Fig. 1. Block diagram of PS structure based on the proposed ELA method.

Fig. 2. Three-symbol ELA implementation process.

Fig. 3. Four probability distributed PAM-4 signals based on three-symbol ELA. (a) P1, (b) P2, (c) P3, and (d) P4.

Fig. 4. NGMI curves of uniform and PS PAM-4 signals over AWGN.

Fig. 5. Experimental setup of 25 Gbaud PAM-4 system. (a) Measured S21 parameter of the AWG; (b) measured end-to-end channel response with/without S21 compensation.

Fig. 6. Structure diagram of FFE.

Fig. 7. Measured BER curves for 25 Gbaud uniform and PS-PAM-4 signals. The inset shows the CCDF curves for different distributions.

Fig. 8. Eye diagrams of PS-PAM-4 signals with various probability distributions (−20 dBm): (a) P1, (b) P2, (c) P3, and (d) P4.