Two-Step RACH for L1/L2 Inter-Cell Mobility
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Solution Overview
Problem
Current 5G NR technology faces challenges in improving inter-cell mobility efficiency, particularly in reducing handover latency and optimizing PCI selection processes, which are crucial for seamless user equipment (UE) communication across different base stations.
Innovation Solution
A two-step Random Access Channel (RACH) based method using Layer 1 (L1)/Layer 2 (L2) messaging is implemented, allowing User Equipment (UE) to initiate a PCI selection process by determining and selecting a target Physical Cell Identifier (PCI) that meets specific thresholds, transmitting random access messages, and monitoring responses to complete the handover procedure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a network-initiated PCI selection process is used, then reliability of handover is improved, but handover latency increases and device complexity increases
Solution Approach 1:
The patent inverts the traditional network-initiated PCI selection approach by enabling the UE to autonomously select and initiate handover to a target PCI. The UE measures downlink signals, selects a target cell meeting criteria, and initiates the RACH procedure without network command, thereby reducing handover latency while maintaining reliability through uplink signal verification.
Solution Approach 2:
The UE performs self-service by autonomously measuring downlink signal quality, selecting a target PCI, and initiating the handover procedure. The UE independently completes the PCI selection process without requiring network control, reducing dependency on network signaling and minimizing handover latency.
2Reliability
If a network-initiated PCI selection process is used, then reliability of handover is improved, but device complexity and network complexity increase
Solution Approach 1:
The UE autonomously performs PCI selection by measuring downlink signals, evaluating target cells against selection criteria, and initiating handover without network intervention. This self-service approach reduces both device complexity and network complexity by eliminating the need for complex network-controlled selection algorithms and extensive signaling.
Solution Approach 2:
The patent extracts the PCI selection function from the network control plane and relocates it to the UE's physical layer. By separating the selection decision-making from network management, the system reduces the complexity burden on both the device and network while maintaining handover reliability through uplink verification.
3Reliability
If traditional multi-step RACH procedure is used, then reliability is improved, but handover latency increases
Solution Approach 1:
The patent segments the traditional four-step RACH procedure into a simplified two-step process: (1) UE transmits a preamble and uplink signal in one message, and (2) the network responds with a single acknowledgment. This segmentation removes intermediate steps (msg2 and msg3), reducing access latency while maintaining reliability through the combined preamble-uplink signal transmission.
Solution Approach 2:
The patent merges the preamble transmission and uplink signal into a single message (msg1), combining multiple functions into one transmission step. This merging eliminates the need for separate message exchanges, reducing the number of round-trip delays while preserving the reliability benefits of both preamble-based random access and uplink signal verification.
Data Source
AI summary
Method and apparatus to configure a UE to initiate a two-step RACH process to perform a PCI selection process using L1/L2 messaging. The apparatus determines at least one target PCI in preparation of performing a PCI selection procedure. The apparatus selects a first target PCI that meets a threshold to initiate the PCI selection procedure. The apparatus transmits a first random access response message to the first target PCI. The apparatus monitors for a second random access response message from the first target PCI.