Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
2000-09-29
2003-04-08
Bocure, Tesfaldet (Department: 2661)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C455S442000
Reexamination Certificate
active
06546058
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communication systems, particularly to a plurality of methods for reducing the average transmit power from a sectorized base station.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone system, wireless local loop (WLL), satellite communication system such as GLOBALSTAR, or personal communications system (PCS), a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed. pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile units. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the mobile unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
A method and system for providing communication with the mobile unit through more than one base station during the handoff process are disclosed in U.S. Pat. No. 5,267,261, issued Nov. 30, 1993, entitled “MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention which is incorporated herein by this reference. Using this system, communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered as a “soft” handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile unit is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a cellular, WLL, GLOBALSTAR, or PCS controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station and the Active Set contains only the first base station. The mobile unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile unit communicates a message to the first base station identifying the new base station. A system controller decides whether to establish communication between the new base station and the mobile unit. Should the system controller decide to do so, the system controller sends a message to the new base station with identifying information about the mobile unit and a command to establish communications therewith. A message is also transmitted to the mobile unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The mobile unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The system controller receives this message through at least one of the base stations with which the mobile unit is communicating. The controller may decide to terminate communications through the base station having a weak pilot signal strength.
The system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communications are to be terminated. The base stations through which communication is established send the message to the mobile unit. The system controller also communicates information to the base station to terminate communications with the mobile unit. The mobile unit communications are thus routed only through base stations identified in the new Active Set.
Because the mobile unit is communicating with the end user though at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the end user. A soft handoff provides significant benefits in its inherent “make before break” communication over conventional “break before make” techniques employed in other cellular communication systems.
A typical cellular, WLL, GLOBALSTAR, or PCS system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. The process of simultaneous communication with two sectors of the same base station is called softer handoff. The process of soft handoff and the process of softer handoff are the same from the mobile unit's perspective. However the base station operation in softer handoff is different from soft handoff. When a mobile unit is communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the system controller. Because the two sectors of a common base station share circuitry and controlling functions, a variety of information is readily available to sectors of a common base station that is not available between independent base stations. Also two sectors of a common base station send the same power control information to a mobile unit (as discussed below). In satellite communication system such as GLOBALSTAR, most users will be in continuous softer handoff.
In a cellular, WLL, GLOBALSTAR, or PCS system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmitter power of each mobile unit is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each mobile unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit hi
Gilhousen Klein S.
Padovani Roberto
Baker Kent D.
Bocure Tesfaldet
Kalousek Pavel
Qualcomm Incorporated
Wadsworth Philip
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