A Comparison of 25 Gbps NRZ PAM-4 Modulation Used in Reference, Legacy, Premium Backplane Channels

摘要

Standards bodies are now examining how to increase the throughput of high-density backplane links to 25 Gbps.One method for achieving this is to construct premium backplane links utilizing advanced materials and connectors.Another approach is to re-use legacy backplanes by employing PAM-4 signaling at half of the baud rate. For PAM-4to offer an advantage over NRZ, the signal-to-noise ratio (SNR) at the slicer input, i.e. after equalization, must be～9.5 dB better than NRZ to overcome loss of separation between signal levels. This paper will examine 25 GbaudNRZ and 12.5 Gbaud PAM-4 signaling across varying levels of channel insertion loss and crosstalk. The paperprovides a reliable reference for engineers to use when considering when it is appropriate to use NRZ signaling at 25Gbaud and when it is appropriate to use PAM-4 signaling at 12.5 Gbaud for successful high-density backplaneoperation.