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Lab 8.11 — 16-QAM implementation trade-offs

Lab 8.11 — 16-QAM bridge: BER, EVM and implementation limits

Goal

Bridge QPSK to higher spectral efficiency with normalized Gray 16-QAM:

  • map/demap four bits per symbol with thresholds at {-2, 0, +2}/√10;
  • measure BER and EVM versus Eb/N0;
  • expose gain/phase imbalance sensitivity;
  • quantify fixed-point quantization and saturation;
  • use 16-QAM as an alternative OFDM payload constellation.

Run

python blocks/block_08_modulation_and_synchronization/python/lab_8_11_16qam_tradeoffs.py

The default sweep compares 200,000 bits per Eb/N0 point. The OFDM bridge uses 52 active subcarriers: QPSK carries 104 bits per OFDM symbol and 16-QAM carries 208.

Artifacts

docs/assets/lab811_16qam_ber.png
docs/assets/lab811_16qam_imbalance.png
docs/assets/lab811_16qam_fixed_point.png
docs/assets/lab811_16qam_metrics.json

Interpretation

16-QAM doubles payload density relative to QPSK but reduces decision margin. EVM therefore exposes gain/phase imbalance and numeric saturation before BER necessarily becomes large. Increasing word length reduces quantization error, while extra bits cannot repair clipping caused by insufficient headroom.