Direct detection of bipolar pulse amplitude modulation

Pulse amplitude modulation (PAM) is a widely employed digital modulation format. PAM formats are generally classified as bipolar PAM (BPAM) if using both positive and negative amplitude levels, and unipolar PAM (UPAM) if using only non-negative amplitude levels. While BPAM formats are in principle more energy efficient, they are not compatible with conventional direct detection (DD) schemes, so that UPAM formats are usually preferred for short-reach optical communications. In this work, we propose a novel DD scheme that employs oversampling at two samples per symbol to extract both the amplitude and phase information from the received optical signal, enabling the detection of BPAM signals. The proposed scheme uses a single photodetector, has no special requirements in terms of bandwidth, and requires only some minimal additional processing compared to a conventional scheme. A theoretical analysis, confirmed by numerical simulations, shows that, in the presence of optical amplifier noise, the proposed BPAM/DD transmission technique provides large optical signal-to-noise ratio gains compared to a more conventional UPAM/DD transmission and a good tolerance to group velocity dispersion and non-ideal filtering.