Understanding PAM4 and how it is enabling higher data rates

With the ever-increasing demands of data usage driven by cloud services, online gaming, video streaming and the availability of advanced data centers there is a growing need for faster optical solutions. To make this possible we need to consider the way that we transmit data, and where the technical challenges are to be able to meet this demand.

NRZ practically limits data transmission to 28 Gbps

The traditional modulation used for optical communication is Non-Return-to-Zero (NRZ) which uses two levels representing binary 0 and binary 1. If we use 100G LR4 as an example it operates at a symbol rate of 28 GBaud (number of state changes per second) which with NRZ’s 1 bit per symbol gives us a data rate of 28 Gbps. Using the LAN-WDM band at 1295 nm – 1310 nm we can combine 4 lambdas at 28 Gbps and the result is a combined data rate of 100 Gbps.

PAM4 – Making 200G and 400G possible

To be able to cost effectively achieve bit rates at 56 Gbps or higher we need to increase the amount of bits per symbol that are transmitted. This is where 4th level Pulse Amplitude Modulation (PAM4) enters the picture. By modulating the amplitude of the signal to a total of four states instead of NRZ’s two, we are now able to represent the binary values of 00, 01, 10 and 11. The downside of this is that it degrades the optical signal to noise (OSNR) ratio and requires a more sophisticated digital signal processor (DSP) in the receiver which increases cost. At the same time newer and more advanced lasers that are entering the market also allow us to double the symbol rate to 50 GBaud. Using these two technologies together can allow for 100 Gbps single lambda transmission and together with CWDM4 or LWDM combined data rates up to 400G.

The diagram below, also known as an “Eye Diagram”, is from an oscilloscope showing the signal quality which is used to test the laser in the transceiver. It shows the degradation of the OSNR with PAM4 compared to NRZ, highlighting the need for a more advanced processor and lasers.

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