Systematic Performance Comparison of (Duobinary)-PAM-2,4 Signaling under Light and Strong Opto-Electronic Bandwidth Conditions

Abstract

We present a systematic comparison of PAM-2 (NRZ), Duobinary-PAM-2, PAM-4, and Duobinary-PAM-4 (duo-quaternary) signaling in the context of short-reach photonic communications systems using a Mach–Zehnder modulator as transmitter. The effect on system performance with a relaxed and constrained system’s opto-electronic bandwidth is analyzed for bit rates ranging from 20 to 116 Gb/s. In contrast to previous analyses, our approach employs the same experimental and simulation conditions for all modulation formats. Consequently, we were able to confidently determine the performance limits of each format for particular values of bit rate, system bandwidth, transmitter chirp, and fiber dispersion. We demonstrate that Duobinary-PAM-4 is a good signaling choice only for bandwidth-limited systems operating at relatively high speed. Otherwise, PAM-4 represents a more sensible choice. Moreover, our analysis put forward the existence of transition points: specific bit rate values where the BER versus bit rate curves for two different formats cross each other. They indicate the bit rate values where, for specific system conditions, switching from one modulation to another guarantees optimum performance. Their existence naturally led to the proposal of a format-selective transceiver, a component that, according to network conditions, operates with the most adequate modulation format. Since all analyzed modulations share similar implementation details, signaling switching is achieved by simply changing the sampling point and threshold count at the receiver, bringing flexibility to IM/DD-based optical networks.

High-Speed IM/DD Transmission Using Partial Response Signaling With Precoding and Memoryless Decoding

“…Given a constrained analog bandwidth, a PR signal with a reduced effective bandwidth (i.e., bandwidth measured at a typical reporting threshold of e.g., 3 dB or 10 dB) can be transmitted at a higher symbol rate than a conventional PAM [5]. Moreover, a PR signaling with appropriate digital lowpass filtering at transmitter can enable sampling rate to decrease below 1 sample/symbol (Sps), also referred as faster-than-Nyquist (FTN) technique [6][7]14].…”

“…By changing the encoding filter, which is an extension of the pulse shaping concept beyond the zero ISI criterion, various spectral shapes can be achieved to reduce undesired signal impairments caused by a channel, thus improve the signal quality [3,4]. Moreover, PR signaling can reduce the effectively required analog bandwidth when compared to a conventional pulse amplitude modulation (PAM) at the same symbol rate, hence increase the maximum symbol rate supported in a system with a given analog bandwidth [5][6][7]. In addition, it can better utilize transmitter output power, reducing digital pre-emphasis caused attenuation at frequencies close to DC [8].…”