Achievable Rates for Short-Reach Fiber-Optic Channels with Direct Detection

TL;DR

This paper investigates the achievable data rates in short-reach fiber-optic channels with direct detection, considering chromatic dispersion and oversampling, and compares different modulation schemes and pulse shapes for spectral efficiency.

Contribution

It introduces a method to compute achievable rates accounting for channel memory and demonstrates the advantages of real-alphabet modulations and frequency-domain pulses over traditional approaches.

Findings

Real-alphabet modulations offer significant energy gains.

FD-RC pulses outperform TD-RC in spectral efficiency.

FD-RC achieves higher rates when spectrum sharing or filtering is considered.

Abstract

Spectrally efficient communication is studied for short-reach fiber-optic links with chromatic dispersion (CD) and receivers that employ direction-detection and oversampling. Achievable rates and symbol error probabilities are computed by using auxiliary channels that account for memory in the sampled symbol strings. Real-alphabet bipolar and complex-alphabet symmetric modulations are shown to achieve significant energy gains over classic intensity modulation. Moreover, frequency-domain raised-cosine (FD-RC) pulses outperform time-domain RC (TD-RC) pulses in terms of spectral efficiency for two scenarios. First, if one shares the spectrum with other users then inter-channel interference significantly reduces the TD-RC rates. Second, if there is a transmit filter to avoid interference then the detection complexity of FD-RC and TD-RC pulses is similar but FD-RC achieves higher rates.