Lab 8.10 — OFDM PAPR, clipping and spectral regrowth¶

Goal¶

Extend the OFDM mini-link with a transmitter-focused experiment that connects waveform statistics to real RF and fixed-point constraints:

estimate the OFDM peak-to-average power ratio (PAPR) distribution;
plot the complementary cumulative distribution function (CCDF);
apply magnitude clipping at several A/RMS ratios;
measure the resulting PAPR, EVM, BER and out-of-band energy;
explain why digital back-off and peak dBFS matter before the AD9363 transmit path.

Why this lab matters¶

A QPSK constellation can look robust while the time-domain OFDM waveform still contains large peaks. Those peaks can cause fixed-point overflow, DAC clipping or RF power-amplifier compression. Reducing PAPR by hard clipping lowers the peaks, but it also increases EVM and spectral regrowth and can eventually create bit errors.

Processing chain¶

flowchart LR
    BITS[random bits] --> MAP[QPSK mapper]
    MAP --> ALLOC[subcarrier allocation]
    ALLOC --> IFFT[oversampled IFFT]
    IFFT --> CLIP[magnitude clipping]
    CLIP --> AWGN[AWGN]
    AWGN --> FFT[FFT]
    FFT --> METRICS[PAPR / EVM / BER / OOB]

Run¶

python blocks/block_08_modulation_and_synchronization/python/lab_8_10_ofdm_papr_clipping.py

Generated artifacts¶

docs/assets/lab810_ofdm_papr_ccdf.png
docs/assets/lab810_ofdm_clipping_tradeoff.png
docs/assets/lab810_ofdm_spectral_regrowth.png
docs/assets/lab810_ofdm_papr_metrics.json

Default experiment¶

FFT=64;
52 occupied subcarriers;
4× oversampling for PAPR estimation;
4000 OFDM symbols for the CCDF;
300 OFDM symbols and 31,200 compared bits per clipping point;
SNR = 24 dB;
clipping ratios from 3.0 down to 0.7 times RMS amplitude.

With the deterministic default seed, the unclipped signal has roughly 7 dB median PAPR and about 10 dB at the 99th percentile. Severe clipping reduces average PAPR to about 1 dB, but raises EVM above 25%, increases out-of-band energy and produces nonzero BER.

Metrics and interpretation¶

PAPR describes peak stress on the digital and RF path.
EVM exposes in-band constellation distortion before BER necessarily changes.
BER proves whether the digital decisions still work.
Out-of-band/in-band power is a compact spectral-regrowth indicator; it is not a standards-compliant ACLR measurement.

This combination reinforces a key course rule: no single metric is sufficient. A waveform can have BER=0 while EVM and spectral quality are already degrading.

Report checklist¶

[ ] Include the PAPR CCDF.
[ ] Report median and 99th-percentile PAPR.
[ ] Compare unclipped and clipped EVM.
[ ] Report BER with the number of compared bits.
[ ] Include the spectral-regrowth plot.
[ ] Recommend a safe digital back-off strategy for a future AD9363 measurement.