Physical Resource Block

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.