Analysis of the Time-Frequency Localization Property of the Filter Banks for Optical OFDM/OQAM Systems

Orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) relaxes the orthogonal condition from the complex domain to the real field. Therefore, the cyclic prefix (CP) inserted between consecutive optical OFDM blocks, for combating the inter-symbol interference (ISI) induced by chromatic dispersion (CD) and polarization-mode-dispersion (PMD), could be removed for optical OFDM/OQAM to promote the system spectral efficiency. The ISI and the inter-carrier-interference (ICI) could be combated by using the filter banks with good time-frequency localization (TFL) property. Thus, the accuracy of channel estimation and system transmission performance largely depend on the TFL property of the filter banks for optical OFDM/OQAM. Recently, numerical experimental demonstrations and theoretical discussions of optical OFDM/OQAM have been reported. However, to our best knowledge, discussions on the TFL property of the filter banks have not been studied for optical OFDM/OQAM to date. In this paper, we systematically analyze the TFL property of the filter banks for optical OFDM/OQAM for the first time. We evaluate the TFL property of the filter banks with energy concentration, orthogonal basis, and isotropic property. Three typical filters as the Gaussian filter, the extended Gaussian function (EGF) filter and the isotropic orthogonal transfer algorithm (IOTA) filter have been employed for comparison. According to our theoretical analysis, with the use of the IOTA filter, optical impairments and channel estimation complexities could be decreased evidently thanks to the promising isotropic property. As shown in the numerous Montel Carlo simulations of the coherent optical OFDM/OQAM, system robustness against IMI, phase noise induced interference (PNII) and nonlinear effect could be improved evidently with the use of the IOTA filter.