Resource Element

Description

A Resource Element (RE) is the fundamental time-frequency grid unit in Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) as used in 3GPP LTE and NR. It represents the smallest individually addressable physical resource for data transmission. An RE is defined by a single subcarrier in the frequency domain and a single OFDM symbol (including cyclic prefix) in the time domain. The entire available transmission bandwidth and transmission time interval are divided into a two-dimensional grid of these REs. The collection of REs across the system bandwidth and a specified time duration (e.g., one slot) forms a Resource Grid.

How it works is central to physical layer processing. Each RE can carry a complex-valued modulation symbol. This symbol could represent user data from a transport channel (after channel coding, scrambling, and modulation), control information (like Downlink Control Information, DCI), or a reference signal (pilot). The mapping of these different types of information onto specific REs within the grid is strictly defined by the standard. For example, in LTE, the Cell-Specific Reference Signals (CRS) are mapped to predetermined RE positions, which are punctured into the grid—meaning data is not transmitted on those REs. Similarly, control channels like the Physical Downlink Control Channel (PDCCH) occupy specific OFDM symbols at the start of a subframe.

Its role is foundational for resource allocation, scheduling, and signal generation. The network scheduler operates by allocating groups of REs, specifically Resource Blocks (RBs), to different users. An RB is a larger unit, typically 12 consecutive subcarriers in frequency and one slot (e.g., 7 or 14 OFDM symbols) in time, thus containing 12 * (symbols per slot) REs. By manipulating the content of REs, the system creates all physical channels and signals. The precision of RE definition enables advanced features like frequency-selective scheduling, beamforming (where the same RE carries different data for different spatial layers), and accurate channel estimation using reference signals placed at known RE locations.

Purpose & Motivation

The Resource Element exists to provide a granular, standardized, and flexible unit for mapping digital information onto the OFDM waveform. It solves the fundamental problem of how to partition the continuous time-frequency transmission resource into discrete, manageable units that can be independently modulated and assigned. Before OFDM/OFDMA, systems like GSM used a Time Division Multiple Access (TDMA) structure based solely on time slots, which was less flexible for handling frequency-selective fading.

The motivation for defining the RE stems from the advantages of OFDM technology, which is robust against multipath interference and enables efficient multi-user access. The RE is the atomic element that makes fine-grained resource allocation possible. It allows the scheduler to assign resources with high spectral efficiency, giving some users specific subcarriers that are experiencing good channel conditions while avoiding those in a deep fade. This frequency-domain scheduling is a key performance differentiator of LTE and NR compared to earlier CDMA-based 3G systems.

Historically, its introduction with LTE in Release 8 (and conceptually from earlier OFDM studies) established a clean, scalable physical layer model that could evolve from LTE through 5G NR. The RE provides a consistent framework for introducing new reference signals (e.g., CSI-RS, DMRS), new control channel structures (e.g., NR's more flexible PDCCH), and supporting variable numerology (different subcarrier spacings) in NR—where the absolute time duration of an OFDM symbol and RE changes with the chosen numerology, but the conceptual definition remains constant.