Design and simulation of 1.28 Tbps dense wavelength division multiplex system suitable for long haul backbone
DDesign and simulation of 1.28 Tbps dense wavelength divisionmultiplex system suitable for long haul backbone
Akinwumi A. Amusan ∗ and Elizabeth A. Amusan ∗† Abstract
Wavelength division multiplex (WDM) system withon / off keying (OOK) modulation and direct detection(DD) is generally simple to implement, less expensiveand energy efficient. The determination of the possibledesign capacity limit, in terms of the bit rate-distanceproduct in WDM-OOK-DD systems is therefore crucial,considering transmitter / receiver simplicity, as well asenergy and cost efficiency. A 32-channel wavelength di-vision multiplex system is designed and simulated over1000 km fiber length using Optsim commercial simula-tion software. The standard channel spacing of 0.4 nmwas used in the C-band range from 1.5436-1.556 nm.Each channel used the simple non return to zero - on /off keying (NRZ-OOK) modulation format to modulatea continuous wave (CW) laser source at 40 Gbps us-ing an external modulator, while the receiver uses a DDscheme. It is proposed that the design will be suitablefor long haul mobile backbone in a national network,since up to 1.28 Tbps data rates can be transmitted over1000 km. A bit rate-length product of 1.28 Pbps.km wasobtained as the optimum capacity limit in 32 channeldispersion managed WDM-OOK-DD system.
Introduction
The need for fast and reliable exchange of infor-mation has increased further in our present society. Thereliable operation of industries, businesses and banks;vehicles and transportation systems; household enter-tainment electronics and the global flow of news andknowledge rely on advanced telecommunication infras-tructure. Numerous services and applications of infor-mation and communication technology in medical diag- ∗ Department of Electrical and Electronics Engineering, ElizadeUniversity, Ilara-mokin, Nigeria. [email protected],[email protected] † Department of Computer Science and Engineering, Ladoke Ak-intola University of Technology, Ogbomoso, Nigeria. [email protected] nosis and treatment, traffic safety and guidance, as wellas, the Internet of things are emerging, stretching theneeds for high-capacity communications even further[1]. By 2020, the mobile communication system willmove to 5G, in which the users data rate will be at least1 Gbps (Giga bits per second), with connection densityreaching 1 million connections per square km, end-to-end latency in milliseconds level, traffic volume densityof tens of Gbps per square km, mobility greater than500 km per hour and peak data rate in tens of Gbps [2].This implies that the backbone of the mobile and dataservice providers will require more fiber connections,since fiber optics has the potential to provide the hugebandwidth capacity required for communication in ourpresent society. In fiber optics systems, light pulses aresent through an optical fiber, and the signals are propa-gated by total internal reflection between a high refrac-tive index core and low refractive index cladding. Al-though, fiber systems can meet the huge bandwidth de-mand, yet the fiber bandwidth needs to be harnessed bymodulating into higher data rates before transmission.Dense-wavelength-division-multiplexing (DWDM) anderbium-doped fiber amplifier (EDFA) technologies havebeen used to increase the capacity by using simple on/ off keying (OOK) format and direct detection up to10 Gbps per channel [3]. In DWDM, several mod-ulated light carriers with wavelengths spaced at con-stant channel spacing (often less than 1 nm) are com-bined and transmitted over the same fiber cable. Multi-terabit capacities over a single fiber was achievable withuse of this DWDM technology and it is likely that thenext generation DWDM systems will require 40 Gbpsor higher channel bit rate [4, 5]. Furthermore, multi-level modulation formats with single or multicarrier, po-larization multiplexing, new detection schemes, novelfiber and Raman amplification technologies have beenextensively investigated for higher data rates transmis-sion up to 500 Gbps and 1 Tbps per channel [6]. Mul-tilevel modulation formats with coherent detection isspectrally efficient, however it requires more energy fortransmission than using simple modulation format withdirect detection. It was reported that transmission of 40 a r X i v : . [ ee ss . SP ] O c t bps by 5 channels, with non return to zero on / offkeying (NRZ-OOK) modulation was power efficient af-ter multiple fiber spans, as compared to dual polariza-tion quadrature phase shift keying (DP-QPSK) modula-tion, in which, more than 90 % of the total power con-sumption was due to re-amplification, re-shaping andre-timing for propagation distance more than 600 km[7]. The NRZ-OOK modulation format with direct de-tection is generally less expensive, because the trans-mitter and receiver configuration are simple to imple-ment and the obtainable data rate and transmission dis-tance may be sufficient in some propagation scenario,such as, for fiber connections from the mobile nationalcore network to external gateway, core network to basestation subsystems as well as within the base stationsubsystems. It is shown here in this work that it is pos-sible to transmit up to 1.28 Tbps (Tera bits per second)using NRZ-OOK and direct detection, with 40 Gbps oneach wavelength channel, for 32 channels, over 1000km fiber span, which can be cost effective design forlong haul mobile backbone in a national network. Related works
The modulation formats of carrier suppressedreturn-to-zero (CSRZ), duo binary return-to-zero(DRZ), and modified duo binary return-to-zero(MDRZ) were analyzed using pre, post and symmet-rical dispersion compensation schemes with respectto Q - value and eye opening penalty for differenttransmission distances and signal input powers varyingfrom -15 to10 dBm [8]. It was concluded that faithfultransmission of 32 channels by 40 Gbps (1.28 Tbps)over 1450 km was possible using MDRZ modulationformat and symmetrical dispersion compensationscheme. However, MDRZ modulation format usuallyhas a complex transmitter and receiver configuration[8]. The performance analysis of DWDM System fordifferent modulation schemes with varying channelspacing was reported in [9]. For 40 Gbps by 16 DWDMchannels or 40 Gbps by 32 DWDM channels, it wasreported that CSRZ scheme with 100 GHz channelspacing could reach transmission coverage up to 4,000km for 16 and 32 channels DWDM system, because itis highly tolerant to nonlinear effects. MDRZ schemewith 75 GHz channel spacing exhibits transmissioncoverage up to 4,000 km and 4,500 km for 16 and 32channels respectively, but with degraded signal due toeffect of inter-symbol interference (ISI) [9]. However,many of such advanced modulation scheme is complexto implement at the transmitter and receiver side[8]. NRZ-OOK scheme seems to be the most simpleto implement. The analysis and compensation of polarization mode dispersion (PMD) in single channeland 32 - channel DWDM system was reported in[10]. The simulation was carried out for 4-channel (40Gbps), 8-channel (80 Gbps), 16-channel (160 Gbps)WDM systems and 32-channel (320 Gbps) DWDMfiber optic system with each channel having the datarate of 10 Gbps. The work focused on mitigationof polarization mode dispersion by deterministicdifferential group delay (DDGD) method for singlechannel and polarization maintaining fiber (PM) fibermethod for multichannel compensation. The DWDMsystem with capacity up to 10 Gbps by 32 channels,10 Gbps by 16 channels and 100 Gbps by 16 channelswere simulated only over 100 km fiber span [10]. Thesimulation of DWDM systems with total capacity upto 1.28 Tbps over five spans of 50 km fiber (250 kmlength) and spectral efficiency approaching 0.4 bps/Hzwas reported [11]. The impact of signal-to-noise ratioon parameters such as channel spacing, fiber length,dispersion, and number of channels were investigated[11]. The signal to noise ratio improved as the channelspacing was increased. However, in addition to fiberspan increase, there is need to improve the spectraefficiency, and as such the channel spacing needs to beoptimal. Hoshida et al. [3] presented a performancecomparison of modulation formats of non return-to-zero (NRZ), CSRZ, and bit-synchronous intensitymodulated differential phase shift keying (IM-DPSK)format in 75-GHz (0.6 nm) spaced WDM long-haultransmission systems. It was concluded that NRZformat is good for shorter transmissions distances upto 1000 km, and therefore is attractive with its virtuessuch as simple and low-cost transmitter and receiverconfiguration with small dependence on fiber type interms of nonlinear tolerance. CS-RZ format is lessattractive in highly spectral efficient systems withless than 75-GHz (0.6 nm) spacing. IM-DPSK wasconcluded to be the best choice among the three fortransmission distance beyond 1000 km [3]. Here inthis work, a 40 Gbps by 32 channel DWDM system isdesigned and simulated using channel spacing of 0.4nm (50 GHz) corresponding to a spectra efficiency of0.8 bps/Hz. The NRZ - OOK simple modulation formatis used to achieve a faithful transmission distancereaching 1000 km.
Methodology
RSoft OptSim TM software from Synopsys Inc isused for the system simulation. The software solvesthe nonlinear Schrdinger equation using a time domainsplit-step algorithm [7]. The software contains mod-els for different optical devices and the analyzing tools, ultiplexer
39 km SMF DCF
EDFA SMF DCFDCMDCMDCM
From eachTransmitter channels
Channel 1 Channel 2 Channel 32 17.9 km demultiplexer To eachreceiver channels
Channel 1 Channel 2 Channel 32
EDFA
Figure 1: Illustration of the simulated DWDM systemsuch as the laser, external modulator, multiplexer / de-multiplexer, single mode fiber (SMF), dispersion com-pensation fiber (DCF), dispersion compensated mod-ule (DCM), erbium doped fiber amplifier (EDFA), opti-cal filters, optical receiver (photodetector), the spectrumanalyzer, power meter, bit error rate tester, eye diagramanalyzer, and property map. The software accuracy de-pends on the number of the simulated bits, in which, theQ-factor uncertainty (error) is usually less than 0.28 dBfor at least 8000 bits [7]. At first, the transmission limitfor single channel 40 Gbps data rate in a single modefiber with fiber loss of 0.2 dB/km is obtained by settingthe dispersion and non-linearity parameters to be zeroand obtaining a faithful transmission with appreciableeye opening and bit error rate less than 10 − . This trans-mission limit was determined to be about 55 km, whichcorresponds to the amplifier spacing of the system. Thedispersion limit of the single mode fiber was obtained tobe about 8 km, which was determined by setting the dis-persion parameter to regular value for single mode fiber(18 ps / nm-km). Material dispersion is a property ofthe fiber in which the different wavelength componentspropagates at different speed, which could lead to in-tersymbol interference. Dispersion compensation fiberis therefore necessary in the system for long haul trans-mission, since the system is greatly dispersion limited.Corning SMF 1550 is used as the single mode fiber,while Corning Vascade S 1000 is used as the dispersioncompensating fiber. The required amount of dispersioncompensation is determined by the following equation[12]: ( D ( λ SMF ) × L SMF ) + ( D ( λ DCF ) × L DCF ) = λ SMF ) and D( λ DCF ) are the dispersion pa-rameters in ps / (nm-km) for single mode fiber (SMF)and dispersion compensation fiber (DCF) respectively,while L
SMF and L
DCF are lengths of the SMF and DCFrespectively. The SMF has a positive dispersion param-eter (+18 ps / (nm-km) for Corning SMF 1550), which is compensated by DCF with large negative dispersionparameter (-38 ps / (nm-km) for Corning Vascade S1000).The simulation is then extended to a DWDM sys-tem consisting of 32 wavelength channels in which eachwavelength is modulated at a standard data rate of 40Gbps. The wavelength range is chosen from 1.5436 to1.556 nm which falls within the C band range, whilethe channel spacing was chosen to be 0.4 nm. Thewavelength spacing corresponds to a frequency spac-ing of 50 GHz which gives a spectra efficiency of 0.8bps/Hz for 40 Gbps. The simulated system is shownin Figure 1. This starts from the transmitters for eachchannels, to multiplexer, single mode fiber (SMF), dis-persion compensation fiber (DCF), erbium doped fiberamplifier (EDFA), demultiplexer and then to optical re-ceivers for each channels. Detailed description of eachchannel is shown in Figure 2.Figure 2: Single channel transmitterA pseudo random bit sequence (PBRS) at data rateof 40 Gbps is sent to the electrical generator. The outputof the electrical generator is subsequently used to mod-ulate the continuous wave laser using the external mod-ulator. A predispersion compensation module (DCM)is used to precompensate for material dispersion thatwill occur in the fiber and this gives a modulated op-tical output for each channel. This output is then sent tohe multiplexer. The multiplexer (Figure 3) combinesthe modulated output from each channel (32 channels)to obtain a cumulative data rate of 1.28 Tbps and theoutput is transmitted over the fiber.Figure 3: Transmission link showing SMF, DCF andEDFA loop is used to combine the SMF, inline DCFand EDFA until the transmission distance of 1000 kmis reached. A total of 18 loops of SMF, DCF and EDFAare required to reach transmission distance of 1000 km.The receiver section is shown in Figure 4. The outputfrom the fiber is connected to a demultiplexer at the re-ceiving end. This separates the multiplexed wavelengthchannels into individual channels and the optical signalsare directly detected using photo detectors.Figure 4: Receiver section showing the multiplexersand the photodetectors
Results and Discussions
Careful dispersion compensation was done to ob-tain appreciable eye-opening and bit error rate less than10 − in all the channels for 1000 km transmission dis-tance. A pre-dispersion compensation module (DCM)was also used in all the channels to compensate forthe final dispersion measured with the dispersion mapat the receiver end. The dispersion for SMF length of39 km was therefore balanced with DCF length of 17.9 km resulting to total fiber lengths of 56.9 km (amplifierspacing). The measured dispersion with respect to fiberlengths is shown in Figure 5.Figure 5: Measured dispersion with respect to fiber dis-tanceThe dispersion varied from a large positive value(due to SMF dispersion) and decreased linearly as a re-sult of compensation with DCF. The final dispersion at1000 km is shown in Figure 6.Figure 6: Final measured dispersion at 1000 km dis-tanceIn order to obtain an appreciable eye opening andbit error rate less than 10 − in all channels, the finaldispersion varies between -4 to 0.6 ps-nm. The sys-tem could tolerate more negative dispersion than posi-tive dispersion. In addition, the received spectrum is areplica of the transmitted spectrum (as shown in Figures7a and 7b).This is an indication of low bit error rates in allthe channels, if the received spectrum is a replica of a) Transmitted spectrum(b) Received spectrum Figure 7: Transmitted and the received spectra at 1000km for the 32 channelstransmitted spectrum, since inter channel cross talk isavoided after careful dispersion compensation. The re-ceived bit error rates (BER) in all channels as a functionof the laser power (transmitter) is shown in Figure 8.It can be observed that the BER decreased as thelaser power is increased up to an optimal power level,where the BER becomes independent of the laser powerincrease. Transmitting too high power is usually detri-mental to the system since this can induce noise in thereceived spectra and inter channel cross talk due to fibernon - linearity [13]. The transmitted optical power as afunction of fiber length is shown in Figure 9.A -12 dBm laser transmitter power was found to beoptimum in all channels to obtain a BER less than 10 − in the channels. A decrease in the optical power alongthe line was due to fiber loss, in which erbium dopedfiber amplifier (EDFA) is used to periodically amplifythe signal at EDFA spacing of approximately 57 km. Figure 8: Received BER as a function of laser transmit-ter powerFigure 9: Optical transmitter power as a function offiber lengthThe received BER as a function of EDFA gain is shownin Figure 10.The optimum EDFA gain was -16dB. High BERwas obtained for over 18 dB EDFA gain due to stimula-tion of fiber non-linearity from high optical power. An-other critical criterion to obtain a faithful transmissionin all the channels is the choice of WDM demultiplexerfilter parameters, such as the demultiplexer filter band-width and the demultiplexer filter spacing. Tight opticalfiltering is necessary at the transmitter and the receiverside, in order to limit inter-channel interference [14].The demultiplexer filter spacing separates the mul-tiplexed signal into individual wavelength channels,while the demultiplexer filter bandwidth (full width halfmaximum bandwidth) selects the modulated data alongwith each wavelength channel. The BER as a functionof demultiplexer filter spacing and filter bandwidth areigure 10: Received BER as a function of EDFA gainFigure 11: Received BER as a function of demultiplexerspacingshown in Figures 11 and 12 respectively.Figure 12: Received BER as a function of demultiplexerbandwidth The demultiplexer filter spacing is chosen to beequal to the channel spacing used for the DWDM.Choosing too large filter spacing will lead to inter-bandcross talk in some channels and too low spacing willresult to insufficient filtering (cutoff) in some channels.It can be observed that the BER was below 10 − forthe entire wavelengths at 0.4 nm filter spacing whichcorresponds to the given DWDM channel spacing. Inaddition, all the wavelength channels gave BER below10 − at 0.3 nm filter bandwidth. The eye diagramopenings were identical in all the channels and exam-ples of the eye diagram opening for channel 1 and chan-nel 32 are shown in Figures 13a and 13b. (a) Channel 1(b) Channel 32 Figure 13: Optimized eye diagram for channel 1 andchannel 32
Conclusion