Abstract
We propose 9-ary quadrature amplitude modulation (9-QAM) data recovery for polarization multiplexing-quadrature phase shift keying (PM-QPSK) signal in presence of strong filtering to approach Nyquist bandwidth. The decision-directed least radius distance (DD-LRD) algorithm for blind equalization is used for 9-QAM recovery and intersymbol interference (ISI) compression. It shows the robustness under strong filtering to recover 9-QAM signal rather than QPSK. We demonstrate 112 Gb/s spectrum shaped PM-QPSK signal by wavelength selective switch (WSS) in a 25-GHz channel spacing Nyquist wavelength division multiplexing (NWDM). The final equalized signal is detected by maximum likelihood sequence decision (MLSD) for data bit-error-ratio (BER) measurement. Optical signal-to-noise ratio (OSNR) tolerance is improved by 0.5 dB at a BER of 1x10(-3) compared to constant modulus algorithm (CMA) plus post-filter algorithm.
Highlights
Doubling spectral efficiency (SE) is an interesting and attractive topic
We propose 9-ary quadrature amplitude modulation (9-QAM) data recovery for polarization multiplexing-quadrature phase shift keying (PM-QPSK) signal in presence of strong filtering to approach Nyquist bandwidth
It shows the robustness under strong filtering to recover 9-QAM signal rather than QPSK
Summary
Doubling spectral efficiency (SE) is an interesting and attractive topic. Nyquist pulse is generated to achieve the Nyquist limit of SE for a given baud rate [1, 2]. The data sampled at the time of T, which is in the center of the symbol, will be recovered. It suffers from severe intersymbol interference (ISI) under strong filtering. We propose to recover the data sampled at the time of T/2, which is between adjacent symbols. The constellation is 9-ary quadrature amplitude modulation (9-QAM) like for spectrum shaped QPSK signal. The radius of the ideal center constellation point is zero for the 9-QAM signal. The decision-directed least radius distance (DD-LRD) algorithm has been proposed for 16-QAM [11], which is phase independent. We demonstrate 112 Gb/s spectrum shaped polarization multiplexing-quadrature phase shift keying (PMQPSK) signal with a 25-GHz bandwidth WSS in the Nyquist wavelength division multiplexing (NWDM) channel. The final equalized signal is detected by maximum likelihood sequence detection (MLSD) [7] for data bit-error-ratio (BER) measurement
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