Real-Time Implementation of Digital Signal Processing for Coherent Optical Digital Communication Systems
Post on 23-Sep-2016
Embed Size (px)
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010 1227
Real-Time Implementation of Digital SignalProcessing for Coherent Optical Digital
Communication SystemsAndreas Leven, Senior Member, IEEE, Noriaki Kaneda, Senior Member, IEEE, and Stephen Corteselli
AbstractDigital signal-processing-based coherent optical com-munication systems are widely viewed as the most promising next-generation long-haul transport systems. One of the biggest chal-lenges in building these systems is the implementation of signalprocessors that are able to deal with signaling rates of a few tensof giga-samples per second. In this paper, we discuss implementa-tion options and design considerations with respect to hardwarerealization and DSP implementation.
Index TermsDigital signal processors, optical fiber communi-cation, quadrature phase-shift keying.
COHERENT communication systems have dominated theworld of wireless communication almost since its begin-nings. Coherent systems, or more exactly phase-coherent sys-tems, offer a number of benefits over noncoherent systems .Nonetheless, practical optical coherent communication systemsbecame feasible only recently.
Coherent optical communication systems have been a mat-ter of intense research in the 1980s and early 1990s of pastcentury. At that time, main motivation was the higher sensitiv-ity coherent receivers promised. Technical difficulties inhibitedrapid transition into commercial systems. With the advent of theerbium-doped fiber amplifier, which offered comparable sensi-tivities with direct-detection systems, the main driving force fordeveloping optical coherent systems disappeared.
Todays motivation to revive coherent concepts in opticalcommunication is twofold. First, coherent receivers enable reli-able data transmission with much higher spectral efficiency thanconventional direct-detection systems, and second, coherent re-ceivers can compensate for linear impairments, most notably,polarization-mode dispersion (PMD) to a degree that is out ofreach for conventional systems.
Also, the technical difficulties that the first generation ofcoherent systems in optical communications faced have beenlessened. This is caused by two developments. First of all, the
Manuscript received January 15, 2010; revised February 12, 2010; acceptedFebruary 28, 2010. Date of publication May 18, 2010; date of current versionOctober 6, 2010.
A. Leven is with Alcatel-Lucent Bell Laboratories, 70435 Stuttgart,Germany (e-mail: email@example.com).
N. Kaneda and S. Corteselli are with Alcatel-Lucent Bell Laboratories,Murray Hill, NJ 07974 USA (e-mail: firstname.lastname@example.org;email@example.com).
Digital Object Identifier 10.1109/JSTQE.2010.2044977
symbol rate to carrier frequency ratio of modern optical com-munication systems approaches the ratio that is commonly usedin wireless systems. For a system that transmits at data rate of100 Gb/s in two polarization orientations utilizing QPSK sig-naling, the symbol rate is 25 GBd. With a carrier frequency ofroughly 200 THz, the symbol rate to carrier frequency ratio is1.25e-3. This indicates that it is possible for optical systems toachieve similar phase noise to symbol rate ratios, as in wirelesssystems.
Second, the performance of digital signal processing (DSP)equipment has been improved dramatically over the past twodecades, which makes it feasible to implement the complexsignal processing steps required to synchronize to the receivedsignal in digital domain. Implementations of optical coherentreceivers have been demonstrated in CMOS-based application-specific ICs (ASICs)  or field-programmable gate arrays(FPGA) , .
Albeit a coherent optical communication system can utilizesingle or multiple carrier [e.g., orthogonal frequency-divisionmultiplexing (OFDM)] transmitter and any modulation format,with QPSK being the most popular and higher order quadrature-amplitude modulation (QAM) and phase-shift keying (PSK)systems under investigation, this paper will concentrate onsingle-carrier frequency-domain-equalized systems, which hasbecome more popular in the wireless domain as well . Themodulation format discussed here will be QPSK. Phase co-herence between a data signal and the reference is typicallyestablished at the receiver side. As this paper is concerned withthe implementation of coherent systems, it will concentrate onreceiver design.
The paper is organized as follows. After a short review of thebasic architecture of optical coherent system, we will discusshardware implementation options. Then, we will discuss spe-cific challenges for the implementation of signal processing atmultiple gigabit per second signaling rates. In Section IV, ex-emplarily some of the algorithms and their implementation willbe described. Finally, some measurement results of a real-timecoherent receiver will be discussed.
II. OPTICAL COHERENT-SYSTEM ARCHITECTUREFig. 1 shows a generic block diagram of a coherent system.
The transmitter on the left side of the diagram consists of adata source, digital-to-analog converters (DACs), and driveramplifiers. Coherent systems often use polarization-division
1077-260X/$26.00 2010 IEEE
1228 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 5, SEPTEMBER/OCTOBER 2010
Fig. 1. Generic coherent system.
multiplex. In this case, the continuous wave signal of the trans-mit laser is split into two, and then, modulated independentlyby two optical in-phase/quadrature (I/Q) modulators. Thetwo signals are then combined in two orthogonal polarizationorientations.
In Fig. 1, the data source is simply displayed as a black box.In a real system, this black box comprises a number of com-plex functions. Besides the functions that are also performedin a classical system, such as data aggregation, coding, andframing, additional steps need to be performed in a transmit-ter for complex modulation formats, as it is typically used ina coherent system. First of all, the data have to be mapped toconstellation points and, in case of multiple carrier systems,to frequencies. Often, the data are also differentially precodedto cope with phase slips during receiver-side carrier synchro-nization. In a second step, the mapped data might be processeddigitally, for instance, it might be predistorted to compensatefor the nonlinearity of amplifiers and modulators, or it mightbe precompensated for deterministic fiber effects, for instance,chromatic dispersion (CD).
The processing of the earlier described data results in fourdigital data streams that subsequently need to be converted intoanalog data. In case of single-carrier QPSK signaling, each datastream carries only a single bit per symbol, and therefore, doesnot require a DAC. This reduces the complexity, and therefore,power consumption of the transmitter significantly. But evenfor multicarrier systems  or modulation formats with higherorder than QPSK, the performance requirements with respectto resolution and conversion speed are typically less restrictivefor the transmit DAC than for the receive AD converter (ADC).For instance, for a 16-QAM transmitter without preprocessing,only 2 bits (four levels) at a conversion speed equivalent to thesymbol rate are required, while at the receiver side, typically68 bits at a sampling rate of twice the data rate are needed.Technology and architecture choices are similar to the ones ofthe ADC, which will be discussed later.
The I/Q modulator most widely used today is a double-nestedMachZehnder modulator  based on LiNbO3 . However, othermore compact solutions are in development based, for instance,on electroabsorption modulator structures .
The receiver consists of a local oscillator (LO) laser, an opticalhybrid, a photoreceiver array, an ADC array, the digital signalprocessor (DSP), and a data sink, which typically comprises adecoder and a client interface.
The 90 optical hybrid mixes the received signal with thesignal of the LO laser and a 90 phase-shifted copy of the LO
laser signal . The mixing of the signal with the LO referenceis performed for each polarization separately. Preferably, theoutput of the hybrid provides differential signals for suppressionof the direct-detection terms.
Optical hybrids have been demonstrated in different designsand technology platforms. Design-wise, optical hybrids can begrouped into actively controlled devices that require a phasecontrol to maintain the 90 phase difference and passive de-vices that assure a 90 phase difference by design. Active de-vices typically consist of two splitters, one for the receivedsignal and one for the LO signal, and two signal combiners, onefor the in-phase component and one for the quadrature com-ponent . The phase of the signal in one arm of the LOsplitter needs to be adjusted by a tunable phase shifter to be90 out of phase with respect to the signal in the second arm.The phase shift can be controlled by utilizing thermal tuningor electrooptic tuning.
Passive hybrids are designed such that the signals alwaysinterfere with a phase difference of about 90. These can beimplemented, for instance, as a Michelson interferometer or amultimode interference coupler (MMI) . The advantage ofpassive hybrids is obvious; no control signal has to be generatedand distributed to the hybrid device. Nevertheless, the phaseaccuracy is often not sufficient so that a phase correction needsto be implemented in the digital signal processor.
After photodetection and linear amplification, the signalsneed to be converted from AD domain. The ADC performanceis still one of the bottlenecks that determine total data rate ofDSP-based optical coherent systems. ADCs with sampling rateof mor