Codebook Subset Restriction Signaling for 5G NR Precoder Selection
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Solution Overview
Problem
The transition from LTE to 5G New Radio (NR) requires new mechanisms for codebook subset restriction and precoder selection due to differences in co-phasing behavior and increased signaling overhead, particularly with the introduction of two parameters (φn and θp) for 2D DFT beams, which are not efficiently handled by existing beam-based rank-agnostic CBSR approaches.
Innovation Solution
The proposed solution involves dividing precoder codebooks into two groups, where the first group uses a common component (vl1,m) for ranks 1, 2, 5, 6, 7, 8, and the second group uses a different component ({tilde over (v)}l2,m) for ranks 3 and 4, with a mapping mechanism that restricts precoders based on these components, reducing signaling overhead by applying beam-based rank-agnostic CBSR within each group.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If beam-based rank-agnostic CBSR approach is used in LTE, then signaling overhead is reduced, but it cannot be directly re-used for NR due to different beam-based quantities in precoders for different ranks
Solution Approach 1:
The codebook is segmented into multiple groups, each group containing codebooks for specific ranks that share the same beam-based quantity. This allows the beam-based CBSR approach to be applied within each segment while accommodating different beam quantities across segments, thus reducing signaling overhead within groups while maintaining adaptability across all ranks.
Solution Approach 2:
Different beam-based quantities are assigned to different rank groups locally. Ranks 1-2 use one beam-based quantity, while ranks 3-4 use another, allowing each local group to be optimized for its specific rank requirements while maintaining overall system efficiency and reducing signaling overhead within each local context.
2Reliability
If pre-LTE full dimension multiple-input, multiple-output (FD-MIMO) CBSR approach based on PMI-based per-rank CBSR is used, then codebook restriction is achieved, but signaling overhead increases around 8 times compared to LTE
Solution Approach 1:
Codebooks for different ranks that share the same beam-based quantity are merged into the same group. This allows a single CBSR indication to restrict multiple codebooks simultaneously, achieving effective codebook restriction across different ranks while minimizing signaling overhead through the merging of restriction criteria.
Solution Approach 2:
The CBSR mechanism is designed to be universal across multiple ranks by using rank-grouping based on shared beam-based quantities. A single CBSR indication serves multiple functions by restricting codebooks across different ranks that belong to the same group, thereby achieving broad codebook control with minimal signaling overhead.
3Measurement precision
If two parameters (φn and θp) are used to co-phase the two-dimensional discrete Fourier transform (2D DFT) beams in NR, then co-phasing accuracy is improved, but new mechanisms are needed for codebook restriction of co-phasing and rank
Solution Approach 1:
The codebook is segmented into groups based on shared beam-based quantities, and within each group, CBSR indications restrict codebooks that share the same co-phasing characteristics. This segmentation allows accurate co-phasing control through two parameters while keeping the restriction mechanism relatively simple by operating within segmented groups rather than across all codebooks.
Solution Approach 2:
The system changes the parameter organization by grouping codebooks according to shared beam-based quantities and co-phasing characteristics. This parameter reorganization allows the two co-phasing parameters to be effectively utilized while simplifying the restriction mechanism through parameter-based grouping rather than individual codebook restriction.
Data Source
AI summary
A user equipment (UE) receives, from a network node, codebook subset restriction (CBSR) signaling comprising a bitmap. The bitmap comprises a plurality of bits. A bit of the plurality of bits mapped to a first index indicates whether a vector with the first index from a first set of vectors is restricted. Bits of the plurality of bits mapped within a particular window indicate whether a vector with a second index from the second set of vectors is restricted. The UE restricts precoders selectable from a plurality of codebooks based on the restricted vectors.