BER-Aware Wavelength Allocation Schemes for Long-Reach PON Employing AWG-Based Remote Node

Lei ShiI, Avishek NagI, Debasish DattaII, and Biswanath MukherjeeI

I. Networks Lab, Department of Computer Science, University of California, Davis, CA 95616, USA II. Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology Kharagpur, 721302, India.

E-mail: {leishi, anag, bmukherjee}@ucdavis.edu; ddatta@ece.iitkgp.ernet.in

Abstract—The use of arrayed-waveguide grating (AWG) and erbium-doped-fiber amplifier (EDFA) enables long-reach passive optical network (LR-PON) to provide enormous bandwidth over large distance, but they also deteriorate receivers’ bit-error rate (BER) performance. In this paper, the effects of AWG and EDFA on BER are studied with both short-term and long-term distance-aware wavelength allocation schemes, to balance the BER among ONUs at different distance. Simulation results show that average BER improves and the relative standard deviation decreases.

Keywords - Long-reach passive optical network; Wavelength-division multiplexing； Bit-error rate; Arrayed-waveguide grating; Wavelength Allocation.

I. INTRODUCTION

By extending the coverage span of passive optical networks from the traditional 20 km range to over 100 km, Long-Reach Passive Optical Network (LR-PON) was proposed as a cost-effective solution for providing broadband access spanning large areas. LR-PON consolidates the large number of active sites and simplifies the hierarchy of telecom networks, thus reducing the network capital expenditure and its operational cost [1]. The low energy profile of LR-PON also matches with new interest in energy savings and green communication [2].

A typical “tree-and-branch” LR-PON contains three parts, optical line terminal (OLT), optical network unit (ONU), and remote node (RN), which resides between the OLT and ONUs. While the point-to-multipoint topology of LR-PON resembles that of a traditional PON, the RN design is different and more complex. Because wavelength-division multiplexing (WDM) is used to exploit the vast bandwidth of fibers, the arrayed-waveguide grating (AWG) is deployed in the RN, serving as a passive wavelength router. Power splitters could be placed after the AWG in RN, or inserted in the drop section to increase the capacity by enabling statistical sharing of bandwidth in one wavelength among multiple ONUs. Optical amplifiers, usually multi-stage erbium-doped-fiber amplifiers (EDFA) are also used at the RN to compensate for the huge power loss due to long transmission distance in feeder section and high split ratio (see Fig. 1).

Ratnam et al. reported the loss of power and inter-channel crosstalk characteristics of AWG and showed that different channels (synonymous with wavelengths, with each channel /wavelength emerging from a unique AWG output port) experience different amount of signal attenuations in AWG, leading to different bit-error rate (BER) at the receiver side [3]. The central channel performs better than the side channels. On the other hand, the distance difference in the drop section varies

significantly from ONU to ONU. This could cause difference in received powers and BER too. A far-away ONU may have worse BER than an ONU close to the RN. Finally, the use of EDFA generates amplified spontaneous emission (ASE) [4], which is a major source of noise. How the ASE noise could deteriorate the BER is also a function of channel numbers and distances in drop section, and thus varies from ONU to ONU.

These observations inspire us to study LR-PON’s BER performance, leading to the design of novel wavelength allocation (WA) schemes, where the goal is to improve the BER performance of all ONUs, balancing their values by compensating for the far-away ONUs with better channels. As the BER is improved and balanced among all channels, simpler and less expensive receivers could be used, which in turn, could reduce the complexity and lower the cost of the whole network. This would also relax the dynamic range of ONU receivers as the power loss among different channels also get balanced through compensation. While in this paper, we focus on the downstream direction, similar scheme can also apply to the upstream direction, as the AWG is a reciprocal device.

II. BER CALCULATION

The AWG is a diffracting device which separates a WDM signal into its component wavelengths. The amount of power focused on an output port is based on the device parameters. It is shown in [3] that the power at the output ports follows a Gaussian profile with the maxima at the central port. Due to the interplay between this characteristic of the AWG with the Lorentzian power spectrum of the source lasers, the output signal power and the signal-dependent noise variances follow a distinctive pattern across the AWG output ports, resulting in a

Fig. 1. Architecture of a typical LR –PON.

BER which is a strong function of the AWG port number and hence the wavelength associated with that port.

The signal power at the ith port is given by:

∆ / tan∆

The crosstalk power at the ith port due to the two adjacent ports (i+1)th and (i-1)th is given by:

∆ / tan ∆∆

tan ∆∆

∆ / tan ∆∆

tan ∆∆

where is the receiving signal power of ith channel without considering the AWG property. Assume all channels have the same transmission power , we have:

where is the insertion loss during the feeder section, including OLT multiplexer AWG and other connectors, is the propagation loss per km, depicts the loss in the drop sections like the 3dB coupler loss at ONU (half of the power needs to go to the RSOA for generating upstream signal; see Fig. 1) and the insertion and split loss at the power splitter, is the gain of EDFA, and are the distance of feeder section (same for all channels) and drop section of ith channel. The detailed derivation of Eqns. (1) and (2) are given in [3] and the different AWG parameters, their meanings and typical values could also be found in there.

The power of the spontaneous noise caused by ASE could be computed by (4), with gain G, spontaneous noise factor and optical bandwidth . Similar to the signal, the receiving spontaneous noise power at the ONU is subject to the AWG property and loss in the drop section, as depicted in (5).

2 1

∆ / tan∆

5

The BER calculation requires the signal and crosstalk powers for the ith port as well as the noise variances at the ith port. Please see Eqns. (17) - (22) in [3] for the definitions of thermal noise variance , shot noise variances for ‘0’ , and for ‘1’ , relative intensity noise (RIN) for “0” and for “1” , crosstalk variance , the signal-crosstalk variance _ , and crosstalk-crosstalk variance _ .

The signal-spontaneous, spontaneous-spontaneous, and crosstalk-spontaneous noise variances are given respectively by Eqns. (6) – (8), where Rλ is the photodetector responsivity (= 0.8 Amp/Watt), is the probability of the data symbol being ‘1’ and is the polarization mismatch factor (= 0.5).

_ 2 6

_ 2 7

_ 2 8

Therefore, the total noise variances due to ‘0’ and ‘1’ reception are given by Eqns. (9) and (10).

_ _ (9)

_ _

_ _ _ (10)

The decision threshold is given by ′ ′

′ ′ ,

where the noise variances are kept independent of the beat-noise components to keep the receiver electronics simple. Assuming Gaussian statistics and defining as laser extinction ratio (=0.1), the BER at the ONU receiver side is given by:

erfc√

erfc√

(11)

III. BER-AWARE WAVELENGTH ALLOCATION SCHEME

In this section, wavelength allocation (WA) schemes are proposed for a LR-PON with N wavelengths. Let the split ratio be M (same for all wavelengths), so a total of N*M ONUs can be accommodated in the network. We consider both the short-term and long-term scenarios: in the short-term scenario, WA is done on a one-time basis with the knowledge of all ONUs’ distances to the RN. The network will not accept new subscribers after the WA and thus remain unchanged. In the long-term scenario, on the contrary, new subscribers emerge gradually over time, requiring the network to allocate new resources periodically, so the WA evolves.

For both scenarios, a simple random WA scheme will let the network planner to randomly choose wavelengths for each ONU, regardless of their distances to the RN. As BER is not taken into consideration, the allocation may have a relatively larger BER difference among the ONUs.

An ONU-distance-aware (ODA) scheme can be used in the short-term scenario to achieve the lowest average BER. In this scheme, we first sort the ONUs according to their distance to RN, and then allocate wavelengths in the order of decreasing distance. We always allocate the best wavelength available to the ONU being considered; so an ONU could not be allocated a worse wavelength than those ONUs closer to the RN. To show that the lowest average BER is reached in this scheme, we consider two ONUs that are allocated on different wavelengths. If a switch is made in the WA between the two, the loss of BER for the far-away ONU to use a worse wavelength outweighs the gain of BER for the near ONU to use a better wavelength due to the characteristic of AWG, thus the average BER increases. Actually, further experiments show that no matter what WA we start with, we would get the same WA as we get in the ODA scheme if we repeatedly do wavelength switch between two ONUs that could lower the overall average BER until no such switch can be made.

The ODA scheme could also balance the BER among ONUs and reduce the variance as it compensates the far-away ONUs with better wavelength channels. However, it cannot be proved that the ODA scheme reaches the lowest relative standard deviation in BER. Therefore, a heuristic A* searching scheme aimed at balancing the BERs is proposed where the search starts from the WA under ODA scheme and one wavelength switch between two ONUs is made from the current WA at every step. The path cost function is set to the

product of a constant coefficient and the number of steps taken. The heuristic estimate function is set to the difference of relative standard deviation between the current WA and the WA under ODA scheme.

In the second scenario, where new ONUs emerge as time elapses, WA needs to be periodically updated. The challenge lies in the fact that the WA for existing ONUs cannot be changed for practical reasons, so the ODA scheme may not work well since the best channels would be allocated to the early birds, leaving the risk of having poor BER for future far-away ONUs. Our way to solve the problem is to conservatively allocate wavelengths according to realistic BER goals, which is set differently for each distance range. In general, we pick the wavelength that just achieves the BER goal for each ONU and in case none of the wavelengths meet our goal, the one that has closest BER to the goal is chosen. Similar to the ODA scheme, ONUs are sorted according to their distances to the RN and the allocation follows a decreasing distance order. It is obvious that an easy goal would waste the capability of center wavelengths, while a too-hard goal makes the scheme regress to the ODA scheme. To set the correct BER goal, we first generate dummy ONUs with distance chosen randomly according to the distribution of existing ONUs’ distances to the RN, until the maximum number of ONU allowed is reached. A partial WA is performed through the ODA scheme for both the real ONU that waits wavelength allocation and the dummy ONUs. The BER goal at each distance range is obtained by averaging the BERs of ONUs in that distance range. The BER goals should be updated each time before the real WA is carried out.

IV. NUMBERICAL EXAMPLES

With respect to different distances to the RN, Fig. 2 shows the downstream BER characteristics for different wavelengths at the ONU side of a LR-PON with 10 Gbps data rate, 5 dBm transmitter for each channel, 0.2 dB/km propagation loss over 90 km feeder section and 0-10 km drop section, 18 dB gain for EDFA, 6 dB insertion loss for AWG, 1 dB insertion loss for the splitter, and 1 dB insertion loss for other connectors. Split ratio under one wavelength is set to 1:16 and a total of 17 wavelengths are numbered from -8 to 8, with channel 0 being the center channel. Note that the BER is measured after applying RS(255,239) forward error correction (FEC) coding.

For the short-term scenario, we found that the ODA scheme and the A* scheme provide the same WA and thus perform equally well. To show the effectiveness of these two schemes, random WA scheme and the worst wavelength allocation scheme (opposite of ODA scheme, allocating best wavelength to nearest ONUs) are used for comparison. As shown in Fig. 3, ODA/ODA-A* schemes have lower average BER and their relative standard deviations (RSD) are much smaller than the other two (32% compared to 134% / 149%).

For the long-term scenario, we simulate through a period of one year, where the number of ONU starts at 80 and increase randomly before reaching over 250 in the end. WA is updated on a monthly basis. The average BER of ODA scheme and conservative scheme are compared, and their average BERs are shown in Fig. 4. The ODA scheme can reach lower average BER in the first few months but gets worse quickly thereafter, while that of the conservative ODA scheme holds still throughout the year. The Conservative ODA scheme also

provides more balanced BERs among ONUs, as indicated by having a much lower and relative standard deviation.

Fig. 2. BER performance of different wavelengths at different distances.

Fig. 3. BER ranges of various WA schemes (in short-term scenario).

Fig. 4. Average BER and RSD trends as LR-PON grows.

V. CONCLUSION

We studied effective wavelength allocation schemes in Long-Reach Passive Optical Networks (LR-PON). Although the arrayed-waveguide grating (AWG) and erbium-doped-fiber amplifier (EDFA) enable the LR-PON to provide enormous bandwidth over large distance, they also deteriorate receivers’ bit-error rate (BER) performance. We studied the effects of AWG and EDFA on BER with both short-term and long-term distance-aware wavelength allocation schemes to balance the BER among ONUs at different distances, improving the average BER and decreasing the relative standard deviation.

REFERENCES [1] H. Song, B.-W. Kim, and B. Mukherjee, “Long-Reach Optical Access

Networks: A Survey of Research Challenges, Demonstrations, and Bandwidth Assignment Mechanisms,” IEEE Communications Surveys & Tutorials, vol.12, no.1, pp. 112-123, First Quarter, 2010.

[2] L. Shi, S.-S. Lee, H. Song, and B. Mukherjee, “Energy-Efficient Long-Reach Passive Optical Network: A Network Planning Approach Based on User Behaviors,” IEEE Systems Journal, vol. 4, no. 4 , pp. 449-457, Dec. 2010.

[3] J. Ratnam, S. Chakrabarti, and D. Datta, “Impact of Transmission Impairments on Demultiplexed Channels in WDMPONs Employing AWG-Based Remote Nodes,” IEEE/OSA Journal of Optical Communications and Networking, vol. 2, no. 10, pp. 848-858. Oct. 2010

[4] E. Desurvire and J. R. Simpson, “Amplification of spontaneous emission in erbium-doped single-mode fibers,” IEEE/OSA Journal of Lightwave Technology, vol. 7, no. 5, pp. 835-845, May 1989.

1.E‐12

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1.E‐10

1.E‐09

1.E‐08

1.E‐07

‐8 ‐7 ‐6 ‐5 ‐4 ‐3 ‐2 ‐1 0 1 2 3 4 5 6 7 8

BER

Wavelength Number

Drop section = 10km

Drop Sectoin = 5km

Drop section = 1km

10‐9.41 10‐9.0510‐9.87

1.E‐12

1.E‐11

1.E‐10

1.E‐09

1.E‐08

1.E‐07

Random Worst ODA/ODA‐A*

BER

Average

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

0

5E‐11

1E‐10

1.5E‐10

2E‐10

2.5E‐10

0 1 2 3 4 5 6 7 8 9 10 11 12

Relative

 Standard 

Deviaton (RSD

)

Aeverge BER

Month

Average:ODA

Average:ODA‐Conservative

RSD:ODA

RSD:ODA‐Conservative