TDECQ, the acronym for “transmitter dispersion and eye closure quaternary,” is the primary metric used to gauge the quality of an optical PAM4 transmitter. It’s a power penalty metric that determines how much extra power is required at the receiver, from the transmitter, relative to an ideal transmitter, to achieve the required symbol error ratio (SER).
That extra power is necessary to compensate for both non-ideal transmitter waveforms and the impact of chromatic and/or modal dispersion. It’s like the transmitter dispersion penalty (TDP) metric used in non-return-to-zero (NRZ) optical communications standards. It was developed in the IEEE 802.3bs standard project and improved and modified in the IEEE 802.3cd, cu and db standards. It’s currently being refined again in the IEEE 802.3dj project.