PAM4

PAM4, or four-level pulse-amplitude modulation, is a modulation format that encodes two bits per symbol by selecting one of four distinct amplitude levels for each transmitted signal. In channels with fixed bandwidth, PAM4 offers about twice the information per symbol compared with binary NRZ, so achieving a given bit rate requires roughly half the symbol rate.

The primary advantage of PAM4 is higher spectral efficiency, allowing higher data rates without increasing bandwidth. The tradeoffs include increased susceptibility to noise, amplitude jitter, and crosstalk, which shrink the distance between levels and raise error rates if not mitigated. These challenges are addressed with forward error correction, robust channel engineering, and advanced digital signal processing.

PAM4 typically demands more complex transmitter and receiver hardware, including higher-resolution digital-to-analogue and analogue-to-digital converters and improved linearity. It also relies on sophisticated equalization and timing recovery to manage inter-symbol interference in both optical and electrical channels. Modern systems employ error-correcting codes and DSP-based approaches, such as equalizers and decision-feedback techniques, to maintain link reliability.

Applications and deployment: PAM4 is used in high-speed data communication systems, particularly in short-reach optical transceivers for data centers and high-density backplanes, as well as in several Ethernet and optical networking standards targeting multi-hundred gigabit-per-second links. It serves as one of the enabling modulation formats for advancing throughput while trading off increased system complexity and tighter noise margins.