Fallback DCI Repetition Indication for 5G RACH Coverage
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Solution Overview
Problem
Current 5G NR wireless communication systems face challenges in enhancing coverage for the random access channel (RACH) procedure, particularly for Msg2 and Msg3, due to limitations in scheduling with fallback DCIs that cannot support repetition.
Innovation Solution
The proposed solutions involve explicitly indicating the number of repetitions for Msg2 and Msg3 in fallback DCIs and providing an indication of an applicable waveform rather than relying on remaining minimum system information (RMSI). This includes using reserved bits in DCIs to indicate repetitions and enabling transform precoding for Msg3 transmissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fallback DCIs are used for scheduling Msg2 and Msg3, then the RACH procedure can be initiated, but coverage enhancement through repetition is not supported
Solution Approach 1:
The DCI structure is segmented by dedicating specific reserved bits to indicate the number of repetitions. This allows the fallback DCI to carry both scheduling information and repetition configuration independently, resolving the contradiction between maintaining fallback functionality and enabling coverage enhancement through repetition.
Solution Approach 2:
The DCI explicitly indicates the number of repetitions in advance before the actual transmission occurs. This preliminary indication allows the UE to prepare for repeated transmissions and properly configure the coverage enhancement procedure, enabling reliability improvement without compromising adaptability.
2Ease of operation
If RMSI is used to indicate waveform information, then system information is provided, but explicit waveform selection for Msg3 is not available
Solution Approach 1:
Waveform indication information is extracted from the general RMSI and placed directly into the fallback DCI structure. This extraction allows the waveform selection to be explicitly indicated in the scheduling message itself, making waveform information readily available without relying on separate RMSI interpretation, thus preventing information loss while maintaining ease of operation.
3Reliability
If transform precoding is not enabled for Msg3, then scheduling is simpler, but coverage enhancement is limited
Solution Approach 1:
The system dynamically enables or disables transform precoding based on the repetition indication carried in the fallback DCI. When repetitions are indicated (coverage enhancement mode), transform precoding is enabled to improve reliability. When no repetitions are indicated, transform precoding can be disabled for simpler operation. This dynamic adaptation resolves the contradiction between reliability enhancement and device complexity.
Data Source
AI summary
Providing coverage enhancements for a user equipment (UE) includes decoding a random access channel (RACH) preamble received from a user equipment (UE) via a physical random access channel (PRACH). A number of repetitions associated with transmitting a random access response (RAR) over a physical downlink shared channel (PDSCH) may be determined in response to the RACH preamble. A transmission for the UE to be sent over a physical downlink control channel (PDCCH) may be encoded. The transmission may indicate the number of repetitions to the UE using one or more reserved bits of a downlink control information (DCI).