Carrier phase estimation scheme for faster-than-Nyquist optical coherent communication systems

Abstract

We propose a modified-Viterbi and Viterbi phase estimation (VVPE) carrier phase recovery scheme that shows an effective capability of reducing the frequent and accumulated cycle slips induced by inter-symbol interference (ISI) in a faster-than-Nyquist (FTN) optical coherent communications system. In a 28-Gbaud FTN polarizationdivision multiplexed quadrature phase-shift keying optical communication system, the comparison of the proposed modified-VVPE scheme and the conventional VVPE scheme is carried out. It is proved that the proposed modified-VVPE scheme can effectively overcome the challenge of ISI induced error carrier phase estimation, which leads to a better bit error ratio performance.OCIS The rapid growth in internet applications leads to the ever-increasing demand for higher spectral efficiency in optical transmission systems. Spectral efficiency can be increased by advanced modulation formats like quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (16QAM), or reducing of channel spacing. Advanced modulation formats are widely used because they can increase spectral efficiency by several times, but they need a higher optical signal-to-noise ratio (OSNR), which limits the transmission distance [1][2][3][4][5] . Recently, faster-thanNyquist (FTN) signaling has attracted much attention because this technology can compress the occupation bandwidth to a fraction of the bandwidth that the Nyquist signal occupies [6][7][8][9] . Meanwhile, in an FTN system the symbol period between two adjacent pulses is lower than that of the corresponding Nyquist system, with the result that inter-symbol interference (ISI) is introduced [10][11][12][13] . Fortunately, we can use a detection method such as the maximum a posteriori (MAP) or maximum likelihood sequence estimation (MLSE) to overcome this challenge on the coherent receiver side [12][13][14][15][16] . With the recent advancement of high-speed digital signal processing (DSP), DSP-based methods have been introduced for high-speed optical systems and have achieved great success. Carrier phase recovery (CPR) is the important part of the DSP process, whose function is to remove the phase noise (PN) induced by the transmitter (Tx) laser and the local oscillator (LO) at the receiver (Rx) end. Many CPR algorithms have been proposed such as the Viterbi and Viterbi phase estimation (VVPE) algorithm, blind phase search (BPS) algorithm, and pilot-assistedcarrier phase recovery (PA-CPR) scheme [17][18][19] . Among them, the VVPE algorithm has a low complexity but a not so good performance. The BPS algorithm has the best performance, but the complexity is the highest. These two methods cannot work well in the FTN system because of ISI induced by FTN signaling, showing a large possibility of a new kind of accumulated cycle slips that will degrade the correcdt estimation of PN. This situation becomes worse especially in the systems with larger linewidth of the Tx and LO lasers and severe ISI due to denser compression of bandwidth by FTN. In our previous work, ...