ISI Sensitivity of PAM Signaling for Very High-Speed Short-Reach Copper Links

This paper presents a comparative study, analyzing the potentials of pulse amplitude modulation (PAM) for implementing next generation high-speed copper wireline links. This comparative study analyzes the sensitivities of PAM2, PAM4 and PAM8 to inter-symbol interference (ISI), using four different channels (with their loss gradually increased) running at 28 Gb/s, 56 Gb/s, 112 Gb/s and 224 Gb/s. Each case is examined with and without equalization to properly assess the inherent limitations of the modulation schemes in the presence of ISI. This study shows that although higher order PAM offer a better spectral efficiency, this does not translate into the best performance in terms of signal integrity. Depending on the severity of ISI and the data rate, a lower order PAM can outperform a higher order PAM, especially for the low-loss cases.