Handoff control in an asynchronous CDMA system

Abstract

In a CDMA system in which the base stations are not each time aligned with one another, the handoff process accommodates the handoff by allowing for frame alignment. For example, frame alignment may be accomplished through the use of a selected set or may be remote unit aligned without reference to external sources. In addition, the neighbor list may included additional entries.

What is claimed is:  
     
         1 . A method of handoff control for a wireless remote unit having an established communications link with a first base station, comprising the steps of: 
 transmitting a message to a network controller identifying a second base station having signal strength sufficient to establish communication;    receiving a message from said network controller via said first base station identifying said second base station as a selected base station;    monitoring an overhead channel from said second base station in order to determine a frame synchronization of said second base station; and    transmitting said frame synchronization to said network controller.    
     
     
         2 . The method of  claim 1 , further comprising the step of receiving a message from said network controller via said first base station identifying said second base station as an active base station.  
     
     
         3 . The method of  claim 1 , wherein said first base station and said second base station are asynchronous with respect to one another.  
     
     
         4 . The method of  claim 1 , further comprising the step of diversity combining signals transmitted by said first and said second base stations.  
     
     
         5 . The method of  claim 1 , further comprising the step of receiving a message from said network controller via said first base station comprising a neighbor list from which said second base station is selected.  
     
     
         6 . The method of  claim 5  wherein said neighbor list comprises a series of entries, each entry corresponding to a base station with a high probability of having signal strength sufficient to establish communication, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.  
     
     
         7 . The method of  claim 6 , wherein said entries further comprise a window size over which a search should be performed.  
     
     
         8 . The method of  claim 7 , wherein said window size implicitly carries information concerning a relative class of synchronization between said reference base station and said base station to which said entries correspond.  
     
     
         9 . The method of  claim 7 , wherein said window size carries information concerning whether said base station corresponding to said entry is frame synchronized with said reference base station.  
     
     
         10 . A handoff control apparatus in a wireless remote unit, said remote unit having an established communication link with a first base station, said apparatus comprising: 
 means for transmitting a message to a network controller identifying a second base station having signal strength sufficient to establish communication;    means for receiving a message from said network controller via said first base station identifying said second base station as a selected base station;    means for monitoring an overhead channel from said second base station in order to determine a frame synchronization of said second base station; and    means for transmitting said frame synchronization to said network controller.    
     
     
         11 . A method of handoff control for a wireless remote unit having an established communications link with a first base station, comprising the steps of: 
 receiving a message from said remote unit identifying a second base station having signal strength sufficient to establish communication;    transmitting a message to said remote unit via said first base station identifying said second base station as a selected base station;    receiving a message identifying a frame synchronization of said second base station; and    establishing communication with said remote unit via said second base station such that transmissions from said first base station and transmissions from said second base station arrive at said remote unit approximately synchronized.    
     
     
         12 . The method of  claim 11 , further comprising the step of transmitting a message to said remote unit via said first base station identifying said second base station as an active base station.  
     
     
         13 . The method of  claim 11 , wherein said first base station and said second base station are asynchronous with respect to one another.  
     
     
         14 . The method of  claim 11 , further comprising the step of diversity combining signals received from said remote unit via said first and said second base stations.  
     
     
         15 . The method of  claim 11 , wherein said step of transmitting a message identifying said second base station as a selected base station is executed only if resources are available at said second base station to support communication with said remote unit.  
     
     
         16 . A wireless remote unit having an established communications link with a first base station, comprising: 
 means for receiving a message from said remote unit identifying a second base station having signal strength sufficient to establish communication;    means for transmitting a message to said remote unit via said first base station identifying said second base station as a selected base station;    means for receiving a message identifying a frame synchronization of said second base station; and    means for establishing communication with said remote unit via said second base station such that transmissions from said first base station and transmissions from said second base station arrive at said remote unit approximately synchronized.    
     
     
         17 . In a communication system in which a remote unit communicates with other users via at least one base station, and in which each of at least two or more base stations among a plurality of base stations within said system transmits a unique pilot signal, a remote unit transceiver comprising: 
 a pilot signal measurement circuit which measures strength of pilot signals received from a set of neighboring base stations;    a controller which generates a first signal strength message when a measured pilot signal of a target base station from among said set of neighboring base stations exceeds a first predetermined level;    a remote unit transmitter which transmits said first signal strength message to at least one base station with which said remote unit is currently communicating, said first signal strength message identifying said target base station; and    a demodulator which receives a first direction signal from said at least one base station and, in response to said first direction signal, monitors a forward link transmission from said target base station to determine a frame synchronization of said target base station.    
     
     
         18 . The remote unit transceiver of  claim 17  wherein said controller generates a relative frame synchronization message to convey said frame synchronization and said remote unit transmitter transmits said relative frame synchronization to said at least one base station.  
     
     
         19 . In a spread spectrum communication system having a plurality of base stations and in which a remote unit communicates with another system user via at least one base station, a method for directing communications between said remote unit and said base stations comprising the steps of: 
 providing to said remote unit an active list identifying one or more base stations through which active communication is established;    receiving from said remote unit a candidate list identifying at least one target base station;    determining an availability of system resources at said target base station;    providing to said remote unit a selected list identifying said target base station;    receiving from said remote unit an alignment message identifying synchronization information concerning said target base station;    directing said target base station to establish communication with said remote unit in accordance with said synchronization information; and    providing to said remote unit a second active list identifying said target base station.    
     
     
         20 . The method of  claim 19 , wherein said selected list comprises just one entry.  
     
     
         21 . The method of directing communications of  claim 19 , further comprising the steps of providing to said remote unit a neighbor list comprising a series of entries, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.  
     
     
         22 . The method of directing communications of  claim 21 , wherein said entries further comprise a window size over which a search should be performed.  
     
     
         23 . The method of directing communications of  claim 22 , wherein said window size implicitly carries information concerning a relative class of synchronization between said reference base station and said base station to which said entries correspond.  
     
     
         24 . The method of directing communications of  claim 22 , wherein said window size carries information concerning whether said base station corresponding to said entry is frame synchronized with said reference base station.  
     
     
         25 . A network controller in spread spectrum communication system in which a remote unit communicates with another system user via at least one base station and wherein each base station transmits an identifying pilot signal, said network controller comprising: 
 means for providing to said remote unit an active list identifying one or more base stations through which active communication is established;    means for receiving from said remote unit a candidate list identifying at least one target base station;    means for determining an availability of system resources at said target base station;    means for providing to said remote unit a selected list identifying said target base station;    means for receiving from said remote unit an alignment message identifying synchronization information concerning said target base station;    means for directing said target base station to establish communication with said remote unit in accordance with said synchronization information; and    means for providing to said remote unit a second active list identifying said target base station.    
     
     
         26 . A method of time alignment in a wireless communications system in which a remote unit is capable of communication with one or more base stations simultaneously, said method comprising the steps of: 
 receiving a first forward link transmission from a first base station having a first frame alignment;    receiving a second forward link transmission from a second base station having a second frame alignment wherein said second frame alignment comprises information concerning frame boundaries and excludes information concerning an absolute frame number;    selecting a first arbitrary frame alignment;    combining said first forward link transmission and said second forward link transmission according to said first arbitrary frame alignment creating a combined signal;    determining whether a performance indication of said combined signal is within expected limits; and    combining said first forward link transmission and said second forward link transmission using a second arbitrary frame alignment if said performance indication is not within the expected limits.    
     
     
         27 . A method of time alignment in a wireless communications system in which a remote unit is capable of communication with one or more base stations simultaneously, said method comprising the steps of: 
 receiving a first forward link transmission from a first base station having a first frame alignment;    receiving a second forward link transmission from a second base station having a second frame alignment wherein said second frame alignment comprises information concerning frame boundaries and excludes information concerning an absolute frame number;    combining said first forward link transmission and said second forward link transmission according to a first frame alignment hypothesis to determine a first performance indication;    combining said first forward link transmission and said second forward link transmission according to a second frame alignment hypothesis to determine a second performance indication; and    comparing said first and second performance indications in order to determine a most likely absolute frame alignment.    
     
     
         28 . An apparatus for time alignment in a wireless communications remote unit capable of communication with one or more base stations simultaneously, said apparatus comprising: 
 means for receiving a first forward link transmission from a first base station having a first frame alignment;    means for receiving a second forward link transmission from a second base station having a second frame alignment wherein said second frame alignment comprises information concerning frame boundaries and excludes information concerning an absolute frame number;    means for selecting a first arbitrary frame alignment;    means for combining said first forward link transmission and said second forward link transmission according to said first arbitrary frame alignment;    means for determining whether a performance indication is within expected limits; and    means for combining said first forward link transmission and said second forward link transmission using a second arbitrary frame alignment if said performance indication is not within the expected limits.    
     
     
         29 . An apparatus for time alignment in a wireless communications system in which a remote unit is capable of communication with one or more base stations simultaneously, said apparatus comprising: 
 means for receiving a first forward link transmission from a first base station having a first frame alignment;    means for receiving a second forward link transmission from a second base station having a second frame alignment wherein said second frame alignment comprises information concerning frame boundaries and excludes information concerning an absolute frame number;    means for combining said first forward link transmission and said second forward link transmission according to a first frame alignment hypothesis to determine a first performance indication;    means for combining said first forward link transmission and said second forward link transmission according to a second frame alignment hypothesis to determine a second performance indication; and    means for comparing said first and second performance indications in order to determine a most likely absolute frame alignment.    
     
     
         30 . In a spread spectrum communication system in which a remote unit communicates with another system user via at least one base station, a method for directing communications between said remote unit and said base stations comprising the steps of: 
 providing to said remote unit a neighbor list identifying one or more base stations;    providing to said remote unit an active list identifying one or more base stations through which active communication is established;    receiving from said remote unit a candidate list identifying at least one target base station;    determining an availability of system resources at said at least one target base station; and    providing to said remote unit an active list identifying said at least one target base station;    wherein said neighbor list comprises a series of entries, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.    
     
     
         31 . The method of  claim 30 , wherein said entries further comprise a window size over which a search should be performed.  
     
     
         32 . The method of  claim 31 , wherein said window size implicitly carries information concerning a relative class of synchronization between said reference base station and said base station to which said entries correspond.  
     
     
         33 . The method of  claim 31 , wherein said window size carries information concerning whether said base station corresponding to said entry is frame synchronized with said reference base station.  
     
     
         34 . In a spread spectrum communication system in which a remote unit communicates with another system user via at least one base station, an apparatus for directing communications between said remote unit and said base stations comprising: 
 means for providing to said remote unit a neighbor list identifying one or more base stations;    means for providing to said remote unit an active list identifying one or more base stations through which active communication is established;    means for receiving from said remote unit a candidate list identifying at least one target base station;    means for determining an availability of system resources at said at least one target base station; and    means for providing to said remote unit an active list identifying said at least one target base station;    wherein said neighbor list comprises a series of entries, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.    
     
     
         35 . In a spread spectrum communication system in which a remote unit communicates with another system user via at least one base station, a method for directing communications between said remote unit and said base stations comprising the steps of: 
 receiving at said remote unit an active list identifying one or more base stations through which active communication is established;    receiving at said remote unit a neighbor list identifying one or more base stations;    measuring at said remote unit a signal strength of a pilot signal transmitted by each base station having an entry on said neighbor list;    transmitting a first message from said remote unit, said first message identifying a candidate list comprising an entry corresponding to at least one target base station; and    receiving at said remote a new active list of base station comprising an entry corresponding to said at least one target base station;    wherein said neighbor list comprises a series of entries, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.    
     
     
         36 . The method of  claim 35 , wherein said entries further comprise a window size over which a search should be performed.  
     
     
         37 . The method of  claim 36 , wherein said window size implicitly carries information concerning a relative class of synchronization between said reference base station and said base station to which said entries correspond.  
     
     
         38 . The method of  claim 36 , wherein said window size carries information concerning whether said base station corresponding to said entry is frame synchronized with said reference base station.  
     
     
         39 . In a spread spectrum communication system in which a remote unit communicates with another system user via at least one base station, an apparatus for directing communications between said remote unit and said base stations comprising: 
 means for receiving at said remote unit an active list identifying one or more base stations through which active communication is established;    means for receiving at said remote unit a neighbor list identifying one or more base stations with a high probability of having signal strength sufficient to establish communication;    means for measuring at said remote unit a signal strength of a pilot signal transmitted by each base station having an entry on said neighbor list;    means for transmitting a first message from said remote unit, said first message identifying a candidate list comprising an entry corresponding to at least one target base station; and    means for receiving at said remote a new active list of base station comprising an entry corresponding to said at least one target base station;    wherein said neighbor list comprises a series of entries, said entries comprising information identifying a reference base station and a PN offset wherein a timing of said reference base station is used as a reference timing for said PN offset.