A Novel Approach to Enhance the Performance of

Ring Based WDM PON Mukesh Kumar Gupta, Student Member, IEEE, Prathmesh Pravin Dali, Ghanashyam Singh, Member, IEEE

Department of Electronics and Communication Engineering, Malaviya National Institute of Technology, Jaipur.

Email:- mkgupta06@gmail.com

Abstract-In this paper, we have analysed a ring-based wavelength-division-multiplexing passive optical network

(WDM-PON), which provides fiber-fault protection by using dual

ring architecture as well as Rayleigh backscattering (RB) noise mitigation. For protection and restoration against fiber fault, the dual fiber ring is used. Here, we discuss and analyse the characteristics of the downstream signals using 10 Gb/s ON-OFF

keying (OOK), 10 Gb/s dicode-coded NRZ. Moreover, we also investigate the upstream traffic using the 2.5Gb/s OOK generated by the reflective semiconductor optical amplifier-based

optical network unit. The ring-based WDM-PON with dicode­coded NRZ gives improved bit error rate (BER), hence reach of PON can be increased.

Keywords- Dicode-coded NRZ, jiber-fault protection, ON­OFF keying (OOK), ring access, optical wavelength division multiplexed (WDM) network, Optical network unit (ONU), Fiber bragg grating (FRG).

I. Introduction In the future fiber access techniques, for increased demand

of the broadband services, Passive Optical Networks (PON) are the promising solution [1], [2]. For these ultra-high speed networks, wavelength division multiplexing (WDM) technique is more efficient than that of the time sharing technique. Use of directly modulated lasers at central office (CO) [3], [4] and Reflective Semiconductor Optical Amplifier (RSOA) at the Optical network Unit (ONU) for remodulation and reuse, seeded from the CO [5], [6], is the most desired architecture for future ultra-high speed, high capacity WDM­PONs[7]. However, the Rayleigh Backscattering (RB), which is generated by the downstream signal, is the major issue in the colourless WDM-PONs. To mitigate the effect of RB, different techniques have been developed, such as using phase and bias-current dithering, utilizing wavelength-shifting technique, employing advanced modulation formats and double laser bands source, etc. [8]-[12]. These techniques make network more complex and costly and mainly used in tree based WDM PON.

The effect of RB noise on PON is the major issue, which can be minimized by ring based WDM-PON. In the ring based WDM PON, RB is generated in opposite direction of upstream signal that reduce the effect of RB noise [13]. Hence the ring­based WDM-PON regarded as one of the important architectures for next-generation PON [2], [14]-[17], the architecture is shown in fig. I.

978-1-4799-3140-8/14/$31.00 ©2014 IEEE

I -- Protection fiber I

Fig. 1. Dual ring WDM-PON architecture

Next generation WDM-PON must provide higher data rate with high quality of service. If the fiber fault is occurred between CO and ONUs, then data traffic cannot reach affected ONU, which leads to loss of data. Hence, Protection management is the major issue in PON access [18]-[21]. Fig. 1 represents dual ring architecture, which is one of the best sol uti on for th e fi ber protecti on [13].

In this paper, we have studied the performance of the ring based WDM-PON system with RB noise mitigation and fiber protection. In the system architecture, dual fiber ring architecture has been used for the fiber protection in which each ONU can select its fiber path automatically. In this paper, we have compared the system performance for the 10Gb/s downstream On-Off Keying (OOK) and dicode-coded NRZ signal. For upstream characterization 2.5 GB/s OOK has been used.

II. Dicode-coded NRZ The dicode-coding is one of the simplest dc-balanced

correlative level (CL) coding. Dc-balanced CL codes upshifts the lower frequency components to upper frequency, which reduces the overlap between signal and interferer spectra.

329

�D-mw, Uv ' . ' ' KYLOO ' Binary Data : : :Dicode�Codcd data , , , : : Dirrer�ntial Coded d:lta : � - - - - - - - - - - - - - - - - - - c:::::: ______________ �

Diffcl"(:'ntial code generala.' Dicodc-codc generator

Fig. 2. Dicode-coded NRZ generation \,

��--------�----------'0 lOG frequenc:yltnl

(a)

100 frequetq' CItl.I (b)

Fig. 3. Power Spectra of (a) uncoded NRZ (b) dicode-coded NRZ

200

Fig. 2 illustrates the dicode-coded NRZ coding technique. In order to treat the correlated signal levels independently in the decoding process, the original binary message needs to be differentially coded prior to the CL encoding. The differential encoder consists of two gates and one delay element while dicode coder consists of one gate and a delay element as shown in fig. 2. Because of the precoding we can observe in given example that there is one to one relation in dicode coded data and original binary data. That is 0 is mapped to 1 and -1 and 1 is mapped to 0, which reduces receiver complexity, and imposes no bandwidth redundancy unlike 8 bl I 0 b code [22] and Manchester code [23].

From fig. 3, we can see that the dicode-coded NRZ has less baseband components as compared to un coded NRZ that is major portion of lower frequency component has moved to higher frequency components. Since RB noise affects the dc component of the upstream signal by using dicode-coded signal, we can mitigate the effect of RB noise.

III. Simulation setup and

discussion Fig. 1 shows the general architecture of the dual ring WDM­PON used in simulation setup. We have used three ONUs, at each ONU there are three optical circulator, two in clockwise and one in anticlockwise, two FBGs for separation of data signal, and upstream continuous wave (CW) seeding light from multiplexed data stream as shown in fig. 4. The data signal and CW seeding signal received at ONU is separated by filter and passed to receiver and RSOA, respectively. The seeding light is used for generation of upstream signal by remodulating it using RSOA.

Fig. 4. Simulation Setup of dual ring WDM-PON, SMF: single mode fiber, Rx: receiver, Tx: transmitter, OC: optical circulator, RSOA: reflective semiconductor optical amplifier, pc: power combiner, SW: switch, ONU: optical network unit, FBG: fiber bragg grating, MUX: WDM multiplexer, DEMUX: WDM demultiplexer.

In simulation each ONU is placed at 5km distance from others, that is we have used a 5 Km SMF to connect ONUs with each other and to CO. A CW laser of wavelength I 550.6nm, I 552.6nm, 1554.6nm is used for seeding RSOA and the data is transmitted at IOGb/s at wavelengths 1512.2nm, 1522.2nm, 1532.2nm for ONU1, ONU2and qNU3, respectively. We have used a PRBS sequence of length 2 - I as data. The upstream signal of 2.5Gb/s is remodulated using OOK at ONU.

In this simulation, we are observing the performance of OOK and dicode-coded NRZ modulation and its effect on the system parameters. The input data is modulated with a 27_1 PRBS using OOK, and its BER and the Q factor at receiver of all three ONUs are noted as mentioned in the Table 1. Now by the same procedure we have analyzed the effect of the dicode­coded NRZ and listed the parameters in Table I. We can observe from the results that in our level coding techniques the dicode-coded NRZ is superior. System BER is improved approximately 10 times than that of the OOK. It is also observed that Q factor for dicode-coded NRZ is better than the OOK.

To analyze the effect of fiber length on BER and Q factor in optical system we performed simulation using a CO and a single ONU. In which the single CW seeding light with wavelength 1550.6 nm and a data at wavelength 1512.2nm is used. Here, we are varying the length of the SMF which is connected between CO and ONU to carry out our simulation work. 15 Km , 20 Km , 25 Km and 30 Km SMF fibers have been used to observe the effect.

Table I. BER and Q Factor measurements for NRZ and dicode-coded NRZ at -IOdBm

Sr. NRZ Dicode-Coded NRZ User No. NO BER Qfi,ctor BER Qfi,ctor

1 ONU I 9. 48 x 10'\\ 6. 39 8. 37 x 10.12 6. 72

2 ONU 2 4. 66 x 10.8 5 . 33 4.55 x 10.9 5. 74

3 ONU 3 1.41 x 10.7 5. 13 2. 27 x 10.8 5. 46

330 20 J 4 International Conference on Signal Propagation and Computer Technology (ICSPCT)

Table 2. BER and Q Factor measurements for NRZ and dicode-coded NRZ at -4 dBm

Sr. Length NO (Km)

I 15

2 20

3 25

4 30

-6

-11

-16 bQ g -21 0:: � -26

-31

-36

-41 15

NRZ Dicode-Coded NRZ

BER Qfactor BER Qjilctor

1. 02 x 10-36 12. 6 2. 00 x 10-40 13. 25

2. 63 X 10-24 10. 1 9. 90 x 10-27 10. 63

4. 00 x 10-16 8. 09 8. 13 x 10-18 8. 51

4.30 X 10-11 6. 48 4. 94 x 10-12 6. 8

__ NRZ

___ Dicode-Coded NRZ

NRZ at 30Km Dicode·Coded NRZ at 30Km 20 25 30

Length (Km)

Fig. 5. BER performance of NRZ and Dicode-coded NRZ at -4 dBm

The Table 2 lists the observation taken from a single ONU configuration at -4dBm power. From Table 2 and fig. 5, we can observe that the dicode-coded NRZ is better than NRZ even at higher distances.

IV. Conclusion We have analyzed and simulated a ring-based WDM-PON

providing both RB noise mitigation and fiber-fault protection. We analyzed the characteristics of downstream signal using 10 Gb/s OOK and dicode-coded NRZ, and it is observed that there is significant improvement in BER and Q factor while using dicode-coded NRZ . Cost and complexity of receiver at the user side is less as compared to other coding techniques such as DPSK, OFDM.

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