Abstract
We propose a novel frequency-domain adaptive equalizer in digital coherent optical receivers, which can reduce computational complexity of the conventional time-domain adaptive equalizer based on finite-impulse-response (FIR) filters. The proposed equalizer can operate on the input sequence sampled by free-running analog-to-digital converters (ADCs) at the rate of two samples per symbol; therefore, the arbitrary initial sampling phase of ADCs can be adjusted so that the best symbol-spaced sequence is produced. The equalizer can also be configured in the butterfly structure, which enables demultiplexing of polarization tributaries apart from equalization of linear transmission impairments. The performance of the proposed equalization scheme is verified by 40-Gbits/s dual-polarization quadrature phase-shift keying (QPSK) transmission experiments.
Highlights
The next-generation optical network is going to employ digital coherent optical receivers, which enable high spectral efficiency by the use of multi-level optical modulation formats, dense wavelength-division multiplexing, and polarization multiplexing [1,2]
(2) We can demultiplex polarization tributaries and compensate for polarization-mode dispersion (PMD). (3) The initial sampling phase, which has been decided by free-running analog-to-digital converters (ADCs), is adjusted optimally during the filter-tap adaptation process, as far as clock frequencies are synchronized between the transmitter and the receiver
Removing the gradient constraint can reduce the complexity of the equalizer as far as the input sequence satisfies some specific conditions; by using such unconstraint frequency-domain equalizer (FDE) algorithm, tap-weight vectors do not converge to the Wiener solution as the number of block iterations approaches infinity [16]
Summary
The next-generation optical network is going to employ digital coherent optical receivers, which enable high spectral efficiency by the use of multi-level optical modulation formats, dense wavelength-division multiplexing, and polarization multiplexing [1,2] Those receivers allow compensation for linear transmission impairments such as group-velocity dispersion (GVD) and polarization-mode dispersion (PMD), by using adaptive equalization in the electrical domain [3,4]. (3) The initial sampling phase, which has been decided by free-running ADC, is adjusted optimally during the filter-tap adaptation process, as far as clock frequencies are synchronized between the transmitter and the receiver This is because the sampled waveform is continuously time-shifted so that sampling instance for the symbolspaced sequence comes to the best positions in symbol duration.
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