Robust Multilevel Coherent Optical Systems With Linear Processing at the Receiver

Abstract

Abstract-This paper investigates optical coherent systems based on polarization multiplexing and high-order modulations such as phase-shift keying (PSK) signals and quadrature amplitude modulations (QAM). It is shown that a simple linear receiver processing is sufficient to perfectly demultiplex the two transmitted streams and to perfectly compensate for group velocity dispersion (GVD) and polarization mode dispersion (PMD). In addition, in the presence of a strong phase noise of the lasers at the transmitter and receiver, a symbol-by-symbol detector with decision feedback is able to considerably improve the receiver robustness with a limited complexity increase. We will also discuss the channel estimation and the receiver adaptivity to time-varying channel conditions as well as the problem of the frequency acquisition and tracking. Finally, a new two-dimensional (polarization/time) differential encoding rule is proposed to overcome a polarization-ambiguity problem. In the numerical results, the receiver performance will be assessed versus the receiver complexity.Index Terms-Electrical equalization, group velocity dispersion (GVD), intersymbol interference (ISI), optical coherent transmission systems, phase-shift keying (PSK), polarization mode dispersion (PMD), quadrature amplitude modulation (QAM).

OFDM versus Single-Carrier Transmission for 100 Gbps Optical Communication

“…The SC system considered in this paper is described in detail in [18]. As a discrete-time sufficient statistic is obtained through oversampling, the main component is represented by a two-dimensional fractionally-spaced linear equalizer which, as shown in [18] (where the most recent works on linear equalization for coherent optical systems are reported and analyzed), is able to perfectly compensate for GVD and PMD provided it has a sufficient number of taps.…”

“…As a discrete-time sufficient statistic is obtained through oversampling, the main component is represented by a two-dimensional fractionally-spaced linear equalizer which, as shown in [18] (where the most recent works on linear equalization for coherent optical systems are reported and analyzed), is able to perfectly compensate for GVD and PMD provided it has a sufficient number of taps. The adoption of the asynchronous strategies for detection and for the adjustment of the equalizer taps described in [18] allows also to remarkably increase the robustness to phase noise. In the following, we will denote by the number of equalizer taps and by the number of past decisions used by the asynchronous detection strategy [18], [20].…”

“…The adoption of the asynchronous strategies for detection and for the adjustment of the equalizer taps described in [18] allows also to remarkably increase the robustness to phase noise. In the following, we will denote by the number of equalizer taps and by the number of past decisions used by the asynchronous detection strategy [18], [20]. Assuming, as in [18], that two samples per symbol interval are employed, GVD and PMD will be perfectly compensated when is larger than or equal to the duration of the channel impulse response.…”

“…Last, but not least, OFDM exhibits several energy and spectral overheads, such as the overhead due to the cyclic prefix (CP), and some technological drawbacks, such as the requirement of analog-to-digital and digital-to-analog converters with an higher resolution than that required in SC systems. On the other hand, SC transmissions require a fractionally-spaced two-dimensional linear equalizer to compensate for linear impairments (GVD and PMD) [18]. For a single carrier system, 0733-8724/$26.00 © 2010 IEEE this equalizer represents the most computational demanding and technological challenging block.…”

“…Specifically, we will compare, in terms of performance, complexity, and overall state-of-the-art feasibility, an OFDM communication system with the single-carrier communication scheme described in [18], for which a parallel and computationally efficient implementation is also discussed here for better comparison with OFDM. In both cases we will assume the adoption of polarization multiplexing and a quaternary phase-shift keying (QPSK) modulation format, as this is going to be the format used in long-haul 100 Gbps links.…”