Foundation: why OFDM, and what a waveform must do
Start from the problem. A radio channel smears symbols in time (multipath echoes) and the smearing gets worse with bandwidth. Orthogonal Frequency-Division Multiplexing (OFDM) solves this by slicing one wide, fragile channel into thousands of narrow subcarriers, each so slow that the echoes fall inside a short guard interval — the cyclic prefix (CP). Equalisation then becomes a single complex multiply per subcarrier. This is the bedrock 5G NR is built on, and 6G inherits it.
5G NR uses CP-OFDM in the downlink and adds DFT-spread-OFDM (DFT-s-OFDM) as an uplink option for lower peak-to-average power ratio (PAPR), improving power-amplifier efficiency and coverage. These are well-specified, well-understood techniques — the concrete part of the 6G physical layer you can learn in full today.
3gpp.org — Release 20 2026-06-15The 6G waveform decision (so far)
Every generation re-asks "do we need a new waveform?". For 6G, dozens of candidates were proposed — OTFS, AFDM, OTSM, FM-OFDM and more — each promising advantages for high mobility or sensing. After study, RAN1 landed on continuity.
3GPP RAN1 has agreed, as a study-phase working assumption, to use CP-OFDM and DFT-s-OFDM as the baseline/benchmark waveform for 6G — while explicitly leaving the door open to enhancements (e.g. spectral shaping) for specific use cases such as sensing or sub-THz. This narrows the field but is not a frozen specification.
the-mobile-network.com 2025-08 secondaryWaveforms like OTFS (delay-Doppler domain) and AFDM (affine frequency-division multiplexing) are attractive for very high mobility and joint sensing, and continue to be studied and benchmarked against the OFDM baseline. Presenting them is fair; presenting any of them as "the 6G waveform" would not be — none has been selected.
keysight.com 2025-07 secondaryWhy keep OFDM? Three practical reasons recur in the literature: it composes cleanly with massive MIMO; the ecosystem (chips, test, algorithms) is mature; and a familiar waveform makes a unified 5G/6G frame — and therefore smooth coexistence and migration — far easier.
| Waveform | Signal domain | PAPR | Multipath | High mobility | Sensing | 6G status |
|---|---|---|---|---|---|---|
| CP-OFDM | Frequency | High | Excellent | Moderate | Good | candidate Baseline DL+UL |
| DFT-s-OFDM | Frequency (spread) | Low | Good | Moderate | Limited | candidate Baseline UL option |
| OTFS | Delay-Doppler | Moderate | Good | Excellent | Good | candidate Under study |
| AFDM | Chirp (DAFT) | Moderate | Good | Excellent | Good | candidate Under study |
Numerology, concretely
5G NR's flexibility comes from one simple scaling rule. The basic subcarrier spacing is 15 kHz; each numerology μ doubles it. Doubling the spacing halves the OFDM symbol duration, which shortens latency but also shortens the cyclic prefix — so wider spacing suits short-range, high-frequency, low-delay-spread deployments. This is exactly the lever 6G reaches for when it moves into wider bandwidths at higher frequencies.
For numerology μ, subcarrier spacing Δf = 15·2μ kHz and the useful OFDM symbol duration is 1/Δf. A slot always holds 14 OFDM symbols, so higher μ packs more slots into a millisecond. This established 5G relationship is the template 6G extends.
3gpp.org 2026-06-15| μ | SCS (kHz) | Useful symbol (μs) | Normal CP (μs) | Slots / 1 ms | Slot duration (μs) | Status |
|---|---|---|---|---|---|---|
| 0 | 15 | 66.67 | 4.69 | 1 | 1000.00 | foundation |
| 1 | 30 | 33.33 | 2.34 | 2 | 500.00 | foundation |
| 2 | 60 | 16.67 | 1.17 | 4 | 250.00 | foundation |
| 3 | 120 | 8.33 | 0.57 | 8 | 125.00 | foundation |
| 4 | 240 | 4.17 | 0.29 | 16 | 62.50 | foundation |
| 5 | 480 | 2.08 | ~0.14 | 32 | 31.25 | candidate |
| 6 | 960 | 1.04 | ~0.07 | 64 | 15.63 | candidate |
A recurring 6G study goal is a frame structure that reuses 5G's slot/symbol concepts so the two generations can share spectrum and hardware and migrate gracefully. Wider-bandwidth numerologies, the exact slot timing, and how 6G coexists with 5G in the same band are open questions inside the 6GR study and TR 38.914.
3gpp.org 2026-03-01Channel coding, initial access — direction, not detail
Two more physical-layer pillars carry over conceptually. 5G NR uses LDPC codes for data and Polar codes for control; whether 6G keeps, tunes, or augments these is under study, so we teach the roles (a foundation) without asserting 6G specifics. Likewise initial access — synchronisation signals and broadcast — has a fixed job (let a cold UE find and read a cell) that 6G must still perform; its 6G design is being shaped now.
The jobs are generation-independent: forward error correction protects the bits; synchronisation + broadcast lets a UE acquire timing, frequency and the minimum system information. 6G will need both. The exact 6G codes and sync design are not specified — learn the mechanism in Foundations and watch the tracker for the 6G choices.
3gpp.org 2026-03-01Where this connects
The physical layer is shaped from above and below: the targets on the vision page say how fast and how low-latency it must be, and the bands on the spectrum page say where it operates — wider bandwidth at higher frequency is exactly why numerology matters. AI-native processing and sensing then ride on top of this waveform.