Abstract
The use of digital signal processors (DSP) to equalize coherent optical communication systems based on spatial division multiplexing (SDM) techniques is widespread in current optical receivers. However, most of DSP implementation approaches found in the literature assume a negligible mode-dependent loss (MDL). This paper is focused on the linear multiple-input multiple-output (MIMO) receiver designed to optimize the minimum mean square error (MMSE) for a coherent SDM optical communication system, without previous assumptions on receiver oversampling or analog front-end realizations. The influence of the roll-off factor of a generic pulse-amplitude modulation (PAM) transmitter on system performance is studied as well. As a main result of the proposed approach, the ability of a simple match filter (MF) based MIMO receiver to completely eliminate inter-symbol interference (ISI) and crosstalk for SDM systems under the assumption of negligible MDL is demonstrated. The performance of the linear MIMO fractionally-spaced equalizer (FSE) receiver for an SDM system with a MDL-impaired channel is then evaluated by numerical simulations using novel system performance indicators, in the form of signal to noise and distortion ratio (SNDR) loss, with respect to the case without MDL. System performance improvements by increasing the transmitter roll-off factor are also quantified.
Highlights
The increasing demand of higher bit rates, combined with the environmental requirement of energy-efficient communication systems, is driving the development of ultra-highcapacity fiber optic communications
Multiple contributions to adaptive multiple-input multiple-output (MIMO) equalizers using the flexibility of digital signal processors (DSP) have been developed [7,10,11], where normally the equalization is divided into two parts: The first one, with an invariant chromatic dispersion (CD) compensation for each of the polarizations; and a second one, with an adaptive 2 × 2 MIMO linear equalizer to resolve the crosstalk between the modes [12]
We show that the generalized linear MIMO minimum mean square error (MMSE) receiver, for channels with negligible mode-dependent loss (MDL), can be simplified to a matched filter MIMO receiver, which completely eliminates the inter-symbol interference (ISI) and crosstalk introduced by the channel
Summary
The increasing demand of higher bit rates, combined with the environmental requirement of energy-efficient communication systems, is driving the development of ultra-highcapacity fiber optic communications. A PAM transmitter with square-root raised cosine pulses; a complete long-haul optical channel with SDM; and a MIMO receiver based on the FSE approach with oversampling of two. To this end, an ensemble of thousands of random optical channels has been generated and the system performance is evaluated by means of the signal to noise and distortion ratio (SNDR) loss at the receiver output w.r.t., the one of an optimal equivalent system without.
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