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Abstract:In order to improve the spectral efficiency of coherent optical communication systems, it has recently been proposed to make use of the orthogonal frequency-division multiplexing offset quadrature amplitude modulation (OFDM-OQAM). Multiple optical channels spaced in the frequency domain by the symbol rate can be transmitted orthogonally, even if each channel overlaps significantly in frequency with its two adjacent channels. The solutions proposed until now in the literature unfortunately only address a single polarization communication, and therefore do not benefit from the capacity gain reached when two polarizations are used to transmit independent information signals. The aim of the present paper is to propose a receiver architecture that can decouple the two polarizations. We build an equalizer per channel at twice the symbol rate and optimize it based on the minimum mean square error (MMSE) criterion. We demonstrate the efficiency of the resulting system compared to the Nyquist wavelength-division multiplexing (N-WDM) system both in terms of performance and complexity. We also assess the system sensitivity to transmit synchronization errors and show that system can even work under significant synchronization errors.

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Filter-bank multi-carrier (FBMC) modulations have recently been considered for the emerging wireless communication systems as a means to improve the utilization of the physical resources and the robustness to channel time variations. FBMC divides the overall frequency channel in a set of subchannels of bandwidth proportionally decreasing with the number of subchannels. If the number of subchannels is high enough, the bandwidth of each subchannel is small enough to assume that it is approximately flat. On the other hand, space-division multiple access (SDMA) is a recognized technique to support multiple access in the downlink of a multi-user system. The user signals are precoded at the base station equipped with multiple antennas to separate the users in the spatial domain. The application of SDMA to FBMC is unfortunately difficult when the channel is too frequency selective (or when the number of subchannels to too small) to assume flat subchannels. In that case, the system suffers from inter-symbol and inter-subchannel interference, besides the multi-user interference inherent to SDMA. State-of-the art solutions simply neglect the inter-symbol/subchannel interference. This article proposes a new SDMA precoder for FBMC capable of mitigating the three sources of interference. It is constructed per subchannel in order to keep an acceptable complexity and has the structure of a filter applied on each subchannel and its neighbors at twice the symbol rate. Numerical results demonstrate that the precoder can get rid of all the interference present in the system and benefit therefore from the diversity and power gains achievable with multiple antenna systems.

We study the performance of multicarrier offset modulation and root-raised-cosine shaped multicarrier modulation with aggregate 32.5 GBd symbol rate and show that offset modulation is preferable for non-zero rolloff factors. OCIS codes: (060.0060) Fiber optics communications; (060.1660) Coherent Communications IntroductionIn coherent optical communication systems, bandwidth-limited multicarrier (MC) transmission can be achieved using conventional quadrature amplitude modulation (QAM) on root-raised-cosine (RRC) shaped subcarriers (MC-QAM). Subcarrier spacing equal to the symbol rate results in crosstalk for any rolloff factor > 0. MC offset quadrature amplitude modulation (MC-OQAM) [1] was recently proposed as an alternative that achieves subcarrier spacing equal to the symbol rate. MC-OQAM uses RRC pulse shaping, but the spectra of adjacent channels overlap without resulting in crosstalk between neighboring subcarriers thanks to a half-symbol time delay between the inphase and quadrature components of the signal on each subcarrier. Hence, non-zero rolloff factors become theoretically accessible without penalty.Similar to orthogonal frequency division multiplexing (OFDM), transmission impairments are bound to break the subcarrier orthogonality of MC-OQAM [2]. The benefit of MC-OQAM therefore strongly depends on the ability to recover subcarrier orthogonality using digital signal processing (DSP). Recently, a 224-Gb/s MC-OQAM communication system with seven 16-Offset-QAM (16OQAM) subcarriers was demonstrated in a back-to-back configuration using a modified blind decision-directed equalization on each subcarrier [3]. In fact, the standard DSP algorithms for coherent optical communications cannot be used in MC-OQAM due to the half-symbol time delay.In this paper, we propose to use training sequences to estimate the communication channel and use a new algorithm to track the phase in MC-OQAM. We compare MC-OQAM with conventional MC-QAM modulations so as to assess the benefits of crosstalk mitigation via OQAM. The channels are modulated at an aggregate symbol rate of 32.5 GBd, to make 100 Gb/s net bit-rate using QPSK or Offset-QPSK (OQPSK) modulation on each subcarrier.