DFT-s-OFDM
Discrete Fourier Transform-spread OFDM: an OFDM variant with lower peak-to-average power ratio, used as an optional NR uplink waveform for coverage-limited scenarios.
DFT-s-OFDM is the answer to OFDM's one real weakness: its high peak-to-average power ratio. You take the data symbols, run them through a DFT first to "spread" each one across the subcarriers, and only then feed them into the normal OFDM machinery. The result behaves more like a single-carrier signal — the peaks are tamer — so the power amplifier can run closer to saturation without distorting. This is the same idea LTE called SC-FDMA on its uplink.
Why it lives on the uplink: it's the handset's battery and the handset's small, cheap power amplifier that suffer from high PAPR, while the base station can afford a bigger, more linear amplifier on the downlink. Lower PAPR means the phone can transmit at higher effective power before its amplifier clips, which directly buys you uplink coverage at the cell edge. In NR it's an option rather than the default — the network can switch a UE from CP-OFDM to DFT-s-OFDM specifically in coverage-limited situations, trading some scheduling flexibility (it's single-layer and the resource allocation is less flexible) for that extra reach.
Frequently asked questions
- Why does the uplink use DFT-s-OFDM instead of plain OFDM?
- Because of power efficiency at the device. Ordinary OFDM has a high peak-to-average power ratio, which forces a phone’s power amplifier to back off and waste headroom to avoid distortion. DFT-s-OFDM spreads the signal so the peaks are lower, letting the handset transmit at higher effective power for the same amplifier. That extra power translates directly into better uplink coverage, which is exactly what you need at the cell edge where the uplink is usually the limiting link.
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