Repetitive RACH Preamble Transmission for Reliable Random Access
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Solution Overview
Problem
Existing wireless communication systems face bottlenecks in establishing connections due to unsuccessful reception of the first random access channel (RACH) message by the base station, often due to inefficient transmission parameters or poor link conditions, leading to reduced reliability and efficiency in random access procedures.
Innovation Solution
The user equipment (UE) transmits the first RACH message multiple times using a subset of preamble sequences over designated RACH occasions, accompanied by a shortened random access response (RAR) window, enhancing the likelihood of successful detection and decoding by the base station.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the UE transmits the random access request message only once, then the transmission delay is reduced, but the reliability of successful detection by the base station deteriorates
Solution Approach 1:
The UE transmits multiple copies of the random access request message in advance across different occasions before the RAR window expires. This preliminary repeated transmission ensures that the base station has multiple opportunities to detect and decode the message, improving reliability without waiting for failure and retransmission.
Solution Approach 2:
The random access request message is transmitted periodically at multiple RACH occasions with a defined periodicity. This periodic transmission pattern increases the probability that at least one transmission will be successfully received by the base station, thereby improving detection reliability while maintaining time efficiency.
2Reliability
If the RAR window length is extended to accommodate single transmission, then the time for random access procedure increases, but the detection opportunity is improved
Solution Approach 1:
Multiple transmissions of the random access request message are performed in advance during the RAR window period. By the time the RAR window expires, the base station has already had multiple detection opportunities, allowing the RAR window to be shortened while maintaining reliable message delivery.
Solution Approach 2:
The random access request message transmission continues across multiple RACH occasions throughout the RAR window period. This continuous transmission approach ensures that the base station receives the message reliably while the shortened RAR window minimizes the overall procedure duration.
3Reliability
If multiple RACH preambles are used for transmission, then the detection probability increases, but the complexity of preamble selection and management increases
Solution Approach 1:
The total set of RACH preambles is segmented into multiple subsets, with each subset assigned to specific RACH occasions. This segmentation simplifies UE implementation by providing clear rules for preamble selection based on the current occasion, reducing the complexity of managing multiple preambles while maintaining high detection probability through diverse preamble usage.
Data Source
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AI summary
Methods, systems, and devices for wireless communications are described. A user equipment (UE) may transmit a first random access channel (RACH) message multiple times over corresponding RACH occasions until a second RACH message is received from a base station in a random access response (RAR) window. The multiple times the first RACH message is transmitted may include preamble sequences that are selected from a subset of preamble sequences configured for this repetitive first RACH message transmission scheme. Additionally, a subset of UEs may use this repetitive first RACH message transmission scheme based on a condition of a link between the UE and the base station, an exposure condition of the UE, other measurements, etc. In some cases, a shortened RAR window length may be configured for this repetitive first RACH message transmission scheme.