Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-03-22
2001-11-27
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S225000, C370S332000, C455S436000, C455S437000, C455S522000
Reexamination Certificate
active
06324207
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to a method and apparatus for handing-off a remote station between two base stations.
DESCRIPTION OF THE RELEVANT ART
A spread-spectrum communications system uses message data. Message data requires transmission without error. When a mobile station moves from a source-base station to a target-base station, the chip sequence used for the channel containing the digital data has to be handed-off so as to not interrupt communications so as to produce errors.
Multiple coverage areas are employed by mobile communications systems to accommodate communications over a wide geographic region. Each geographic area has several base stations to provide radiotelephones within the base site coverage area with a number of radiotelephone communication channels. Simulcast communication systems and cellular communication systems provide hand-offs between coverage areas. Simulcast communication systems involve linking together the respective coverage areas of several communication sites to form a large geographic area coverage. As a mobile radio moves between coverage areas, a conversation on the mobile radio is maintained through simultaneous reception and broadcasting of the conversation between several sites on the same channel.
In a FDMA cellular communications system, base site equipment periodically measures the signal strength of the radiotelephone during the conversation and, once the signal strength reaches a relatively low threshold, the same base site equipment sends a message to the adjacent base sites to determine which base site's area the radiotelephone is entering. The radiotelephone is then instructed by the base site equipment associated with the coverage area into which the radiotelephone is moving to communicate on a selected channel.
SUMMARY OF THE INVENTION
A general object of the invention is a method and apparatus for handing off a remote station between two base stations without interrupting communications between the remote station and the base stations.
The present invention, as embodied and broadly described herein, provides a method and apparatus for handing off a remote station between two base stations in a spread-spectrum-communications system. The spread-spectrum-communications system has a plurality of base stations and a plurality of remote stations within a geographical area. A particular remote station communicates with a source-base station using spread-spectrum modulation. The remote station has a matched filter or correlator for despreading a first received-spread-spectrum signal transmitted from the source-base station. The remote station transmits data to the source-base station at a first data rate and a first power level.
The method comprises the steps of monitoring at the output of the matched filter or correlator of the remote station, a first signal quality of the first received-spread-spectrum signal transmitted from the source-base station. The first signal quality may be probability of error, power level, signal-to-noise ratio, or other signal quality as is well known in the art. The method includes scanning a plurality of received-spread-spectrum signals radiated from the plurality of base stations. A plurality of signal qualities for the plurality of received-spread-spectrum signals, respectively, are stored, and according to the method of the present invention, the remote station selects from the plurality of received-spread-spectrum signals, using the plurality of signal qualities from the received plurality of received-spread-spectrum signals, a second received-spread-spectrum signal. The second received-spread-spectrum signal has a second signal quality. Typically the second received-spread-spectrum signal is chosen from the plurality of received-spread-spectrum signals because it has a signal quality which is better than the remaining plurality of signal qualities.
The method includes the step of determining when the first signal quality falls below a predetermined threshold and/or below the second signal quality. By falling below the second signal quality is meant, for example, that the second signal quality might have a probability of error which is less than the probability of error of the first signal quality. Upon the occurrence of this event, the remote station determines it is time to handoff and sends a request to a central control unit for handoff to the target-base station.
Upon requesting handoff to the target-base station, the remote station stores data that would normally be transmitted during the period of time during which the handoff occurs. Thus, during the period of time over which handoff occurs, data need not be transmitted from the remote station to the source-base station. Similarly, the source-base station may store data during the period of time during which handoff occurs, and thus data need not be transmitted from the source-base station to the remote station.
The steps further include, from the remote station, initiating handoff to the target-base station. The steps include transmitting, from the remote station, an RS-access-burst signal having a plurality of RS segments. Each RS segment has a plurality of RS symbols carrying differentially encoded BS power-control information. The plurality of RS segments has a plurality of RS power levels, increasing in time, respectively.
The steps further include queuing, upon requesting the handoff process to the target-base station, RS data for transmission from the remote station. At the target-base station, the steps include receiving the RS-access-burst signal at an RS detected-power level. From the target base station the steps include transmitting a BS-access-burst signal having a plurality of BS segments/Each BS segment has a plurality of BS symbols carrying differentially encoded RS power-control information. The plurality of BS segments has a plurality of BS power levels, increasing in time, respectively.
In response to the remote station detecting the BS-access-burst signal, the steps include differentially encoding the plurality of RS symbols with BS-power control information including power level for the target-base station. In response to the target base station detecting the RS-access-burst signal, the steps include differentially encoding the plurality of BS symbols with RS-power control information including power level for the remote station.
At the remote station, the steps include receiving the BS-access-burst signal from the target-base station. At target-base station, the steps include the RS-access-burst signal the remote station.
The queued RS data are transmitted at a second RS data rate from the remote station to the target-base station. The second RS data rate is greater than the first RS data rate. Thus, the queued RS data is to the target-base station.
Similarly, the queued BS data are transmitted at a second BS data rate from the target-base station to the remote station. The queued BS data are transferred at a second BS data rate. The second BS data rate is greater than the first BS data rate. The second BS data rate and second RS data rate may be greater tan the first BS data rate and the first RS data rate, respectively, due to sending packets at a higher data rate, or due to using parallel spread-spectrum channels, to effectively realize a faster data rate.
After the queued RS data are transferred, the method then resumes at the first RS date rate, by transmitting, from the remote station to the target-base station, in response to the queued RS data being transferred to the target-base station, at the first RS data rate. Similarly, in response to the queued BS data being transferred to the remote station, the steps include transmitting, from the target-base station to the remote station, at the first BS data rate.
Upon handoff, the target base station is synchronized in the remote station.
Similarly, data stored at the source-base station may be transferred to the target-base station, and transmitted to the remote station in a simi
Kanterakis Emmanuel
Parsa Kourosh
Yuen Elmer
David Newman Chartered
Golden Bridge Technology Inc.
Tse Young T.
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