PAPR — Peak-to-average power ratio
RF and system
Where it sits
What it is
What it is. The ratio between the highest instantaneous power of a waveform and its average power. Not a 3GPP-defined quantity — it is a property of the signal that the requirements in TS 38.101-1 respond to.
Why OFDM has a bad one. An OFDM symbol is the sum of hundreds of independently modulated subcarriers (frame structure §1.4). Occasionally they all peak together, and the sum is far above the average.
Why that costs money. A power amplifier must stay linear up to the peak, but it delivers the average. The gap is back-off: the amplifier is run well below its maximum, wasting both efficiency and battery. Exceed the linear region and the constellation compresses — which shows up directly as EVM.
Which is why the uplink has a second waveform. DFT-spread OFDM (TS 38.211 clause 6.3.1.4) spreads each symbol across the subcarriers before the IFFT, giving a signal closer to single-carrier and a markedly lower PAPR. It costs the frequency-selective scheduling that CP-OFDM allows — a trade frame structure §9.2 sets out.
The asymmetry is the point. PDSCH is CP-OFDM only. A base station can afford back-off; a phone running on a battery cannot, and the amplifier is the largest consumer in it.
Read on
This concept was first written up in ref-system, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: