Talha Rahman

Next generation elastic optical networks: The vision of the European research project IDEALIST

In this work we detail the strategies adopted in the European research project IDEALIST to overcome the predicted data plane capacity crunch in optical networks. In order for core and metropolitan telecommunication systems to be able to catch up with Internet traffic, which keeps growing exponentially, we exploit the elastic optical networks paradigm for its astounding characteristics: flexible bandwidth allocation and reach tailoring through adaptive line rate, modulation formats, and spectral efficiency. We emphasize the novelties stemming from the flex-grid concept and report on the corresponding proposed target network scenarios. Fundamental building blocks, like the bandwidth-variable transponder and complementary node architectures ushering those systems, are detailed focusing on physical layer, monitoring aspects, and node architecture design.

Reduced Complexity Digital Back-Propagation Methods for Optical Communication Systems

Next-generation optical communication systems will continue to push the ( bandwidth · distance) product towards its physical limit. To address this enormous demand, the usage of digital signal processing together with advanced modulation formats and coherent detection has been proposed to enable data-rates as high as 400 Gb/s per channel over distances in the order of 1000 km. These technological breakthroughs have been made possible by full compensation of linear fiber impairments using digital equalization algorithms. While linear equalization techniques have already matured over the last decade, the next logical focus is to explore solutions enabling the mitigation of the Kerr effect induced nonlinear channel impairments. One of the most promising methods to compensate for fiber nonlinearities is digital back-propagation (DBP), which has recently been acknowledged as a universal compensator for fiber propagation impairments, albeit with high computational requirements. In this paper, we discuss two proposals to reduce the hardware complexity required by DBP. The first confirms and extends published results for non-dispersion managed link, while the second introduces a novel method applicable to dispersion managed links, showing complexity reductions in the order of 50% and up to 85%, respectively. The proposed techniques are validated by comparing results obtained through post-processing of simulated and experimental data, employing single channel and WDM configurations, with advanced modulation formats, such as quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM). The considered net symbol rate for all cases is 25 GSymbol/s. Our post-processing results show that we can significantly reduce the hardware complexity without affecting the system performance. Finally, a detailed analysis of the obtained reduction is presented for the case of dispersion managed link in terms of number of required complex multiplications per transmitted bit.

On the Mitigation of Optical Filtering Penalties Originating From ROADM Cascade

Wavelength selective switches (WSSs) that are integrated in reconfigurable optical add-drop multiplexers (ROADMs) induce penalties on the optical signal due to tight optical filtering, which increases as several ROADMs are cascaded in a meshed network. In this letter, we propose and analyze possible configurations for the mitigation of these penalties in optical domain using optical wave shaper (WS). Including one WS in every ROADM node allows transmission of 28 and 32 GBd signals, which are QPSK, 8-QAM, or 16-QAM modulated, through a cascade of 32 and 14 WSS filters, respectively. With an average bandwidth of 33 GHz per WSS, an optical signal to noise ratio penalty below 1 dB at BER=1×10-3 is observed.

Technology Options for 400 Gb/s PM-16QAM Flex-Grid Network Upgrades

In this letter, we report on 400 Gb/s polarization multiplexed 16-state quadrature amplitude modulation (PM-16QAM) transponder variants and flex-grid network upgrade configurations. We address transponder subcarrier granularity, and demonstrate that the performance improvement, from dual-carrier to quad-carrier super-channel configuration, is limited to ~ 1.4 dB (in Q-factor, at power spectral density of 10-1 mW/GHz), at the cost of doubled hardware requirements. In view of that, we establish the performance improvements, for a dual-carrier 400 Gb/s PM-16QAM transceiver, as a function of increasing forward error correction overhead (FEC-OH) and spectral inversion based super-channel fiber nonlinearity compensation (SNLC-SI). We show that increasing the FEC-OH improves the transmission performance, at the cost of significant power consumption requirements, alternatively, employing SNLC-SI, at a lower FEC-OH, is a more power efficient solution. In particular, for homogeneous and heterogeneous launch power based network configurations, SNLC-SI enables ~ 23% and ~ 45% reach improvements at maximum considered FEC-OH (45%). At a fixed distance, it enables ~ 25% and ~ 50% power savings, respectively, compared with FEC-OH employing linear compensation only.

Ultralong haul 1.28-Tb/s PM-16QAM WDM transmission employing hybrid amplification

In order to cope with the foreseeable capacity crunch next-generation optical transmission systems aim to utilize higher order quadrature amplitude modulation formats to achieve spectral efficiency (SE) higher than the current commercial systems. In particular, transmission rates as high as 1 Tb/s are envisioned, employing superchannel configuration achieved by closer than standard 50 GHz placement of Nyquist filtered wavelength-division multiplexed (WDM) channels. Moreover, increase in symbol rate of each subcarrier in a superchannel is desired to reduce the number of components per Tb/s and, consequently, overall cost. In this regard, we addressed a series of challenges namely intersymbolinterference (ISI) induced by low-pass filtering of digital-to-analog converter (DAC), intrasuperchannel crosstalk penalties, and selected suitable forward error correction (FEC) code considering limitations of electronic components. Digital preemphasis is employed to mitigate DAC induced ISI, a subcarrier spacing of 1.2 × symbol rate is chosen to limit crosstalk penalties below 0.5 dB in Q2 and a FEC overhead of 23% is established limiting transponder count to four, achieving 1 Tb/s net data rate. The superchannel is assigned a 200-GHz optical spectrum to achieve a SE of 5.0 b/s/Hz, and WDM transmission performance is evaluated over three different kinds of fibers: standard single-mode fiber (SSMF), large area pure silica core fiber (LAPSCF), and large effective area fiber (LEAF), having span lengths of 95/121, 82/164, and 81 km, respectively. The maximum reach of 1-Tb/s superchannel with 8 × 100-Gb/s WDM channels at pre-FEC threshold of 3.37 × 10-2 was found to be 1110, 1921, and 789 km for SSMF, LAPSCF, and LEAF, respectively. Further improvement in transmission performance is achieved by employing hybrid EDFA-Raman amplification, and achievable distance was extended to 2054, 2952, and 1341 km for SSMF, LAPSCF, and LEAF, respectively, at pre-FEC threshold. Mitigation of a nonlinear phase noise employing single-channel digital back propagation resulted in extension of maximum reach up to 2262, 3349, and 1530 km for SSMF, LAPSCF, and LEAF, respectively.

Digital Pre-Emphasis in Optical Communication Systems: On the Nonlinear Performance

Digital signal pre-processing at the transmitter is a key enabler for next-generation optical transceivers. One of the major challenges faced by these transponders is the limited resolution and -3 dB electrical bandwidth of the digital-to-analog converters (DAC), severely limiting the transmission performance. On the other hand, these spectrally efficient flexible transport networks are extremely sensitive to nonlinear channel impairments, essentially limiting the maximum system reach. In this paper, we employ a simple digital pre-emphasis (DPE) algorithm to mitigate DAC-induced signal distortions, and further report on the impact of DPE algorithm on nonlinear transmission performance. We demonstrate, both numerically and experimentally, that DPE not only counteracts DAC low-pass response, allowing for higher baud rate transmission, but also enables better nonlinear channel tolerance. In particular, we first establish the maximum transmittable baud rates in back-to-back configuration for polarization multiplexed m-state quadrature amplitude modulation (PM-mQAM) formats, allowing up to 50, 46 and 44 Gbaud transmission for PM-4QAM, PM-8QAM, and PM-16QAM, respectively assuming typical DAC specifications of 16 GHz -3 dB bandwidth and 5.5 effective number of bits. Furthermore, we show that the performance improvements from DPE increase with increasing modulation order and decreasing span count, with maximum improvements of up to ~2.2 dB. In a conventional standard single mode fiber transmission link with lumped amplification, employing DPE, we report maximum reach of up to ~10 000, ~5000, and ~3000 km, for PM-4QAM, PM-8QAM, and PM-16QAM, respectively. Moreover, at pre-FEC bit-error-rate, relative reach improvements from DPE are found to be up to 21%, 25%, and 27%, respectively. Finally, we show that DPE gains are constant beyond a minimum required channel spacing, with minimum spacing requirements of 1.1 × baud rate, 1.15 × baud rate, and 1.2 × baud rate for PM-4QAM→PM-16QAM, respectively.

Long-haul terabit transmission (2272km) employing digitally pre-distorted quad-carrier PM-16QAM super-channel

We experimentally demonstrate long-haul WDM transmission of 36GBaud four-subcarrier Terabit PM-16QAM super-channel. Digital pre-distortion enables ~50% reach improvement for both LAPSCF and SSMF fiber-types, with maximum recorded reach of 2272km and 949km, respectively.

Flex-grid optical networks: spectrum allocation and nonlinear dynamics of super-channels

Flex-grid optical networks have evolved as a near-future deployment option to facilitate dynamic and bandwidth intense traffic demands. These networks enable capacity gains by operating on a flexible spectrum, allocating minimum required bandwidth, for a given channel configuration. It is thus important to understand the nonlinear dynamics of various high bit-rate super-channel configurations, and whether such channels should propagate homogenously (uniform channel configuration) or heterogeneously (non-uniform channel configuration), when upgrading the current static network structure to a flex-grid network. In this paper, we report on the spectrum allocation strategies based on the impact of inter-channel fiber nonlinearities, for PM-16QAM channels (240Gb/s, 480Gb/s and 1.2Tb/s) -termed as super-channels, propagating both homogenously, and heterogeneously with 120Gb/s PM-QPSK, 43Gb/s PM-QPSK, and 43Gb/s DPSK traffic. In particular, we show that for high dispersion fibers, both homogenous and heterogeneous spectrum allocation enable similar performance, i.e. the nonlinear impact of hybrid traffic is found to be minimal (less than 0.5dB relative penalties). We further report that in low dispersion fibers, the impact of spectrum allocation is more pronounced, and heterogeneous traffic employing 120Gb/s PM-QPSK neighbors enables the best performance, ~0.5dB better than homogenous transmission. However, the absolute nonlinear impact of co-propagating traffic is more significant, compared to high dispersion fibers, with maximum performance penalties up to 1.5dB.

Vendor-interoperable elastic optical interfaces: Standards, experiments, and challenges [Invited]

This paper aims to review the status of standardization activities on the black link (BL) and corresponding alien wavelength concepts. It discusses experimental work on filterless optical networks conducted within a dedicated Deutsche Telekom project. The general prospects and existing challenges concerning elastic extensions of the BL are also assessed. Furthermore, we present relevant work on control and management plane interoperability comprising generalized multi-protocol label switching and transport software defined networking aspects. In the second part of the article, we report on the latest dual-vendor experiments on data plane interoperability in terms of digital signal processing technology for next generation 1.28 Tb/s PM-16QAM super-channels. Finally, we present a network reachability analysis based on the widely used Gaussian noise model in the context of data plane interoperability. This analysis estimates the network-wide impact of the single-vendor versus the dual-vendor approach.

Next generation optical nodes: The vision of the European research project IDEALIST

As traffic demands become more uncertain and newer services continuously arise, novel network elements are needed to provide more flexibility, scalability, resilience and adaptability to todays optical networks. Considering these requirements, within the European project IDEALIST the investigation of elastic optical networks is undertaken with special focus on next generation optical node architectures. As an evolution of existent ROADMs and OXCs, these optical nodes will establish a new paradigm in which the network requirements will be efficiently addressed considering various emerging dimensions. In this article, we describe the drivers, architectures, and technologies that will enable these novel optical nodes. In addition, multivendor traffic interoperability, optical defragmentation, and node cascadability are also described as considerations in the node design.