We study the limitation of electrical bandwidth in QPSK optical coherent receiver with decision-aided maximum likelihood (DAML) carrier phase estimation. Before signal sampling, a low-pass rectangular filter is utilized to model the bandwidth limitation of electronic devices in the receiver. On one hand the limited bandwidth can reduce the power of additive noise, on the other hand it would distort the signal since inter-symbol interference (ISI) between adjacent symbols occurs. In this paper we discuss the effects on signal amplitude and phase caused by ISI and find original distribution of QPSK constellation points will shift due to ISI. When adjacent symbol interference, that is, the ISI effect of one previous symbol, is taken into account, the distribution changes into the form of 16-QAM, while changing as 4K+1-QAM if K previous symbols are considered. The impact on the subsequent DAML algorithm and the final bit error rate (BER) calculation due to constellation points shifts are analyzed intensively. Monte-Carlo (MC) simulation results show that 0.7 to 0.9 times symbol rate is a reasonable bandwidth range for different optical signal to noise ratios (OSNRs), and optimum bandwidth is often inside this range. Results also reveal that filter-induced ISI would degrade the BER performance of DAML receiver when OSNR is over 10 dB.

Date Published: 5 August 2015
PDF: 9 pages
Proc. SPIE 9619, 2015 International Conference on Optical Instruments and Technology: Optoelectronic Devices and Optical Signal Processing, 961905 (5 August 2015); doi: 10.1117/12.2193456

Published in SPIE Proceedings Vol. 9619:
2015 International Conference on Optical Instruments and Technology: Optoelectronic Devices and Optical Signal Processing
Yi Dong; Chao Lu; Jian Wu; Zhaohui Li, Editor(s)