CP-OFDM vs DFT-s-OFDM
5G NR has two uplink waveforms, and the difference between them comes down to peak power. CP-OFDM (Cyclic Prefix OFDM) is the default — NR uses it for the downlink always, and as the standard option on the uplink. It carries high spectral efficiency, maps cleanly onto multi-layer MIMO, and is easy to schedule across the band. The catch is a high peak-to-average power ratio (PAPR): the signal has sharp peaks, so the device's power amplifier has to back off and can't run flat out.
DFT-s-OFDM — also called SC-FDMA, or "transform-precoded" PUSCH — is an uplink-only option that adds a DFT spreading step before the OFDM modulator. That smooths the peaks, drops the PAPR, and lets the amplifier push more power. The network turns it on for power-limited UEs at the cell edge, trading away MIMO layers for reach. It runs single-stream only.| Aspect | CP-OFDM | DFT-s-OFDM |
|---|---|---|
| Used where | Downlink always; the default uplink waveform too. | Uplink only (PUSCH/PUCCH); an option, not the default. |
| PAPR | High — sharp signal peaks across the subcarriers. | Low — the DFT spread smooths peaks toward a single-carrier shape. |
| Power-amplifier efficiency | Lower — the PA backs off to stay linear over the peaks. | Higher — less back-off, so the PA delivers more usable output power. |
| Coverage / cell-edge | Shorter uplink reach when the UE is power-limited. | Better — extra transmit power extends the cell-edge uplink budget. |
| MIMO layers | Multi-layer (up to 4 uplink layers) spatial multiplexing. | Single layer — transform precoding rules out spatial multiplexing. |
| Spectral efficiency | Higher — flexible subcarrier mapping plus MIMO gains. | Lower — single stream, and allocation sizes are constrained. |
| How it's enabled | Default — transformPrecoder set to disabled (or absent). | RRC/DCI sets transformPrecoder = enabled on the PUSCH grant. |
| Inherited from | New multi-carrier design carried across the whole NR air interface. | The same single-carrier idea as the LTE SC-FDMA uplink. |
| Resource allocation | Flexible — type 0 (non-contiguous) or type 1 (contiguous) RBs. | Contiguous RBs only, and the count limited to products of 2/3/5. |
| Complexity | Straightforward IFFT/FFT chain at both ends. | Adds a DFT spread at the transmitter and de-spread at the receiver. |
| Typical use | High-throughput uplink with good signal; all downlink traffic. | Cell-edge or low-SINR uplink where coverage beats peak rate. |
Why PAPR matters for the uplink
A multi-carrier signal like CP-OFDM adds together hundreds of subcarriers. Now and then those subcarriers line up in phase and the combined waveform spikes far above its average level — that gap is the peak-to-average power ratio (PAPR).
The problem is the power amplifier. To reproduce those peaks without clipping and splattering energy into neighbouring channels, the PA has to operate in its linear region, which means backing the average power down well below the peak. On a handset that back-off wastes battery and caps how much power reaches the antenna — and the uplink is usually the link that limits cell range, because a phone can't match a base station's transmit power.
DFT-s-OFDM attacks the peaks directly. The DFT spreading step before the IFFT turns the signal back into something close to single-carrier, so the PAPR drops by a few dB. Less back-off means the PA runs closer to full output, and that extra power is exactly what a cell-edge device needs.When the network switches a UE to DFT-s-OFDM
The choice is the network's, not the device's. The gNB signals it per UE through the transformPrecoder parameter — set in RRC config, or selected by the DCI that grants the PUSCH. With it disabled the uplink is CP-OFDM; enabled, it becomes DFT-s-OFDM.
The trigger is almost always coverage. When a UE drifts to the cell edge, its SINR falls and it starts running short on transmit power. The scheduler reads that — from power headroom reports, measured uplink quality, and path loss — and flips the waveform to claw back a few dB of PA output. A UE sitting close to the cell with plenty of headroom stays on CP-OFDM, because there's no reason to give up MIMO layers and spectral efficiency it can actually use.
So the two waveforms aren't rivals so much as a coverage switch: CP-OFDM for throughput, DFT-s-OFDM when the link budget gets tight.
Why the downlink is always CP-OFDM
DFT-s-OFDM only ever shows up on the uplink. On the downlink, NR uses CP-OFDM with no option to switch — and the reason is the asymmetry between the two ends of the link.
A base station has a generous power budget, mains supply, and room for a high-quality linear PA, so the PAPR penalty that hurts a handset barely registers. What the downlink wants instead is throughput, and CP-OFDM delivers it: clean MIMO spatial multiplexing across many layers, flexible scheduling of users across the band, and easy mixing of different services in the same slot. Spending all that to shave PAPR the base station can afford anyway would make no sense.
The single-carrier shape of DFT-s-OFDM is the opposite trade — peak power in exchange for MIMO and flexibility. That bargain only pays off on a power-limited handset uplink, which is exactly where 5G keeps it.
The bottom line
Frequently asked questions
- Is DFT-s-OFDM the same as SC-FDMA?
- Effectively yes. DFT-s-OFDM (Discrete Fourier Transform spread OFDM) is the same single-carrier-like scheme that LTE used on its uplink under the name SC-FDMA. 5G NR carries it forward as an optional uplink waveform, where it is also called transform-precoded PUSCH. The names refer to the same DFT-spreading technique.
- Why does 5G use two uplink waveforms?
- Because no single waveform is best in every condition. CP-OFDM gives high spectral efficiency and multi-layer MIMO when the signal is strong, but its high PAPR forces the device PA to back off. DFT-s-OFDM lowers PAPR so a power-limited UE at the cell edge can transmit harder and stay connected. The network picks whichever fits the UE.
- Does DFT-s-OFDM support MIMO?
- It supports beamforming and transmit diversity, but not spatial multiplexing — transform precoding restricts it to a single layer (one stream). If you want multi-layer uplink MIMO in NR, the UE has to run CP-OFDM. That single-layer limit is one of the trade-offs you accept for the lower PAPR and better coverage.
- What is PAPR and why does it matter?
- PAPR is the peak-to-average power ratio — how far the signal's instantaneous peaks rise above its average level. High PAPR forces a power amplifier to back off to stay linear, which wastes battery and limits transmit power. On a handset uplink that directly shrinks coverage, which is why the low-PAPR DFT-s-OFDM waveform exists.
- Which waveform gives better uplink coverage?
- DFT-s-OFDM. Its lower PAPR lets the device amplifier run with less back-off, so more usable power reaches the antenna and the cell-edge uplink budget improves by a few dB. CP-OFDM has the edge on throughput and MIMO when signal is good, but it does not reach as far when the UE is power-limited.
Related terms
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