Soft handoff in ofdma system

Abstract

Soft handoff in an OFDMA system is disclosed. If the pilot signal strength for a base station exceeds the defined threshold, the base station is added to an active set list. Subcarriers in a plurality of orthogonal frequency division multiplexing (OFDM) symbols are divided and allocated into subchannels. The OFDM symbols are divided and multiplexed. A soft handoff zone with a first dimension of the subchannels and a second dimension of the divided and multiplexed OFDM symbols is defined. The soft handoff zone has subcarriers with a subchannel definition, for example, an identical permutation.

1 - 11 . (canceled) 
     
     
         12 . A method for operating a base station controller in order to facilitate soft handoffs for a subscriber station, the method comprising:
 receiving an active set list from a subscriber station via a first base station among a plurality of base stations, wherein the active set list indicates one or more of the base stations that have been identified by the subscriber station as being sufficiently strong for communication with the subscriber station;   in response to determining that the active set list includes more than one base station, sending an alert to the base stations included in the active set list, wherein said alert initiates a soft handoff mode with respect to the subscriber station;   receiving data destined for the subscriber station from a network; and   transmitting the data to two or more of the base stations included in the active set list, wherein the two or more base stations transmit the data to the subscriber station.   
     
     
         13 . The method of  claim 12 , further comprising:
 receiving an update to the active set list from the subscriber station;   in response to determining that the updated active set list includes only one base station, terminating the soft handoff mode with respect to the subscriber station, wherein the one base station on the active set list continues to serve the subscriber station after said termination.   
     
     
         14 . The method of  claim 12 , wherein said transmitting the data to the two or more base stations includes multicasting the data to the two or more base stations. 
     
     
         15 . The method of  claim 12 , wherein the data includes a sequence of data packets, wherein said transmitting the data to the two or more base stations includes dividing the data packets into sub-packets and transmitting each sub-packet to a corresponding one of the two or more base stations. 
     
     
         16 . The method of  claim 12 , wherein each of the two or more base stations generates and transmits a corresponding stream of OFDM symbols, wherein each of the OFDM symbols includes a common set of subcarriers, wherein each stream of OFDM symbols includes a plurality of zones, wherein each of the zones extends in a time dimension over more than one of the OFDM symbols and includes one or more subsets of the subcarriers, wherein the zones include a soft handoff zone, wherein a definition of the one or more subsets of subcarriers belonging to the soft handoff zone is shared by the two or more base stations. 
     
     
         17 . The method of  claim 16 , wherein the definition of the one or more subsets of subcarriers belonging to each zone is specified by a respective subcarrier permutation, wherein a first subcarrier permutation corresponding to the soft handoff zone is used by the two or more base stations to define the one or more subsets belonging to the soft handoff zone. 
     
     
         18 . The method of  claim 17 , wherein the first subcarrier permutation is a distributed subcarrier permutation or an adjacent subcarrier permutation. 
     
     
         19 . The method of  claim 16 , wherein the soft handoff zone includes a first data region, wherein the two or more base stations simultaneously transmit said data in the first data region to facilitate RF combining at the subscriber station. 
     
     
         20 . The method of  claim 16 , wherein the soft handoff zone includes a plurality of data regions, wherein the two or more base stations transmit said data in respectively different ones of the data regions to facilitate soft combining at the subscriber station. 
     
     
         21 . A base station controller comprising:
 a processor; and   memory storing program instructions, wherein the program instructions, when executed by the processor, cause the processor to:
 receive an active set list from a subscriber station via a first base station among a plurality of base stations, wherein the active set list indicates one or more of the base stations that have been identified by the subscriber station as being sufficiently strong for communication with the subscriber station; 
 in response to determining that the active set list includes more than one base station, send an alert to the base stations included in the active set list, wherein said alert initiates a soft handoff mode with respect to the subscriber station; 
 receive data destined for the subscriber station from a network; and 
 transmit the data to two or more of the base stations included in the active set list, wherein the two or more base stations transmit the data to the subscriber station. 
   
     
     
         22 . The base station controller of  claim 21 , wherein the program instructions, when executed by the processor, further cause the processor to:
 receive an update to the active set list from the subscriber station; and   in response to determining that the updated active set list includes only one base station, terminate the soft handoff mode with respect to the subscriber station, wherein the one base station on the active set list continues to serve the subscriber station after said termination.   
     
     
         23 . The base station controller of  claim 21 , wherein said transmitting the data to the two or more base stations includes multicasting the data to the two or more base stations. 
     
     
         24 . The base station controller of  claim 21 , wherein the data includes a sequence of data packets, wherein said transmitting the data to the two or more base stations includes dividing the data packets into sub-packets and transmitting each sub-packet to a corresponding one of the two or more base stations. 
     
     
         25 . The base station controller of  claim 21 , wherein each of the two or more base stations generates and transmits a corresponding stream of OFDM symbols, wherein each of the OFDM symbols includes a common set of subcarriers, wherein each stream of OFDM symbols includes a plurality of zones, wherein each of the zones extends in a time dimension over more than one of the OFDM symbols and includes one or more subsets of the subcarriers, wherein the zones include a soft handoff zone, wherein a definition of the one or more subsets of subcarriers belonging to the soft handoff zone is shared by the two or more base stations. 
     
     
         26 . The base station controller of  claim 25 , wherein the definition of the one or more subsets of subcarriers belonging to each zone is specified by a respective subcarrier permutation, wherein a first subcarrier permutation corresponding to the soft handoff zone is used by the two or more base stations to define the one or more subsets belonging to the soft handoff zone. 
     
     
         27 . The base station controller of  claim 26 , wherein the first subcarrier permutation is a distributed subcarrier permutation or an adjacent subcarrier permutation. 
     
     
         28 . The base station controller of  claim 25 , wherein the soft handoff zone includes a first data region, wherein the two or more base stations simultaneously transmit said data in the first data region to facilitate RF combining at the subscriber station. 
     
     
         29 . The base station controller of  claim 25 , wherein the soft handoff zone includes a plurality of data regions, wherein the two or more base stations transmit said data in respectively different ones of the data regions to facilitate soft combining at the subscriber station. 
     
     
         30 . A non-transitory computer-readable memory medium storing program instructions for operating a base station controller in order to facilitate soft handoffs for a subscriber station, wherein the program instructions, when executed by a processor, cause the processor to:
 receive an active set list from a subscriber station via a first base station among a plurality of base stations, wherein the active set list indicates one or more of the base stations that have been identified by the subscriber station as being sufficiently strong for communication with the subscriber station;   in response to determining that the active set list includes more than one base station, send an alert to the base stations included in the active set list, wherein said alert initiates a soft handoff mode with respect to the subscriber station;   receive data destined for the subscriber station from a network; and   transmit the data to two or more of the base stations included in the active set list, wherein the two or more base stations transmit the data to the subscriber station.   
     
     
         31 . The memory medium of  claim 30 , wherein each of the two or more base stations generates and transmits a corresponding stream of OFDM symbols, wherein each of the OFDM symbols includes a common set of subcarriers, wherein each stream of OFDM symbols includes a plurality of zones, wherein each of the zones extends in a time dimension over more than one of the OFDM symbols and includes one or more subsets of the subcarriers, wherein the zones include a soft handoff zone, wherein a definition of the one or more subsets of subcarriers belonging to the soft handoff zone is shared by the two or more base stations.