What causes high PAPR in OFDM and what techniques reduce it?

OFDM's high Peak-to-Average Power Ratio (PAPR) occurs when subcarrier phases align, creating large peaks. High PAPR requires linear amplifiers with large back-off, reducing power efficiency. Reduction techniques: Clipping (simple but causes distortion and spectral regrowth), Coding (select codewords with low PAPR, reduces rate), Selected Mapping (SLM, transmit sequence with lowest PAPR from multiple phase rotations), Partial Transmit Sequences (PTS, optimize phase of subcarrier groups), Tone reservation (reserve subcarriers for peak reduction), and DFT-spreading (SC-FDMA in LTE uplink). Trade-offs involve complexity, rate loss, and distortion.