Description
Orthogonal Frequency Division Multiplexing (OFDM) is a sophisticated modulation and multiplexing technique that forms the backbone of modern broadband wireless systems like LTE and NR. At its core, OFDM transforms a frequency-selective wideband channel into a collection of many narrowband, flat-fading subcarriers. A high-rate serial data stream is split into numerous lower-rate parallel streams, each modulating a separate subcarrier. The critical innovation is the orthogonality of these subcarriers: they are spaced precisely at the reciprocal of the symbol duration, ensuring that at the peak of one subcarrier's waveform, all other subcarriers have zero crossings. This orthogonality allows the subcarriers to overlap in the frequency domain without causing Inter-Carrier Interference (ICI), leading to very high spectral efficiency.
The practical implementation of OFDM relies on the Inverse Fast Fourier Transform (IFFT) at the transmitter and the Fast Fourier Transform (FFT) at the receiver. The transmitter maps the parallel data symbols onto the subcarriers and performs an IFFT to generate the time-domain OFDM symbol. A Cyclic Prefix (CP) is then prepended to each symbol. The CP is a copy of the last portion of the OFDM symbol appended to its beginning. This guard interval mitigates Inter-Symbol Interference (ISI) caused by multipath propagation, as long as the delay spread of the channel is shorter than the CP duration. At the receiver, after removing the CP, the FFT operation converts the signal back to the frequency domain, where simple one-tap equalization per subcarrier can compensate for channel effects, simplifying receiver design significantly compared to single-carrier systems in wideband channels.
In 3GPP systems, OFDM parameters such as subcarrier spacing and symbol duration are carefully chosen. For LTE, a fixed 15 kHz subcarrier spacing was adopted. 5G NR introduced flexible numerology, supporting multiple subcarrier spacings (e.g., 15, 30, 60, 120 kHz) scaled by powers of two, allowing optimization for different frequency bands and use cases. OFDM's resilience to multipath, its efficient use of spectrum, and its compatibility with advanced antenna techniques like MIMO make it an indispensable technology for achieving the high data rates and reliable connectivity required by modern mobile broadband.
Purpose & Motivation
OFDM was adopted by 3GPP starting with LTE (Release 8) to overcome the limitations of the Wideband Code Division Multiple Access (WCDMA) used in 3G UMTS. WCDMA, a single-carrier spread spectrum technology, struggled with high Peak-to-Average Power Ratio (PAPR) and required complex equalizers to handle the severe inter-symbol interference in wideband, multipath channels. These factors limited achievable data rates and spectral efficiency as demand for mobile data grew exponentially.
The primary motivation for OFDM was its inherent robustness to frequency-selective fading caused by multipath propagation. By dividing the channel into narrow subcarriers, a deep fade affects only a small subset, and error correction coding can easily recover the data. This eliminates the need for complex time-domain equalizers. Furthermore, its orthogonality and efficient FFT-based implementation make it highly scalable for wide bandwidths. OFDM also provides a natural fit for frequency-domain scheduling, allowing the network to allocate the best subcarriers to different users dynamically, and for Multiple Input Multiple Output (MIMO) spatial multiplexing, which is crucial for boosting capacity. Its adoption enabled the leap from Mbps to Gbps data rates, forming the foundation for 4G and 5G performance targets.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (37 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 introduced for OFDM-related functions, including a correction for the bitwidth calculation of frequency domain resource assignment in DCI fields. This release also addressed UCI multiplexing and removed the 62MHz frequency separation restriction for LTE LAA downlink operations, refining carrier and channel edge definitions for unlicensed spectrum.
- Correction on inter-frequency neighbour cell measurements TS 36.300CR1252
- CR to TS 37.145-2: mirror of operating band and frequency range declaration from NR, Rel-15 TS 37.145CR0133
- Correction of wrong implementation on frequency domain resource assignment bitwidth TS 38.212CR0006
- Correction to UCI multiplexing TS 38.212CR0008
- On bitwidth calculation for DCI fields using RRC parameter indicating maximum number of MIMO layers per serving cell TS 38.212CR0011
- Removal of 62MHz frequency seperation restriction for LTE LAA DL operations TS 36.300CR1226
In Release 16, specific enhancements and corrections were made to NR MIMO, along with fixes for procedures like dynamic frequency domain resource allocation and CG-UCI multiplexing. Corrections were also applied to the definition of CSI-RS based intra-frequency and inter-frequency measurements. Furthermore, technical specifications for NB-IoT were updated regarding the frequency offset between anchor and non-anchor carriers for TDD standalone operation.
- Introduction of Enhancements on NR MIMO TS 38.212CR0027
- Corrections for NR MIMO after RAN1#100-e TS 38.212CR0033
- Corrections in TS 38.212 for NR MIMO TS 38.212CR0042
- Corrections to MIMO enhancements TS 38.212CR0053
- Corrections to MIMO enhancements TS 38.212CR0054
- RRC IE name fix to dynamic frequency domain resource allocation type selection (Rel-15 origin) TS 38.212CR0056
+ 5 more changes
In Release 17, specific enhancements and corrections were made to the OFDM-based MIMO procedures for NR, including refinements to intra-UE multiplexing and semi-static channel occupancy. These updates built upon the framework for multi-antenna transmission, ensuring proper operation within the defined carrier frequencies and channel bandwidths. The corrections primarily addressed the technical specifications to ensure accurate signaling and power measurements, such as those related to Adjacent Channel Leakage power Ratio (ACLR), within the established physical channel structure.
- Introduction of Further enhancements on MIMO for NR TS 38.212CR0089
- Corrections on Further enhancements on MIMO for NR in TS 38.212 TS 38.212CR0106
- Corrections on Further enhancements on MIMO for NR in TS 38.212 TS 38.212CR0113
- Corrections on intra-UE multiplexing and semi-static channel occupancy TS 38.212CR0136
- CR to 37.145-2 to modify AAS BS OTA Spurious emissions limits for co-existence with systems operating in other frequency bands in R17 TS 37.145CR0302
In Release 18, the primary evolution for OFDM-based transmission was the introduction of MIMO evolution for both downlink and uplink, which included specific enhancements like defining the PTRS-DMRS association for 8-transmitter uplink MIMO. This release also contained necessary corrections and refinements to these new MIMO procedures within the core specification documents to ensure proper implementation. The work focused on advancing multi-antenna techniques within the existing OFDM framework to improve spectral efficiency and link robustness.
- Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
- Introduction of MIMO evolution for Downlink and Uplink TS 38.300CR0742
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0167
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0185
- Corrections on Rel-18 MIMO Evolution for Downlink and Uplink in 38.212 TS 38.212CR0200
- CR on PTRS-DMRS Association for 8 Tx UL MIMO TS 38.212CR0204
+ 2 more changes
In Release 19, the primary advancements for OFDM-based systems are detailed under the "Rel-19 NR MIMO Phase 5" work item, which introduces new multi-antenna techniques. This release also includes a specific clarification regarding the restriction of using only a single Sub-Carrier Spacing (SCS) per frequency layer. The changes are focused on enhancing MIMO performance and providing operational clarifications within the existing OFDM framework.
- Introduction of Rel-19 NR MIMO Phase 5 TS 38.212CR0220
- Introduction of Rel-19 MIMO Phase 5 TS 38.300CR1021
- Corrections on Rel-19 NR MIMO Phase 5 TS 38.212CR0231
- Corrections on Rel-19 NR MIMO Phase 5 TS 38.212CR0237
- Correction of Rel-19 MIMO Phase 5 TS 38.300CR1084
- Correction of Rel-19 MIMO Phase 5 TS 38.300CR1104
+ 1 more changes
Explore further
Broader topics and technologies where OFDM plays a role.
Defining Specifications
3GPP specifications that define or reference OFDM, 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 |
| TS 25.123 vj00 | Radio Resource Management for TDD | Rel-19 |
| TS 25.133 vj00 | UTRAN RRM Requirements for FDD | Rel-19 |
| TR 25.912 vj00 | Evolved UTRA and UTRAN Technical Report | Rel-19 |
| TS 36.104 vj10 | Base Station (BS) radio transmission and reception | Rel-19 |
| TS 36.116 vj00 | E-UTRA Relay RF Requirements | Rel-19 |
| TS 36.117 vj00 | E-UTRA Relay RF Test Methods & Requirements | Rel-19 |
| TS 36.133 vj20 | E-UTRA RRM Requirements | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.201 vj00 | LTE Physical Layer General Description | 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 |
| TR 36.791 vg00 | E-UTRA 2.4 GHz TDD Band for US | Rel-16 |
| TS 36.825 vd00 | Study on Additional LTE TDD Configurations | Rel-13 |
| TS 36.884 vd10 | MMSE-IRC Receiver Performance for LTE BS | Rel-13 |
| TR 36.902 v931 | SON Use Cases and Solutions for LTE | Rel-9 |
| TS 37.141 vj10 | RF Test Methods for Multi-Standard Radio Base Stations | Rel-19 |
| TS 37.145 vj10 | AAS Base Station Conducted Conformance Testing | Rel-19 |
| TS 37.802 va10 | MSR BS RF Requirements for Non-Contiguous Spectrum | Rel-10 |
| TR 37.829 vi00 | Technical Report | Rel-18 |
| TR 37.900 vj00 | Multi-Standard Radio (MSR) Base Station Requirements | Rel-19 |
| TR 37.901 vf10 | UE Application Layer Data Throughput Performance | Rel-15 |
| TR 37.911 vj00 | 3GPP 5G NTN Self-Evaluation Report | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.201 vj00 | NR Physical Layer General Description | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | 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 38.774 vj00 | Rel-19 LP-WUS/WUR RF Requirements TR | Rel-19 |
| TR 38.812 vg00 | Study on NOMA for NR | Rel-16 |
| TR 38.858 vi20 | Technical Report on Evolution of NR Duplex Operation | Rel-18 |
| TR 38.869 vi00 | Study on low-power wake up signal and receiver for NR | Rel-18 |
| TR 38.878 vi40 | Technical Report on Advanced Receiver for MU-MIMO | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |
| 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 |