Upgrading the Transmission Capacity

8/10/2019 Upgrading the Transmission Capacity

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Upgrading the Transmission Capacity of Local Area Networks byImproving the Receiver Performance

Daniel Naves de Luces Fortes*, Maria José Pontes, Maria Thereza Miranda Rocco Giraldi

Instituto Militar de Engenharia – Electrical Engineering Department SE/3, Praça General Tibúrcio, 80 – PraiaVermelha, Rio de Janeiro, RJ, Brazil, 22290-270

ABSTRACT

In this paper we analyze by simulation a measured amplified single channel 40 Gb/s communication system composed by standard fiber. Two different commercial simulation tools are used to perform the calculations. Experimental resultswere reproduced in such distinct simulation environments that allowed calibrating and predicting the system

performance when new parameter settings are taken into evaluation. The simulation results indicate that at least 10 dBless power is required to achieve the same BER simply using an optimized receiver. That result enabled to have the sameBER and also allowed removing the EDFA. Since one device such as an EDFA could be eliminated of the layout, withless optical power being required in the receiver, fiber spans as short as lengths that eliminate the dispersion

compensation have been considered.

Keywords: Optical network, high capacity system, local area network, reliability issues

1. INTRODUCTION

The existing voice and data traffic operating at higher bit rates in a broadband communication network are bringing adramatic growth to the traffic concentration nodes [1]. As a consequence, components such as hubs that concentrate mostof the traffic and are connected to other few core nodes are arriving closer and closer to the local customer accessnetwork [2]. In addition, the search for higher bandwidths is coming together with the expansion of the number ofchannels and bit rate as well [3]. It gives rise to WDM transmission systems that supports both long haul and

metropolitan network backbones [4]. As multi-channel 10 Gb/s solution is getting sufficiently mature to the current backbone demands over the last few years, a 40 Gb/s long haul WDM commercial link should also happen in a nearfuture [5].

A suitable immediate application for 40 Gb/s in this scenario is in local area networks very short reach spans, whichseems to be an excellent solution to core nodes interconnection, as soon as those nodes are supposed to be in a restrictedurban area within a few kilometers and require a high traffic load.

Among the main advantages of 40 Gb/s short links used in local area networks from 4 × 10 Gb/s or 16 × 2.5 Gb/s [6], isthe fact it is easier to manage a single high bit rate if compared to an n × lower bit rate link. An important issue regardsthe equipment size reduction and physical space requirements. As an example, if we consider a 40 Gb/s WDM local arealink implementation based on 2.5 Gb/s channels, it will be required a 16-channels mux/demux and 16 modulated opticalsources. Otherwise, 40 Gb/s single channel solution can be released with only one modulated optical source and no useof mux/demux. This leads to an equipment size reduction and consequently, less physical space requirement. It is still

possible to mention a higher transmission capacity can be achieved with less frequent upgrades.

In this paper it is analyzed by simulation a measured single channel 40 Gb/s communication system with short distancefiber spans, where no dispersion compensation techniques were applied.

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The first part of the present work consists in simulating a reported measured 40 Gb/s NRZ experimental setup. AnErbium Doped Fiber Amplifier (EDFA) operating as a booster is included in a 1.38 km and a 2.6 km G.652 (StandardFiber - STD) links [7].

Considering the simulation tools are an important resource on the system design, since they allow the designer to studymany scenarios in a cost efficient way, the simulations are performed using two commercial software: Transmission &

Component Maker from Virtual Photonics Inc. and OptiSystem 3.1 from Optiwave Corporation. The main point of thefirst part of such simulations is to adjust the calculated results to be as close as possible to the measured ones.Experimental results were reproduced in such distinct simulation environments that allowed calibrating and predictingthe system performance when new parameter settings are taken into evaluation.

Once the simulation parameters were properly adjusted, in the second part of this paper we propose a new systemconfiguration that includes a photodetector, which characteristics were selected mainly attracted by its superior reported

performance [8]-[9]. That result also allowed removing the EDFA. The simulation results indicate that at least 10 dB less power is required to achieve the same BER simply using an optimized photodetector.

The layout of the discussed optical systems as well as both simulators characteristics and main parameters will bedescribed in details in Section 2. In Section 3 the simulation results from both simulators will be compared to theexperimental results. And finally in Section 4 we have the conclusions.

2. SIMULATION SETUP

In a laboratory test environment, BER measuring procedure basically consists in inject a pre-defined bit sequence in thechannel input, receive the information at the channel output and compare both transmitted and received bits. It issupposed to be a continuous procedure, which may take hours or even days to be completed, depending on the bit rateand BER level to be achieved. The higher the bit rate and/or the lower BER target are, the slower will be the process.

BER value is the ratio between the bits presenting errors and the total number of transmitted bits. It is not possible toreproduce this procedure in the simulation domain, because it would require too much computer resources andsimulation time. So the two simulation tools used, calculates the BER by analyzing the noise behavior of a short bitsequence and assuming its distribution as a Gaussian function.

The simulation schemes that reproduce the experimental setup for the two simulators are represented in Fig.1 and Fig. 2,respectively.

The first layout has been configured in such a way that each element is set to behave as close as possible to thecomponents found at the experimental setup [7].

In both schemes, the transmitter is composed by a continuous wave laser operating at 1549.4 nm, which linewidth is setas 0.9 MHz. An external Mach-Zehnder modulator with extinction ratio of 12 dB and insertion loss equal to 9 dB wasmodulated by a PRBS bit sequence coded by a NRZ pulse coder that enabled a bit rate operation of 40 Gb/s.

In order to provide sufficient number of samples to the Gaussian BER estimation method utilized, the time acquisitionwas set to 0.2 µs, which results in 8-kbit length sequence.






Figure 1: First simulation layout that reproduces the experimental results used for both VPI and Optiwave simulators.

Figure 2: Second simulation layout that uses an optimized PIN at the receiver.

BER

CW

PRBS NRZ

PIN

LASER EXTERNAL

MODULATOR

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Figure 3: Simulated and measured results to the back-to-back configuration.

The ideal EDFA model has been used. Its gain was set to 27 dB and noise figure to 4.7 dB. A trapezoidal optical filteralso centered at 1549.4 nm with bandwidth of 0.8 nm is used to reduce EDFA amplified spontaneous emission (ASE)noise. Standard fiber chromatic dispersion and attenuation were respectively 17 ps/nm.km and 0.2 dB/km. A variableattenuator is used to sweep the received power, enabling to obtain the BER variation as a function of the optical received

power.

The receiver that can be dealt as a subsystem is composed by a PIN photodetector, with its parameters adjusted toachieve the specified performance. A low pass 4TH order Bessel electrical filter was used.

In the layout shown in Fig. 2, the basic elements are set up in a similar way than that presented in Fig.1, except for theEDFA and the optical filter that are no longer necessary. The parameter settings are exactly the same as previouslydescribed, except for the PIN receiver thermal noise. As the PIN photodetector is one of the most critical devices withina system, our proposal is to use a PIN with a better performance as reported in [8]-[9]. An essential parameter thatseverely affects the PIN photodetector performance is its thermal noise. Therefore, the thermal noise was the selected

parameter to be used in the performance analysis of the receiver and determine how it affects the links operating in localarea networks at 40Gb/s.

3. RESULTS AND DISCUSSION

Using the first layout and the experimental parameter values given by [7] as the component settings, the simulations performed with both simulators were compared to the experimental results. Figure 3 shows the back-to-back results forsimulated and experimental data. A good agreement between calculated and measured results can be observed in Fig.3.

The difference observed for BER smaller than 10−14 can be attributed to the probabilistic models and numericalapproximations used by each simulator.

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Experimental

VPI Simulation

OptiWave Simulation

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Received Optical Power (dBm)






Figure 4: Results obtained to 1.38 km of fiber length and 6 dBm of signal input power.

In order to evaluate the agreement between simulated and measured results for the links with short fiber length present inlocal area networks, systems containing 1.38 km and 2.6 km of standard fiber were considered.The results shown in Fig. 4 and Fig. 5 contain the simulated and experimental values to 1.38 km and 2.6 km,respectively. A signal power of +6 dBm was considered in these cases. A significant difference can be observed in thesimulated results obtained using the two different simulators. However, if a rigorous analysis on the fiber propagationmodel is done for both simulators, this difference can be easily explained. Even more, if the noise treatment used in bothsimulators is taken into consideration, a quite significant difference will be observed.

Figure 5: Results obtained to 2.6 km of fiber length and +6 dBm of signal input power.

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Figure 6: Simulated results obtained to both simulators for back-to-back configuration and considering the adjusted receiver.

Assuming there are different models being used in both simulators and considering the simulated and experimentalresults intersects over a small interval we will consider acceptable the agreement obtained between experimental andmeasured results. Perhaps, the simulations should be performed until find the parameter settings or combination ofelements that could result in a better agreement.

Taking the layout shown in Fig. 2, the simulations were performed with both simulators setting a different value to the

thermal noise of the PIN photodetector. The signal input power was fixed to +6 dBm and results that maximize thereceiver performance could be determined. The PIN detector performance can be compared with previous simulations,where adjusts on the thermal noise were not performed.

Figure 6 shows the simulated results obtained for both simulators to the back-to-back configuration using the high performance PIN photodetector. By analyzing the BER calculated to the back-to-back configuration for the receiver withand without adjusted thermal noise, i.e. Fig. 3 and Fig. 6 respectively, it can be noted the same BER can be reached withless input signal power at the receiver in the case of Fig. 6.

Considering a BER of 10−12, for instance, the adjusted layout requires around 10 dB less power to the receiver input. Inthat case, the EDFA and also the optical filter are no longer necessary in the setup. This is an additional advantage of theadjusted receiver subsystem.

Further simulations were also performed to different fiber lengths. Fig. 7 shows the results obtained to the adjustedreceiver and 1.38 km and 2.6 km of fiber length using both simulators.

It can be seen a good agreement between the results obtained with both simulators. The result for 4 km has beensimulated, as it is the calculated dispersion limit of this system [5], for both simulators a low BER was observed.

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Figure 7: Simulated results obtained to the adjusted receiver, 6 dBm of signal input power and fiber lengths of 1.38 km and 2.6 km.

The eye diagram is another figure of merit considered to evaluate the system performance. Figure 8 shows the result tothe 2.6 km fiber link. This result was obtained with simulation performed only in VPI software.

As can be seen in Fig. 8, multiple lines are overlapped what closes the eye and degrades the system performance. Eventhough the fiber length is being selected to prevent group velocity dispersion effects, the superposition in the eye diagrammay indicate the presence of nonlinear effects. It can also be observed, a power oscillation at the upper crossing levelthat should occur due to the photodetector thermal and shot noise. Both simulators account this power variation in BERcalculation algorithms.

As observed in further simulation results, the presence of a large power oscillation effect was obtained when EDFAs areincluded in the layout. In that case ASE noise was the main responsible for an additional noise contribution.

Figure 8: Eye diagram after 2.6 km of STD fiber for the optimized PIN receiver.

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4. CONCLUSIONS

In this paper we analyze by simulation a measured amplified single channel 40 Gb/s communication system composed by standard fiber used in local area networks. A set of measured results using NRZ modulation over one input signalchannel coupled to an amplified link was reproduced by simulation using two different commercial tools. Experimental

results were reproduced in such distinct simulation environments that allowed calibrating and predicting the system performance when new parameter settings are taken into evaluation.

The simulated results presented acceptable accordance with the measured ones. A receiver optimization, by using ahigher performance PIN, was evaluated, and simulation results indicated a 10 dB less optical received powerrequirements, with no optical amplification needed.

To fiber lengths longer than 4 km, the results for calculated fiber propagation indicate that dispersion compensation becomes necessary, since the bit rate of 40 Gb/s is being considered.

Since one device such as an EDFA could be eliminated of the layout, with less optical power being required in thereceiver, fiber spans as short as lengths that eliminate the dispersion compensation have been considered.

ACKNOWLEDGMENT

The authors would like to acknowledge the partial financial support provided by FAPERJ and GIGA Project.

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3. Y. Mochida, N. Yamaguchi, and G. Ishikawa, Technology-oriented review and vision of 40-Gb/s-based opticaltransport networks, IEEE J. Lightwave Technology, vol. 20, no. 12, pp. 2272-2281, 2002.

4. M. S. Borella, J.P. Jue, D. Banerjee, B. Ramamurthy, B. Mukherjee, Optical components for WDM lightwavenetworks, Proc. IEEE, vol. 85, no. 8, pp. 1274-1307, 1997.

5. R. DeSalvo, A.G. Wilson, J. Rollman, D.F. Schneider,L.M. Lunardi, S. Lumish, N. Agrawal, A.H. Steinbach, W.Baun, T. Wall, R. Ben-Michael, M.A. Itzler, A. Fejzuli, R.A. Chipman, G.T. Kiehne, K.M. Kissa, Advancedcomponents and sub-system solutions for 40 Gb/s transmission, IEEE J. Lightwave Technology, vol. 20, no. 12, pp.2154-2181, 2002.

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channel transmission of NRZ 40 Gb/s through G. 652, G. 653 and G. 655 fibers for short-reach applications ,SBMO/IEEE Int. Microwave Optoelectronics Conference - IMOC 2003, Post-Deadline paper, pp. PD47-PD52, Fozdo Iguaçu – PR, Brazil, September 2003.

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271-275, November 2002.9. X. Tian, A.P. Freundorfer, L. Roy, Noise analysis of a photoreceiver using a P-I-N and GaAs HBT distributed

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