Glossary entry

TDECQ

Transmitter dispersion eye closure quaternary: the standardized penalty, in dB, that a PAM4 optical transmitter's imperfections impose on an idealized reference receiver. Defined in IEEE 802.3 for 100G-and-faster lanes; the pass/fail transmitter metric for DR, FR, and LR module classes.

Fiber & telecom Lab practiceUpdated September 2026

A PAM4 transmitter can fail in many ways at once: limited bandwidth smears symbols into their neighbors, noise blurs the levels, jitter blurs the crossings, and a compressed modulator squeezes the upper eyes. TDECQ (transmitter dispersion eye closure quaternary, IEEE 802.3bs clause 121) collapses all of it into a single penalty: the extra optical power, in dB, that this transmitter would need over an ideal one to reach the same error ratio at a standardized receiver.

The measurement is a fixed recipe rather than a judgment call. The captured waveform is passed through a fourth-order Bessel-Thomson filter with a 3 dB bandwidth of half the symbol rate (53.125 GHz for 106.25 GBd), then through a 5-tap, symbol-spaced feed-forward equalizer whose taps are chosen to minimize the result. Amplitude histograms are taken in two narrow windows 0.05 UI either side of the chosen sampling instant, and the analysis asks: how much Gaussian noise of RMS $\sigma_G$ can be added, referred to the equalizer input, before the symbol error ratio reaches $4.8 \times 10^{-4}$ ? An ideal transmitter with the same outer OMA could tolerate $\sigma_\text{ideal} = \mathrm{OMA}_\text{outer} / (6 \, Q_t)$ with $Q_t = 3.414$ , so

\mathrm{TDECQ} = 10 \log_{10} \!\left( \frac{\sigma_\text{ideal}}{\sigma_G} \right).

The SER target is not arbitrary: the RS(544,514) forward error correction used by these links corrects up to a pre-FEC bit error ratio of $2.4 \times 10^{-4}$ , and with Gray-coded PAM4 a symbol error is one bit error in two bits, so the equivalent symbol error ratio is $4.8 \times 10^{-4}$ . TDECQ is therefore a statement about where the transmitter leaves the link relative to the FEC cliff, made with the receiver held fixed.

Two properties follow from the definition and are worth internalizing. First, TDECQ is referenced to OMA, not to average power, so lowering the extinction ratio at fixed OMA does not change it at all; what low ER costs is average laser power, which is a separate line in the power budget. Second, because the reference equalizer sits inside the measurement, TDECQ prices equalization honestly: white noise at the input is amplified by the tap norm, so a transmitter that forces the FFE to work hard pays a noise-enhancement penalty even after the eye is reopened. Conformance limits for the DR and FR classes sit near 3.4 dB, with the exact ceiling set per PMD in the relevant clause; the companion metric RLM bounds the level-spacing nonlinearity that a linear equalizer cannot repair.

References: IEEE Std 802.3bs-2017, clause 121.8.5 (TDECQ definition); IEEE 802.3cd and the 802.3dj project extend the method to 100G and 200G per lane. The PAM4 Eye & TDECQ Explorer computes TDECQ live from a synthesized waveform with adjustable impairments, and Measuring TDECQ on a PAM4 transmitter is the bench procedure.