Contention-Free Random Access Recovery in LTE Networks
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Solution Overview
Problem
In LTE wireless networks, the contention-free random access procedure is prone to failures due to incorrect timing advance updates, especially at high velocities, leading to delays and potential call drops during handovers, as spurious peaks can be erroneously detected, causing the UE to fail in synchronizing with the network.
Innovation Solution
The radio access node is configured to detect the failure of the random access procedure by awaiting a predetermined number of retries for the third message and then ordering a restart of the procedure with the same contention-free random access preamble, minimizing delays and ensuring successful handovers by using a Hybrid Automatic Repeat Request (HARQ) mechanism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the RAN uses spurious peaks for timing estimation, then the random access procedure can be completed, but the timing advance update becomes incorrect causing synchronization failure
Solution Approach 1:
The patent implements a feedback mechanism where the RAN monitors whether the UE successfully completes the random access procedure by detecting the third message. When synchronization fails despite receiving a random access response, the RAN detects this through the absence of the expected third message and triggers a restart of the procedure, using feedback from the procedure outcome to correct the timing estimation error.
Solution Approach 2:
The patent applies preliminary action by restarting the random access procedure with a new preamble before the UE can return to idle mode or attempt alternative access methods. The RAN proactively initiates a new contention-free random access procedure with a different preamble from the restricted set, preventing the propagation of the incorrect timing advance and ensuring successful synchronization before proceeding with handover or data transmission.
2Ease of operation
If the UE returns to idle mode after random access response, then the procedure is considered complete, but delays occur when the procedure actually failed
Solution Approach 1:
The patent uses feedback from the RAN side to determine whether the random access procedure was truly successful. The RAN monitors for the third message from the UE as confirmation of successful synchronization. Only when this feedback confirms success does the UE return to idle mode or proceed with handover, preventing premature termination of failed procedures and reducing unnecessary delays.
Solution Approach 2:
The patent enables the random access procedure to self-correct through automatic restart mechanisms. When the RAN detects failure through missing third messages, it automatically initiates a new procedure without requiring higher-layer intervention or manual configuration changes, allowing the system to self-service the synchronization issue and reduce overall handover delay.
3Reliability
If a restricted set of preambles is used, then the risk of detecting incorrect preambles is reduced, but delays still occur at high velocities around 350 km/hour
Solution Approach 1:
The patent applies preliminary action by pre-configuring the UE with a restricted set of preambles that have reduced cyclic shifts, which minimizes the risk of spurious peak detection. This preliminary configuration is optimized for high-velocity scenarios, and when combined with the automatic restart mechanism, ensures that even if one preamble fails, the next attempt uses a different preamble from the restricted set, reducing overall delay compared to using the full preamble set.
Solution Approach 2:
The patent changes the parameters of the preamble set by using a restricted set with fewer cyclic shifts instead of the full set. This parameter modification reduces the probability of spurious peak detection while maintaining sufficient preamble availability. The combination of this parameter change with the automatic restart mechanism creates a robust solution for high-velocity scenarios where Doppler effects are significant.
Data Source
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AI summary
In order to provide a more robust contention-free random access procedure for use in a cellular wireless network and specifically, a contention-free random access procedure that is able to recover more quickly in case of failure a method for a radio access node (10) that is arranged in a wireless network and communications with user equipment, UE (20) and a radio access node are proposed. The method includes: receiving a first random access message (40) from a UE, the first message containing a pre-assigned contention-free random access preamble and representing a start of a random access procedure; responding to the received first message by sending a second message (50) to the UE, the second message containing time alignment information to enable a scheduled transmission of a third message (60) by the UE; ascertaining that no third message is received from the UE within a predetermined period of time and transmitting a command (80) to the UE ordering a restart of the random access procedure with the same pre-assigned contention-free random access preamble.