Random Access Response Window Extension via RA-RNTI Segmentation
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Solution Overview
Problem
In high band wireless communications, large subcarrier spacings for random access preambles can result in short random access response windows, making it difficult for user equipment (UE) to capture responses from base stations, leading to missed timing information and increased latency.
Innovation Solution
The described techniques modify the random access radio network temporary identifier (RA-RNTI) formula to extend the random access response window by limiting the number of slots used for PRACH transmission, restricting time and frequency domain RACH occasions, and using separate RA-RNTIs for normal and supplementary uplink carriers, allowing the RA-RNTI to exceed 16 bits, and incorporating sub-system frame number timing information in downlink control messages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If large subcarrier spacings are used for random access preambles in high band communications, then the system can support larger bandwidth and handle phase noise more effectively, but the random access response window becomes too short for UE to capture responses reliably
Solution Approach 1:
The random access response window is segmented into multiple sub-windows, each monitored with different RA-RNTI values corresponding to different slot ranges. This allows the UE to systematically search through the entire response window even when individual sub-windows are too short to capture responses reliably, thereby maintaining reliability while supporting large subcarrier spacings.
Solution Approach 2:
The UE determines and monitors multiple sub-windows in advance throughout the random access response window, rather than waiting for a single window to expire. This preliminary monitoring approach ensures that responses are captured as soon as they arrive, reducing latency while maintaining reliability despite short individual window durations.
2Reliability
If the random access response window is extended to allow reliable response capture, then response reliability improves, but the time resources for other transmissions are reduced and latency increases
Solution Approach 1:
The response window is divided into multiple sub-windows that can be monitored in parallel with different RA-RNTI values. This segmentation allows the system to maintain a long overall response window for reliability while enabling early detection and processing of responses in individual sub-windows, thereby reducing effective latency.
Solution Approach 2:
The UE monitors multiple sub-windows periodically throughout the random access response window at regular intervals. This periodic monitoring approach ensures continuous coverage of the extended window while maintaining efficient resource usage, balancing reliability requirements with latency constraints.
3Reliability
If multiple sub-windows are monitored with different RA-RNTI values, then response capture reliability improves, but the complexity of the random access procedure increases
Solution Approach 1:
The same monitoring mechanism is used for all sub-windows, with the only variation being the RA-RNTI value assigned to each sub-window. This universal approach allows the UE to handle multiple sub-windows using a single standardized procedure, minimizing the increase in complexity while achieving improved reliability through multi-sub-window monitoring.
Solution Approach 2:
The UE determines all RA-RNTI values for different sub-windows in advance before initiating the random access procedure. This preliminary determination simplifies the actual monitoring process by having all necessary identifiers ready, reducing runtime complexity while enabling comprehensive multi-sub-window monitoring for improved reliability.
Data Source
AI summary
Methods, systems, and devices for wireless communications are described. The method includes receiving a random access configuration for the UE, the random access configuration indicating a subcarrier spacing for random access preambles of a random access procedure and a periodicity of slots including random access channel occasions for transmission of the random access preambles, where the periodicity of the slots is based on the subcarrier spacing for the random access preambles, determining a random access radio network temporary identifier based on the periodicity of the slots and a slot index for a slot in which the UE is to transmit a random access preamble, and transmitting, in the slot, the random access preamble indicating the determined random access radio network temporary identifier.