PAM-4 implementation study for future high-speed links

<!--HTML-->With the ever-increasing amount of data from HEP experiments, the transmission rates must keep up. To mitigate the exponential growth of the total loss due to the increased frequency, the 4-Level Pulse-amplitude Modulation (PAM-4) could be envisaged, allowing to reach 56 Gbps or even 112 Gbps in extremely high-end applications. A system using PAM-4 encoders and transceivers has been built based on FPGA as a proof-of-concept to demonstrate potential future links. In this talk, the PAM-4 modulation will be introduced, the performance of this initial system will be presented and the challenges for future links will be discussed.