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QPAR: a Quasi-Passive and Reconfigurable Node for Green Next-Generation Optical Access Networks Yingying Bi, Jing Jin, Ahmad R. Dhaini, and Leonid.G. Kazovsky Photonics and Networking Research Laboratory (PNRL) Department of Electrical Engineering, Stanford University, CA, USA [email protected] Abstract—Passive optical network (PON) is regarded as a promising solution for the broadband bandwidth bottleneck problem. However, due to its passive nature, legacy PON is limited by the static power distribution, which makes it power inefficient. To address this problem, we propose QPAR [4], a Quasi-Passive and Reconfigurable node, which enables dynamic power and wavelength assignment so as to save optical power budget in PON. In this paper, we study the power gains that can be achieved in PON employing QPAR, as well as different factors that may facilitate or prevent real QPAR deployments. We conduct extensive simulations to demonstrate the merits of QPAR. Results show that QPAR can achieve high optical power saving by intelligently redistributing the unnecessary power assigned to “close” optical network units (ONUs) in the network. The saved power can either be used to connect more ONUs, or extend the network reach without increasing the optical power budget. Keywords—energy efficiency; optical latching switch; quasipassive; optical access networks; PON. I. INTRODUCTION Access networks connect subscribers directly to their service providers. They are regarded as the bottleneck to highspeed broadband services, encompassing Internet access, highdefinition video streams, and cloud computing services. An important class of optical access solutions is time-division multiplexed passive optical networks (TDM-PONs). In legacy TDM-PONs, the feeder fiber is connected from the optical line terminal (OLT) residing at central office (CO), to the remote node (RN) from which the distribution fiber is connected to a set of optical network units (ONUs). An RN can be a passive power splitter used to broadcast the signal to all ONUs in a point-to-multiple-point (P2MP) manner. Due to the rigid power splitting ratio, PONs can only connect subscribers who are geographically close to the CO (< 25 km). Recently, long-reach PON (LR-PON) solutions have been proposed. In LR-PON, the feeder fiber’s reach is extended by deploying optical amplifiers [1]. The deployment range of the distribution sections remains in the range of 5-10 km due to the employment of passive splitters. In addition, because the bandwidth of a PON is shared among subscribers in the time domain, the bandwidth-per-user is limited by the splitting ratio and individual bandwidth upgrades are virtually precluded [2]. As a future-proof technology of FTTx, active optical networks (AONs) can provide better performance in those aspects. In AONs, signals are directed to specific customers in a point-to-point (P2P) manner via electrically powered switching equipment, such as routers or switches/aggregators [2]. By introducing flexibility through switches, AONs can extend the network reach to almost 70 km without requiring repeaters. This enables a pay-as-you-grow bandwidth upgrade. However, AON nodes are composed of active components and therefore have high energy consumption, high maintenance costs, and are less reliable than TDM-PONs. QPAR is a Quasi-Passive and Reconfigurable node designed to combine the attractive features of both TDM-PON and AON, so as to extend the network reach and incur high optical power budget savings [4]. QPAR can operate over legacy PONs. It performs dynamic/adaptive power and wavelength distribution depending on the ONUs’ geographical locations and bandwidth requirements. QPAR is quasi-passive; it only requires power during reconfiguration, which can be injected remotely from the CO. Thus, it preserves the passive feature of PON since the reconfiguration may only occur every several weeks or months. In this paper, we study the power budget gains that can be achieved using QPAR and discuss the several factors that may impact the deployment of such a promising device. The rest of the paper is organized as follows. Section II presents a detailed overview of QPAR including its architecture and functions. It also discusses the feasibility of QPAR and how it can be a green solution for next-generation PONs. Section III discusses the method used to determine the total power budget required in PON, as well as the individual ONU power. Section IV presents a simulation study, in which we demonstrate the advantages of QPAR, and determine the salient factors that could impact its deployment. The paper is concluded in Section V. Fig. 1. PON with QPAR. II. QPAR: A QUASI-PASSIVE AND RECONFIGURABLE NODE As illustrated in Fig. 1, QPAR is a branching device located in the remote node replacing the passive splitter in legacy TDM-PONs, or a WDM coupler in future wavelength-division multiplexed passive optical networks (WDM-PONs). QPAR can act as an optical power splitter with a controllable power splitting ratio. It offers adaptive power distribution for different geographical users’ distributions. QPAR can also act as a dynamic wavelength router by providing flexible wavelength allocation depending on the users’ bandwidth requirements, thereby enabling an easy pay-as-you-grow bandwidth upgrade. module consists of 1×2 and 2×2 optical latching switches (OLSs), which maintain the quasi-passive feature of the node. 3dB couplers are utilized to generate different power levels in the power splitting module. Each wavelength has the identical and independent structure, so-called single wavelength module. A. Functions of QPAR The main functions of QPAR are (1) splitting the input power into multiple levels; and (2) transmitting signals at different wavelengths dynamically. QPAR can combine these two functions simultaneously in a quasi-passive manner. That is, it only consumes power during reconfiguration. Hence, it does not require steady–state power. B. Architecture of QPAR To realize the functions of QPAR, we designed and built a QPAR node consisting of four modules as depicted in Fig. 2: 1) De-multiplexing module; 2) Power splitting module (PSM); 3) Space routing module (SRM); and 4) Multiplexing module. Fig. 2. QPAR Architecture [4]. The input signal is first de-multiplexed into separate wavelengths, and then each wavelength is independently directed into the corresponding PSM where the required power levels are generated. Subsequently, the different power levels are routed by the SRM to the multiplexing module, where they are combined with other wavelengths. In such a configuration, we can achieve any combination of wavelength and power levels at all output ports. QPAR has four dimensions: the number of input ports Nin, which corresponds to the number of fiber connections from the OLT; the number of wavelengths Nλ; the number of power levels Np; and the number of output ports Nout, which corresponds to the number of ONUs. For the case of a single feeder fiber, Nin=1. Such a device is denoted as Nλ×Np×Nout. A power level is defined as the fraction of total transmission power assigned to each ONU. The number of power levels is counted from the full power to the smallest granularity. For example, if QPAR can offer the following set of power levels {P, 1/2P, 1/4P, 1/8P}, then Np=4. QPAR can be reconfigured to output zero power to certain ports. However, for simplicity, “zero power” is not included in the number of power levels. C. QPAR Implementation with Discrete Components QPAR can be implemented with discrete components. As illustrated in Fig. 3, the proposed and experimentally demonstrated 2×2×2 QPAR [4,6] has two wavelengths, two power levels (full and half power levels) and two outputs. Arrayed waveguide gratings (AWGs) are used to realize the de-multiplexing and multiplexing functions. The space routing Fig. 3. Illustraion of 2×2×2 QPAR with discrete components [4]. OLSs are indispensible for realizing the quasi-passive operation. Among various optical switches based on different latching mechanisms, Micro-Electro-Mechanical Systems (MEMS)-based OLSs are widely recognized to be ubiquitous, cost-efficient, and compatible with current CMOS technology. In most MEMS-based OLSs, vertical micro-mirrors are attached to a shuttle, which is suspended by straight flexible beams and positioned by electrically actuated comb drives [5]. Every time the micro mirror needs to switch from one state to another, a short voltage pulse is applied across the comb drives to actuate it into or out of the optical path. However, each state is maintained mechanically using suspension beams without requiring power. Compared with the Magneto-Optical (MO) OLS, MEMS OLS-based QPAR has smaller insertion loss, larger extinction ratio and lower power consumption. Although it has longer switching time, the latter is still shorter than the traffic restoration time, which is usually in the order of tens of milliseconds [7]. D. Power Efficiency Gains using QPAR In the design and deployment of PONs, energy efficiency has become an increasingly important aspect in its operations, due to the high costs of power and the increasing awareness of global warming and climate change. In legacy PONs, the passive power splitter distributes the signal power sent by the OLT evenly among the ONUs. Therefore the power budget at the CO is decided based on the ONU with the longest distance from the OLT, which results in two main disadvantages: (1) the ONUs with shorter distances receive more power than their minimum need; and (2) no power is conserved for potentially new ONUs. This implicates adding a new PON, thereby tremendously increasing power and operational costs. Thus, legacy TDM-PONs have limited network coverage and inflexible network dimensioning. Nextgeneration access networks should offer enhanced potential for node consolidation through different capabilities with respect to reach and user count per feeder fiber. QPAR, as an intelligent power distributor, flexibly distributes the optical power according to the users’ distances from the CO, so that the required optical power budget can be reduced compared to the passive power splitter. However, this optical power budget saving contributed by the power redistribution is a tradeoff with the following two factors: (1) the extra insertion loss (IL) caused by the increased complexity of QPAR’s architecture; and (2) electrical power used to control the optical latching switches in QPAR. However, the quasi-passive nature of QPAR makes it consume power only during reconfiguration on a per-state-changing basis. Given the low frequency of reconfiguration, we ignore the effect of the second factor and focus on the power efficiency of QPAR in the optical domain in this paper. MEMS based OLSs can be integrated on the silicon-oninsulator (SOI) based PLC platform. The silica-based PLC and SOI based PLC use different types of wafers denoted as silicon A and silicon B, respectively in Fig. 5. This results in two main disadvantages. On one hand, the integration plan is complex and unpractical with too many cascaded wafers. On the other hand, the coupling loss between different wafers is still a significant source of QPAR insertion loss. QPAR allows for lower total transmission power budget at the OLT, thus being a green solution. Alternately, the saved power can either be used to support more users, or extend the network reach. In addition, the quasi-passive nature of QPAR makes it much more energy efficient than AON since it only requires power during network reconfiguration (i.e., when new users are connected and/or when more working wavelengths are added or dropped). Therefore, QPAR not only provides more flexibility than TDM-PON, but it also does it at much lower cost than AON, which makes it the perfect candidate for next-generation optical access networks. E. Feasibility of QPAR Unlike the simple structure of passive splitters, QPAR requires a large number of components. This becomes extremely high for larger dimensions of QPAR, especially when implemented using discrete components. As shown in Fig. 4, the total number of components increases dramatically with the increase of outputs. Meanwhile, a potentially large insertion loss of QPAR may cancel the power saving or even exceed the power budget, thus making the discrete component implementation improper. Integrated implementations based on planar lightwave circuits (PLC) can be a promising solution, since the insertion loss of a PLC device is very low due to little absorption and scattering in the waveguide. The PLC also provides high fiber coupling efficiency [8]. Fig. 5. QPAR with integrated components. The functions of the 3dB coupler and OLS can be integrated into a single tri-state MEMS switch [3]. One tri-state MEMS switch can replace the function of a single wavelength module, which makes QPAR more compact with fewer components. Fig. 4 shows how this solution can significantly reduce the number of components in QPAR, thereby making it promising for real PON deployments. For much larger dimensions of QPAR, multi-state MEMS switches can be cascaded in conjunction with conventional MEMS based OLSs, as shown in Fig. 6. Since both the multistate MEMS switch and conventional MEMS-based OLS can be integrated on the same SOI wafer, the integration plan is simplified and has less coupling loss between wafers. Fig. 6. Fig. 4. Number of components vs. number of outputs. F. QPAR Implementation with Integrated Components Fig. 5 shows how QPAR can be realized using integrated components. When the dimensions of QPAR become larger, multiple layers of power splitting module and space routing module need to be cascaded to generate more power levels and output branches. The passive components including both AWGs and power splitters can be manufactured with the silicabased planar lightwave circuits (PLC) technology. The active III. QPAr with integrated components using muti-state MEMS. OPTIMAL OPTICAL POWER BUDGET IN PON Determining the total power budget required in PON, and consequently the minimum power level for each ONU, is the core theme of QPAR. Finding the required PON power budget can be formulated as a minimization problem. To facilitate the formulation, we first define the following parameters, as illustrated in Fig. 7: N: Number of ONUs; di: Distance between QPAR and ONU i; Dmax: Maximum QPAR-ONU distance; R: Set of power levels, such that R ϵ [Rmin, Rmax], where Rmin and Rmax are the minimum and maximum power levels; We evaluate the power budget gains using QPAR with and without insertion loss. The impact of the power levels and users’ distribution is also investigated. A. QPAR witout Insertion Loss Pin: QPAR input power. 1) Power Budget Saving with Different Power Levels The main difference between QPAR and a passive power splitter is that QPAR can provide multiple power levels. To demonstrate the effect of the number of power levels, we conduct simulations with N=32 and Dmax=70km. The minimum power ratio varies from 1/32 to 1/256. At each minimum power ratio of 1/2n, two cases are compared: (1) n+1 power levels [1, 1/2, 1/4, …, 1/2n], and (2) 2n+1 power levels [1, 1/√2, 1/2, (1/√2)3, 1/4, …, 1/2n]. Fig. 7. Illustration of parameters used to determine min ONU power. We formulate the minimization problem as follows: Find: PQPAR,min = min Pin Subject to: 𝑟! + 𝑟! + ⋯ + 𝑟! ≤ 1 𝑟! , 𝑟! , … , 𝑟! ∈ 𝑅 𝑃! = (𝑑! ∙ 𝛼!"#$% + 𝑆! )!" 𝑃!" ∙ 𝑟! ≥ 𝑃! 𝑖 = 1,2, … , 𝑁 As shown in Fig. 8, as the number of power levels increases, more power budget can be saved in the network. This is because with more power levels offered by QPAR, the probability to meet the ONU’s power requirement becomes higher, which minimizes the waste of unused power. In the ideal case, QPAR will provide infinite power levels, which means that the power ratio can be continuously reconfigured (i.e., QPAR with continuous power levels). 1 2 3 4 where ri denotes the power ratio assigned to ONU i; αfiber= 0.275 dB/km, representing the loss of single-modefiber (SMF); Si is the sensitivity of the optical receiver for ONU i; and Pi is the minimum required power for ONU i to fulfill a specified signal-to-noise-ratio (SNR). Constraint (1) ensures the sum of allocated power ratios for all ONUs is less than or equal to one. Depending on the implementation of QPAR, the set of available power levels R may be different. For example, a QPAR implemented with discrete OLSs and 3dB couplers, 𝑅 = 𝑟! 𝑅!"# ≤ 𝑟! ≤ 𝑅!"# , 𝑟! = 1 2! , 𝑛 ∈ 𝒩 . Constraint (2) ensures the allocated power ratios are implementable. To reach a certain S/N ratio, there exists a minimum required sensitivity for the receiver at the ONU. By including the additional fiber loss in the distribution section, Constraint (3) gives the minimum required power at the output of QPAR for each ONU i. Finally, Constraint (4) ensures the power allocated to each ONU is greater than or equal to its minimum required power. Fig. 8. Power budget saving vs. number of power levels. Fig. 9 shows how QPAR with continuous power levels requires lower power budget than that with discrete power levels. For example, with N=32 and Dmax=60km, the extra saved power is about 1.9 dB. Upon solving the minimization problem and (re)configuring the state of QPAR, the actual power assigned by QPAR for each ONU i at the corresponding output port will be determined by: 𝑃!"#$,!"# ×𝑟! . IV. SIMULATION RESULTS To highlight the benefits of QPAR in terms of power budget saving and find the determinant factors that lead to its practical employment, we implement a solver for the minimization problem presented in Section III, and conduct extensive simulations. Fig. 9. Effect of power levels on the power budget. 2) Power Budget Saving with Different User Distributions To study the impact of users’ distribution on power budget saving, we run the solver for 500 times with N=8 randomly distributed ONUs, and Dmax=70 km. Assuming the minimum power ratio is 1/128, the corresponding power budget saving is calculated for all 500 cases. The histogram of power budget saving is shown in Fig. 10. As observed, for example, the power saving achieved by QPAR is 4 dB, 60 % of the times. To better understand the relationship between the power budget saving and users’ distribution, 10 samples of the lowest and highest power budget saving (from Fig. 10) are selected, as shown in Figs. 11 (a) and (b), respectively. Based on these samples, we can conclude that QPAR is especially power efficient when few users are located far away from it. In the worst-case scenario, when all the users are located close to each other, QPAR converges to a passive splitter. Fig. 13. Effect of user distribution on power budget. 3) Increased Number of ONUs and Extended Reach The maximum number of ONUs and network reach are limited by the power budget, which is determined by the type of transceiver used in the network; a typical value of 32dB for Class C+ optics is set as reference in the analysis. Assuming that the feeder fiber is 10km with 0.275dB/km attenuation, the remaining power budget for the distribution section will be 29.25dB (as shown in Fig. 9). This limits the maximum network reach of 32 users’ QPAR with discrete power levels to be around 65km. Fig. 10. Histogram of power saving. Fig. 14 shows that by employing QPAR with discrete power levels, how the number of supported users can be increased by almost 100%; and using QPAR with continuous power levels, we can increase the number of ONUs by almost 200%. On the other hand, the reach of the distribution section using QPAR with discrete power levels can be increased by about 30%, and by about 60% using QPAR with continuous power levels. Fig. 11. Top 10 geographical distributions with left) lowest and right) highest power saving. To highlight the impact of QPAR on the networking dimensioning in PON, we consider QPAR with continuous power levels and a determnistic user density function D = xn, where x is the normalized QPAR-to-ONU distance; n is a parameter used to control the users’ distribution. With randomly distributed users, n=1. As shown in Fig. 12, a larger value of n skews the user distribution towards QPAR (i.e., D=0). Our conclusion based on Figs. 10 and 11 is clearly asserted in Fig. 13, where the power budget and parameter n are negatively correlated. Fig. 14. Number of users vs. maximum network reach. Fig. 12. Geographical distribution for D= xn: left) n=0.5, right) n=2. B. QPAR with Insertion Loss Our experimental study in [4] shows that the average insertion loss of a single OLS is about 0.5 dB, which can lead to high insertion loss in QPAR, especially with large dimensions. For example, with a 1x7x32 QPAR (i.e., connecting 32 ONUs), the estimated insertion loss is about 13.5 dB, of which 5.5 dB are from the OLSs and 8 dB are contributed by the AWGs. This will result in a higher power budget requirement as depicted in Fig. 15. Hence, QPAR won’t be able to operate within a Class C+ interface, and will no longer be more advantageous than the passive splitter. However, the realization of a QPAR with insertion loss lower than 5.1 dB (as shown Fig. 15) can still maintain the “upperhand” of QPAR. V. CONCLUSIONS In this paper we explored the power efficiency gains achieved with PON using QPAR, a novel Quasi-Passive and Reconfigurable node that enables dynamic power and wavelength assignment. The realization of QPAR using discrete and continuous power levels has also been explored, and the impact of different designs on the power consumption has been investigated. Results show that QPAR can conserve high optical power, which can be used to increase the number of users by almost 100%, and extend the network by almost 30% with the same power budget. The effect of users’ distribution is also studied, which shows that the QPAR node is especially useful when few users are distributed far away from the remote node. With all its benefits, QPAR can be an ideal candidate for flexible deployment and operation of green next-generation optical access networks. ACKNOWLEDGMENT Fig. 15. Power budget with or without insertion loss. In Fig. 16, we show how QPAR with continuous power levels can tolerate more insertion loss (about 2 dB), by requiring less power budget. This work is funded by the Center for Integrated Systems (CIS) at Stanford University. Ahmad R. Dhaini is sponsored by the Natural Sciences and Engineering Research Council of Canada (NSERC) and NSF. REFERENCES [1] [2] [3] [4] Fig. 16. Effect of power levels on maximum insertion loss. Finally, Fig. 17 shows how a users’ distribution with n=2 (i.e., ONUs are closer to QPAR) provides 3.4 dB tolerance of insertion loss more than n=0.5. Thus, QPAR is more plausible to find application for users’ distributions with long tails and skewed towards the RN. [5] [6] [7] [8] Fig. 17. Effect of user distribution on maximum insertion loss. Huan Song; Byoung-whi Kim; Mukherjee, B., "Long-reach optical access networks: A survey of research challenges, demonstrations, and bandwidth assignment mechanisms," Communications Surveys & Tutorials, IEEE , vol.12, no.1, pp.112,123, First Quarter 2010 KEYMILE Report, “AON vs. PON - A comparison of two optical access network technologies and the different impact on operations,” KEYMILE International GmbH, 2008. SH Yen, SW Wong, S Das, N Cheng, J Cho, S Yamashita, O Solgaard, LG Kazovsky, “Photonic Components for Future Fiber Access Network”, IEEE Journal on Selected Areas in Communications 28 6, 928-935, 2010. Y. Bi, J. Jin, L. G. Kazovsky, "Quasi-Passive Reconfigurable Optical Node: First Experimental Demonstration," in Proc. Conference on Lasers and Electro-Optics (CLEO), May 2012, Paper CTh1H.5. W. Noell, P. A. Clerc, F. Duport, C. Marxer, N. de Rooij, "Novel process-insensitive latchable 2 × 2 optical cross connector for singleand multimode optical MEMS fiber switches," in Proc. 2003 IEEE/LEOS International Conference on Optical MEMS, pp. 49-50, 1821 Aug. 2003. Jin, J., Y. Bi, M D. Leenheer, L. G. Kazovsky, J. Perin, and M. R. N. Ribeiro, "Quasi-Passive and Reconfigurable Optical Node: Implementations with Discrete Latching Switches", IEEE Photonics Conference (IPC) 2012, Burlingame, California, U.S.A., pp. 917-918, September 2012. Y. S. Didosyan, H. Hauser, W. Fiala, J. Nicolics, W. Toriser, "Latching type optical switch," Journal of Applied Physics, vol. 91, no. 10, pp. 7000-7002, May 2002. Takahashi, Hiroshi "Planar lightwave circuit devices for optical communication: present and future," Proceedings of SPIE, vol. 5246, pp. 520-531, 2003.