1. INTRODUCTION:

Fiber optic communication is an intensely high-speed growing technology, motivated by the increasing need for global expansion of the internet. The vast bandwidth potential of optical fibers leads to the accommodation of the escalating amount of network traffic. Thus in near future, the efficient use of bandwidth is of ultimate importance. The WDM technique is the most promising solution for the successful utilization of the optical spectrum and the efficient construction of the coming generation of Broadband Optical Networks The onset of WDM technology has a significant influence on the advancement of high transmission networks [1, 2].

However WDM optical networks experience serious physical impairments during transmission which consist of linear as well as non linear impairments, the most essential issue being the inter-channel crosstalk. Nonlinear crosstalk leads to exchange of power among the channels due to fiber nonlinearities and degrades the system performance. Cross Phase Modulation (XPM), Four Wave Mixing (FWM) etc are the examples of such type of crosstalk. At the same time, now-a-days certain nonlinear effects are being exploited for various practical and useful purposes. Researchers have demonstrated all-optical half-adder and half-subtracter based on exploitation of non-linear effects such as cross gain modulation (XGM) & FWM [3, 4, 5, 6]. Also certain nonlinear devices like Ultra-fast Non-Linear Interferometer (UNI) and Highly Nonlinear Fiber (HNLF) have been reported for the implementation of several All-Optical digital logic gates and modules [7, 8]. Crosstalk can occur in a perfectly linear channel too, due to the imperfect nature of various WDM components like de-multiplexers and optical filters [9, 10].

For data rates greater than 10Gbps, the lethal effects of nonlinearity and even dispersion must be dealt with in order to achieve efficient transmission in WDM system over any appreciable link length. Management of dispersion, using alternating fiber segments having opposite dispersion values, keeps the overall added dispersion near to the ground and suppresses most nonlinear effects. The systems that are dispersion-managed, exploit single-mode fiber (SMF) and dispersion compensating fiber (DCF). The positive dispersion of SMF can be recompensed by the large negative dispersion of DCF [11-13]. Anu Sheetal, et al [11] have simulated 16 channel 40Gb/s DWDM system with 25GHz channel spacing over a transmission distance of 2000km using CSRZ, DRZ and MDRZ modulation formats. For this, they have analyzed the performance of the system for pre, post and symmetrical dispersion compensation schemes using DCF by varying the signal input power.

A single-line-rate (SLR) system operates on a single bit rate. Growing traffic diversity and the rising need for bandwidth may set up light paths on the networks with multiple line rates called mixed-line-rate (MLR) optical networks. MLR networks are more gainful and cost-effective than SLR networks [14]. In this modern world, the concept of MLR in optical WDM networks is widely used in both the research community and the industry. Fig. 1 shows an MLR network sustaining 10, 40, and 100Gbps bit rate within the same fiber using different wavelengths. In MLR system, additional high-speed channels are allocated next to the existing legacy channels thus it is cost-efficient. MLR system is power-efficient as limited power consumption takes place due to the deployment of transponders using well-organized modulation formats [15].

Fig. 1 Mixed Line Rate optical network [15]

Iyer Sridhar, et al [15], compared power efficiency of various MLR and SLR solutions and also investigated the trade-off that exists between spectral and power efficiency in a WDM network. Results showed that for high transmission capacities, a combination of 100Gbps transponders and 40Gbps regenerators obtained the highest power efficiency. Nag. A, et al [16] examined the trade-off between modulation format, bit rate and cost of a transponder with respect to its transmission reach. The results revealed that there is a trade-off between cost of a transceiver and its optical reach. Batayneh. M, et al [17] studied the effect of transmission reach for mixed-line-rate signals (10/40/100Gb/s) on the cost of the optical network so as to determine the most favorable transmission reach of each bit rate. The results illustrated the network’s cost can be minimized by using short transmission reach, and that the best possible transmission reach relies on traffic features and on the transmission reach values of different bit rate signals. Chowdhury. P, et al [18] carried out a relative study of MLR and SLR energy efficiencies and investigated that MLR networks reduce consumption of energy. Cavdar. C, et al [19] put forth a design strategy of optical network with guaranteed signal quality. A mathematical model has been formulated that includes both linear and nonlinear Physical Layer Impairments together with the aim of energy efficiency. A mixed integer linear programming (MILP) model where energy efficiency and quality of transmission are considered is prepared. The energy and cost efficient MLR network upgrade problem is examined by Nag. A, et al [20], and it is seen that the amount of disruptions and interference affects energy-efficient and cost efficient upgrade in MLR networks. However, the study we performed has not been carried out in the literature so far. We investigate the transmission reach capacity of different advanced modulation formats. We also examine the QoT at different bit rates, channel spacing for different power levels over varying link lengths taking DCF into account. In our study, the investigations are performed in both MLR and SLR optical networks.

The article is organized as follows: Section 1 includes introduction part. The new term MLR provides us large number of benefits as compared to traditional communication systems. It also gives idea about crosstalk due to nonlinearities, and the use of DCF. A brief literature survey is also included in this part. Section 2 illustrates WDM link model used in the simulation. Section 3 covers the simulation results and discussions. Section 4 concentrates on the conclusion inferred from this work and future recommendations.

2. OPTICAL LINK MODEL AND SIMULATION SETUP:

This part includes the link model of the WDM system design used, the various components used along with their respective parameters, and the simulation arrangement. The WDM link architecture considered in the study is used for both SLR and MLR systems and is shown in Fig.2 below.

Fig. 2 WDM optical network model [15]

It corresponds to an 8-channel system consisting of 8 transmitters (TX), an 8×1 multiplexer (MUX), an optical pre-amplifier, a single-mode fiber (SMF), a DCF, a 1×8 splitter, 8 band pass filters, the bandwidths of whose can be changed accordingly to suit the respective transmitters and 8 optical receivers (RX) to receive the corresponding signals for analysis accordingly. The length of the DCF is decided according to that of SMF by considering dispersion coefficient as under [21]:

Table 1 System parameters used in the study for both MLR and SLR.

SMF (km)	DCF (km)	Bit rate (Gbps)	Channel spacing (nm)	Operating wavelength for 8 transmitters (1-8) (nm)
10	2	10	1nm	1546-1553
30	6	20	1.5nm	1544-1554.5
50	10	40	2nm	1542-1556

Where, D_SMF= Dispersion Coefficient of Single Mode Fiber, L_SMF = Length of Single Mode Fiber, D_DCF = Dispersion Coefficient of Dispersion Compensating Fiber and L_DCF= Length of Dispersion Compensating Fiber [21]. The system parameters used in this study are presented in Table 1. Table 1 shows the various link lengths, bit rates and the channel spacing we use in our

simulation. Our simulations consider the effect of both linear and nonlinear PLIs, QoT and transmission reach of advanced modulation formats. This is the novelty of our work. Optical amplifier leads to amplified spontaneous emission (ASE) noise which is a major cause of linear crosstalk and its effect is considered as in [15, 21]. We consider FWM effect as the major nonlinear effect for 10Gbps SLR [15, 22].

Fiber loss and dispersion are the two limiting factors in the design of WDM systems, which are comparatively well understood, and can be overcome by optical amplifiers and dispersion compensation techniques respectively, like we have used DCF in this paper. The effect of fiber nonlinearities, on contrary, have not been examined and analyzed properly in spite of good literature collection involving extensive studies dealing with the nonlinear effects on system performance [23-30]. Also, in the examinations of nonlinear effects on WDM systems, most works focus on the individual effects of nonlinearities and ASE noise on various system parameters. Today’s transmission system of high speed WDM networks requires a comprehensive considerateness of the combined nonlinear effects. Also the interaction of nonlinear effects with the added ASE noise and their impact on Q-factor is studied [22]. However, when higher bit rate(s) (i.e., 40Gbps or more) are introduced, the major nonlinear crosstalk is due to nonlinear phase noise, consisting of self-phase modulation (SPM) and cross phase modulation (XPM). Among the above mentioned nonlinear effects, the intensity-controlled signal (OOK signal) transmission and quality of transmission (QoT) at the receiver is adversely affected by XPM-related amplitude noise [15, 31]. The BER estimation model for OOK modulation formats is explained as follows:

In the absence of inter-symbol interference (ISI), bit error rate (BER) of a signal for OOK systems is related to the Q-factor as [32]

BER =

The Q parameter, under the assumption of Gaussian distribution for the intensity of PLIs, is given as [33]

Where, M is the receiver sensitivity and is given as M = 2B₀ T; B₀ is the optical filter bandwidth; and T is the symbol time [32].

For the various operations to perform, we use six different modulation formats as under [35]:

1) Return-to-Zero ON-OFF Keying (RZ OOK) Format

2) Carrier Suppressed Return-to-Zero (CS RZ) Format

3) Carrier Suppressed Non-Return-to-Zero (CS NRZ) Format

4) Duo-Binary Non-return-to-Zero (DB NRZ) Format

5) Modified Duo-Binary Non-Return-to-Zero (MDB NRZ) Format

6) Modified Duo-Binary Return-to-Zero (MDB RZ) Format

In order to look into the channel spacing issues in various SLR designs, the WDM system is arranged to consist of all 10Gbps, all 20Gbps, or all 40Gbps modulation formats. The various cases performed in our study for SLR are illustrated in Table 3 below. For examining the channel spacing issues in the case of MLR system, we considered various cases like 10Gbps RZ-OOK, 20Gbps CS RZ, 40Gbps DB NRZ, 10Gbps CS NRZ, 20Gbps MDB NRZ, 40Gbps MDB RZ and so on. We operated at 1nm, 1.5nm and 2nm channel spacing, starting from 1546nm to1553nm, 1544nm to 1554.5 and 1542 to 1556 frequency for 1-8 transmitters respectively (Table 1). Wavelength of the central channel value is set to 1550nm. At the receiving side, for evaluating the QoT, wavelengths are arranged such that the QoT of channel numbers 2–7 (i.e., λ₂, λ₃, λ₄, λ₅, λ₆, and λ₇) is affected by both linear and nonlinear crosstalk from the neighboring channels. This means that, channel 2 (λ₂) is affected by crosstalk due to the signal on channel 1 (λ₁) and on channel 3 (λ₃). Similarly, channel 7 (λ₇) is impacted by crosstalk due to signal on channel six (λ₆) and on channel 8 (λ₈). In a symmetric manner, all other channels are affected. Further, the values of these channels are such that they allow QoT detection with a fixed quality. We use MATLAB and a software called opti-system, version 7.0 that provided us with the look-alike environment for physical realization of a WDM optical network. We measure the QoT of the received signal using the measured value of Q-factor. The transmitters are selected on the basis of the modulation format and bit rate required for the various SLR and MLR designs. The OptSim transmitter and receiver block diagrams for RZ-OOK, CS RZ, CS NRZ, and DB NRZ, MDB NRZ, and MDB RZ are given as in [34]. The transmitter is based on the use of an electro-optical modulator and is driven by rectangular RZ signals. No optical filters are used on the transmitter side. As a booster, a pre-amplifier is used before SMF with a fixed output power level of 10dBm. For transmission purpose, we use SMF as a medium, and for the compensation of dispersion, a DCF is used. The value of DCF is set according to the aforementioned formula for the calculation purpose. The SMF parameters used are given in Table 2 below [15, 22, 31, 32].

Table 2 SMF parameters used in the study.

Parameters of SMF used	Value
Dispersion	16.75ps/nm/km
Attenuation	0.2db/km
Core area	50 μm²
Nonlinear refractive index	2.7×10⁻²⁰ m²/W

Eight Gaussian optical filters of the same type with suitable bandwidth according to their respective transmitters are used at the receiver after power splitter for various modulation formats before detection. At the receiver side, detection is carried out using eight optical receivers connected with individual BER analyzers. In our simulations, the required BER value is set to 10⁻¹², i.e., a Q-factor value of 7 or 16.9 dB on the electrical scale has been deemed as required value.

In our simulation, we use a 1×8 power splitter, eight band pass filters with suitable bandwidths and eight optical receivers connected to the BER analyzers. The main components included at the transmitting side for each transmitter are: drivers, DSP, an optical modulator and laser (On–Off); and at each receiver for optical to electrical conversion we have: an optical receiver after optical Gaussian filter and BER analyzer.

3. RESULTS AND DISCUSSIONS:

In this part we discuss the simulations and various results obtained through them.

3.1 SLR Analysis:

We first performed the analysis of SLR and for that we used eight modulation formats as: two RZ OOK, CS RZ, CS NRZ, DB NRZ, two MDB NRZ and MDB RZ. We performed various cases as mentioned in Table 1 and Table 3 using 0dBm (20dBm after pre-amplification) and 5dBm (25dBm after pre-amplification) power levels at laser diode individually. The SLR results are shown in Fig. 3 below.

Table 3 Various cases performed in our simulation for SLR at two different power levels over three different link lengths.

Power/bit rate	10km SMF	10km SMF	10km SMF	30km SMF	30km SMF	30km SMF	50km SMF	50km SMF	50km SMF
0dBm	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps
0dBm	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps
0dBm	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps
5dBm	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps	1nm 10Gbps	1nm 20Gbps	1nm 40Gbps
5dBm	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps	1.5nm 10Gbps	1.5nm 20Gbps	1.5nm 40Gbps
5dBm	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps	2nm 10Gbps	2nm 20Gbps	2nm 40Gbps

Fig.3 shows SLR results at 0 and 5dBm. In Fig.3 (a), the Q-factor is highest at 2nm 0dBm 10Gbps 10km for CS NRZ format and is lowest at 1nm 0dBm, 40Gbps 10km for again CS NRZ. At 2nm 40Gbps the Q-factor is highest for CS RZ and MDB RZ formats. Thus it can be seen that among eight modulation formats used, at 40Gbps, the crosstalk affects CS RZ and MDB RZ the least over a 10km SMF. However over a 30km SMF, the Q-factor is highest in case of RZ OOK at 1542nm, 2nm spacing, 10Gbps, 30km SMF as in Fig.3 (a). For 40Gbps, the Q-factor is maximum in case of MDB RZ and thus is least affected by distortion among all other modulation formats. It is seen that the Q-factor is maximum for RZ OOK at 1544nm over a 50km SMF at 10Gbps bit rate. For 1nm and 2nm channel spacing, at 40Gbps the MDB NRZ at 1547 and 1552nm respectively survives the best compared to other modulation formats.

From Fig. 3 (b), at power level 5dBm, it can be seen that the highest quality factor is that of MDB RZ at 1556nm, 20Gbps over a 10km SMF. The signal having lowest Q-factor is CS NRZ at 40Gbps if spacing between the channels is 1nm. CS RZ shows equal Q-factor at 1.5nm and 2nm at 10Gbps bit rate over a 10km SMF. In Fig.3 (b), MDB RZ at 20Gbps survives the best if the channel spacing is kept to be 1.5nm over a 30km SMF. The signals which are affected by crosstalk the most are CS NRZ and RZ OOK even at 2nm spacing at 20Gbps bit rate. In Fig.3 (b), CS RZ shows a good quality of transmission followed by MDB RZ. CS RZ is adversely affected by distortion, thus showing low Q-factor.

Thus different modulation formats show different Q-factor and hence the QoT. The eye diagrams for 1nm channel spacing at 10Gbps 50km, 40Gbps 10km and 40Gbps 50km SMF for various modulation formats are shown below in Fig.4 and Fig.5 respectively. CS NRZ and MDB NRZ 2 show clear eye diagrams compared to others for 1nm spacing (Fig.4). Also, from Fig.4 and Fig.5, it is seen that a bit rate of 10Gbps shows a clear eye pattern and hence has a good QoT even over longer distance of 50km. A 40Gbps system survives best at short distances of up to10km rather than a longer distance of 50km. Thus, for efficient transmission we can use lower bit rates for longer distances and higher bit rates for shorter distances.

Fig. 3 SLR analysis at10, 30 and 50km SMF using (a) 0dBm (b) 5dBm power levels for 1, 1.5 and 2nm channel spacing individually plotting 8 modulation formats along X-axis and Q-factor along Y-axis.

Fig.4 Eye diagram analysis for SLR with 1nm channel spacing, 0dBm power, 10Gbps bit rate over 50km SMF for RZ OOK 1, RZ OOK 2, CS RZ, CS NRZ, DB NRZ, MDB NRZ 1, MDB NRZ 2 and MDB RZ modulation formats.

Fig. 5 Eye diagram analysis for SLR with 1nm channel spacing, 0dBm, 40Gbps bit rate over 10km SMF for modulation formats, RZ OOK, CS RZ, CS NRZ, DB NRZ, MDB NRZ and MDB RZ.

At the end results show that at less power level, signals with 10Gbps bit rate show maximum transmission reach even over longer link lengths at large channel spacing.

3.2 MLR Analysis

After the analysis of SLR, MLR analysis is carried out for same eight modulation formats. We performed various cases, using power 0dBm (20dBm after pre-amplification) and 5dBm (25dBm after pre-amplification) at 1nm, 1.5nm, 2nm channel spacing over 10, 30 and 40 km SMF:

Case 1:- first four transmitters at 10Gbps, next four at 20Gbps

Case 2:- first four transmitters at 20Gbps, next four at 10Gbps

Case 3:- at bit rate of 10, 20Gbps following in succession

Case 4:- at bit rate of 20, 10Gbps following in succession

Case 5:- at bit rate of 10, 20, 40Gbps following in succession

Case 6:- at bit rate of 10, 40, 20Gbps following in succession

These cases lead to the following results:

Case 1:

From Fig.7 (a, b, c), it is seen that RZ OOK and MDB NRZ have the highest

Q-factor at frequencies 1547nm and 1551nm having bit rates equal to 10Gbps and 20Gbps respectively.

Case 2:

For case 2, CS RZ survives best at 20Gbps.

Case 3:

and for case 3, MDB RZ survives best at 20Gbps

Case 4:

For case 4, CS RZ shows the best QoT at 20Gbps.

Cases 5 and 6:

the QoT gets worse at 40Gbps due to the crosstalk. In Fig 7 (d), at 5dBm lasing power along a link length of 50km, the Q-factor of CS RZ at 10Gbps in case 3 is highest.

This means that when power level is increased to 5dBm, the CS RZ signal shows high QoT than at 0dBm transmitting power. At 5dBm power, CS NRZ, DB NRZ and MDB NRZ show worst response at the optical receiver according to their Q-factor shown in Fig.7 (d, e, f). Thus these modulation formats are most affected by the crosstalk, which degrades their QoT.

Fig. 6 Eye diagram analyzing MLR at 2nm channel spacing over 50km SMF for case 1, using modulation formats, RZ OOK 1, RZ OOK 2, CS RZ, CS NRZ, DB NRZ, MDB NRZ 1, MDB NRZ 2 and MDB RZ.

Fig. 6 shows the Eye diagrams analyzing MLR, case 1 for various modulation formats at 2nm channel spacing over 50km SMF. Eye patterns are clear at 10Gbps bit rate than at 20Gbps. This shows that at 10Gbps, MLR system survives the best and has better transmission reach.

Fig. 7 Mixed line rate analysis with 1nm, 1.5nm and 2nm channel spacing, using 0 and 5dBm power levels over 50km SMF by plotting 8 modulation formats along X-axis and Q-factor along Y-axis.

4. COMPARISON OF MLR AND SLR:

MLR networks are more gainful and cost-effective than SLR networks as mentioned earlier. MLR network sustains 10, 40, and 100Gbps bit rate within the same fiber using different wavelengths. MLR system is power-efficient too as mentioned before. If we compare SLR with MLR, the performance of an MLR design, in terms of the Q-factor, is better than that of the 10Gbps SLR but cannot secure better transmission efficiency performance, compared to 20Gbps and 40Gbps SLR. Fig.8 shows the comparison between SLR and MLR optical networks along 50km SMF for cases 1, 2, 3, 4, 5 and 6. Q-factor is worst if we consider 40Gbps bit rate.

Fig. 8 SLR-MLR comparison (a) SLR at 10, 20Gbps and MLR at cases 1, 2 at 1nm (b) SLR at 10, 20Gbps and MLR at cases 3, 4 at 1nm (c) SLR at 10, 20, 40Gbps and MLR at cases 5, 6 at 1nm (d) SLR at 10, 20, 40Gbps and MLR at cases 5,6 at 2nm spacing, over 50km fiber.

5. CONCLUSION:

In our work, we have dealt with the Q-factor and wavelength assignment for various SLR and MLR designs. For high capacities, we find that a combination of 10, 20 and 40Gbps will obtain the highest Q-factor. With regard to the link length and Q-factor, performance of an MLR system is always better than the performance of a 10Gbps SLR system; and lower compared to a 20 and 40Gbps SLR system.

The results demonstrate that at less power level, signals with 10Gbps bit rate show maximum transmission reach even over longer link lengths at large channel spacing. However if power level is increased, Q-factor and hence transmission reach is increased at low channel spacing and high bit rate which is the requirement of WDM systems today except for the increased power level. Thus, there exists a tradeoff between channel spacing, bit rate and power level used. Also certain modulation formats do not follow the aforementioned. Thus, it is our main effort to study various modulation formats and use them accordingly as we have attempted. Also QoT can be improved either by using shorter links with bit rate of up to 40Gbps or longer links with bit rate of 10Gbps and by increasing the channel spacing in both SLR and MLR systems. Finally, the results indicate that focusing on Q-factor or QoT alone may result in extra bandwidth utilization. In this study, we have chosen only a particular demand distribution for the MLR design. In future work, we will consider various other demand distributions between the 10, 20, and 40Gbps requests and extend the study.

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Received on 25.06.2017 Accepted on 15.01.2018

Research J. Engineering and Tech. 2018;9(2): 150-160

DOI: 10.5958/2321-581X.2018.00021.1