This modulation uses just two signal levels (1 or 0) to represent the digital information, with a negative voltage equating to 0 and a positive voltage representing a 1. PAM4 utilizes four discrete signal levels (0, 1, 2, 3) with each representing 2 bits of information. Since we have two bits of information sent in the same time as one bit using NRZ, PAM4 can use a reduced bandwidth channel.
Pulse Amplitude Modulation 4-level (PAM4), A fiber optic link must convert electrical energy into an optical signal, transmit the optical signal over the fiber link, then convert the optical signal back into an electrical signal. Modulation is the method chosen to encode and decode information during these signal conversions.
PAM4 Modulation utilizes four different signal levels during data transmission, with each symbol representing two bits of information. At each of the four levels, the data bits are logically presented in combinations of 00, 01, 10, or 11. Gray coding is often used to manage the swing between transitions to help reduce bandwidth issues.
Power Consumption for PAM4 modulation is relatively high, due to the equalization and pre-compensation used at both ends of the line to minimize BER. This high energy utilization can also lead to power & cooling concerns, especially for 800Gb pluggable optics.
