NTN RACH Access Timing With Extended RAR Window Identification
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Solution Overview
Problem
The large propagation delays in non-terrestrial networks (NTN) cause issues with the random access channel (RACH) procedure, particularly in determining the intended recipient of the random access response (RAR) due to extended differential delays and ambiguities in the RAR window, making it difficult for user devices to gain initial access.
Innovation Solution
The RACH procedure is enhanced by extending the start and length of the RAR window based on propagation delays, using NTN-RNTI to scramble the CRC of DCI format 1_0, and implementing blind retransmissions of PRACH preambles and RARs to improve transmission reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the RAR window is extended to accommodate maximum differential delay in NTN, then user devices can receive RAR within the window, but ambiguities arise in determining if a RAR is intended for the user device due to multiple RARs with the same RA-RNTI from different system frames
Solution Approach 1:
The patent segments the RAR identification process by introducing a frame number field that divides the identification into two parts: RA-RNTI for basic identification and frame number for temporal distinction. This allows the RAR window to be extended while maintaining unambiguous identification of the intended RAR recipient.
Solution Approach 2:
The frame number field acts as an intermediary element that mediates between the extended RAR window duration and the RAR identification process. It provides the additional information needed to distinguish RARs from different system frames, resolving the ambiguity without requiring changes to the fundamental RAR structure.
2Duration of action of moving object
If the RAR window is extended to accommodate long propagation delays in NTN, then user devices can receive RAR, but the RAR window contains multiple RARs generated in response to multiple user devices with the same RA-RNTI transmitting PRACHs using identical time-frequency resources in different system frames
Solution Approach 1:
The patent applies preliminary action by pre-configuring the frame number field in the DCI format 1_0 to indicate the system frame number of the PRACH transmission. This allows user devices to proactively identify the correct RAR by comparing the indicated frame number with their transmission timing, simplifying the identification process despite the extended RAR window.
Solution Approach 2:
The frame number field provides feedback information about which system frame the PRACH transmission occurred in. This feedback mechanism allows user devices to correlate their transmission timing with the received RAR, enabling accurate identification even when multiple RARs are present in the extended window.
3Reliability
If blind retransmissions of PRACH preambles and RARs are implemented, then transmission reliability is improved in the face of long propagation delays, but transmission time and resource consumption increase
Solution Approach 1:
The patent implements periodic action through blind retransmissions of PRACH preambles and RARs at predetermined intervals. This periodic retransmission strategy ensures that transmissions are repeated until successful delivery, improving reliability in the face of long and variable propagation delays in NTN without requiring complex real-time adjustments.
Data Source
AI summary
Methods and systems to enhance NR RACH procedure to accommodate non-terrestrial networks (NTN) are disclosed. The length of the RAR window may be extended. In one aspect, a gNB of the NTN may perform blind retransmissions of the RAR message scheduled by DCI within the RAR window to user equipment to improve transmission reliability of the RAR message for NTN. The number of blind retransmission and the transmission pattern may depend on the PRACH reception condition, an uplink channel condition, or may be pre-configured. In one aspect, the gNB may extend the K1 value and K2 value that determine the delays between uplink and downlink transmissions to align the time domain duplex (TDD) uplink-downlink configuration due to the long propagation delays associated with the NTN. In one aspect, the gNB may broadcast or multicast RAR window size extension values to the UEs based on the orbital altitude of the satellites.