PRACH Preamble Design for Link-Budget-Limited UE Random Access
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Solution Overview
Problem
Link-budget-limited user equipment (UE) devices face challenges in successfully initiating the random access procedure due to poor antenna performance or location-related coverage issues, leading to inadequate reception of the Physical Random Access Channel (PRACH) in wireless communication systems.
Innovation Solution
Enhanced random access procedures for link-budget-limited UE devices involve transmitting a PRACH preamble with larger subcarrier spacing and temporal width, multiple instances of Zadoff-Chu sequences, and repeated transmissions with specific timing configurations, along with modified random access response messages to improve decoding probability, and using Doppler shift-based sequence selection for frequency diversity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional PRACH formats are used by link-budget-limited UE devices, then the device can maintain standard transmission parameters, but the probability of successful PRACH detection by the base station is insufficient due to poor coverage
Solution Approach 1:
The patent changes key parameters of the PRACH preamble including increasing subcarrier spacing from conventional values to larger values (e.g., 15kHz to 30kHz or 60kHz), increasing temporal width by extending the preamble duration, and increasing the number of Zadoff-Chu sequence instances from 1-2 to 3 or more. These parameter changes enhance the signal robustness and detection probability for link-budget-limited devices in poor coverage areas
Solution Approach 2:
The PRACH preamble is segmented into multiple instances of Zadoff-Chu sequences transmitted at different time instances. Instead of using a single conventional preamble, the patent divides the preamble into multiple segments (e.g., 3 or more instances) that are transmitted sequentially, allowing the base station to combine these segments for improved detection reliability
2Reliability
If the PRACH preamble uses larger subcarrier spacing and temporal width, then the robustness of transmission is improved for link-budget-limited devices, but the resource consumption increases
Solution Approach 1:
The patent applies partial action by selectively using enhanced PRACH formats only for link-budget-limited UE devices rather than all devices. The base station can identify link-budget-limited devices and configure them with enhanced formats (larger subcarrier spacing, more instances) while conventional devices use standard formats, thus optimizing resource usage while providing enhanced reliability where needed
3Reliability
If multiple instances of Zadoff-Chu sequences are embedded in the PRACH preamble, then the base station's probability of successful decode increases, but the complexity of the preamble structure increases
Solution Approach 1:
The patent uses Zadoff-Chu sequences as a universal building block for PRACH preambles. The same Zadoff-Chu sequence generation mechanism and correlation-based detection method are used for both conventional single-instance preambles and the enhanced multi-instance preambles. This multi-functionality allows the system to handle both formats using the same underlying technology, reducing the effective complexity increase despite the structural differences
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed solution significantly increases the probability of successful random access procedure completion for link-budget-limited UE devices by enhancing the robustness of PRACH preamble transmission and reception, even in challenging coverage areas.
Implementation Method 1
the PRACH preamble may have larger subcarrier spacing and/or larger temporal width than defined for conventional PRACH formats
Implementation Method 2
a larger number of instances of a selected Zadoff-Chu sequence may be embedded within the PRACH preamble than in conventional PRACH formats
Implementation Method 3
using Doppler shift-based sequence selection for frequency diversity
Data Source
AI summary
Enhanced random access procedures for link-budget-limited user equipment (UE) devices are disclosed. A user equipment device may transmit a first message containing a Physical Random Access Channel (PRACH). The PRACH contains instances of a Zadoff-Chu sequence, and may be transmitted repeatedly as part of a single random attempt, to facilitate correlation data combining at the base station. The available Zadoff-Chu sequences may be partitioned among a plurality of sets, each set being associated with a respective Doppler shift range (or frequency hop pattern or time repetition pattern). A UE device may signal Doppler shift (or other information) to the base station by selection of one of the sets. The first PRACH transmission and the following PRACH transmission may occur in consecutive subframes. A UE device may select from a special set of Zadoff-Chu sequences (different from a conventional set of sequences), to signal its status as a link-budget-limited device.