Description
The Physical Resource Block (PRB) is the smallest element of resource allocation that can be scheduled to a user equipment (UE) in the downlink or uplink of LTE (E-UTRA) and NR (New Radio) systems. It represents a contiguous block of resources in the frequency-time domain. In the frequency domain, a PRB consists of 12 consecutive subcarriers. In the time domain, it spans one slot, which comprises a configurable number of OFDM symbols (e.g., 7 or 14 symbols for normal and extended cyclic prefix in LTE, and flexible numerology in NR). The product of these dimensions defines the total number of resource elements (REs) within a PRB, each RE being one subcarrier for one symbol period.
Architecturally, the PRB is the central construct of the Orthogonal Frequency Division Multiple Access (OFDMA) and Single-Carrier FDMA (SC-FDMA) schemes used in LTE and NR. The entire system bandwidth is divided into a set of available PRBs. The scheduler in the base station (eNodeB in LTE, gNodeB in NR) dynamically allocates specific PRBs to different UEs based on factors like channel quality, QoS requirements, and traffic load. This granular allocation enables multi-user diversity and frequency-selective scheduling, where users are assigned resources on the parts of the spectrum where their channel conditions are best.
How it works involves mapping higher-layer data and control information onto the physical layer resource grid. Transport blocks from the Medium Access Control (MAC) layer are channel coded, modulated, and then mapped onto the resource elements of the allocated PRBs. Control channels like the Physical Downlink Control Channel (PDCCH) and reference signals (e.g., Cell-Specific Reference Signals in LTE, Demodulation Reference Signals in NR) are also mapped onto specific REs within the PRB structure. The power level per PRB, as defined in specifications, is a key parameter for link adaptation and interference management.
In NR, the concept evolved with the introduction of flexible numerology. The subcarrier spacing (SCS) and slot duration are not fixed but scale with the numerology (μ). Therefore, the absolute bandwidth of a PRB (12 * SCS) and its duration change accordingly. This allows NR to efficiently support diverse service types, from enhanced mobile broadband (eMBB) with wide PRBs to ultra-reliable low-latency communications (URLLC) with shorter, more numerous PRBs in time. The PRB remains the atomic unit of scheduling, but its dimensions are adaptable to the deployment scenario.
Purpose & Motivation
The PRB was created to provide a standardized, efficient, and flexible unit for radio resource management in OFDMA-based cellular systems. Prior to LTE, 3G UMTS used code division multiple access (CDMA), where resources were primarily distinguished by spreading codes, making fine-grained frequency-domain scheduling impossible. The shift to OFDMA in LTE required a new paradigm for dividing and allocating the shared time-frequency resource among users.
The PRB solves the problem of granular resource allocation. By breaking the spectrum into small, schedulable blocks, it enables the system to exploit frequency-selective fading—allocating resources to users on their best frequencies. This dramatically improves spectral efficiency and user throughput compared to wideband allocation. It also facilitates advanced techniques like fractional frequency reuse for interference coordination in heterogeneous networks.
Furthermore, the PRB provides a common reference for defining channel bandwidths, power levels, and performance requirements. Specifications define parameters like 'Transmitted power per allocated RB' to ensure consistent RF performance. The PRB grid also structures the placement of essential signals like reference signals and synchronization signals, ensuring predictable network behavior. Its design in LTE (from Release 8) and subsequent enhancement in NR (from Release 15) was motivated by the need for a scalable resource unit that could support ever-increasing data rates, diverse latency requirements, and a wide range of frequency bands from sub-1 GHz to millimeter wave.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (129 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, specific corrections and enhancements were made to the PRB (Physical Resource Block) function, including a correction on the mapping from virtual to physical resource blocks and a CR on the distribution of total PRB usage. Furthermore, Release 15 introduced an implementation of UL PRB to DL PRB center offset for TDD NB-IoT to improve resource coordination between E-UTRA and NR cells.
- CR to TS 36.141: Implementation of UL PRB to DL PRB center offset for TDD NB-IoT TS 36.141CR1189
- E-UTRA - NR Cell Resource Coordination TS 36.300CR1122
- Support maximum 8 SS/PBCH blocks for unpaired spectrum beyond 2.4GHz TS 38.213CR0006
- 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
- Minor corrections to services provided by physical layer TS 36.302CR1195
+ 19 more changes
In Release 16, specific corrections were made to the PRB (Physical Resource Block) function, primarily focusing on the VRB-to-PRB mapping procedure for DCI format 1_2. Additionally, corrections were applied to PDSCH PRB bundling notation and to the dynamic frequency domain resource allocation type selection, refining the control of downlink physical channel resources. These updates ensured more precise and reliable resource allocation and mapping for enhanced physical layer procedures.
- Introduction of Physical Layer Enhancements for NR URLLC TS 38.212CR0026
- Correction to NR-DL-PRS-ResourcesCapability field description TS 37.355CR0331
- Correction on RIM RS resource and set ID mapping TS 38.211CR0069
- Correction to VRB-to-PRB mapping for DCI format 1_2 TS 38.211CR0079
- RRC IE name fix to dynamic frequency domain resource allocation type selection (Rel-15 origin) TS 38.212CR0056
- Correction on SRS resource set configuration in TS 38.212 TS 38.212CR0070
+ 30 more changes
In Release 17, specific clarifications and corrections were made to Physical Resource Block (PRB) related procedures, including PUCCH resource determination for multiplexing HARQ-ACK feedback and corrections to SRS resource sets for PUSCH repetition. The release also introduced updates for positioning reference signals, such as adding missing values for the `dl-prs-ResourceSetPeriodicityReq-r17` parameter. Furthermore, it included corrections to resource coordination parameters for CSI-RS and guard periods for antenna switching.
- CR to TS36.141 on introduction of upper 700MHz A block TS 36.141CR1325
- CR to TS38.104 on introduction of upper 700MHz A block TS 38.104CR0365
- Introduction of new attributes "Resource Coordination Only" in ANR TS 36.300CR1390
- Addition of missing values for dl-prs-ResourceSetPeriodicityReq-r17 TS 37.355CR0464
- Clarification on the field description of dl-prs-ResourceSetPeriodicityReq TS 37.355CR0477
- CR on the description of the SRS resource set indication for PUSCH repetition TS 38.212CR0117
+ 23 more changes
In Release 18, specific corrections were introduced to refine PRB-related resource handling, including corrections on guardband PRB handling and on the mapping of PSFCH to physical resources. The release also addressed physical resource allocation for sidelink and PDSCH in dedicated spectrum less than 5 MHz, ensuring proper contiguous RB-based resource allocation and frequency resource determination for resource pools. These updates provided more precise definitions for the use of radio resources within the physical layer.
- CR to TS 38.104 on additional narrowband blocking requirement for Band n100 TS 38.104CR0612
- (NR_NTN_enh-Core) CR for TS 38.108, Correction on OTA out-of-band blocking requirement for SAN type 2-O TS 38.108CR0095
- (NR_NTN_enh-Core) CR for TS 38.181, Correction on OTA out-of-band blocking requirement for SAN type 2-O TS 38.181CR0049
- 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
- Correction on the frequency resource of a resource pool for SL-U TS 38.213CR0627
+ 20 more changes
In Release 19, specific enhancements were made to the counting and configuration of CSI-RS resources, including simultaneous NZP-CSI-RS resource counting with NES, and clarifications on the maximum number of SRS-RSRP and CLI-RSSI measurement resource sets. Furthermore, corrections were applied to PDSCH resource mapping and the removal of the request for CSI-RS resource configuration for Early CSI acquisition was implemented.
- 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
- Introducing SR resources in LTM cell switch MAC CE [LTM_enh_SR] TS 38.300CR1054
- CR to TS 38.104 correcction on SBFD BS blocking TS 38.104CR0770
- 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
+ 7 more changes
Explore further
Broader topics and technologies where PRB plays a role.
Defining Specifications
3GPP specifications that define or reference PRB, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TR 25.912 vj00 | Evolved UTRA and UTRAN Technical Report | Rel-19 |
| TS 28.627 vj00 | SON Policy NRM IRP: Requirements | Rel-19 |
| TS 32.521 vb10 | SON Policy NRM IRP Requirements | Rel-11 |
| TS 36.108 vj10 | Satellite Access Node RF Requirements | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.181 vj30 | E-UTRA RF Test Methods for Satellite Access Node | Rel-19 |
| TS 36.211 vj10 | LTE Physical Layer Specification | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.216 vj00 | LTE Relay Node Physical Layer | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.314 vj00 | E-UTRA Radio Measurements Specification | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 36.747 ve00 | Enhanced CRS and SU-MIMO IM Performance Requirements | Rel-14 |
| TS 36.755 vf00 | US 600 MHz LTE Band 71 Technical Report | Rel-15 |
| TS 36.766 vf00 | LTE BS Interference Cancellation Receiver Study | Rel-15 |
| TR 36.791 vg00 | E-UTRA 2.4 GHz TDD Band for US | Rel-16 |
| TS 36.863 vc00 | CRS Interference Mitigation for Homogeneous Networks | Rel-12 |
| TS 36.887 vc00 | Energy Saving Enhancement for E-UTRAN Study | Rel-12 |
| TR 36.902 v931 | SON Use Cases and Solutions for LTE | Rel-9 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TR 37.880 vh20 | High-power UE for fixed-wireless/vehicle use | Rel-17 |
| TS 38.101 vj31 | NR User Equipment Radio Transmissions | Rel-19 |
| TS 38.104 vj20 | NR Base Station RF Requirements | Rel-19 |
| TS 38.108 vj20 | NTN NR Satellite Access Node RF Requirements | Rel-19 |
| TS 38.174 vj10 | NR Integrated Access and Backhaul Radio Spec | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.181 vj10 | NR Satellite Access Node RF Testing | Rel-19 |
| TS 38.211 vj10 | NR Physical Channels and Modulation | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.521 vj20 | NR Physical Layer UE Conformance Testing | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TS 38.551 vi30 | User Equipment (UE) Multiple Input Multiple Output (MIMO) Over-the-Air (OTA) performance | Rel-18 |
| TS 38.741 vj00 | NTN L-/S-band for NR Technical Specification | Rel-19 |
| TS 38.755 vj10 | NR FR1 DL Fragmented Carriers Study | Rel-19 |
| TR 38.785 vh00 | UE radio transmission for enhanced NR sidelink | Rel-17 |
| TR 38.786 vi20 | Technical Report for NR Sidelink Evolution | Rel-18 |
| TS 38.787 vj00 | UE Radio Transmission for Sidelink CA in ITS Band | Rel-19 |
| TS 38.793 vj00 | Simultaneous Rx/Tx Band Combinations TR | Rel-19 |
| TR 38.830 vh00 | NR Coverage Enhancements Study | Rel-17 |
| TS 38.831 vg10 | UE RF Requirements for FR2 Enhancements | Rel-16 |
| TR 38.839 vh00 | Simultaneous Rx/Tx band combinations | Rel-17 |
| TR 38.858 vi20 | Technical Report on Evolution of NR Duplex Operation | Rel-18 |
| TS 38.863 vj10 | NR NTN RF and Co-existence Spec | Rel-19 |
| TR 38.868 vh00 | Optimizations of pi/2 BPSK uplink power in NR | Rel-17 |
| TR 38.881 vi00 | Technical Report on Lower MSD for Inter-band CA/EN-DC/DC | Rel-18 |
| TR 38.886 vg30 | NR V2X UE Radio Transmission & Reception | Rel-16 |
| TR 38.892 vi00 | Technical Report | Rel-18 |
| TR 38.894 vi00 | Technical Report | Rel-18 |
| TR 38.900 vf00 | Channel Model Study for >6 GHz | Rel-15 |
| TR 38.901 vj10 | Channel Model for 0.5-100 GHz | Rel-19 |
| TR 38.903 vj00 | Test Tolerances & Measurement Uncertainties | Rel-19 |