintaining respective first and second voice paths with modulated signals uplinked from the first and second radio transceivers to the common baseband processing unit while communications with the mobile station are handed off from the first radio transceiver to the second radio transceiver.
In one aspect, the method includes the steps of maintaining a first voice path at a first frequency and a first time slot between the mobile station and the common baseband processing unit through the first radio transceiver; while the mobile station is tuned to the first voice path, establishing the second voice path at a second frequency in a second time slot between the common baseband processing unit and the second radio transceiver, with at least one of the second frequency and the second time slot being different from the first frequency and the first time slot, respectively; and transmitting a handoff order to the mobile station from the common baseband processing unit through the first radio transceiver, the handoff order being to switch to the second voice path at the second frequency and the second time slot.
In one aspect, the method further includes the step of monitoring both the first and second voice paths at the same time to determine which path is currently carrying voice data from the mobile station.
In one aspect, the method further includes the step of monitoring both the first and second voice paths at the same time to determine when to route voice data carried by the second voice path from the mobile station through to a switching system operably associated with the communication system.
In one aspect, the method further includes the step of transmitting, at the same time, downlinked voice signals intended for the mobile station via both the first voice path from the first radio transceiver and the second voice path from the second radio transceiver.
It is an object of the invention to provide an improved method for controlling handoff of communications with a mobile station from a first radio transceiver to a second radio transceiver in a communication system.
It is another object of the invention to provide a method and apparatus for hard handoffs that minimizes loss of voice communications and degradation in voice quality.
Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.
REFERENCES:
patent: 5483668 (1996-01-01), Malkamaki et al.
patent: 0 281 111 (1988-09-01), None
patent: 0 577 322 (1994-01-01), None
patent: 281 177 (1995-02-01), None
Bexten Ronald L.
Freeze John Mark
Ericsson Inc.
Sobutka Philip J.
Trost William G.
Wood Phillips VanSanten Clark & Mortimer
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