Common Resource Block

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.

Key Features

• Absolute frequency reference grid anchored at point A
• Continuous numbering across entire carrier bandwidth
• 12 subcarriers per CRB with configurable subcarrier spacing
• Supports multiple numerologies (15, 30, 60, 120, 240 kHz SCS)
• Enables mapping between PRBs and absolute frequency resources
• Foundation for bandwidth part configuration and management

Evolution Across Releases

Defining Specifications