Description
The Common Resource Block (CRB) serves as the absolute reference grid for physical resource allocation in 5G New Radio (NR), defined in 3GPP specification 38.211. Each CRB is composed of 12 consecutive subcarriers in the frequency domain, with the subcarrier spacing (SCS) determining the exact bandwidth of each CRB. The CRB grid is anchored at a common reference point known as 'point A,' which serves as the absolute frequency reference for the entire carrier bandwidth. This point A is defined as the center of subcarrier 0 of the lowest-numbered CRB (CRB 0) and remains fixed regardless of bandwidth part (BWP) configurations or user equipment (UE) capabilities.
CRB indexing starts from 0 and extends across the entire carrier bandwidth, providing a continuous numbering scheme for all available resource blocks. This common grid enables the network to manage resources consistently across different UEs with varying bandwidth capabilities. When scheduling resources for a specific UE, the gNodeB maps the allocated Physical Resource Blocks (PRBs) within the UE's active bandwidth part to the corresponding CRB indices. This mapping is crucial for maintaining alignment between the network's resource management and the UE's configured bandwidth, ensuring that both entities reference the same physical resources despite operating within different bandwidth constraints.
The relationship between CRBs and PRBs is fundamental to 5G resource allocation. While CRBs provide the absolute reference grid spanning the entire carrier, PRBs represent the relative resource blocks within a specific bandwidth part. The conversion between PRB indices and CRB indices is defined by an offset parameter (N_start_BWP) that indicates where the BWP starts relative to the common reference point. This architecture allows for flexible bandwidth adaptation while maintaining a consistent global resource management framework. The CRB concept also supports carrier aggregation scenarios, where multiple component carriers each have their own CRB grid anchored at their respective point A references.
In practical implementation, CRBs are used for various physical layer functions including synchronization signal block (SSB) transmission, physical downlink shared channel (PDSCH) allocation, physical uplink shared channel (PUSCH) scheduling, and reference signal placement. The common reference grid ensures that different physical channels and signals maintain proper frequency alignment, which is critical for maintaining orthogonality and minimizing interference. The CRB architecture also supports the mixed numerology feature of 5G NR, where different subcarrier spacings can coexist within the same carrier, with each numerology having its own CRB grid defined relative to the common point A reference.
Purpose & Motivation
The Common Resource Block concept was introduced in 5G NR Release 15 to address the limitations of LTE's resource block architecture, which was less flexible for supporting diverse deployment scenarios and spectrum bands. In LTE, resource blocks were defined relative to the carrier center frequency, which created challenges for supporting wide carrier bandwidths and flexible spectrum allocations. The CRB architecture with its absolute reference point (point A) provides a more robust foundation for managing resources across varying bandwidth configurations and carrier aggregation scenarios.
A key motivation for the CRB concept was to support the enhanced flexibility requirements of 5G NR, including support for multiple numerologies, variable bandwidth parts, and diverse spectrum ranges from sub-6 GHz to millimeter wave. The common reference grid enables consistent resource management across these diverse scenarios, ensuring that scheduling decisions, interference coordination, and radio resource management algorithms can operate with a unified understanding of the available spectrum resources. This is particularly important for dynamic spectrum sharing and spectrum aggregation scenarios where different bandwidth parts may be allocated to different services or operators.
The CRB architecture also addresses the need for improved coexistence with LTE and other radio access technologies. By providing a clear, absolute reference for resource allocation, the CRB system enables better interference management between 5G NR and legacy systems operating in adjacent spectrum. This was a significant improvement over previous approaches that relied on relative references, which could lead to ambiguity in resource coordination between different network elements and technologies. The standardized CRB grid has become fundamental to 5G's ability to efficiently utilize fragmented spectrum and support diverse deployment scenarios.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (56 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the foundational framework for Common Resource Blocks (CRBs) was established, defining them as the basic frequency-domain units numbered from 0 upwards for each subcarrier spacing configuration. The release also introduced the concept of interlaced resource blocks, where CRBs are grouped into specific interlace patterns, and defined their role as the contiguous subset from which Bandwidth Parts (BWPs) are configured. Furthermore, corrections and alignments were made to ensure consistent mapping between virtual, physical, and common resource blocks for procedures like PDSCH, PUSCH (including configured grant transmissions), and SRS resource mapping.
- CR on PDSCH mapping to virtual resource blocks TS 38.211CR0006
- Correction on physical resource mapping for PUSCH with configured grant TS 38.211CR0008
- Correction to frequency-domain starting position for SRS resource mapping TS 38.211CR0009
- Correction on mapping from virtual to physical resource blocks TS 38.211CR0012
- Correction on PDSCH resource allocation scheduled by PDCCH in Type 0 common search space TS 38.211CR0018
- Correction on uplink resource allocation type 1 TS 38.214CR0010
+ 5 more changes
In Release 16, the Common Resource Block (CRB) framework was refined to support new functions like sidelink resource pool determination based on PSBCH and the definition of a Common MBS frequency resource as a contiguous set of CRBs. Enhancements also included corrections for PDSCH resource mapping with RE-level granularity and clarifications for the resource and port occupation of duplicate CSI-RS resources. These updates provided a more robust foundation for features such as precise positioning with DL PRS and reliable sidelink communications.
- Correction on RIM RS resource and set ID mapping TS 38.211CR0069
- CR on Interference Measurement Resource for L1-SINR TS 38.214CR0139
- Correction on sidelink resource pool determination based on PSBCH TS 38.214CR0141
- CR on DL PRS resource prioritization for UE measurements TS 38.214CR0148
- Corrections related to the sidelink resource reservation period TS 38.214CR0164
- Correction to pre-emption condition for Mode-2 resource allocation TS 38.214CR0165
+ 9 more changes
In Release 17, specific corrections and clarifications were made to procedures involving Common Resource Blocks (CRBs) to improve reliability and precision. These included corrections to the resource selection mechanism indicated by higher layers and to the determination of time-domain resource allocation for a PUSCH scheduled by an RAR UL grant. Furthermore, clarifications were provided for the field in SCI format 2-C indicating the time offset of the first resource of each tuple with respect to the reference slot.
- Correction on slot offsets of CSI-RS resource pairs for MTRP TS 38.214CR0320
- Correction on frequency resource for CSI-RS for tracking in TS 38.214 TS 38.214CR0351
- Correction on a minimum guard period between two SRS resources for antenna switching TS 38.214CR0363
- Correction on Type 1 configured grant PUSCH transmission associated with two SRS resource sets TS 38.214CR0365
- Correction on duplicated part on UE procedure for determining a resource conflict between TS 38.213 and TS 38.214 TS 38.214CR0390
- CR on power control parameters for multiple aperiodic SRS resource sets TS 38.214CR0395
+ 9 more changes
In Release 18, specific corrections were made to the Common Resource Block (CRB) framework, particularly for sidelink and uplink procedures. These included corrections for contiguous RB-based resource allocation and for the mapping of the Physical Sidelink Feedback Channel (PSFCH) to physical resources. Additionally, refinements were introduced for resource allocation in the time domain for multi-PXSCH scheduling and for slot offset calculations related to PUSCH transmissions.
- Correction on mapping PSFCH to physical resources TS 38.211CR0141
- CR on PSCCH DMRS sequence generation in a dedicated SL PRS resource pool TS 38.211CR0148
- CR on PDSCH resource mapping for dedicated spectrum less than 5 MHz TS 38.214CR0518
- Correction on TRIV/FRIV resource indication of SL-U TS 38.214CR0548
- Correction on contiguous RB based resource allocation TS 38.214CR0550
- Correction to Sidelink resource allocation mode 2 TS 38.214CR0568
+ 4 more changes
In Release 19, the CRB function saw clarifications and corrections related to the counting of simultaneous NZP-CSI-RS resources and the maximum number of SRS-RSRP and CLI-RSSI measurement resource sets. These changes refined the procedures for managing simultaneous L1 CLI-RSSI and L1 SRS-RSRP measurement resources. Additionally, corrections were made to PDSCH resource mapping to ensure accurate resource allocation within the common resource block grid.
- TEI19 Counting of CSI-RS resource referred by N CSI reporting settings [SimCSI_count] TS 38.214CR0681
- TEI19 Simultaneous NZP-CSI-RS resource counting with NES [SimCSI_countNES] TS 38.214CR0689
- Correction on PDSCH resource mapping TS 38.211CR0178
- Clarification on the number of simultaneous L1 CLI-RSSI and simultaneous L1 SRS-RSRP measurement resources TS 38.214CR0745
- Correction on the maximum number of SRS-RSRP measurement resource sets and CLI-RSSI measurement resource sets TS 38.214CR0746
Explore further
Broader topics and technologies where CRB plays a role.
Defining Specifications
3GPP specifications that define or reference CRB, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.211 vj10 | NR Physical Channels and Modulation | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |