Spectrally Efficient Frequency Division Multiplexing (SEFDM) For Optical Communication System

The fast growth of Internet applications such as voice, video, and gaming has led to a hugedemand on the bandwidth of optical networks. To satisfy this increasing demand, extensiveresearch has been conducted to increase the spectral efficiency (SE) both in access and core fiberoptic networks. Among these pioneering works, Optical orthogonal frequency divisionmultiplexing (OFDM), Nyquist frequency wavelength-division multiplexing (NWDM), havegained much attention as the attractive candidates for future optical communication systems. Forboth of them, orthogonal signaling is usually adopted to ensure the absence of inter-symbolinterference (ISI) and also the absence of interference from adjacent channels. However, if theorthogonality condition must be satisfied and given the present hardware limitations, the onlyway to increase the spectral efficiency is to increase the constellation cardinality, thus employingmodulation formats more sensitive to the nonlinear effects. Therefore, the research of savingoptical systems spectrum by relocating the sub-carriers and relaxing the orthogonality rule hasbegun to be a new attractivtion. Spectrally efficient FDM (SEFDM) systems has been proposedin wireless communications to further improve spectral efficiency for future communicationsystems. It offers significant bandwidth gains by relaxing the orthogonality condition at theexpense of receiver complexity. In this paper, we investigate the application of this frequencypacking technique to optical communication System under coherent and incoherent(direct-detected) detection, respectively. The simulative and experimental results show that thistechnique provide a significant spectral efficiency increase and represent a viable alternative toovercome the theoretical and technological issues related to the use of high-order modulationformats.Two receiver architectures will be considered in this paper:(i) a tradeoff-complexity SEFDMsymbol detector for achieving a semi-optimum BER performance, and (ii) a proper electronicequalization algorithm for compensating the channel distortion and phase noise. AlthoughSEFDM system fulfills the objective of saving the spectrum, the deliberate collapse oforthogonality generates significant interference between the subcarriers that leads the detectionof SEFDM signals to be challenged. Based on the pioneering research on the detection forSEFDM signals, we proposed a combination of ID with FSD algorithm, which can not onlyachieve a satisfying BER performance with a fixed and reduced complexity, but also make thedetction better-suited for application in the real system. Meanwhile, due to the signal suffersfrom intercarrier interference (ICI) resulting from the non-orthogonal structure of the subcarriers,the conventional compensation algorithm for optical OFDM system may not suitable for anymore. Here we demonstrate time domain based methods for channel and phase estimation, which are capable of delivering good accuracy of estimations.In this paper, we firstly experimentally demonstratied these optical SEFDM systems that savebandwidth in both the electricaland optical domains. It is experimentally shown that forbandwidth saving up to20%, we can achieve the same performance as O-OFDM. This is the firstexperimental verification of20%optical faster than the Nyquist rate. Furthermore, forapproximately the same spectral efficiency,4QAM O-SEFDM outperforms standard8QAM by1.6dB. These results indicate that the O-SEFDM systems may be used in future multi-carriersystems to improve spectral efficiency when small error performance degradation may betolerated.