NTN Challenges for Random Access (Continued: Timer Conflicts)

In competitive random access, after a terminal (UE) receives a RAR message and sends a request for RRC connection establishment, whether it receives permission to establish the connection is crucial for determining the success of the competition. In the NTN scenario, the duration of the contention resolution timer presents another challenge for the terminal (UE).

I. Timer Challenges: During the RACH process, after the terminal (UE) sends the RRC connection request MSG3, it waits for the contention resolution message MSG4 to determine whether its random access attempt was successful. The duration for which the UE listens for MSG4 is controlled by the ra-ContentionResolutionTimer – this timer starts immediately after MSG3 is sent.

In NTN systems, the distance between the UE and the satellite base station is much greater, resulting in significantly higher round-trip delays compared to terrestrial systems. While the maximum configurable value of the ra-ContentionResolutionTimer can theoretically cover these longer delays, this approach is inefficient and may unnecessarily consume power at the UE. NTN typically requires energy-efficient operation, especially in remote or battery-constrained applications. Therefore, the default settings of the ra-ContentionResolutionTimer must be adjusted to better accommodate NTN propagation delays while conserving UE power.

II. Potential Solution: One solution is to introduce an offset for the start of the ra-ContentionResolutionTimer in the NTN scenario. The timer would not start immediately after MSG3 transmission, but only after an offset period that accounts for the expected round-trip delay in NTN.

This adjustment ensures that the timer is only active during the time period when MSG4 is expected to be received; by aligning the timer with the NTN-specific delay, the UE can avoid unnecessary monitoring during periods when MSG4 is unlikely to arrive. This saves power consumption and ensures compatibility with the longer latency of NTN. The advantages of offset-based timer adjustment include:

• Power Efficiency: The UE only monitors when a message is actually likely to arrive, thus reducing unnecessary power consumption.
• Adaptability to Different Orbits: The offset can be configured according to the type of NTN (GEO or LEO), as the propagation delay differs significantly between these systems.
• Scalability: This method can adapt to NTNs of different scales and propagation delay characteristics without requiring significant modifications to the standard conflict resolution process.
• Robustness: Aligning the timer with the actual delay prevents the conflict resolution timer from timing out prematurely, which could otherwise lead to unnecessary retransmissions or failures in NTN communication.