Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-10-29
2001-04-10
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S142000, C375S150000, C375S152000, C370S412000, C455S436000
Reexamination Certificate
active
06215811
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 unit 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 first base station to a second 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 unit between two base stations without interrupting communications between the remote unit and the base stations.
The present invention, as embodied and broadly described herein, provides a method and apparatus for handing off a remote unit 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 units within a geographical area. A particular remote unit communicates with a first base station using spread-spectrum modulation. The remote unit has a matched filter for despreading a first received-spread-spectrum signal transmitted from the first base station. The remote unit transmits data to the first base station at a first data rate and a first power level.
The method comprises the steps, at the remote unit, of monitoring at the output of the matched filter of the remote unit, a first signal quality of the first received-spread-spectrum signal transmitted from the first 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 unit 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 unit determines it is time to handoff and sends a request to a central control unit for handoff to the second base station.
Upon requesting handoff to the second base station, the remote unit 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 unit to the first base station. Similarly, the first base station may store data during the period of time during which handoff occurs, and thus data need not be transmitted from the first base station to the remote unit.
The steps further include, from the remote unit, initiating handoff to the second base station. Upon handoff, the second received-spread-spectrum signal is synchronized in the remote unit. The remote unit then transmits to the second base station the stored data at a second data rate and at a second power level. Typically the second data rate is greater than the first data rate, and the second power level is greater than the first power level. After the stored data is transmitted to the second base station, the second base station continues to communicate with the remote unit at the first data rate and the first power level. After the stored data is transmitted to the second base station, the second base station and the remote unit may continue to communicate at the second power level and/or second data rate, or at a different power level and data rate, dependent on geographical factors. Thus, the data stored at the remote unit is transferred to the second base station.
Similarly, data stored at the first base station may be transferred to the second base station, and transmitted to the remote unit in a similar fashion, at a higher data rate and a higher power level than is normally used for transmitting to the remote unit.
The method steps may be repeated for any number of handoffs, with the first base station being that base station communicating with the remote unit at a given time, and the second base station being whatever base station of the plurality of base stations is selected as having the best signal quality.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
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Chartered David Newman
Golden Bridge Technology Inc.
Tse Young T.
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