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Next-Generation Photonic Transport Network Using Digital Signal Processing Next-Generation Photonic Transport Network Using Digital Signal Processing Yasuhiko Aoki Hisao Nakashima Shoichiro Oda Paparao Palacharla Coherent optical-fiber transmission technology using digital signal processing is being actively researched and developed for use in transmitting high-speed signals of the 100-Gb/s class over long distances. It is expected that network capacity can be further expanded by operating a flexible photonic network having high spectral efficiency achieved by applying an optimal modulation format and signal processing algorithm depending on the transmission distance and required bit rate between transmit and receive nodes. A system that uses such adaptive modulation technology should be able to assign in real time a transmission path between a transmitter and receiver. Furthermore, in the research and development stage, optical transmission characteristics for each modulation format and signal processing algorithm to be used by the system should be evaluated under emulated quasi-field conditions. We first discuss how the use of transmitters and receivers supporting multiple modulation formats can affect network capacity. We then introduce an evaluation platform consisting of a coherent receiver based on a field programmable gate array (FPGA) and polarization mode dispersion/polarization dependent loss (PMD/PDL) emulators with a recirculating-loop experimental system. Finally, we report the results of using this platform to evaluate the transmission characteristics of 112-Gb/s dual-polarization, quadrature phase shift keying (DP-QPSK) signals by emulating the factors that degrade signal transmission in a real environment. 1. Introduction amplifiers—which is becoming a limiting factor In the field of optical communications in system capacity—can be efficiently used. using digital signal processing, research One means offered for resolving this issue is and development of long-haul transmission adaptive modulation technology that applies the technology using high-speed signals of the most appropriate modulation format and signal- 100-Gb/s class such as 100-Gbit Ethernet has processing algorithm for current conditions such been quite active.1) In the future, transmission as the transmission distance between transmit speeds in optical communications systems will and receive nodes and the required bit rate. need to reach the 400-Gb/s level and eventually Adaptive modulation technology has already come 1 Tb/s to cope with the trend toward higher to be partially used in wireless communications, input/output (I/O) bit rates in the servers and but when it is applied to optical communications network devices making up data centers. Thus, systems, the transmission path and frequency a major issue of a next-generation optical slots between transmit and receive nodes should communications system is how to extend the be set in accordance with whatever modulation transmission distance of high-speed signals format has been selected as optimal given the while also achieving high spectral efficiency optical transmission characteristics at the time so that the amplification bandwidth of optical of operation. In this regard, a system parameter FUJITSU Sci. Tech. J., Vol. 48, No. 2, pp. 209–217 (April 2012) 209 Y. Aoki et al.: Next-Generation Photonic Transport Network Using Digital Signal Processing is needed to judge whether transmission is An optical communications system using feasible between certain transmit/receive nodes digital signal processing presents a different for each modulation format. Moreover, the scenario. At the transmitter, the system drives parameter should be carefully designed to ensure an optical modulator using a digital to analog that transmission quality is guaranteed when converter (DAC). At the receiver, the system adaptive modulation is applied. The optical uses an analog to digital converter (ADC) to transmission characteristics should be evaluated convert the analog-based optical-electric-field for a variety of transmission-path states while information obtained from the optical signal and emulating signal-degradation factors under a local oscillator into a digital signal and to then conditions approximating an actual environment. perform signal processing and demodulation. Aiming for a large-capacity, flexible photonic This means that optical signal generation and network, we simulated the use of adaptive reproduction can be performed with respect to modulation in optical transmitters and receivers any modulation format by changing the signal to evaluate its effectiveness for expanding system processing algorithms and parameters stored capacity and to test the effectiveness of adaptive in the transmitter’s and receiver’s digital signal modulation technology on a photonic network. processors without having to replace the optical We developed a transmission evaluation components used in those devices. platform2) with a digital coherent receiver3) that Figure 1 compares the configuration used is based on a field programmable gate array for existing photonic networks with that of a (FPGA). It enables a variety of modulation photonic network using an adaptive modulation formats and signal processing algorithms to format. The former is based on a fixed bit rate be loaded and evaluated. We evaluated the and fixed modulation format while the latter transmission characteristics of 112-Gb/s dual- is capable of varying the modulation format polarization, quadrature phase shift keying in accordance with the transmission distance (DP-QPSK) on a recirculating-loop experimental and transmission requirements. Adaptive system that emulates transmission-degradation modulation is an effective technique for efficiently factors.4) accommodating high-speed signals (above 100 Gb/s). The system can select the appropriate 2. Photonic networks using modulation format and the associated signal digital signal processing processing algorithm that is optimal for Optical communications systems using communication between the transmitter and wavelength division multiplexing (WDM) receiver in accordance with the transmission are widely used in current networks. These distance and required bit rate and then vary systems feature an equipment configuration channel arrangement accordingly. With adaptive that multiplexes optical signals transmitted modulation, the use of a modulation format under a specific modulation format and bit rate having a high bit/symbol ratio on relatively short against a frequency grid arranged with either transmission paths raises spectral efficiency, 50- or 100-GHz spacing as defined by ITU-T while the use of a format with a low bit/symbol Recommendation G.694.1. The main reason the ratio on relatively long transmission paths can modulation format and bit rate are fixed is that connect transmit and receive nodes without the characteristics of optical components used having to use a regenerative repeater. In short, in the optical transmitters and receivers are adaptive modulation enables flexible photonic optimal for a specific modulation format and bit network operation that can cope with different rate. optical fiber transmission paths and equipment 210 FUJITSU Sci. Tech. J., Vol. 48, No. 2 (April 2012) Y. Aoki et al.: Next-Generation Photonic Transport Network Using Digital Signal Processing OADM node OADM Wavelength arrangement is fixed regardless of modulation system nodes 40 Gb/s 10 Gb/s 100 Gb/s 100 Gb/s 40 Gb/s 10 Gb/s. 40 Gb/s Hub node OADM nodes OADM node 40 Gb/s 100 Gb/s 10 Gb/s 100 Gb/s 10 Gb/s Wavelength grid (50-GHz spacing) Uses different optical components depending on modulation format and bit rate (a) Current configuration (operation under a fixed grid) OADM node Wavelength arrangement adapts to different modulation systems OADM in units of minimum spacing. Hub node nodes 2×200 Gb/s 2×200 Gb/s 1×400 Gb/s 4×100 Gb/s 200 Gb/s 16QAM QPSK 256QAM QPSK OADM OADM nodes node 200 Gb/s 400 Gb/s 100 Gb/s 400 Gb/s 100 Gb/s Wavelength grid (6.25-GHz spacing) Uses common optical components for different modulation formats and bit rates (b) Future configuration (adaptive modulation + flexible grid spacing) Figure 1 Figure 1 Photonic networkPhotonic architecture network using architecture digital using signal digital processing. signal processing. layouts. ratio (OSNR) deemed necessary at reception with respect to that format’s maximum transmission 3. Capacity enhancement in a distance was used as an index for deciding photonic network by adaptive whether to select that format. Spectrum slots modulation were assigned to the transmission path between We evaluated the effectiveness of a transmit and receive nodes on the basis of the photonic network using adaptive modulation by frequency slots required for each modulation conducting a simulation experiment. format given its symbol rate (DP-QPSK: 1) Experimental confi guration and conditions 25 Gbaud; DP-16QAM: 12.5 Gbaud; DP-64QAM: For this simulation, we examined a multi- 8.3 Gbaud). A transmission path was assigned ring network consisting of ten nodes, as shown by an algorithm that fi rst determined whether in Figure 2 (a). Traffi c demand between nodes 64QAM—which has the highest spectral had a mesh pattern, and the distance between effi ciency—could be used to transmit signals adjacent nodes was defi ned in terms of link between transmit and receive nodes without a cost, as indicated in the fi gure. We assumed a re-shaping, re-timing, and re-amplifi cation (3R) fi xed bit rate of 100 Gb/s and used DP-QPSK, regenerative repeater.
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