QCL Reference Indicators for Low-Latency 5G Beam Switching
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Solution Overview
Problem
The challenge in 5G New Radio (NR) systems is to efficiently perform beam indication while minimizing overhead, interference, and latency, especially when using a mix of different reference signal types such as SSB, p-CSI-RS, and a-CSI-RS, without a single solution that fits all scenarios.
Innovation Solution
The network provides a UE with a QCL Reference Indictor (QRI) indicating that a transmitted RS is spatially quasi-co-located with a scheduled transmission, allowing the UE to use the same RX beam for reception, and flexible beam indication is achieved through explicit or implicit signaling based on prior measurements, associating QRIs with RSIs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple reference signal types (SSB, p-CSI-RS, a-CSI-RS) are used for beam indication, then beamforming flexibility and system robustness are improved, but signaling overhead and complexity increase
Solution Approach 1:
The QCL Reference Indicator (QRI) is designed as a universal signaling mechanism that can indicate spatial QCL relationships for multiple reference signal types (SSB, p-CSI-RS, a-CSI-RS) through a single unified indicator structure. This allows the same QRI framework to serve different RS types without requiring separate indication mechanisms for each, thereby reducing overall signaling overhead while maintaining flexibility across diverse beamforming scenarios
Solution Approach 2:
The network pre-configures QRI to RSI associations through higher layer signaling (RRC or MAC-CE) before dynamic beam indication is needed. This preliminary configuration establishes a mapping table that the UE can store and use for rapid beam switching, eliminating the need for repeated detailed signaling during time-critical beam management operations and thus reducing real-time overhead
2Reliability
If dynamic beam switching is performed frequently to maintain link budget, then communication performance is improved, but latency increases
Solution Approach 1:
The UE stores QRI to RSI mapping associations in advance through higher layer signaling, creating a ready-to-use lookup table. When beam switching is needed, the UE can immediately retrieve the appropriate RSI from the stored mappings based on the received QRI, avoiding time-consuming search or calculation processes and thus reducing beam switching latency while maintaining reliable link budget
Solution Approach 2:
The QRI serves as an intermediary indicator that indirectly points to the actual reference signal resources (RSI) through pre-established mappings. This two-stage indication mechanism (QRI → stored mapping → RSI) allows the network to dynamically switch beams while the UE efficiently translates the indication to actual beam configuration using pre-fetched information, reducing the time penalty of frequent beam switching
3Measurement precision
If explicit signaling is used to associate QRIs with RSIs, then beam indication accuracy is improved, but overhead increases
Solution Approach 1:
The system uses partial explicit signaling through higher layer configurations (RRC or MAC-CE) to establish QRI-RSI mappings only when needed, rather than continuously signaling all beam information. The UE stores these partial explicit indications and combines them with dynamic QRI signaling, achieving accurate beam indication while minimizing overhead by signaling only the essential mapping relationships in advance and relying on compact QRI indicators for dynamic beam selection
Data Source
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AI summary
In some embodiments, a network (e.g., a TRP) provides to a UE information indicating that a transmitted "first" RS resource (or "RS" for short) is quasi-co-located (QCL) with a scheduled transmission for the UE (e.g., a "second" RS). The UE may then receive the scheduled transmission under an assumption that the scheduled transmission (e.g., a second RS, such as a demodulation RS (DMRS)) is QCL with the first RS. The UE may receive such QCL information before, after or while receiving the first RS.