Flexible Data-rate and Reach Transceiver Employing Hybrid Modulations and Scrambled Coherent Superposition Talha Rahman(1), Bernhard Spinnler(1), Stefano Calabrò(1), Erik De Man(1), Antonio Napoli(1), Bernd Sommerkorn-Krombholz(1), A. M. J. Koonen(2), C. M. Okonkwo(2), Huug de Waardt(2) (1)
Coriant R&D GmbH, Munich Germany,
[email protected] of Photonics Integration, Eindhoven University of Technology, The Netherlands
(2) Institute
Abstract Subcarrier multiplexing hybrid QAM combined with coherent superposition is proposed to achieve flexibility in spectral efficiency (SE) and reach. Experimental evaluation over SSMF shows scalable SE selection from 4.9 to 1.2bit/s/Hz with the maximum reach from 2000 to 15500km, respectively. Introduction Due to high spectral efficiency (SE) as well as reduced operational costs for ultra long-haul and long-haul data transport applications, coherent optical transmission systems are established as the most feasible solution. Currently commercialized coherent optical transponders employ different orders of quadrature amplitude modulation (QAM) formats to improve SE. Employing classical QAM formats (4QAM, 16QAM, etc.), the data-rate and maximum transmission reach can only be selected in coarse large steps; severely limiting the flexibility in optical networks design as well as the ability to cope with growing dynamic service demands of the operators1. In order to alleviate this limitation, next generation of transponders are expected to support flexible selection of data-rate and maximum reach. In this context, the use of time domain hybrid QAM has been proposed2. More recently, probabilistic constellation shaping has been proposed where transmit constellation distribution is modified to control SE and reach3. As an alternative, the use of digital subcarrier multiplexing (SCM) hybrid QAM was recently proposed and its enhanced tolerance to cascaded optical filtering was also demonstrated4. Compared to other schemes, SCM has the advantage of utilizing standard single carrier receiver digital signal processing (DSP) algorithms with minimal additional complexity in addition to being tolerant to optical fiber nonlinearity5. For multi-core fiber systems, the concept of scrambled coherent superposition (SCS) has been presented where multiple scrambled copies of transmitted signal are digitally superimposed at the receiver to improve signal fidelity6. In this paper, we propose SCM hybrid QAM combined with SCS to achieve flexible SE and reach with a high granularity for standard single mode fiber (SSMF) systems. We employ polarization multiplexed (PM-) quadrature phase
shift keying (QPSK), 8QAM and 16QAM formats as well as their hybrid combinations with SCS to achieve a highly flexible SE and reach transceiver utilizing a fixed- symbol-rate, signal bandwidth and forward error correction (FEC) overhead. After removal of overheads, net datarates in range of 61Gbit/s to 244Gbit/s were transmitted over distances in range of 15500km to 1944km over an erbium doped fiber amplifier (EDFA) only amplified SSMF link with 50GHz spaced 33 total wavelength division multiplexing (WDM) signals. Flexible data-rate transmission scheme In order to employ SCS scheme for multi-core transmission systems, the electrical fields of two or more cores are propagated as an scrambled copy of each other in parallel which are digitally superimposed at the receiver end. For the case of single core fibers (e.g. SSMF), the optical signal can not propagate in parallel spatial channels. Hence, for single core fibers, the dimension of either time, frequency or polarization must be used for the propagation of scarmbled copy of the desired signal. In the following analysis, we have chosen the polarization dimension to propagate the scrambled signal field. The desired data field and the scrambled field are related to each according to the following relation6: = ∙ (1) where Ex,y are the electric fields of X- and Y-pol. and S is the scrambling function. Following the above relation, it is worth noting that the Y-pol. can not be used for independent data transmission since it is propagating a scrambled version of the X-pol. electric field. For a single carrier signal, it translates to 50% data-rate loss. However, employing SCM, only selected digital subcarriers can be propagated with scrambled field on Y-pol. and data-rate loss as well as gain in received signal quality can be tuned with a higher granularity; which is an essential feature for flexible data-rate transceivers. In order to
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further increase data-rate granularity, different digital subcarriers of an SCM signal are modulated with an arbitrary QAM modulation format, enabling a highly flexible data-rate transceiver. The steps of transmitter DSP for the generation of SCM hybrid QAM signals are presented in4. One of the orthogonal polarization can be modulated with scrambled version of the other. The scrambling function can be arbitrarily selected which must be known at the receiver to perform unscrambling. In the current analysis, the scrambling function is a conjugated and circular-shifted signal field divided by the original signal field. A shift value of 100 symbols is used. Experimental Setup The experimental setup is shown in Fig. 1(a). The aggregate symbol-rate of the test signal was fixed to 39GBd however, the number of digital subcarriers varied for different cases. The transmit DSP steps were performed offline and the resulting waveforms were uploaded to digital to analog converters (DACs) which drove a dual polarization IQ-modulator. The IQ-modulator was fed in by an external cavity laser (ECL) set to operate at 193.4THz frequency. A 50% dispersion pre-compensation was applied to the test signal employing digital filter. The resulting dispersion map is shown in Fig. 1(b). From another parallel setup, 32×100Gb/s WDM signals were generated which were evenly distributed to the left and right of test signal. The neighboring signals were all single carrier PMQPSK modulated and were 50GHz spaced. The neighboring signals as well as the test signal were coupled together employing a wavelength selective switch (WSS) which also balanced the optical spectrum. The WDM signal was then fed in to an optical loop which consisted of 4×95km long SSMF spans having an average loss of 19.5dB per span which was compensated by EDFA only amplification. Optical loop also contained a loop synchronous polarization scrambler (LSPS) to evenly distribute polarization dependent loop effects and a WSS to balance optical power among different WDM signals as well as filter out of band amplified
Fig. 2: (a) Rx DSP block diagram for SCS of SCM QAM signals. (b) Received constellation without/ with SCS.
spontaneous emission (ASE) noise. The optical spectra at the input as well as after 40 loop cirulations is shown in Fig. 1(c). After circulations through the loop, the test signal was filtered out of WDM signal employing an optical band-pass filter and was mixed with a local oscillator laser in an integrated coherent receiver (ICR). The electrical outputs of ICR were digitized by an oscilloscope operating at 50Gsamples/s (23GHz bandwidth) and the resulting samples were saved to perform offline receiver DSP. The initial steps of receiver DSP are detailed in4. After de-multiplexing of digital subcarriers, each could be processed in parallel as highlighted in Fig. 2(a). After timing recovery, blind equalization was performed to achieve polarization de-rotation which was followed by carrier recovery employing a digital phase locked loop. In order to apply coherent superposition of E fields in X- and Ypolarization, the signals must first be unscrambled by applying the inverse of scrambling function used at transmitter (S-1); which in our case, was an inverse circular-shift of 100 symbols and conjugate operation. After unscrambling, the signals were digitally superimposed and the resulting constellations of different tested modulation formats are shown in
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Fig. 3: (a) Q-factor vs launch power per channel for PM-QPSK at 7600km. (b) (c)
Fig. 2(b), showing improved constellations. Results and Discussion An optical launch power per channel sweep was performed for PM-QPSK test signal with 6 digital subcarriers at a transmission distance of 7600km and the corresponding Q-factor (calculated from weighted average bit error ratio (BER)[]) is shown in Fig. 3(a). The optimum power in each case if found to be 0dBm and a maximum Q-factor gain of ~3dB is achieved over each launch power when all digital subcarriers employ SCS. The maximum Qfactor gain is achieved at a cost of 50% datarate. However, employing SCS on selected subcarriers, Q-factor gain can be progressively increased with data-rate loss which enables a large granularity in data-rate and reach selection at a fixed symbol-rate as well as FEC. At the pre-FEC Q-factor of 5.24dB (23% FEC overhead), the maximum transmission reach for each case at 0dBm per channel launch power was evaluated and the corresponding results are shown in Fig. 3(b). Without SCS, a maximum transmission reach of 9300km was achieved for a net data-rate of 121.95Gb/s. Increasing the number of subcarriers employing SCS, the maximum reach as well as data-rate loss is progressively increased to a maximum reach of 15500km for data-rate of 60.97Gb/s. Markers on Fig. 3(b) represent the experimental points while the line represent a polynomial fit. The number of digital subcarriers for 8QAM was chosen to be 6, while that for 16QAM was 2. For the hybrid modulation schemes of QPSK-8QAM hybrid (4-8 hybrid) and 8QAM-16QAM hybrid (816 hybrid), the number of digital subcarriers was 4. For hybrid modulation schemes, 2 central subcarriers were modulated with higher order QAM while the edge subcarriers were modulated by a lower order QAM in each case. For these modulation schemes, achievable maximum net data-rates and reach curves are
shown in Fig. 3(c). It is worth noting that the performance of hybrid modulation formats lie between classical QAM formats providing additional granularity in SE and reach for a highly flexible data-rate transceiver. It can be observed from Fig. 3(c) that a lower order QAM always outperforms a higher order one in terms of reach, if both can achieve a given data-rate. The maximum achievable data-rate of lower order formats is however limited by constellation contraint necessitating higher order QAM to increase data-rate and SE. Conclusion In this paper a flexible data-rate transceiver design employing SCM hybrid and non-hybrid QAM is presented. Flexibility in achievable datarate and reach is enhanced by employing SCS of selected digital subcarriers. The presented design employs fixed transmission symbol-rate as well as FEC code; simplifying implementation, operation and upgradability of existing optical networks. Acknowledgements We would like to acknowledge support from Coriant R&D as well as funding from SENDATE-FICUS.
References [1] O. Gerstel et. al., “Elastic optical networking: a new dawn for the optical layer?” IEEE Commun. Mag.,Vol. 50, no. 2, pp. s12–s20 (2012) [2] Q. Zhuge et. al., “Spectral Efficiency-Adaptive Optical Transmission Using Time Domain Hybrid QAM for Agile Optical Networks,” J. Lightw. Technol., Vol. 31, no. 15, pp. 2621–2628 (2013) [3] F. Buchali et. al., "Rate Adaptation and Reach Increase by Probabilistically Shaped 64-QAM: An Experimental Demonstration," J. Lightw. Technol., Vol. 34, no. 7, pp. 1599-1609 (2016) [4] T. Rahman et. al., “Digital Subcarrier Multiplexed Hybrid QAM for Data-rate Flexibility and ROADM Filtering Tolerance,” Proc. OFC, Tu3K.5 (2016) [5] M. Qiu et. al., "Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical
communication systems," Proc. OFC, M2A.5 (2014) [6] X. Liu et. al., "Scrambled coherent superposition for enhanced optical fiber communication in the nonlinear transmission regime," Opt. Express 20, 19088-19095 (2012)
