NTN RACH Procedures with Extended RAR Windows and Frame 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 due to insufficient RAR window duration and ambiguity in decoding RAR messages, especially for user devices with varying propagation delays within the beam footprint.
Innovation Solution
The RAR window duration is extended based on propagation delays, and a NTN-RNTI is used to scramble the CRC of DCI format 1_0, along with blind retransmissions and adjustments to K1 and K2 values to enhance RACH procedures in NTN.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the RAR window duration is extended to accommodate long propagation delays in NTN, then the reliability of RACH procedure is improved, but ambiguities in decoding RAR messages increase due to multiple user devices transmitting PRACH using identical time-frequency resources in different system frames
Solution Approach 1:
The patent segments the RAR window into multiple non-overlapping time intervals, each associated with a specific system frame. This segmentation allows the UE to determine which time interval a received RAR belongs to based on the system frame number, thereby eliminating ambiguity in RAR identification while maintaining the extended window duration needed for NTN propagation delays
Solution Approach 2:
The patent applies preliminary action by having the UE determine the system frame number before attempting to decode RAR messages. This preliminary knowledge of the system frame number allows the UE to pre-determine which time interval to monitor and which RARs are relevant, preventing information loss from the outset rather than attempting to resolve ambiguity after the fact
2Measurement precision
If the RAR window start time is delayed to account for long propagation delays, then the accuracy of RAR reception timing is improved, but the productivity of the RACH procedure decreases due to extended access time
Solution Approach 1:
The patent implements dynamic adjustment of the RAR window start time and duration based on the specific NTN propagation delay characteristics. Rather than using a fixed window, the system dynamically configures the window parameters to match the actual delay conditions, achieving both timing accuracy and efficient access without unnecessary delays
3Adaptability or versatility
If multiple RARs are transmitted within an extended RAR window for different system frames, then the adaptability of the system to NTN conditions is improved, but the device complexity increases due to the need to track and distinguish RARs from multiple frames
Solution Approach 1:
The patent reduces UE processing complexity by having the device determine the system frame number in advance before RAR reception. This preliminary determination allows the UE to pre-configure which time intervals to monitor and which RARs are relevant, eliminating the need for complex tracking and distinction mechanisms during actual RAR processing
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. A NTN-RNTI associated with the time-frequency resources used for the PRACH preamble may be used to scramble the CRC of DCI used for downlink assignment in the RAR. The DCI content may include information on the associated PRACH preamble to assist the UE in distinguishing between RARs generated as a response to PRACH preambles transmitted by different UEs from different system frames. The NTN-RNTI may contain information on the system frames when the UE sends the PRACH preamble. The RA-RNTI associated with the time-frequency resources used for the PRACH preambles transmitted from different frames may be used to scramble different subsets of the CRC of the DCI format 1_0 to assist the UE in distinguishing between RARs generated in response to the different PRACH preambles.