Experimental demonstration of a format-flexible ... - OSA Publishing

Experimental demonstration of a format-flexible single-carrier coherent receiver using data-aided digital signal processing Robert Elschner,1,* Felix Frey,1 Christian Meuer,2 Johannes Karl Fischer,1 Saleem Alreesh,2 Carsten Schmidt-Langhorst,1 Lutz Molle,1 Takahito Tanimura,3 and Colja Schubert,1 1

Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Department Photonic Networks and Systems, Einsteinufer 37, 10587 Berlin, Germany 2 On leave from Technische Universität Berlin, Institut für Telekommunikationssysteme, Einsteinufer 25, 10587 Berlin, Germany 3 On leave from Fujitsu Laboratories, Ltd., 1-1 Kamikodanaka 4-chome, Nakahara-ku, 211-8588 Kawasaki, Japan * [email protected]

Abstract: We experimentally demonstrate the use of data-aided digital signal processing for format-flexible coherent reception of different 28-GBd PDM and 4D modulated signals in WDM transmission experiments over up to 7680 km SSMF by using the same resource-efficient digital signal processing algorithms for the equalization of all formats. Stable and regular performance in the nonlinear transmission regime is confirmed. ©2012 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.1660) Coherent communications; (060.4080) Modulation.

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Received 1 Oct 2012; revised 15 Nov 2012; accepted 16 Nov 2012; published 12 Dec 2012

17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 28786

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1. Introduction Since the future internet traffic is expected to be much more dynamic than in the past, options for the flexibilization of optical networks and elastic networking have recently attracted a lot of attention [1,2]. In particular, the flexible generation and reception of modulation formats with different spectral efficiencies using a single transponder (i.e. software-defined) will allow to optimize the use of the network infrastructure under dynamic bandwidth demands [3]. While the combination of a high-speed digital-to-analog converter (DAC) with a dualpolarization in-phase- and quadrature (DP-IQ)-modulator in the transmitter and a high-speed analog-to-digital converter (ADC) with coherent reception in the receiver in principle already allows for generation and reception of different modulation formats, format-flexible and therefore resource-efficient digital signal processing (DSP) is the key technology for the practical realization of such a transponder. In this context, data-aided equalization algorithms [4–6], have many advantages over popular blind algorithms like the constant-modulus algorithm because they work independently of the modulation format in the payload signal and provide stable performance. In this contribution, we experimentally show the use of data-aided channel estimation in combination with resource-efficient frequency-domain equalization for the wavelengthdivision multiplexed (WDM) transmission of 28-GBd polarization-division multiplexed binary phase-shift keying (PDM-BPSK), polarization-switched quaternary PSK (PS-QPSK) [7–10], PDM-QPSK and PDM 8-ary quadrature amplitude modulation (PDM-8QAM) over an uncompensated ITU-T G.652 standard single-mode fiber (SSMF) link with up to 7680 km length. The different signals were generated by using a high-speed four-channel DAC at the transmitter and provided varying bit rates from 56 Gb/s to 168 Gb/s within the same optical bandwidth as well as different maximal transmission reaches. All formats carried the same header sequence and were equalized offline using the same algorithms.

Fig. 1. (a) Header structure and (b) structure of data-aided digital signal processing.

2. Data-aided digital signal processing The used header structure and the used offline algorithms are shown in Fig. 1. The header was composed of three different sequences (BPSK and QPSK symbols) which were used for frame synchronization, carrier frequency offset (CFO) compensation and channel estimation [5,6]. Each of the sequences in the Y-polarization was a cyclic-shifted copy of the corresponding X-polarization sequence. The header was periodically repeated with a total overhead of 1.17%, corresponding to a negligible required optical signal-to-noise ratio (OSNR) penalty of less than 0.1 dB. In the DSP, the signal was first resampled to two samples per symbol. After front-end corrections [11], the accumulated chromatic dispersion (CD) was

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blindly estimated [12], using a block of 4096 samples and compensated in the frequency domain on the full shot within one step to reduce the length of the channel impulse response and therefore the required length of the header. The following frame synchronization was based on an autocorrelation metric [13], and used the framing sequences in the header consisting of BPSK symbols. Then, the carrier frequency offset was estimated in two dataaided stages [14]. The first stage used the header CFO synchronization sequences consisting of QPSK signals and had an acquisition range of ± 1.5 GHz. The second stage used the pilot sequences for fine adjustment. These sequences carried constant-amplitude-zeroautocorrelation (CAZAC) sequences [5], consisting of 16 QPSK symbols and were primarily used for the channel estimation. The equalizer matrix was directly calculated from the estimated channel matrix using the minimum-mean-square error criterion [5]. The following feed-forward add-and-overlap frequency-domain equalizer [15], used a block size of 1024 samples and an overlap of 25%. The carrier phase estimation for all PSK formats (including PS-QPSK) was conducted using the format-flexible block-based blind feed-forward ViterbiViterbi (VV) algorithm [16]. For PDM-8QAM, we used the blind-phase search (BPS) algorithm [17] with 32 test angles. We also tested this algorithm with the other formats and found a similar performance as for the VV algorithm under our experimental conditions. The used averaging block size was 32 in all cases. Since the BPS algorithm can be in principle used for all types of modulation formats including PSK, QAM and 4D modulation, it enables full format-flexible carrier phase estimation completing the format-flexibility of all algorithms of the DSP shown in Fig. 1(b). After the header removal, decision and demapping, the errors were counted and the resulting bit-error ratio (BER) was converted to a Q-factor.

Fig. 2. Experimental setup for the generation and WDM transmission.

Fig. 3. Drive signals (d3) for PDM-BPSK, PS-QPSK, PDM-QPSK and PDM-8QAM.

Fig. 4. Spectrum for 20 x 28-GBd PDM-BPSK (btb and after 7200-km transmission)

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3. Experimental setup The experimental setup is shown in Fig. 2. At the transmitter, an external cavity laser (ECL) with a linewidth of ~100 kHz was used as a light source for the probe channel at 193.4 THz (1550.115 nm). For the remaining WDM channels, distributed feedback lasers (DFB) with linewidth