Uplink Grant Validity Control in RACH-less Handover
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Solution Overview
Problem
Current RACH-less handover procedures in LTE wireless communication networks face misalignment issues between the target eNB and UE regarding the validity of uplink grants, leading to increased delay and interference, as the target eNB lacks knowledge of when the UE synchronizes to the target cell.
Innovation Solution
Incorporating uplink grants with validity timers based on System Frame Number (SFN) values from the source cell into the HO Command message, allowing the UE to determine when to fallback to the RACH procedure, thus aligning the UE and target eNB on grant validity without requiring the UE to read the target cell's SFN.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the validity timer for uplink grants is based on when the UE synchronizes to the target cell, then the UE can determine grant validity autonomously, but the target eNB cannot know when the UE has synchronized, leading to misalignment between eNB and UE on grant validity timing
Solution Approach 1:
The source eNB transmits the HO Command message to the UE before the handover executes, containing pre-calculated uplink grant validity timing information. This preliminary transmission of timing data allows the UE to determine grant validity without needing to read the target cell's SFN, reducing handover delay while maintaining alignment between eNB and UE on timing.
Solution Approach 2:
The source eNB acts as an intermediary that receives the HO Command from the target eNB, adds or adjusts the uplink grant validity timing information based on its knowledge of the UE's synchronization status, and forwards it to the UE. This intermediary role resolves the information asymmetry between target eNB and UE regarding synchronization timing.
2Measurement precision
If the UE determines uplink grant validity based on target cell SFN, then timing alignment between UE and target eNB is achieved, but the UE must read the target cell's MIB to obtain SFN, increasing handover complexity and delay
Solution Approach 1:
The target eNB pre-calculates the uplink grant validity timing information in the HO Command message based on its knowledge of the UE's expected synchronization time. The source eNB forwards this pre-determined timing information to the UE, which can then determine grant validity without needing to read the target cell's MIB or SFN, thereby maintaining timing precision while reducing procedure complexity.
3Productivity
If the target eNB sets uplink grant validity based on estimated UE synchronization time, then grants can be allocated in advance, but the actual synchronization time is unknown, causing potential misalignment and interference
Solution Approach 1:
The source eNB provides feedback to the target eNB about the UE's synchronization status and timing information. Based on this feedback, the target eNB can accurately determine and communicate the uplink grant validity timing to the source eNB, which then forwards it to the UE. This feedback mechanism ensures reliable alignment between eNB and UE on grant validity timing while maintaining high handover efficiency.
Data Source
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AI summary
Methods for handover of a wireless device from a source cell served by a source base station to a target cell served by a target base station. An example method, implemented in the source base station, comprises obtaining a command for transmission to the wireless device, the command comprising configuration information that comprises an indication of at least one uplink grant time for uplink resources in the target cell, specified in terms of at least one system frame number (SFN) value. The method further comprises transmitting the command to the wireless device. The configuration information in the command comprises information sufficient to enable the wireless device to determine a time or times for accessing uplink resources in the target cell without obtaining SFN values from the target cell. The command may be a handover command for a random access channel-less (RACH-less) handover of the wireless device to the target cell.