Method of enhanced connection recovery and cell selection
An enhanced connection recovery upon lost RRC connection due to radio link failure (RLF) or handover failure (HOF) is proposed. A UE first establishes an RRC connection in a source cell in a mobile communication network. Later on, the UE detects a failure event and starts an RRC reestablishment procedure in a target cell to restore the RRC connection. In a first novel aspect, a fast RLF process is applied to reduce the outage time in the serving cell. In a second novel aspect, an enhanced cell selection mechanism based on cell prioritization information is applied to reduce the outage time in the target cell. In one embodiment, multi-RAT registration is used to steer cell selection.
1. A method, comprising:
establishing a radio resource control (RRC) connection by a user equipment (UE) with a base station in a mobile communication network;
detecting a radio link problem of the RRC connection and thereby starting a first timer associated with the radio link problem;
starting a second timer upon detecting a triggering condition while the first timer is still running;
initiating an RRC reestablishment procedure with a target cell when the second timer expires but before the first timer expires; and
performing the RRC connection reestablishment procedure with the target cell.
2. The method of claim 1 , wherein a channel quality of the target cell is better than a channel quality of a serving cell by a non-negative threshold based on reference signal received power or reference signal received quality (RSRP/RSRQ) measurements.
3. The method of claim 1 , wherein the triggering condition is satisfied when the UE transmits a measurement report to the base station, wherein the RRC reestablishment procedure is initiated when the second timer expires.
4. The method of claim 1 ,
wherein the triggering condition is satisfied when a channel quality of a serving cell is lower than a channel quality of the target cell by a non-negative threshold, and wherein the RRC reestablishment procedure is initiated when the second timer expires.
5. The method of claim 1 ,
wherein the triggering condition is satisfied when a channel quality of a serving cell is lower than a first threshold and a channel quality of the target cell is higher than a second threshold, wherein the RRC reestablishment procedure is initiated when the second timer expires.
6. The method of claim 1 , wherein the RRC reestablishment is initiated before the first timer expires if the target cell is identified as a candidate cell, and wherein the UE receives candidate cell assignment information from the base station.
7. The method of claim 6 , wherein the UE receives cell prioritization information from the base station for selecting the target cell.
8. A method, comprising:
establishing a radio resource control (RRC) connection by a user equipment (UE) with a base station in a mobile communication network;
initiating a first timer when detecting a radio link problem;
selecting a target cell based on cell prioritization information only when the UE is in a high mobility state, wherein the cell prioritization information comprises information for cell selection as well as for cell reselection;
initiating an RRC reestablishment procedure; and
performing the RRC connection setup procedure with the selected target cell.
9. The method of claim 8 , wherein the RRC reestablishment procedure is initiated when the first timer expires.
10. The method of claim 8 , wherein the RRC reestablishment procedure is initiated before the first timer expires.
11. The method of claim 8 , wherein the UE receives the cell prioritization information via a broadcast channel from the base station.
12. The method of claim 8 , wherein the UE receives the cell prioritization information via dedicated signaling from the base station.
13. The method of claim 8 , wherein cells are prioritized based on corresponding frequency layers, and wherein the UE receives cell ID information of prioritized cells.
14. The method of claim 8 , wherein cells of the same frequency layer are prioritized based on corresponding mobility coverage, and wherein the UE receives cell ID information of prioritized cells.
15. The method of claim 8 , wherein cells are prioritized based on radio access technology (RAT), and wherein the UE pre-registers in different RATs to facilitate inter-RAT coordination.
16. A user equipment (UE), comprising:
a radio resource control (RRC) management module that establishes an RRC connection with a base station in a mobile communication network;
a radio link management (RLM) module that detects a radio link problem of the RRC connection and thereby starting a first timer, wherein the RLF module also starts a second timer upon detecting a triggering condition while the first timer is still running, wherein the UE initiates an RRC reestablishment procedure with a target cell when the second timer expires but before the first timer expires; and
a cell selection module that selects a target cell, wherein the UE performs the RRC connection reestablishment with the target cell.
17. The UE of claim 16 , wherein the UE detects the triggering condition when the UE transmits a measurement report to the base station, and wherein the RRC reestablishment procedure is initiated when the second timer expires.
18. The UE of claim 16 , wherein the RRC reestablishment is initiated before the first timer expires if the target cell is identified as a candidate cell.
19. The UE of claim 16 , wherein the UE selects the target cell based on cell prioritization information for the RRC reestablishment procedure or an RRC connection setup procedure.
20. The UE of claim 19 , wherein the UE performs cell prioritization based on at least one of a frequency layer, a mobility coverage, and a radio access technology (RAT) of a cell.
21. The UE of claim 19 , wherein the UE selectively applies the cell prioritization when the UE is in a high mobility state.