Abstract
Digital coherent transceivers have revolutionized optical fiber
communications due to their superior performance offered compared to
intensity modulation and direct detection based alternatives. As systems
employing digital coherent transceivers seek to approach their information
theoretic capacity, the use of multilevel modulation formats combined with
appropriate forward error correction becomes essential. Given this
context, in this tutorial paper, we, therefore, explore the digital signal
processing (DSP) utilized in a coherent transceiver with a focus on
multilevel modulation formats. By way of an introduction, we open by
discussing the photonic technology required to realize a coherent
transceiver. After discussing this interface between the analog optical
channel and the digital domain, the rest of the paper is focused on DSP.
We begin by discussing algorithms that correct for imperfections in the
optical to digital conversion, including IQ imbalance and timing skew.
Next, we discuss channel equalization including means for their
realization for both quasi-static and dynamic channel impairments.
Synchronization algorithms that correct for the difference between the
transmitter and receiver oscillators both optical and electrical are then
discussed and issues associated with symbol decoding highlighted. For most
of the cases, we start with polarization division multiplexed quadrature
phase-shift keying format as a basis and then discuss the extension to
allow for high-order multilevel formats. Finally, we conclude by
discussing some of the open research challenges in the field.
© 2017 IEEE
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