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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.
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Fig. 16. Effect of power levels on maximum insertion loss.
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