PAPR
Understanding PAPR
PAPR is one of the most important system-level challenges in modern wireless communications. OFDM, which dominates 4G/5G, Wi-Fi, and broadcasting, produces signals with high PAPR that stress the PA. The amplifier must handle the peaks without clipping, but operates at average power far below its maximum capability.
PAPR by Signal Type
| Signal | PAPR | Notes |
|---|---|---|
| CW | 0 dB | Constant envelope |
| FM/GMSK | 0 dB | Constant envelope |
| QPSK | 0 dB | Constant envelope |
| 16-QAM (single) | 2.6 dB | Moderate |
| OFDM (64 SC) | ~8 dB | High |
| OFDM (1200 SC) | ~12 dB | Very high |
PAPR (dB) = 10 log10(P_peak/P_avg)
OFDM PAPR (N subcarriers):
PAPR_max = N (all subcarriers aligned)
Practical PAPR (99% CCDF) ~ 8-12 dB
PA efficiency impact:
Without DPD: PA backed off by PAPR
PAE at 10 dB backoff: 5-10%
With DPD: 3-5 dB backoff, PAE 20-30%
Frequently Asked Questions
What is PAPR?
PAPR is the ratio of peak to average signal power. OFDM signals have high PAPR (8-12 dB) because multiple subcarriers can occasionally align, creating large peaks. This forces the PA to operate far below saturation, reducing efficiency.
Why is high PAPR bad?
The PA must be sized to handle the peak power without clipping. If PAPR is 10 dB, the PA must handle 10x more peak power than the average transmitted power. Most of the time, the PA operates inefficiently at the low average power level.
How do you reduce PAPR?
PAPR reduction techniques include clipping and filtering, tone reservation, active constellation extension, DFT-spread OFDM (used in LTE uplink), and peak cancellation. DPD also helps by linearizing the PA so that slight peak compression is tolerable.