5G OFDM

Contents

1. Overview
2. How the OFDM waveform model works in NR
3. CP-OFDM and DFT-s-OFDM in NR
4. Where OFDM appears in real procedures
5. Troubleshooting
6. References
7. FAQ

Overview

In practical language, OFDM splits the channel into many narrow subcarriers and transmits symbols across them in parallel. Instead of trying to send everything through one wide carrier, the system spreads the transmission across a structured grid.

• frequency is divided into many subcarriers
• time is divided into OFDM symbols
• control, data, and reference signals are mapped onto that grid
• real transmissions are read as resource occupancy on the NR time-frequency plane

This is the reason NR resource allocation is usually discussed in terms of subcarriers, symbols, resource blocks, DMRS positions, and slot timing.

Quick interpretation

How the OFDM waveform model works in NR

frequency ->
| sc | sc | sc | sc | sc | sc |

time
  |
  v
| sym |  RE grid where data, control, and reference signals are placed
| sym |
| sym |

OFDM is usually not inspected as an abstract math topic. It is inspected as the resource grid on which PDCCH, PDSCH, PUSCH, DMRS, CSI-RS, and other signals are mapped.

Subcarriers

A subcarrier is one narrow frequency component inside the OFDM waveform. In NR, the spacing between adjacent subcarriers is determined by the active numerology.

OFDM symbols

OFDM symbols are the time units that sit on the grid. Control, data, and reference signals are assigned to selected symbols within a slot.

Cyclic prefix

The cyclic prefix is added to help the receiver handle multipath propagation more robustly. For most everyday NR work, the normal CP case is the main baseline.

Resource elements and resource blocks

The smallest mapping unit is the resource element. A resource block groups 12 subcarriers in frequency over the relevant symbol span in time. This is where resource scheduling, DMRS overhead, and throughput analysis become concrete.

CP-OFDM and DFT-s-OFDM in NR

A practical takeaway is this: if you are reading most downlink traces and many uplink traces, you are effectively dealing with CP-OFDM resource behavior. But it is still important to remember that NR uplink is not limited to only one waveform option.

How OFDM connects to numerology and frame structure

• Numerology defines the subcarrier spacing and symbol timing.
• Frame structure explains how those symbols and slots are organized in time.
• OFDM is where both ideas become the practical grid used by the scheduler and receiver.

This is why these three pages work best together: numerology defines the timing scale, frame structure defines the time hierarchy, and OFDM shows how actual radio resources are mapped.

Where OFDM appears in real procedures

Initial access and synchronization

SSB mapping -> PBCH decoding -> PRACH opportunity -> RRC setup path

OFDM is visible here through symbol-level placement of synchronization and broadcast structures, not just through high-level signaling names. Read it together with SSB, PBCH, PRACH, and the RRC setup flow.

Scheduling and data delivery

PDCCH grant -> PDSCH/PUSCH allocation -> DMRS-assisted decoding -> HARQ feedback

This is the most important daily-use OFDM context. Data throughput, RB allocation, DMRS overhead, and layer usage all show up on the OFDM resource grid. Read it together with PDCCH, PDSCH, PUSCH, DMRS, and HARQ.

Trace and performance analysis

Poor throughput or decode failures often have to be read in terms of symbol usage, DMRS placement, resource allocation, and overhead, which are all OFDM-grid questions.

Troubleshooting

OFDM itself is not usually the isolated cause of a problem, but waveform and grid assumptions often explain why overhead, resource use, or decode margin differ from what was expected.

• active numerology and slot structure behind the waveform timing
• PDSCH or PUSCH allocation size in RBs and symbols
• DMRS and other overhead that reduce usable data REs
• MCS, layer count, and scheduler behavior when judging resource efficiency
• whether uplink behavior is using CP-OFDM or DFT-s-OFDM assumptions
• control-channel limitations before blaming the raw data channel alone

Common mistakes

• thinking OFDM is only theory and not directly relevant to trace analysis
• estimating throughput from bandwidth alone without considering symbol and RE overhead
• ignoring the difference between waveform structure and scheduler policy
• assuming uplink and downlink waveform behavior are always interpreted identically

Troubleshooting clues

References

• 3GPP TS 38.211 - NR physical channels and modulation
• 3GPP TS 38.212 - NR multiplexing and channel coding
• 3GPP TS 38.213 - NR physical-layer procedures for control
• 3GPP TS 38.214 - NR physical-layer procedures for data
• 3GPP TS 38.300 - NR and NG-RAN overall description

FAQ

What is OFDM in 5G?

It is the waveform framework NR uses to map transmissions onto many orthogonal subcarriers over time. It is the basis of the NR resource grid.

Does 5G use CP-OFDM or DFT-s-OFDM?

NR uses CP-OFDM for downlink and for most uplink cases, while DFT-s-OFDM is also available for selected uplink transmissions.

How is OFDM related to numerology?

Numerology sets the subcarrier spacing and symbol timing, so it directly defines the timing scale of the OFDM waveform.

Why is OFDM important for throughput analysis?

Because throughput depends on how many usable resource elements remain after control, DMRS, CSI-RS, and other overhead are mapped onto the OFDM grid.

Why should protocol analysis care about OFDM?

Because many apparent higher-layer issues are shaped by lower-layer resource allocation, reference-signal overhead, decoding quality, and scheduler use of the OFDM grid.

Is OFDM only a PHY concept?

It is a PHY concept, but its effects are visible in MAC scheduling, RRC configuration interpretation, and practical troubleshooting of real network behavior.