Managing system frame numbers (SFNs) for circuit-switched fallback (CSFB)

Abstract

A method of wireless communication includes recording an absolute system frame number (SFN) of a target radio access technology (RAT) and/or recording a relative system frame number (SFN) difference between a serving radio access technology (RAT) and the target RAT. A transmission time interval (TTI) boundary, is determined after redirection, based at least in part on the recorded absolute frame number (SFN) and/or the recorded relative system frame number (SFN) difference.

What is claimed is: 
     
       1. A method of wireless communication, comprising:
 Recording, by a user equipment (UE), an absolute system frame number (SFN) of a target radio access technology (RAT) and/or a relative system frame number (SFN) difference between a serving radio access technology (RAT) and the target RAT; and 
 determining, by the UE, a transmission time interval (TTI) boundary, after redirection, based at least in part on the recorded absolute system frame number (SFN) and/or the recorded relative system frame number (SFN) difference. 
 
     
     
       2. The method of  claim 1 , further comprising skipping blind decoding of a broadcast control channel (BCCH) in the target RAT to read a SFN to determine the TTI boundary. 
     
     
       3. The method of  claim 1 , in which the serving RAT is Long Term Evolution (LTE). 
     
     
       4. The method of  claim 1 , in which the target RAT is time division-synchronous code division multiple access (TD-SCDMA). 
     
     
       5. The method of  claim 1 , further comprising storing the recorded difference. 
     
     
       6. The method of  claim 1 , in which recording the SFN difference occurs while camped in the target RAT. 
     
     
       7. The method of  claim 1 , in which recording the SFN difference occurs during an inter-radio access technology (IRAT) measurement. 
     
     
       8. An apparatus for wireless communication, comprising:
 means for recording an absolute system frame number (SFN) of a target radio access technology (RAT) and/or a relative system frame number (SFN) difference between a serving radio access technology (RAT) and the target RAT; and 
 means for determining a transmission time interval (TTI) boundary, after redirection, based at least in part on the recorded absolute system frame number (SFN) and/or the recorded relative system frame number (SFN) difference. 
 
     
     
       9. The apparatus of  claim 8 , further comprising means for skipping blind decoding of a broadcast control channel (BCCH) in the target RAT to read a SFN to determine the TTI boundary. 
     
     
       10. The apparatus of  claim 8 , in which the serving RAT is Long Term Evolution (LTE). 
     
     
       11. The apparatus of  claim 8 , in which the target RAT is time division-synchronous code division multiple access (TD-SCDMA). 
     
     
       12. The apparatus of  claim 8 , further comprising means for storing the recorded difference. 
     
     
       13. The apparatus of  claim 8 , in which the means for recording the SFN difference occurs while camped in the target RAT. 
     
     
       14. The apparatus of  claim 8 , in which the means for recording the SFN difference occurs during an inter-radio access technology (IRAT) measurement. 
     
     
       15. A non-transitory computer-readable medium having program code recorded thereon, the program code comprising:
 program code to record an absolute system frame number (SFN) of a target radio access technology (RAT) and/or to record a relative system frame number (SFN) difference between a serving radio access technology (RAT) and the target RAT; and 
 program code to determine a transmission time interval (TTI) boundary, after redirection, based at least in part on the recorded absolute system frame number (SFN) and/or the recorded relative system frame number (SFN) difference. 
 
     
     
       16. The non-transitory computer-readable medium of  claim 15 , further comprising program code to skip blind decoding of a broadcast control channel (BCCH) in the target RAT to read a SFN to determine the TTI boundary. 
     
     
       17. The non-transitory computer-readable medium of  claim 15 , in which the serving RAT is Long Term Evolution (LTE). 
     
     
       18. The non-transitory computer-readable medium of  claim 15 , in which the target RAT is time division-synchronous code division multiple access (TD-SCDMA). 
     
     
       19. The non-transitory computer-readable medium of  claim 15 , further comprising program code to store the recorded difference. 
     
     
       20. The non-transitory computer-readable medium of  claim 15 , in which the program code to record is further configured to record the SFN difference while a user equipment (UE) is camped in the target RAT. 
     
     
       21. The non-transitory computer-readable medium of  claim 15 , in which the program code to record is further configured to record the SFN difference during an inter-radio access technology (IRAT) measurement. 
     
     
       22. An apparatus for wireless communication, comprising:
 a memory; and 
 at least one processor coupled to the memory, the at least one processor being configured:
 to record an absolute system frame number (SFN) of a target radio access technology (RAT) and/or to record a relative system frame number (SFN) difference between a serving radio access technology (RAT) and the target RAT; and 
 to determine a transmission time interval (TTI) boundary, after redirection, based at least in part on the recorded absolute system frame number (SFN) and/or the recorded relative system frame number (SFN) difference. 
 
 
     
     
       23. The apparatus of  claim 22 , in which the at least one processor is further configured to skip blind decoding of a broadcast control channel (BCCH) in the target RAT to read a SFN to determine the TTI boundary. 
     
     
       24. The apparatus of  claim 22 , in which the serving RAT is Long Term Evolution (LTE). 
     
     
       25. The apparatus of  claim 22 , in which the target RAT is time division-synchronous code division multiple access (TD-SCDMA). 
     
     
       26. The apparatus of  claim 22 , in which the at least one processor is further configured to store the recorded difference. 
     
     
       27. The apparatus of  claim 22 , in which the at least one processor is configured to record the SFN difference occurs while a user equipment (UE) is camped in the target RAT. 
     
     
       28. The apparatus of  claim 22 , in which the at least one processor is configured to record the SFN difference during an inter-radio access technology (IRAT) measurement.