Description
A Resource Block Group is a fundamental unit for resource allocation in the frequency domain within LTE and 5G NR's OFDMA-based air interface. A Physical Resource Block (PRB) is the smallest unit of resource that can be allocated to a user, defined as 12 consecutive subcarriers for one slot (0.5 ms in LTE). However, signaling the allocation of individual PRBs for every user in every subframe would create excessive control channel overhead. To mitigate this, the standard defines RBGs, where a group of contiguous PRBs is treated as a single allocatable entity. The scheduler in the gNB (or eNB) allocates one or more RBGs to a UE via a bitmap in the Downlink Control Information (DCI).
The size of an RBG (P) is not fixed; it is a function of the system bandwidth. 3GPP specification 36.213 defines a table mapping the number of downlink resource blocks (system bandwidth) to the RBG size. For example, for a system bandwidth of 10 MHz (50 PRBs), the RBG size is typically 3 PRBs. This results in approximately 17 RBGs (50/3, rounded up). The DCI format for resource allocation type 0 uses a bitmap where each bit corresponds to an RBG. A bit set to '1' indicates that the corresponding RBG is allocated to the UE. This method, known as Resource Allocation Type 0, provides a compact representation. For smaller bandwidths or more granular allocations, Resource Allocation Type 1 allows allocation of a subset of PRBs within an RBG, but still uses RBGs as a reference structure.
RBG-based allocation directly impacts scheduling flexibility and efficiency. Larger RBG sizes reduce DCI overhead but decrease scheduling granularity, which can lead to inefficient resource utilization, especially for users with small data packets or at the cell edge requiring frequency-selective scheduling gain. Smaller RBG sizes offer finer granularity but increase signaling cost. The network can configure the preferred allocation type. In 5G NR, the concept is retained and enhanced. NR defines a Resource Block Group size for bandwidth part (BWP)-based scheduling, and the DCI can indicate resource allocation using a resource indication value (RIV) for contiguous allocations or a bitmap for non-contiguous, with the granularity tied to the PRB and the configured BWP.
Purpose & Motivation
The Resource Block Group mechanism was created to solve the critical problem of control channel overhead in OFDMA-based cellular systems like LTE and NR. These systems offer high flexibility by allowing dynamic allocation of frequency resources (PRBs) to multiple users every transmission time interval (TTI). Signaling the exact set of PRBs for each user would require a large number of bits in the downlink control channel (PDCCH), consuming significant resources that could otherwise be used for data transmission and limiting the number of users that can be scheduled simultaneously. RBGs provide a practical compromise between scheduling granularity and signaling efficiency.
Introduced in LTE Release 8, RBGs addressed the limitations of simpler allocation schemes used in prior technologies. 3G UMTS used code division multiple access (CDMA) with spreading factors, which had different overhead characteristics. The shift to OFDMA in LTE demanded a new method for efficient frequency-domain resource indication. The RBG concept allowed the network to trade off between fine-grained, frequency-selective scheduling (beneficial for channel-aware scheduling in frequency-selective fading channels) and the capacity of the control channel. It enabled the support of wider system bandwidths (up to 20 MHz in LTE Rel-8) without a proportional explosion in control signaling.
In 5G NR (from Release 15), the purpose of RBGs remains, but within a more flexible framework. NR supports much wider bandwidths (up to 400 MHz) and diverse use cases. The RBG concept is applied within a Bandwidth Part (BWP), which is a subset of the total carrier bandwidth configured for a UE. This allows for efficient scheduling even for UEs that do not support the full carrier bandwidth. The configurable RBG size within a BWP allows network operators to optimize the overhead/granularity trade-off specifically for the services (e.g., massive IoT with small packets vs. eMBB with large bursts) active on that BWP, which is essential for network slicing and service-specific optimization.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (53 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the changes related to the RBG function primarily involved corrections and alignment for resource allocation procedures. These included specific corrections for uplink resource allocation type 1, PUSCH resource allocation with SP CSI, and the resource mapping for PDSCH. Furthermore, adjustments were made to align frequency domain resource allocation for PUSCH scheduled by a RAR UL grant and to correct resource allocation for configured grant Type 1 uplink transmission.
- E-UTRA - NR Cell Resource Coordination TS 36.300CR1122
- Corrections on resource coordination in stage-2 TS 36.300CR1190
- 36.300 CR on Correction of Physical Layer Resource to Cell Resource TS 36.300CR1211
- Correction on uplink resource allocation type 1 TS 38.214CR0010
- Correction on determination of the resource allocation table for PUSCH with SP CSI TS 38.214CR0011
- Correction on PUSCH resource allocation TS 38.214CR0012
+ 3 more changes
In Release 16, the RBG function was enhanced through corrections and clarifications to PDSCH resource mapping, including mapping with RE symbol level granularity. Specific refinements were also made to the pre-emption condition for Mode-2 resource allocation and to the procedures for PDSCH resource mapping in general. Additionally, corrections addressed the resource and port occupation of duplicate CSI-RS resources, impacting RBG-related configurations.
- 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
- CR on DL PRS resource indication in activation command for semi-persistent SRS for positioning and amount of DL PRS resource sets per TRP TS 38.214CR0171
+ 8 more changes
In Release 17, specific corrections were made to the RBG-related resource selection mechanism indicated by higher layers. Furthermore, a correction was applied to the UE procedure for determining a resource conflict, resolving a duplication between TS 38.213 and TS 38.214. These changes provided clarifications and fixes to existing RBG and resource allocation procedures.
- Introduction of new attributes "Resource Coordination Only" in ANR TS 36.300CR1390
- Correction on slot offsets of CSI-RS resource pairs for MTRP TS 38.214CR0320
- CR on the application of the configuration of group-based beam reporting TS 38.214CR0323
- 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
+ 11 more changes
In Release 18, specific corrections and clarifications were made to the RBG function and related resource allocation procedures. These included a correction for contiguous resource block based resource allocation and a correction to Sidelink resource allocation mode 2. Furthermore, enhancements addressed resource allocation in the time domain for multi-PXSCH scheduling and PDSCH resource mapping for dedicated spectrum less than 5 MHz.
- 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
- CR on active semi-persistent SRS resource configuration and transmission of SRS TS 38.214CR0650
- CR on Rel-18 group based beam reporting in TS 38.214 TS 38.214CR0664
+ 3 more changes
In Release 19, the enhancements for the RBG function specifically addressed counting and clarification for simultaneous measurement resources. This included defining the counting of CSI-RS resources referred by multiple CSI reporting settings and the counting of simultaneous NZP-CSI-RS resources with NES. Furthermore, the release provided clarifications and corrections on the maximum number of simultaneous L1 CLI-RSSI and SRS-RSRP measurement resources and their associated resource sets.
- 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
- 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
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Broader topics and technologies where RBG plays a role.
Defining Specifications
3GPP specifications that define or reference RBG, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | 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 |