TECHNICAL FIELD

The disclosed embodiments relate generally to wireless communication, and, more particularly, to improvement of mobility robustness in the wireless network.

BACKGROUND

In the current wireless communication network, handover procedure is performed to support mobility when UE moves among different cells. For example, in the current new radio (NR) system, only basic handover is introduced. The basic handover is mainly based on LTE handover (HO) mechanism in which network controls UE mobility based on UE measurement reporting. In the basic/normal handover, similar to LTE, the source gNB triggers handover by sending HO request to the target gNB. After receiving HO Request ACK from the target gNB, the source gNB initiates handover by sending HO command with target cell configuration.

Further, in addition to traditional voice and internet data services, the wireless network now needs to support many other services with various QoS requirements. For example, some services require ultra-reliability and low latency, such as remote control, aerial, industrial automation, industrial control, and Augmented Reality (AR) and Virtual Reality (VR). For such services, mobility performance with high reliability and low interruption time are required.

With the increased density of network deployment, the UE may experience more frequent handover due to the number of the cells and the radius of cells. compared to the LTE, the handover in high frequency range of the NR with beamforming may increase interruption time due to beam sweep delays. With smaller coverage, the beamforming also reduces the reliability of the wireless system. When the UE moves or rotates, the UE can experience very fast signal degradation. It may result in higher handover failure and high ping-pong rate. Therefore, handover becomes extremely challenging in high frequency environment than in low frequency. When handover failure or radio link failure (RLF) occurs, the UE needs to perform RRC connection re-establishment, which takes a long time and impairs the user experience dramatically. If RRC connection re-establishment fails, the UE will go to IDLE.

Improvements and enhancements are required for the mobility reliability and the reduction of handover failure rate and radio link failure rate.

SUMMARY

Apparatus and methods are provided for handover robustness. In novel aspect, the UE receives a conditional handover (CHO) command from a source gNB containing a set of candidate cells, wherein each candidate cell is a handover target cell when one or more corresponding handover conditions are verified; detects a handover conditions for a target gNB belonging to the set of candidate cells contained in the conditional HO command, wherein each candidate cell is configured with a corresponding handover condition; and performs a random access towards the target gNB. In one embodiment, the CHO command is carried by RRC reconfiguration message. In one embodiment, the UE performs a conditional HO in response to a conditional HO command in the RRC reconfiguration message, a normal HO in response to a normal HO command in the RRC reconfiguration message, and a reconfiguration in response to a regular RRC reconfiguration message. In one embodiment, the conditional HO command further configures a validity timer that controls a validity of handover conditions for the set of candidate cells. In one embodiment, the validity timer is started upon receiving the conditional HO command, the validity timer is stopped upon detecting at least one events comprising a handover condition is met and a normal handover command is received, and the conditional HO command is set to be invalid upon the expiration of the validity timer. In another embodiment, the conditional HO command further includes a candidate cell ID for each candidate cell, a new C-RNTI, one or more security algorithm identifiers for corresponding selected security algorithms for each candidate cell. In yet another embodiment, the conditional HO command further includes a set of dedicated random access channel (RACH) resources, an association between RACH resources and synchronization signal blocks (SSBs), an association between RACH resources and UE-specification channel state information-reference signal (CSI-RS) configurations, common RACH resources, and candidate cell system information blocks (SIBs). In one embodiment, the UE transmits a HO start indication message to the source gNB upon detecting at least one handover condition for a candidate cell is met.

In another novel aspect, a gNB receives a HO request from a source gNB, transmits HO request ACK message in response to the HO request to the source gNB, and performs an SN STATUS TRANSFER with the source gNB upon detecting one or more preconfigured conditions. In one embodiment, a validity timer is configured for the one or more preconfigured conditions. The gNB releases releasing UE context upon expiration of the validity timer. In another embodiment, the gNB receives an RRC reconfiguration message from the UE and connects with the UE as the target gNB. This summary does not purport to define the invention. The invention is defined by the claims.

