Abstract
Unlike conventional CO-OFDM systems, we show in this paper that reduced-guard-interval (RGI) CO-OFDM systems experience subcarrier-dependent phase noise (PN) from the local oscillator laser. This phenomenon manifests in RGI-CO-COFM systems because the chromatic dispersion (CD) induced walk-off becomes comparable to the OFDM symbol length. We term this phenomenon the dispersion enhanced PN (DEPN). In this work an analytical study of the impact of DEPN on CO-OFDM transmission is conducted. We develop a system-level analytical model and calculate the variance of the dispersion-induced subcarrier-dependent phase rotation term (PRT) using two different distribution patterns of pilot subcarriers (PS). Moreover, we present a bit error rate (BER) estimator to quantify the system performance degradation due to PRT. Numerical simulations are then performed to verify the analytical model. Finally, we propose a grouped maximum-likelihood (GML) phase estimation approach to mitigate the DEPN impairment, and demonstrate a 0.7-1.7 dB SNR improvement at BER=10⁻³ for typical 100 Gb/s RGI CO-OFDM systems.
Highlights
Coherent optical OFDM (CO-OFDM) systems have been actively investigated as a promising candidate for Ethernet transport at 100 Gb/s and beyond [1,2,3]
We show that the dispersioninduced average phase rotation term (PRT) variance is less for uniformly distributed pilot subcarriers (PS)’s (UD-pilot subcarriers (PS’s)) than centered PS’s (C-PS’s), and subsequently UD-PS’s provides a better bit error rate (BER) performance than C-PS’s
We find that CO-OFDM with a smaller number of subcarriers experience less inter-carrier interference (ICI) but more PRT
Summary
Coherent optical OFDM (CO-OFDM) systems have been actively investigated as a promising candidate for Ethernet transport at 100 Gb/s and beyond [1,2,3]. CO-OFDM signals with long symbol durations suffer from a large inter-carrier interference (ICI) from the laser phase noise (PN) Considering these disadvantages, conventional CO-OFDM systems become less attractive than single carrier systems [4]. A reduced-guard-interval (RGI) COOFDM system was proposed to realize 100 Gb/s and beyond transmission [5,6,7,8,9,10] In this novel scheme, CD and polarization mode dispersion (PMD) are compensated before OFDM demodulation [10], which enables the use of a smaller number of subcarriers and achieves a smaller CP overhead than that of the conventional CO-OFDM.
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