They refer to the equalization settings applied to the received signal (RX) and transmitted signal (TX) in optical transceivers. In the optical domain, an equalizer is a device that equalizes the gain response over a particular wavelength range. 3) can be approximated to within 10 percent by ${M}^{x}$, where $x$ is 0. TIA design presents Gain-BW and Noise-BW trade-offs.