2-Step Random Access RO Selection for Low-Latency PRACH-PUSCH Timing
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Solution Overview
Problem
The challenge in 5G wireless communication networks is to minimize latency and reduce the number of Listen-Before-Talk (LBT) procedures during the 2-step Random Access (RA) procedure, particularly in unlicensed spectrum, where the gap between Physical Random Access Channel (PRACH) and Physical Uplink Shared Channel (PUSCH) transmissions can exceed 16 microseconds, leading to increased latency and potential LBT failures.
Innovation Solution
The solution involves configuring a subset of Random Access Channel (RACH) occasions for the 2-step RA procedure, with signaling indicating which ROs are candidates for PRACH transmissions, and ensuring that only the last RO in a slot is used to minimize the gap between PRACH and PUSCH, thereby reducing latency and LBT requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If all RACH occasions are used for 2-step RA procedure, then the coverage and accessibility are improved, but the gap between PRACH and PUSCH transmissions increases leading to higher latency and LBT failures
Solution Approach 1:
The patent segments the RACH occasions into two distinct sets: a first set of RACH occasions configured for 2-step RA procedure and a second set configured for 4-step RA procedure. This segmentation allows the network to control which ROs are available for 2-step RA, ensuring that only those with acceptable gaps to PUSCH transmissions are included, thereby reducing latency while maintaining adequate coverage.
Solution Approach 2:
The patent applies local quality by configuring different properties for different RACH occasions. Specifically, certain ROs are configured with specific time-domain positions that ensure the gap between PRACH and PUSCH transmissions meets the 16 microsecond threshold, while other ROs may have different characteristics suitable for 4-step RA. This localized optimization ensures that 2-step RA occurrences have the required quality (small gap) without compromising overall system capacity.
2Loss of time
If the gap between PRACH and PUSCH is reduced to minimize latency, then the LBT procedures are reduced, but the configuration complexity increases
Solution Approach 1:
The patent introduces dynamic configuration mechanisms where the network can adjust which RACH occasions are allocated for 2-step RA based on current network conditions and requirements. The gNB can dynamically modify the set of first RACH occasions to optimize the gap between PRACH and PUSCH transmissions while managing configuration complexity through standardized signaling procedures.
Solution Approach 2:
The patent changes key parameters such as the time-domain position of RACH occasions, the association between SSBs and RACH occasions, and the configuration of msgA PUSCH resources to control the gap duration. By adjusting these parameters, the system can ensure the gap remains below 16 microseconds, reducing LBT procedures while managing configuration complexity through systematic parameter optimization.
3Loss of time
If separate RACH occasion configurations are used for 2-step and 4-step RA procedures, then the latency is reduced for 2-step RA, but the signaling overhead increases
Solution Approach 1:
The patent implements a unified RACH configuration framework that serves both 2-step and 4-step RA procedures. The same RACH configuration parameters (such as prach-ConfigurationIndex, messageA-PUSCH-Configuration) are used for both procedures, but with different interpretations and associations. This universal approach allows separate optimization for 2-step RA (with dedicated first RACH occasions) while reusing existing 4-step RA configurations, thereby reducing signaling overhead.
Solution Approach 2:
The patent performs preliminary configuration of RACH occasions and their associations with SSBs before the actual RA procedures occur. The network pre-configures which RACH occasions should be used for 2-step RA versus 4-step RA, and pre-associates SSBs with appropriate RACH occasions. This preliminary action reduces the need for dynamic signaling during RA operations, thereby reducing overall signaling overhead while maintaining low latency for 2-step RA.
Data Source
AI summary
Operation in unlicensed spectrum requires Clear Channel Assessment (CCA), where a transmitter must monitor the channel before transmitting, to ensure it is free. An exception is a brief gap that allows for Rx/Tx turn-around, so that an ACK can be transmitted following reception, without performing a new CCA. NR introduces a 2-step Random Access (RA) procedure, where MsgA includes both a PRACH and a PUSCH transmission. To minimize CCA delays in unlicensed spectrum, only those RACH opportunities (ROs) and PUSCH opportunities (POs) that result in a very small delay between the PRACH and PUSCH transmissions of MsgA should be employed. RO configurations are defined for LTE 4-step RA. Embodiments provides methods for selecting which of these be may be shared for 2-step RA, for selecting which should be used in case of collisions, for selecting ROs and POs when the configurations are not shared, and for communicating RO configurations to UEs.