Jessica Fickers

Dual-polarization OFDM-OQAM for communications over optical fibers with coherent detection.

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.

Multicarrier offset-QAM for long-haul coherent optical communications

The availability of high-speed digital-to-analog converters for coherent optical communication systems makes it possible to digitally divide the available electrical bandwidth into subchannels on each optical carrier which enables a parallel implementation of the digital signal processing. In this paper, we use electrical multicarrier offset-QAM modulations to achieve crosstalk-free modulation for subcarriers spaced at the symbol rate. We show that in coherent optical communication systems, several impairments are bound to break the subcarrier orthogonality and cause failures of payload symbol recovering. We propose a dedicated digital signal processing architecture which implements channel estimation and a new algorithm to track the phase in multicarrier offset-QAM modulation. We experimentally compare the latter with conventional multicarrier and single carrier QAM modulation so as to assess the benefits of crosstalk mitigation via offset-QAM modulation. We carry out experiments to test the transmission performance of multicarrier offset-QAM modulation in a dispersion-unmanaged long-haul transmission link.

Interference-free SDMA for FBMC-OQAM

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.

Decision-Feedback Equalization of Bandwidth-Constrained N-WDM Coherent Optical Communication Systems

Nyquist Wavelength division multiplexing (N-WDM), is a promising scheme in order to enhance the spectral efficiency of future coherent optical communication systems. In N-WDM systems, the channel bandwidth and spacing are selected to maximize the spectral efficiency while maintaining acceptable levels of inter-carrier and inter symbol interference. To further increase the spectral efficiency, bandwidth constrained N-WDM, where baudrate is higher than channel bandwidth can be considered. We propose to combine the bandwidth-constrained N-WDM scheme with a decision feedback equalizer (DFE) designed according to the minimum mean square error (MMSE) criterion. We show that the enhanced resilience to the inter symbol interference offered by DFE provides an effective gain on spectral efficiency. We compare system benefits of DFE and maximum a posteriori sequence detection (MAP) for coherent optical receivers, and show that DFE is as efficient as MAP in mitigating inter symbol interference at a considerably lower complexity.

Multicarrier Offset-QAM Modulations for Coherent Optical Communication 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.

Pulse Design Trade-Offs for Spectrum-Efficient PDM-WDM Coherent Optical Transmission Systems

We study the joint effect of pulse spectral rolloff and impulse-response truncation length on the performance of densely packed root-raised-cosine pulse-shaped 32.5 GBaud PDM-QPSK and 16QAM, through extensive bit-error-rate and spectrum measurements.

Polarization Division Multiplexing for SC-FDE Communications over Dispersive Optical Fibers

High order linear modulations, combined with coherent detection and signal processing at the receiver, are nowadays considered to reach the new capacity targets for the communications over the optical fibers. In order to reduce the computational complexity of the algorithms at the receiver, it has recently been proposed to compensate for the chromatic dispersion (CD) and polarization mode dispersion (PMD) in the frequency domain. Orthogonal frequency-division multiplexing (OFDM) and single- carrier with frequency domain equalization (SC-FDE) are two alternative coding strategies to support frequency domain signal processing. Compared to OFDM, SC-FDE is particularly interesting because it potentially facilitates the implementation of the transmitter and it generates a lower peak-to-average power ratio (PAPR) waveform. The goal of the present paper is to demonstrate how the SC-FDE system can further support polarization division multiplexing (PDM) to double the communication capacity. The SC-FDE PDM system is derived analytically and its performance is assessed by numerical simulations. It is shown that a 72 Gbps bit rate can be obtained in a 10 GHz bandwidth.

Application of FBMC in Optical Communications

Abstract This chapter firstly introduces the historical perspectives of using optical coherent communications in order to fulfill the ever-increasing capacity requirements. The optical fiber communication systems are then presented, in which the impairments of optical fiber medium are summarized. The impairments can be efficiently mitigated digitally thanks to the advents of digital signal processing (DSP) in combination to the coherent detection. In the next section, two major approaches to achieve highest spectral efficiencies (SEs), which are based on the use of new shaping techniques and multicarrier, are discussed. Among them, the optical filter-bank multicarrier (FBMC) based on offset quadrature amplitude modulation (OQAM) has shown to be a promising candidate experimentally demonstrated in the long-haul optical transmission system. Finally, this chapter discusses a possible low-complexity equalization for FBMC/OQAM in optical transmissions and open challenges related to the optical implementation of OQAM.