Energy Efficiency of Femtocell Deployment in
Combined Wireless/Optical Access Networks
Authors: Slavisa Aleksic, Margot Deruyck, Willem Vereecken, Wout Joseph,
Mario Pickavet and Luc Martens
c Elsevier (2012). This is an authors’ copy of the work. It is posted here by permission of Elsevier for your personal
⃝
use. Not for redistribution. The definitive version is scheduled for publication in Computer Networks in 2013. The
DOI of the final publication is 10.1016/j.comnet.2012.12.013.
Energy Efficiency of Femtocell Deployment in
Combined Wireless/Optical Access Networks
Slavisa Aleksica , Margot Deruyckb , Willem Vereeckenb , Wout Josephb , Mario Pickavetb , and Luc Martensb
a Vienna
University of Technology, Institute of Telecommunications, Favoritenstrasse 9-11/389, 1040 Vienna, Austria
Email: slavisa.aleksic@tuwien.ac.at
b Department of Information Technology (INTEC) of Ghent University - iMinds, Gaston Crommenlaan 8, B-9050 Gent, Belgium
Email: {margot.deruyck, willem.vereecken, wout.joseph, mario.pickavet, luc.martens}@intec.ugent.be
Abstract
Optical/wireless convergence has become of particular interest recently because a combined radio wireless and
optical wired network has the potential to provide both mobility and high bandwidth in an efficient way. Recent
developments of new radio access technologies such as the Long Term Evolution (LTE) and introduction of femtocell
base stations open new perspectives in providing broadband services and applications to everyone and everywhere, but
the instantaneous quality of radio channel varies in time, space and frequency and radio communication is inherently
energy inefficient and susceptible to reflections and interference. On the other hand, optical fiber-based networks do
not provide mobility, but they are robust, energy efficient, and able to provide both an almost unlimited bandwidth
and high availability.
In this paper, we analyze the energy efficiency of combined wireless/optical access networks, in which LTE technology provides ubiquitous broadband Internet access, while optical fiber-based technologies serve as wireless backhaul
and offer high-bandwidth wired Internet access to business and residential customers. In this contest, we pay a particular attention to femtocell deployment for increasing both access data rates and area coverage. The paper presents
a novel model for evaluating the energy efficiency of combined optical/wireless networks that takes into account the
main architectural and implementational aspects of both RF wireless and optical parts of the access network. Several
hypothetical network deployment scenarios are defined and used to study effects of femtocell deployment and power
saving techniques on network’s energy efficiency in urban, suburban and rural areas and for different traffic conditions.
Keywords: Communication networks, energy efficiency, access networks, optical/wireless convergence
1. Introduction
Nowadays, cellular wireless networks grow rapidly in
both number of subscribers and amount of traffic. According to the International Telecommunication Union
(ITU-T), the number of wireless broadband subscriptions worldwide overtook the total number of fixed
broadband subscriptions for the first time in 2008, and
currently, there are 60% more wireless broadband subscriptions than the wired ones [1]. Since the number
of deployments and the penetration of the 3rd generation (3G) radio technologies has been rapidly increasing and the data rate per user of all 3G technologies has
∗ Received: June 19, 2012
Revised: December 19, 2012
Accepted: December 20, 2012
Preprint submitted to Computer Networks
been continuously improving, the requirements on the
network providing backhaul for the wireless infrastructure are increasing too. New radio interfaces are already
capable of providing hundreds of Mbit/s over a single
radio link between a base station and a user. The introduction of the so called 4th generation radio technologies (4G), which is also known as ”Beyond 3G” (B3G)
and new applications for smart phones and other mobile
devices will lead to even more user traffic and significantly stronger needs for both high capacity and a better
support of advanced software applications.
Due to the current developments, the wireless backhaul is experiencing the major paradigm shift from the
currently commonly used solutions based on relatively
low data-rate Plesiochronous Digital Hierarchy (PDH)
leased lines for connecting 2G/3G cell sides to base station controllers to a more flexible, broadband, and both
January 3, 2013
energy- and cost-effective solution using an fiber-based
network. As optical access networks are able to provide
a very high bandwidth practically almost independent
on distance and a high energy efficiency [2], it seems
that a combination of mobile wireless access with optical access networks is a very promising integrated approach for a sustainable and efficient next generation
ubiquitous access network. Wireless cellular and optical wired access networks provide synergies and complementary features. The main benefits of using a kind
of wireless access are mobility and ubiquity while optical access networks provide high data rates, low power
consumption and robustness.
Despite its ever increasing capacity and ability to
support new bandwidth intensive applications, high energy consumption of the access network infrastructure
demands particular attention. It has been shown that
access networks are the largest contributor to the network related electricity consumption and corresponding
greenhouse gas emissions (GHGs) [3, 4, 5]. Among all
access solutions, mobile access technologies have the
lowest energy efficiency [6], i.e., they consume the highest energy per user and per bit of data transmitted. Although user mobile devices are designed for very low
energy consumption because of limited capacity and
size of batteries, base stations of cellular networks are
not energy efficient. This is due to the fact that base
stations are constantly kept active and because of the
inherently energy-inefficient transmission method used
for wireless access, in which a large percentage of the
power emitted from antenna is not used for communication but rather spread out in the air. Therefore, any effort
in increasing the energy efficiency of the wireless networks could lead to significant energy savings. For example, introduction of femtocells could lead to a more
efficient use of the transmitted power. Femtocell base
stations provide higher data rates with lower power signals because of reduced cell size and increased spectral
efficiency. Thus, the femtocell base stations consume
less energy than, e.g., macrocell or microcell ones for
the same data rate [7, 8, 9, 10]. Concurrently, the limited range means a larger number of base stations to be
deployed, but also a higher flexibility. Different mechanisms such as selective putting in the sleep mode of
individual base stations during their inactivity times can
thus easily be implemented. Indeed, various architectures and algorithms have already been proposed basing
on such methods and approaches to increase the energy
efficiency of the wireless access infrastructure [10, 11].
On the one hand, the increase of both data rate per
user of mobile networks and number of base stations
demands a higher capacity and a larger number of net-
work terminals in the wireless backhaul network. On
the other hand, optical access networks have the potential to provide very high energy efficiency, large distances and high data rate access for a large number of
users [5]. Therefore, a logical path towards future highperformance access networks is an efficient combination of radio and optical access technologies in order
to realize an integrated and efficient wireless/optical access network.
In this paper, we study the energy efficiency of a combined wireless/wired network employing Long Term
Evolution (LTE) for wireless access and a standard optical access technology for wired Internet access and
for connecting the mobile base stations to the network
core. The optical wired access technologies we consider here are either point-to-point (p-t-p) Ethernet or a
kind of Passive Optical Network such as Gigabit PON
(GPON) that is based on the Asynchronous Transfer
Mode (ATM) or EPON based on the Ethernet.
For this purpose, we developed a model that takes
into account the main architectural and implementational aspects of both LTE and optical access. Several hypothetical deployment scenarios for combined
wireless/optical networks are defined with the particular focus on a possible deployment of femtocell base
stations. The femtocell deployment scenarios are analyzed by means of achievable energy efficiency. Although there are several studies that consider energy efficiency of RF wireless [7, 9, 10, 11, 12, 13] and optical
wired [2, 3, 4, 5, 6, 14] access as separate networks,
there is still no study, to the best of the authors’ knowledge, that investigate a combined wireless/optical network form the energy point of view.
The paper is organized as follows. Section 2 reviews current trends in technologies and approaches
for access networks. Section 3 describes the considered architecture of the combined wireless/optical access network and the model that is developed for evaluating the energy efficiency of wireless/optical access
networks. Considered scenarios and main parameters
are presented in Section 4, while some selected results
obtained by the model are presented and discussed in
Section 5. Finally, Section 6 summarizes the paper and
draws conclusions.
2. Current Trends in Access Networks
One of the emerging radio technologies nowadays is
LTE (Long Term Evolution) [15]. In December 2009,
world’s first publicly available LTE-service was started
in Scandinavia [16]. It promises downlink peak rates of
at least 100 Mbit/s. This is achieved by using among
others SOFDMA (Scalable Orthogonal Frequency Division Multiple Access) as multiple access technique.
SOFDMA is derived from OFDMA (Orthogonal Frequency Division Multiple Access) and supports a wide
range of bandwidths to flexibly address the need for various spectrum allocation and application requirements.
LTE supports variable bandwidths from 1.4 MHz to
20 MHz and operates in the 2.6 GHz frequency range.
In the future, LTE will probably also use the 800 MHz
band (digital dividend frequencies). Release 10 of LTE,
also known as LTE-Advanced [17], is a candidate technology for IMT-Advanced, which defines the systems
beyond IMT-2000.
There have been recently a large number of projects
concentrating on fiber-based optical access for broadband transmission of data generally named Fiber-ToThe-x (FTTx). There are different options for FTTx
depending on how near to the subscriber the fiber
reaches. A typical example is the fiber-to-the-home
(FTTH), which means that the optical signal reaches
the end subscriber’s equipment situated in the subscriber home. Other examples are FTTB (Fiber-ToThe-Building), FTTC (Fiber-To-The-Curb), and FTTN
(Fiber-To-The-Node). When looking at the topology of
the access network, it can be either ring or tree or a combination of these two topologies. Additionally, interconnections can be based on unidirectional or bidirectional
fibers in a point-to-point (p-t-p) or point-to-multipoint
(p-t-mp) arrangement. Since recently, the number of
FTTH connections has been remarkably growing. For
example, in the United States, the annual growth rate
was above 100% between 2002 and 2008 [18]. While
still relatively strong, FTTH growth has slowed somewhat since 2008. There are currently about 20.9 million homes in North America where a fiber connection
is technically available and approximately 7 millions
homes actually connected with lit fiber [18]. In Japan,
the number of FTTH connections has exceeded that of
ADSL in January 2007 [19]. In Asia, there are currently
around 78% of the world’s FTTH subscribers. In the
first half of 2009, the growth rates in China and Taiwan
were 167% and 25%, respectively [20].
There are various ways how to combine wireless networks with optical fiber-based access [21, 22]. Maybe
the most intuitive way would be to use the so-called
Radio over Fiber (RoF) method, in which radio frequency signals are modulated directly on the optical carrier and transmitted through a fiber optic link. Different signals, frequency bands, modulation formats and
coding schemes can be combined and transmitted over
a single fiber [22]. Although this technique can potentially provide transparency in the optical domain to
wireless standards and services and a high energy efficiency through minimizing the number of required components between the mobile user and central office of
network provider, it suffers, however, from impairments
on the signal such as noise, distortion, and dispersion.
Additionally, although considered as an option for the
so-called converged next-generation FTTH networks,
the currently deployed optical access infrastructure and
devices do not support this option. Therefore, combining wireless access with a standard FTTH technology
such as GPON, EPON or p-t-p optical Ethernet seems
to be the most probable option for an efficient short and
medium term support of fast penetration of the highspeed LTE technology.
3. Combined Wireless/Optical Access Network
This section presents the architecture of the considered combined wireless/optical access network and the
method used to estimate its energy efficiency.
3.1. Network Architecture
The architecture of the considered access network is
shown in Figure 1. The combined network can be divided into three different planes, i.e., three different networks that coexist in the same spacial area and share
both the customers residing in this area and the available resources in the central office. The bottom plane
represents the wired optical access network that provides very high data rate Internet access to business and
residential customers. We assume here two different
types of wired optical access, namely a point-to-point
network based on optical Ethernet, i.e., an active optical access network with Ethernet switches deployed in
the field, and a point-to-multipoint optical access network, i.e. a passive optical network (PON). The middle plane represents the optical backhaul of the 3G/4G
cellular wireless network, which itself is represented by
the upper plane in the figure. In this study, we consider
the same access technologies for both optical wired access and wireless backhaul networks. We assume that
the network termination equipment belonging to these
two networks can be housed in the same rack unit and
may share the same uplink of the switch in the central
office. However, the network terminal equipment of the
two networks is never connected to the same port of the
aggregation switch because synchronization and service
levels required by the wired and wireless parts of the
network are different. For this reason we assume that
optical network terminals (ONTs) of the wireless backhaul and those of the wired optical access network are
connected to different optical line terminals (OLTs).
3.2. Description of the Network Model
In order to determine both network design parameters
and average data rates per user, we define some technology and topology related parameters and limitations as
depicted in Figure 2. For cellular networks we have to
define the minimum required link budget for a particular
configuration of the base station and selected transmission mode. Using a suitable propagation model we can
determine the range of a base station (DRL ) and the maximum achievable data rate for users that are located at
the distance DRL from the antenna (DRR,max ) that can be
provided by the chosen technology and particular network design. The limited capacities of both links and
switches in the access networks have also to be taken
into account. For example, the finite capacity of the base
station network interface (C BS ), the maximum capacity
of the Ethernet switch (CS W ) in the field in case of an
active optical access network and the upstream data rate
(DRPON,max ) of the feeder fiber in case of a passive optical network limit the achievable data rate per user in an
overload situation. Similarly, the uplink capacity of the
aggregation switch in the central office (CCO ) sets an upper limit on the aggregated data rate within the network
segment that is served by the switch.
The architectural and technological aspects as well
as the limitations described above are considered in the
model when determining the achievable average access
data rates for wired and wireless network users. Additionally, statistical data about network and service usage, i.e., the time dependent network utilization, number of wired and wireless subscribers and technology
penetration represent the input to the model.
As can be seen from Figure 3, the overall model
of the combined wireless/opitical access network is di-
3G/4G
Cellular
Wireless
Network
CO
PON backhaul
Wireless
Backhaul
(Optical)
Active optical p-t-p backhaul
vided into three submodels as shown in Figure 1. The
input parameters of both cellular wireless and optical
wired network models are: population density in the
area under consideration, number of subscribers, network usage, the percentage of subscribers that are active
at each particular time, and user traffic (both DS (DownStream) and US (Up-Stream)). According to the technology and design-related parameters and considering
the usage of the network during the day, we obtain the
achievable aggregate traffic, and thus the achievable average data rate per user for each of the two submodels
(the wireless and the wired access network submodels).
In the case of the submodel for wireless backhaul, its
input parameters are the output values of the wireless
submodel such as the average number of active users
per cell, the number of active base stations and average data rates (both US and DS) per base station. Note
that in case of a coexistence of two or more cell types,
i.e., macro-, micro-, pico- and femtocells, the aforementioned parameters are given per cell type. In order to
keep the complexity of the model as low as possible, we
decided not to model the transmission of data packets
through the network, but instead to deal with time-ofthe-day dependent average data rates per user. Following this approach we are able to perform traffic analysis
without using a packet-based, event-driven simulator,
which would most probably lead to a complex and less
scalable implementation. Hence, in the simulator developed in the course of this study, the amount of traffic
generated and consumed by network users within a time
interval serves to calculate average data rates for this interval, which are then used to determine the volume of
traffic to be processed by network elements. The model
CCO : Capacity of the uplink in the CO
RU,PON : Data rate per user in a PON
DRL
RU,p-t-p : Data rate per user in a p-t-p network
DRR,max
CBS : Capacity of the BS network interface
TBS
DRL: distance of a user form the BS
BS
RU,p-t-p
DRR,max : max. radio link data rate
CSW : Capacity of the switch
Active optical
p-t-p backhaul
CO: Central Office
DRL
DS: Downstream
US: Upstream
DRR,max
DS
TBS
CSW
US
RU,PON
BS
CO
CCO
PON backhaul
RU,p-t-p
CO
PON access network
Optical
Access
Network
Active optical p-t-p access network
Figure 1: Planes of the combined wireless/optical access network
CSW
PON access
network
Active p-t-p optical
access network
RU,PON
Figure 2: Technology and topology related aspects and limitations
average no. of users per cell
average data rate per user (DS,US)
average data rate per BS (DS,US)
no. of BS
Model of wireless backhaul (optical)
technology (coding, structure, synchronization)
no. of OLT/P-t-P cards ONU energy consumption topology
reach
no. of COs
OLT energy consumption
Energy
averge
data rate
Model of optical access network
topology
averge
data rate
technology (coding, structure, synchronization)
no. of OLT/P-t-P cards
ONU energy consumption
OLT energy consumption
reach
Energy
no. of COs
Configuration parameters
Network model
Average data rate provided to users
Model of 3G/4G cellular network
technology (modulation/coding)
Energy
cell size
average no. of users per cell
BS energy consumption
range
coverage
channel bandwidth
Network element models
Total energy consumption (wireless + backhaul + optical)
demanded data rate (DS,US)
no. of subscribers
population density
network usage
network usage
population density
no. of subscribers
demanded data rate (DS,US)
Figure 3: Model overview
is implemented in a modular manner, where first design
and configuration parameters are calculated according
to input parameters such as the number of users, area to
be covered, technology peculiarities, aimed coverage,
etc. Then, aggregate traffic at each network element and
for every time interval during a day is determined and
the intermediate results are exchanged between the network submodels. Thus, this approach allows a quasidynamic modeling of the combined wireless/optical access network while keeping the complexity of the simulator at a reasonable level. Finally, the total average data
rates and energy consumption are calculated and used to
obtain energy efficiency for different deployment scenarios. A detailed description of the scenarios considered in this study can be found in Section 4.
The instantaneous energy consumption of the network is obtained by summing up the energy consumption values of all active network elements within a specified period of time. The average overall energy consumption is then obtained by averaging the values of
energy consumption obtained for different time periods
during the day. Note that the power consumption of enduser equipment is not taken into account because we set
the system boundaries at the network terminals. Thus,
we consider the network equipment including the optical network terminals and radio base stations, but excluding personal computers (PCs), laptops and mobile
devices such as cellular phones, smartphones and tablet
PCs. Additionally, the mobile devices are optimized
for low energy consumption and usually consume just a
small fraction of the energy consumed by base stations
and fixed-access equipment [23]. Since we consider the
use of different power saving methods in the model for
Results
Total network‘s
energy consumption
Resulting achievable
data rate per user
Energy Efficiency
Figure 4: Schematic representation of the model for evaluation of energy efficiency of access networks.
energy consumption, we differentiate between several
levels of power consumption that correspond to several
operation modes of the equipment.
Network element models, as presented in the next two
subsections, are used together with configuration parameters in the network model, in which technological,
topological and configuration parameters are combined
in order to obtain the total network’s energy consumption and corresponding achievable data rate per user for
a given scenario and traffic demand. Energy efficiency
is obtained by dividing the resulting data rate per user
by the energy consumed by a single user. The modelling approach is depicted in Figure 4. The models we
use to estimate power consumption of network elements
for optical wired and cellular wireless networks are presented in the two following subsections.
3.2.1. Model of optical access networks
Energy consumption of optical network elements is
estimated by first defining generic structures of different equipment located at both network provider and user
premisses as shown in Figure 5. A concentration element in the central office (CO) of network providers
usually comprises an uplink interface, a switching module and a number of optical line terminal (OLT) cards.
Optionally, it can comprise cooling equipment. All
other components that may be needed to implement
some additional functions or features are represented
by the block ”Misc”. As already mentioned, we consider in this study the standard p-t-p optical Ethernet
and two options of passive optical networks (PONs),
namely EPON and GPON. GPON uses an ATM (Asynchronous Transfer Mode) based data transport, and thus
is able to provide both constant bit rate (CBR) services
for an efficient transport of cellular traffic and synchronization required by radio access networks. In contrast,
the standard Ethernet based p-t-p and PONs do not inherently support such type of service. However, recent
developments of the carrier-grade Ethernet were forc-
Uplink interface
CO equipment
Misc.
Tx/Rx/PHY/MAC
Synch
OLT/p-t-p port
switch
…
OLT/p-t-p port
…
M 10GE
uplink ports
OLT or P-t-P interface
a)
Tx/Rx/PHY/MAC
N ports
OLT/p-t-p port
Cooling
(if present)
µP + Memory
OLT/p-t-p line card
PHY
SER/
DES
PCS
MAC
Duplexer
(OLT only)
Digital
Optics
CO: Central Office
GE: Gigabit Ethernet
OLT: Optical Line Terminal
Tx: Transmitter
Rx: Receiver
MAC: Media Access Control Protocol
P-t-P: Point-to-Point
PHY: Physical Layer Device
SER/DES: Serializer/Deserializer
PCS: Physical Coding Sublayer
µP: Microprocessor
ONT: Oprical Network Terminal
ONU: Optical Network Unit
SLIC: Subscriber Line Interface Circuit
µP
Digital
Optics
Switch
ONT/ONU/p-t-p SoC
MAC
EU-Style CPE
Synch
Dual SLIC
PHY
Memory
Ethernet
…
Network interface
(optical)
Duplexer
(ONT/ONU
only)
L ports
Ethernet
Misc.
User interfaces
b)
Figure 5: High-level representation of optical access network equipment located at a) network provider premisses and b) customer premisses
ing the inclusion of methods for synchronization in the
Ethernet such as the precision time protocol (PTP), version 2 (IEEE1588-2008) and the Synchronous Ethernet (SyncE) as recommended by ITU-T G.8261. For
this reason, the structure of optical network elements
for wireless backhaul differ from that for wired access
because they must meet stronger synchronization requirements, which can be provided by the inclusion of
an additional component (Synch) that implements both
SyncE and IEEE1588v2 functions.
The energy consumption per network element is calculated by considering consumption values of individual functional blocks that can be implemented either in
a single specific component or as a part of a large integrated circuit (IC) device. The energy consumption of a
functional block is obtained by averaging the consumption values of a number of components currently available on the market. In order to validate our model of network elements, the obtained values for their total power
consumption are compared to those given in technical
specifications of some commercially available systems.
The values used to obtain the total power consumption
per network element are shown in Tables 1 and 2, which
also contain the maximum achievable net line data rates
for each technology, i.e., the specified maximum data
rates excluding the line coding overheads. The values
of line transceivers’ power consumption are typical consumption values for transmission distances of up to 10
km.
The optical network model [2, 5, 24, 25] includes several modules as shown in Figure 6. Parameters used
in the model can be grouped into configuration and
network-related parameters. Configuration parameters
include the number of subscribers per central office, the
capacity limitation of the switch in the field (CS W ) and
the uplink capacity of the central office (CCO ). Networkrelated parameters are on the one hand the networkspecific topology, the maximum reach of the access network, and the power consumption of the corresponding
network elements and on the other hand the maximum
upstream and downstream data rates per subscriber that
are directly influenced by the applied network configuration and its parameters. A combination of the aforementioned parameters and their values represent the input to the model used for calculation of energy efficiency. The output of the model is the energy efficiency
obtained by combining the total power consumption of
the network and the resulting achievable data rates per
subscriber. Thus, the resulting energy efficiency, i.e.,
the amount of information transferred through the access network while consuming one Joule of energy, is
expressed in Gbit per Joule (Gbit/s/Watt = Gbit/Joule).
Note that all data rates are observed at the interface between the physical layer and the data link layer. Thus,
the overheads due to coding and modulation are taken
into account but not the overheads induced by the data
link and the network layer.
The execution of the model for optical wired access
networks is carried out in three steps. In the first step,
the topology, configuration and number of network line
and termination elements are determined on the basis of
the technology used, the number of subscribers, the area
to be covered and the required access data rates. This
first step corresponds to a network design process that
takes into account technological and topological limitations as well as requirements set by network users.
In the second step, achievable data rates are calculated
at each network element according to the user traffic
from both wired and wireless parts of the access network and according to the assumptions made for each
deployment scenario. At the same time, total network
power consumption is obtained by summing up the estimated power consumptions of all active elements. How
both the achievable data rates and the total power consumption are determined will be explained later in this
chapter. The final step consists of determining energy
efficiency by combining the achievable aggregate data
rate and the total network power consumption.
In contrast to passive optical networks, where active
users have to share the available bandwidth of the feeder
fiber, we assume for the p-t-p active optical Ethernet that
all users can achieve the maximum data rate provided by
the technology between CPEs and the Ethernet switch in
the field. The data rate per user in this case is only limited by the maximum capacity of the switch to which
network element
max. data rate
(DL/UL) [Gbit/s]
power
GPON (4 OLT pots)
10/10
47 W
1G-EPON (4 OLT ports)
10/10
45 W
1G-EPON (4 OLT ports) Synch
10/10
51 W
p-t-p 1G Ethernet (8 ports)
10/10
40 W
p-t-p 1G Ethernet (8 ports) Synch
10/10
43 W
typical power consumption breakdown
- uplink
13%
- downlink
39%
- switch
37%
- µP + memory
8%
- misc.
3%
AC-DC inefficiency
15%
additional power for cooling
35%
Table 1: Values of power consumption for optical network elements
at network provider premisses
network element
max. data rate
(DS/US) [Gbit/s]
power
EU type GPON ONT
2.3/1
4.2 W
1G-EPON ONU
1/1
4.1 W
1G-EPON ONU Synch
1/1
6W
p-t-p 1G Ethernet CPE
1/1
3.9 W
p-t-p 1G Ethernet CPE Synch
1/1
5.6 W
they are directly connected and by the maximum uplink
capacity of the corresponding CO. Thus, the finite capacity of the switch between the CO and users sets an
W
upper bound on the achievable data rate , RSU,p−t−p
for
NS W users that are connected to this switch:
rDS + rUS , for CS W > NS W (rUS + rDS )
=
CS W /NS W , otherwise
(1)
Here, rDS and rUS are respectively downstream and
upstream data rates provided by the technology. Additionally, the limitation of the CO uplink can be taken
into account in the following way:
W
RSU,p−t−p
rDS + rUS , for CCO > Nuser (rUS + rDS )
=
CCO /Nuser , otherwise
(2)
where, Nuser is the total number of active users connected to the CO. The resulting achievable data rate per
user is then calculated according to:
RCO
U,p−t−p
W
, RCO
RU,p−t−p = min(RSU,p−t−p
U,p−t−p ).
(3)
Due to the fact that users of passive optical networks
connected to the same OLT have to share both US and
DS data rates, they cannot reach the maximum data rates
listed in Tables 1 and 2 unless there is only one active
user per OLT. While in p-t-p networks both broadcast
and multicast traffic have to be multiplied and transmitted to users in parallel over different links, PONs
can naturally support broadcast and multicast services
by sending only a single data stream to all users. This
property of PON options is addressed by introducing the
broadcast factor, B, which represents the percentage of
the downstream data rate used for broadcast services.
Thus, the bidirectional data rate per user in a PON,
typical power consumption breakdown
- digital optics
35%
- 4 × FE PHY
11%
- ONT (ONU ) SoC
33%
- memory
3%
- dual SLIC
7%
- misc.
11%
AC-DC inefficiency
Network element models
Network side
o Structure
o Functional blocks
o Data base about energy
consumption at the chip level
Network parameters
o Number of users per CO
o Switch capacity
o Uplink capacity of the CO
Network model
o Energy consumption of network elements
o Topology and reach
o Number of active users per OLT port
15%
o Maximum data rates
(upstream and downstream)
Results
Total network‘s energy consumption
Table 2: Values of power consumption for optical network elements
at customer premisses
Configuration parameters
User side
o Structure
o Functional blocks
o Data base about power
consumption at the chip level
Resulting achievable data rate per user
Energy Efficiency
Figure 6: Depiction of the model for evaluation of energy efficiency
of optical networks
RU,PON , can be calculated by considering the following
three cases.
Case 1: In this case, the total bandwidth consumed by
all active users is below the given limit, CCO , which is
the maximum bidirectional uplink capacity of the central office switch, i.e., when CCO > NOLT (rUS + rDS ),
where rUS and rDS represent the maximum upstream
and downstream data rates, respectively. NOLT is the total number of active optical line terminals in the CO. In
this case, all users can be provided with the maximum
data rates in both upstream and downstream directions
and the total achievable data rate of a single user can be
calculated using Equation (4):
R1U,PON = rDS B + [rDS (1 − B) + rUS ]
NOLT
,
Nuser
(4)
Case 2: The second condition reflects the case in
which the bandwidth limitation is reached, but there is
still no effect on the broadcast traffic, i.e., when CCO ≤
NOLT (rUS +rDS ) but (CCO −NOLT ·rUS ,L )/NOLT > rDS · B,
where rUS ,L represents the reduced upstream data rate
due to the limitation of the uplink (CCO ). In this case,
both downstream and upstream data rates are reduced,
but not as much to affect delivering of broadcast services. The achievable data rate per user can be then
calculated by the following formula:
) NOLT
(
R2U,PON = rDS B + rDS ,L − rDS B + rUS ,L
,
Nuser
(5)
where rDS ,L is the reduced downstream data rate due
to the limitation of the uplink.
Case 3: Finally, the third case refers to a strong
reduction in the available bandwidth per user due to
the uplink limitation, where not only the best-effort
but also the prioritized broadcast traffic is throttled.
In this case we have CCO < NOLT (rUS + rDS ) and
(CCO − rUS ,L NOLT )/NOLT < rDS B and the achievable
data rate per user is then:
R3U,PON = rDS ,L + rUS ,L
NOLT
,
Nuser
(6)
The reduced upstream and downstream data rates,
rUS ,L and rUS ,L , are given by Equations 7 - 9:
rDS ,L =
rUS ,L =
NOLT
CCO
(r
US
rDS
),
+1
CCO
− rDS ,L ,
NOLT
(7)
(8)
CCO
= rDS ,L + rUS ,L ,
NOLT
(9)
3.2.2. Model of cellular wireless networks
In case of radio networks, we concentrate on energy
consumption of base stations only because of the following reasons:
• system boundaries of our model are the end points
of the network. We excluded end user equipment such as personal computers (PCs) and mobile user devices. Mobile user devices such as mobile phones, smartphones and tablet PCs are today
primarily computational devices on which user applications are running, similar to desktop PCs and
laptops.
• user mobile devices are already optimized for low
energy consumption because of mobility requirements and limited size and capacity of batteries.
Thus, there is a very limited room for additional
improvement of energy efficiency of mobile devices.
• base stations are not yet optimized for high energy
efficiency. They usually consume a lot of energy
while always operated in on-state in order to provide a high-quality pilot signal.
• It has been shown recently that the energy consumption of mobile terminals is more than two orders of magnitude lower than that of base stations
[23]. Consequently, the main contributor to the energy consumption is the network equipment and
not the terminals.
We applied a similar approach to that used for optical
network elements to model the wireless networks. We
first defined generic structures of base stations [7, 8, 9].
Two examples of generic structure for macrocell and
femtocell base stations are shown in Figures 7a and
7b, respectively. The values we obtained for the total
energy consumption of the two base station types including typical power consumption breakdown by functional blocks are listed in Tables 3 and 4. The cell size
and supported data rate are design parameters that are
calculated for each scenario and network configuration.
The cell size is determined by the coverage range of
a base station, R, which is calculated in the following
manner.
A first step in determining R is the calculation of the
maximum allowable path loss PLmax . The path loss
is the ratio of the transmitted power to the received
power of the signal. PLmax is then the maximum allowable path loss to which a transmitted signal can be
subjected while still being detectable at the receiver. It
includes all of the possible elements of loss that occur due to interactions between the propagating wave
and obstacles in the line-of-sight between the transmitter and the receiver. Based on this PLmax , the range of a
BS can be determined by using an appropriate propagation model. For the macrocell base station, the Erceg C
model was used [26], while for the femtocell base station the model ITU-R P.1238 (residential environment)
was chosen [27]. The number of macrocell and femtocell base stations needed to cover a predefined area
is then determined by dividing the considered area by
the coverage of the macrocell and femtocell base station. The link adaptation is not taken into account. The
network is designed to provide a predefined maximum
data rate over the considered area, so a fixed bandwidth,
modulation and coding scheme is used. An extension of
the model by including the link adaptation function will
be considered in a future work.
An important parameter to define PLmax is the receiver SNR (Signal-to-Noise Ratio) which determines
the minimum required SNR at the receiver for a certain
BER (Bit Error Rate). The receiver SNR depends on the
used modulation and coding rate. The modulation translates the binary bit stream into an analogue signal, while
the coding rate determines the FEC (Forward Error Correction) which is responsible for the correction of errors
occurred. The most important modulation schemes in
wireless communication are QPSK (Quadrature Phase
Shifting Keying) and 16- or 64-QAM (Quadrature Amplitude Modulation). The coding rate is typically a ratio which indicates how many redundant bits are added
per number of information bit rate. The combination
of modulation scheme and coding rate allows us to determine the physical bit rate which is the total number
a)
nsector
nTx
nTx
Transceiver
Transceiver
Transceiver
Transceiver
Transceiver
Power
Amplifier
Digital signal
processing
Macrocell
Cooling
(if present)
Microwave link
(if present)
Rectifier
Misc.
µP
Backhaul
Interface
network element
max. data rate
(DL/UL) [Gbit/s]
LTE Macrocell BS (3 sectors)
0.7/0.2
power
1665.6 W
typical power consumption breakdown
- 3 transceivers
28%
- 3 power amplifiers
43.6%
- 3 digital signal processors
28%
- µP + memory
0.2%
- backhaul interface
0.1%
- misc.
0.1%
AC-DC inefficiency
15%
additional power for cooling
35%
Table 3: Values of power consumption for LTE macrocell base stations
of physically transferred bits per second including useful data as well as protocol overhead. Other important
parameters to determine PLmax are the antenna gains
of both the receiver and transmitting antennas, the input power of the transmitting antenna, the used MIMO
(Multiple Input Multiple Output) mode, etc. For a more
detailed description of the wireless network model the
reader is referred to [7].
network element
LTE Femtocell BS
max. data rate
(DL/UL) [Gbit/s]
power
0.7/0.2
12 W
typical power consumption breakdown
- transceiver
14.4%
- power amplifier
19.2%
- FPGA
16.3%
- µP + memory
21.3%
- backhaul interface
14.4
- misc.
14.4
AC-DC inefficiency
15%
Table 4: Values of power consumption for LTE femtocell base stations
b)
Femtocell
Radio
interface
Data
encryption
FPGA
Transceiver
Misc.
µP
Backhaul
Interface
FPGA: Field Programmable Gate Array
µP: Microprocessor
4. Considered Scenarios
Power Amplifier
Figure 7: High-level representation of cellular wireless access network equipment: a) macrocell base station and b) femtocell base station
This section describes the assumptions made and the
scenarios considered in this study. Several deployment scenarios are defined for a fixed number of inhabitants and different population densities. We assume
Network element models
Configuration parameters
Macrocell BS
Femtocell BS
o Structure
o Functional blocks
o Energy consumption
pro functional block
o Structure
o Functional blocks
o Energy consumption
pro functional block
Network parameters
o Uplink capacity of BS
o Modulation/coding
o Propagation model
Network model
o Energy consumption of base stations (BSs)
o Coverage
o Number of active users per BS
o Maximum data rates
(upstream and downstream)
Results
Total network‘s energy consumption
Resulting achievable data rate per user
Energy Efficiency
Figure 8: Depiction of the model for evaluation of energy efficiency
of wireless cellular networks
24%
22%
20%
Active users [%]
here 1 Million inhabitants and vary the population density within a certain area between three values, namely
pd1 = 4, 000, pd2 = 1, 000 and pd3 = 100 people per
square kilometer, which we in the residual part of the
present paper refer to as urban, suburban and rural areas. Thus, we can now calculate three areas to be covered by the combined wireless/optical network for three
population densities, i.e., A1 = 250 km2 A2 = 1,000 km2
and A3 = 10,000 km2 , respectively.
Activity of network users is considered through defining the activity factor, fa , which shows how many users
are active in average at each particular point of time
of the day. The activity factors are based on statistical data and measurements [28, 29, 30] and follow a
sinusoidal-like curve with a maximum in the afternoon
hours between 6 pm and 9 pm and a minimum in the
early morning hours as shown in Figure 9 . In order to
take into account future changes of user behavior and
developments in network technologies, we assume here
three activity factors and four average data rates. The
three activity factors are obtained by defining the basis
activity factor and multiplying it by a factor of two and
four as shown in Figure 9. The average access data rates
that we assume for users of wireless and wired networks
are shown in Table 5. DR1 represents the average data
rate per active user in current access networks [30, 31],
while DR2 , DR3 and DR4 are projected data rates for
the years 2014, 2016 and 2020 [31]. The ratio between
downstream/upstream data rates is set to 3/2. The penetration of RF cellular access is assumed to be 90% and
that of wired access 28% [32]. The considered input
parameters are summarized in Tables 5 and 6
Since we are interested in energy efficiency of different LTE femtocell deployment scenarios, we change the
area covered by femtocells from 0 % to 80 %. The deployed femtocells are assumed to be almost uniformly
distributed over the covered area, which can be the case
18%
Activity Factor 1
f a1
Activity Factor 2
Activity Factor 3
16%
14%
12%
fa2
10%
8%
f a3
6%
4%
2%
0%
12 AM 2 AM
4 AM 6 AM 8 AM 10 AM 12 PM 2 PM 4 PM 6 PM
8 PM 10 PM 12 AM
Time of the day [hours]
Figure 9: Considered day/night profiles of user activity (activity factors).
wireless access
wired access
US
[Mbit/s]
DS
[Mbit/s]
US
[Mbit/s]
DS
[Mbit/s]
DR1 (current)
0.189
0.126
0.80
0.34
DR2 (2014)
0.742
0.494
1.80
1.20
DR3 (2016)
1.724
1.149
9.00
6.00
DR4 (2020)
6.780
4.520
600.00
400.00
Table 5: Considered average upstream (US) and downstream (DS)
data rates per active user.
in residential areas with quasi-uniform building density.
Perhaps the most probable case is to deploy femtocells in addition to an existing macrocell-based network
in order to provide higher data rates to users. We compare this case with a pure macrocell-based network and
with a network based on femtocells only, where existing
macrocells and microcells are completely replaced by
femtocells. In the scenarios with macrocells, the macrocell coverage is assumed to be 98 %.
5. Results and Discussions
The obtained results for the three areas and various
femtocell deployment scenarios are presented in Figures 10 - 15.
5.1. Femtocell deployment in suburban areas
Figure 10a shows the total energy consumed by
the wireless cellular network with an optical backhaul
based on GPON within a suburban area covering 1,000
km2 . Besides the parameters listed in Tables 5 and 6,
we assumed here an average area of macrocells of 1.87
km2 and a guaranteed data rate of 8.5 Mbit/s and thus,
the number of macrocell base stations (BS) needed for
achieving a coverage of 98% is 526. Both macro and
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
6
fa3
GPON backhaul
Macrocell + Femtocell
5
Femtocell only
Power
Saving
Macrocell only
4
w/o power saving
w power saving
3
Power
Saving
2
1
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.3
Microcell + Femtocell
w/o power saving
0.25
DR4
w/ power saving
0.2
Relative energy savings [%]
Microcell + Femtocell
p-t-p optical Ethernet
f a3
f a3
22%
fa2
20%
fa2
18%
f a1
16%
14%
f a1
12%
10%
0%
10%
20%
30%
40%
50%
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
28%
24%
60%
70%
0.1
DR3
DR1
0
0%
10%
20%
30%
40%
50%
80%
Femtocell deployment [%]
b)
PON (GPON)
f a1
0.05
Femtocell deployment [%]
26%
GPON
0.15
0.12
GPON backhaul
w/o power saving
0.1
f a1
DR3
w/ power saving
Femtocell only
0.08
0.06
Macrocell + Femtocell
0.04
Macrocell only
0.02
0
0%
10%
20%
Femtocell deployment [%]
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 10: Energy efficiency of femtocell deployment with an optical backhaul in suburban area: a) total network energy consumption and
possible energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp (PON) optical backhaul and for three
different user activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell networks for three different average
access data rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks employing either macrocell or
femtocell or a combination of mactrocell and femtocell base stations.
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
8
f a3
GPON
Macrocell + Femtocell
7
Femtocell only
6
Power
Savings
Macrocell only
5
w/o power saving
w power saving
4
3
Power Saving
2
1
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.4
w/o power saving
0.35
DR4
0.2
0.15
DR3
0.1
0.05
DR1
0
0%
10%
20%
Relative energy savings [%]
fa3
f a3
25%
23%
fa2
fa2
21%
f a1
19%
f a1
17%
15%
0%
10%
20%
30%
40%
50%
Femtocell deployment [%]
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
29%
p-t-p optical Ethernet
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
b)
PON (GPON)
f a1
GPON
0.3
0.25
Femtocell deployment [%]
27%
Microcell + Femtocell
w/ power saving
0.12
GPON
w/o power saving
0.1
f a1
DR3
Femtocell only
w/ power saving
Macrocell + Femtocell
0.08
0.06
Macrocell only
0.04
0.02
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 11: Energy efficiency of femtocell deployment with an optical backhaul and a coexisting optical access network in suburban area:
a) total network energy consumption and possible energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp
(PON) optical backhaul and for three different user activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell
networks for three different average access data rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks
employing either macrocell or femtocell or a combination of mactrocell and femtocell base stations.
1 Million inhabitants
population density
4,000
(urban)
1,000
(suburban)
100
(rural)
covered area [km2 ]
250
1,000
10,000
DS/US ratio
3/2
penetration wireless
90%
penetration wired
28%
coverage macro
98%
bandwidth macro
5 MHz
coding rate macro
1/2
coverage femto
0% - 80%
bandwidth femto
5 MHz
coding rate femto
2/3
Table 6: Considered parameters for femtocell deployments in urban,
suburban and rural areas.
femto base stations use the 16-QAM modulation format. Additionally, we assumed for femtocell base stations that there is no more than one active user within a
femtocell at a time.
The energy consumption is ploted versus the percentage of the area covered by femtocells for the three
network realizations using either macrocells only or
macrocells and femtocells or femtocells only.
As can be seen from Figure 10a, a deployment of
femtocells additionally to the existing macrocell based
network leads to an increased total network energy consumption. If we would implement a pure femtocell network in suburban area, it would consume the same energy for a coverage of approximately 17 % as a network based on macrocells only with a coverage of 98 %.
However, femtocells are able to provide a higher data
rate per user, which can lead to a lower energy consumed to transmit a certain amount of data. Thus, femtocell networks, if highly utilized, can achieve a high
energy efficiency measured in Gbit/s per Watt as it is
evident from Figure 10d.
An additional benefit of using femtocells is that various power saving methods can be more easily and more
effectively applied. In this study, we assume the use of
the idle mode procedure based on noise rise as described
in [33] for femtocells and shedding of optical network
terminals (ONTs) for optical access networks as defined
in [34]. In both cases the connection to the network is
retained in order to avoid time-consuming boot-up times
and the need for re-synchronization. When applying
these methods, the power consumption of a femtocell
base station can be reduced from 12 Watt when active to
7.8 Watt in the idle mode while, e.g., a GPON ONT consumes about 2.6 Watt with power shedding in comparison to 4.2 Watt in the active mode. In case of femtocell
base stations, the power savings are achieved by switching off the power amplifier, RF transceiver, and miscellaneous hardware components that implement various
nonessential functionalities for the idle mode such as
data encryption and hardware authentication. However,
a low-power radio sniffer that consumes 0.3 W has to
be switched on in order to detect an active call from a
mobile device to the underlying macrocell [33]. On the
other hand, the power shedding techniques allows large
savings in the optical access network terminals through
switching off the user interface (Eth. PHY and Eth.
MAC) and putting the ONT (ONU) SoC in low activity mode [34].
Utilization of the power saving techniques can indeed
help in reducing the total energy consumption as shown
in Figure 10a. In suburban areas, the achievable reduction of energy consumption is between 16 % and 27 %.
Figure 10b shows the relative savings for the three considered user activity profiles as presented in Figure 9.
The relative energy savins are calculated using the following formula: (E save − Etot )/Etot , where Etot and E save
denote the total network energy consumption and the
amount of energy that can be saved by utilizing the lowpower modes, respectively. If users become more active, a lower number of femtocell base stations can be
put in the idle mode, and thus, less energy can be saved.
For instance, relative power savings of more than 25 %
are possible for 80 % femtocell coverage when considering the user activity corresponding to the profile fa3 .
When more users are concurrently active, which is the
case for fa1 , the maximum achievable reduction of energy consumption in comparison to the case without using low-power modes becomes less than 17 %. If the
p-t-p optical backhaul is used instead of a PON based
one, the achievable relative savings can be further reduced by approximately 2 %.
Figures 10c and 10d take into consideration additionally to energy consumption also the achievable data
rate per user. Here, energy efficiency of different
wireless network deployment scenarios is presented in
Gbit/s/Watt. For high average access data rates the energy efficiency increases with increasing the femtocell
coverage. Thus, although adding femtocell base statoins to the network leads to a higher total energy consumption, the advantage of providing higher data rates
results in a lower energy needed to transmit a certain
amount of data. However, if the high data rate per user
that can be provided by femtocell base stations is not
used, which is the case for the relatively low average
access data rate DR1 , energy efficiency decreases with
increasing the femtocell coverage. The highest energy
efficiency and the largest relative energy savings can be
achieved in a pure femtocell network because high data
rates per user are possible and a large number of base
stations can be put in the idle mode (see Figure 10d).
Energy efficiency of the combined optical wired and
cellular wireless access network is shown in Figure 11.
The same optical access technology (GPON) is assumed
for both the fixed access and the backhaul of the mobile network. The combined wired/wireless access network consumes more energy than the optically backhauled wireless network (see Figures 11a and 10a), but
it is also able to serve more users concurrently with
higher data rates. For example, a network providing
98% coverage with macrocells and 80 % coverage by
femtocells together with an optical wired access based
on GPON with a penetration of 28 % consumes slightly
less than 8 MW (Figure 11a) in comparison to 5.2 MW
in case of the wireless network with optical backhoul
only (see Figure 10a). However, due to the presence of
the wired optical access network, the achievable average
data rates are high even for the case with less deployed
femtocell base stations (Figures 11c and 11d), which
leads to an initial high energy efficiency and large relative power savings at high average access rates. Also
the difference between p-t-mp (PON) and p-t-p optical
access becomes more evident (Figure 11b).
5.2. Femtocell deployment in urban areas
For urban areas of 250 km2 , we calculated the required number of macrocell base stations to be 629 with
an average cell area of 0.39 km2 and a guaranteed data
rate of 8.5 Mbit/s. In this case, QPSK was assumed for
macro and 16-QAM for femto base stations. All other
configuration parameters are reported in Table 5.
In urban areas, a high coverage can be achieved by
deploying less base stations. Thus, the total energy consumption of the optically backhauled LTE network in
urban area is significantly lower than that of a suburban network as it can be seen when comparing Figures
12a and 10a. Concurrently, a lower number of femtocell base stations can be put into the idle mode because
of the high population density, so the chance there is an
active user within a femtocell increases. Therefore, the
relative energy savings are not as high as in the suburban area (see Figures 12b and 10b), but the amount of
transmitted data per unit energy is for a factor of 2 to
3 larger (Figures 12c and 12d in comparison to Figures
10c and 10d).
Similar conclusions can be drown when observing
Figure 13 that shows results for energy efficiency of a
combined optical wired and cellular wireless network
in urban areas. The only difference to the backhaul only
case is that when optical wired access is added the relative power savings become larger. Although high energy savings are possible in the urban area, the relative
savings decrease with increase in femtocell coverage in
case of high user activity, i.e., for user activity profiles
fa1 and fa2 , which is evident from Figure 13b. This is
because as more femtocell base stations are installed,
the total energy consumption increases, while the benefit of using low-power modes decreases due to the increased user activity and longer periods in which femtocell base stations have to be in the active state.
5.3. Femtocell deployment in rural areas
In a rural area of 10,000 km2 , the calculated number of macrocell base stations is 1,816 with an average
cell area of 5.4 km2 and a guaranteed data rate of 2.8
Mbit/s. Similar to the suburban case, we also assumed
here QPSK for macro and 16-QAM for femto base stations.
As one could intuitively expect, a combined optical/wireless access network seems not to be an adequate
option for rural areas due to the very low population
density. As it becomes evident from Figures 14 and 15,
a roll out of an optical access network to serve as backhaul for a femtocell based LTE network would lead to
a very high energy consumption and a low energy efficiency, especially for a large femtocell coverage and low
average access data rates. A network in a rural area with
98 % macrocell coverage, 80 % femtocell coverage, and
28 % GPON penetration would consume about 60 MW
of power (see Figure a), which is 15 times more energy
than needed for powering the network in an urban area
(Figure 13). Furthermore, the energy efficiency measured in Gbit/sec/Watt is for a factor of 5 to 15 lower
in a rural area than in an urban area (Figures 15d and
13d). An interesting effect of decreasing energy efficiency with increasing femtocell coverage can be observed in case of coexistence of macrocell and femtocell base stations in rural areas (Figures 14d and 15d).
This is because in rural areas the energy consumption
increases faster than the achievable average access data
rate when increasing the number of femtocell base stations. Hence, even for very high average access rates the
combined optical wired and cellular wireless network
provides low energy efficiency in rural areas. Consequently, the deployment scenarios for femtocells in rural areas do not bring any advantage regarding both the
total energy consumption and the achievable data rate
per unit power.
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
3
f a3
GPON backhaul
Macrocell + Femtocell
Femtocell only
Power Saving
Macrocell only
2
w/o power saving
w power saving
1
Power Saving
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.8
w/o power saving
DR4
f a1
0.4
0.2
DR3
DR1
0
0%
10%
20%
Relative energy savings [%]
Microcell + Femtocell
fa3
8%
f a3
6%
fa2
fa2
4%
f a1
f a1
2%
0%
0%
10%
20%
30%
40%
50%
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
12%
p-t-p optical Ethernet
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
b)
PON (GPON)
GPON backhaul
0.6
Femtocell deployment [%]
10%
Microcell + Femtocell
w/ power saving
0.4
GPON backhaul
w/o power saving
w/ power saving
0.3
f a1
DR3
Femtocell only
0.2
Macrocell + Femtocell
0.1
Macrocell only
0
0%
10%
20%
Femtocell deployment [%]
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 12: Energy efficiency of femtocell deployment with an optical backhaul in urban area: a) total network energy consumption and possible
energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp (PON) optical backhaul and for three different user
activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell networks for three different average access data
rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks employing either macrocell or femtocell or a
combination of mactrocell and femtocell base stations.
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
4
fa3
GPON
Macrocell + Femtocell
Femtocell only
Power
Saving
Macrocell only
3
w/o power saving
w power saving
2
Power
Saving
Power
Saving
1
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.6
w/o power saving
0.5
Relative energy savings [%]
fa3
p-t-p optical Ethernet
fa3
16%
fa2
fa2
12%
f a1
f a1
10%
0%
10%
20%
30%
40%
50%
Femtocell deployment [%]
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
20%
14%
f a1
DR4
0.3
DR3
0.2
0.1
DR1
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
b)
18%
GPON
0.4
Femtocell deployment [%]
PON (GPON)
Microcell + Femtocell
w/ power saving
0.3
GPON
w/o power saving
0.25
w/ power saving
DR3
f a1
Femtocell only
0.2
Macrocell + Femtocell
0.15
0.1
Macrocell only
0.05
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 13: Energy efficiency of femtocell deployment with an optical backhaul and a coexisting optical access network in urban area: a) total
network energy consumption and possible energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp (PON)
optical backhaul and for three different user activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell networks
for three different average access data rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks employing
either macrocell or femtocell or a combination of mactrocell and femtocell base stations.
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
50
fa3
GPON backhaul
Macrocell + Femtocell
Femtocell only
40
Macrocell only
Power
Savings
w/o power saving
w power saving
30
20
10
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.08
w/o power saving
0.06
DR4
0.04
0.02
DR3
DR1
0
0%
10%
20%
Femtocell deployment [%]
Relative energy savings [%]
Microcell + Femtocell
p-t-p optical Ethernet
f a3
f a1
fa2
29%
fa3
fa2
f a1
27%
25%
0%
10%
20%
30%
40%
50%
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
33%
31%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
b)
PON (GPON)
f a1
Microcell + Femtocell GPON backhaul
w/ power saving
0.04
Macrocell only
GPON
backhaul
DR3
0.02
w/o power saving
f a1
w/ power saving
Macrocell + Femtocell
Femtocell only
0
0%
10%
20%
Femtocell deployment [%]
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 14: Energy efficiency of femtocell deployment with an optical backhaul in rural area: a) total network energy consumption and possible
energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp (PON) optical backhaul and for three different
user activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell networks for three different average access data
rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks employing either macrocell or femtocell or a
combination of mactrocell and femtocell base stations.
Data rate per unit power [Gbit/s/Watt]
c)
Total network energy conumption [MW]
a)
60
GPON
Macrocell + Femtocell
50
f a3
Femtocell only
Macrocell only
40
Power
Savings
w/o power saving
w power saving
30
20
10
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.12
w/o power saving
0.1
Relative energy savings [%]
fa3
PON (GPON)
p-t-p optical Ethernet
f a3
fa2
27%
f a2
f a1
25%
0%
10%
20%
30%
40%
50%
Femtocell deployment [%]
60%
70%
80%
Data rate per unit power [Gbit/s/Watt]
d)
33%
f a1
f a1
0.06
DR4
0.04
0.02
DR3
DR1
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
b)
29%
GPON
0.08
Femtocell deployment [%]
31%
Microcell + Femtocell
w/ power saving
0.04
GPON
DR3
w/o power saving
w/ power saving
0.02
f a1
Macrocell only
Macrocell + Femtocell
Femtocell only
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
Femtocell deployment [%]
Figure 15: Energy efficiency of femtocell deployment with an optical backhaul and a coexisting optical access network in rural area: a) total
network energy consumption and possible energy savings when using low-power modes, b) relative energy savings with p-t-p and p-t-mp (PON)
optical backhaul and for three different user activity factors ( fa1 , fa2 , and fa3 ,), c) energy efficiency of coexisting macrocell and femtocell networks
for three different average access data rates (DR1 , DR2 , and DR3 ), and d) energy efficiency of future optically backhauled LTE networks employing
either macrocell or femtocell or a combination of mactrocell and femtocell base stations.
6. Summary and Conclusions
In conclusion, we presented a novel model for evaluating the energy efficiency of converged wireless/optical
access networks. The model takes into account the
main architectural and implementational aspects of both
wireless and optical parts of the network. Several deployment scenarios for combined wireless/optical networks were defined and possible deployments of femtocell base stations analyzed by means of achievable energy efficiency. We also analyzed possible energy savings by utilizing practically applicable methods that are
based on low-power modes for both femtocell base stations and optical network terminals.
Energy efficiency of a deployment of femtocell base
stations strongly depends on the population density, average access data rates, and the number of concurrently
active users in a certain area to be covered. We concentrated on an implementation basing on the LTE technology for the wireless part and the point-to-point optical Ethernet or passive optical networks (Ethernet-based
EPON and ATM-based GPON) for the optical wired access and wireless backhaul. In general, the combined
wireless/optical access network has the potential to increase the overall energy efficiency of the access area,
especially when assuming future high average access
data rates. In this context, passive optical access networks (PON) are better suited to serve as backaul to
the LTE network. If the femtocell base stations are installed in addition to an existing macrocell-based network, the overall power consumption of the combined
network increases with the increase in femtocell coverage. However, in urban and suburban areas, the introduction of femtocells usually leads to an increased energy efficiency measured as the amount of data that can
be transmitted per unit energy, i.e., in bit/second/Watt ≡
bit/Joule. In rural area, the very high energy consumption of a combined femtocell and macrocell network
leads to a decrease in efficiency in comparison to pure
macrocell network. For example, in urban and suburban
areas, relative energy savings of about 10 % to 27 %
and efficiencies on the order of hundreds of Mbit/s/W
are achievable when utilizing low-power modes. Although in rural areas the use of low-power modes can
lead to high energy savings of up to 30 %, the total energy consumption increases by a factor of 15 and the
energy efficiency is by 5 to 15 times lower than in the
urban area, which makes femtocells less attractive for
rural areas from the energy efficiency perspective.
Thus, the findings of this study can be summarized
as follows. Although deployment of femtocells does
not necessarily lead to a lower total network power con-
sumption, it can certainly result in a significant increase
of both access data rates and energy efficiency in areas
with high population densities such as in city centers or
within business parks and shopping centers. The energy
efficiency can be further increased by utilizing effective
power saving mechanisms and using passive optical networks as wireless backhaul. Another important precondition for fully exploiting the potential of femtocells is
a broad availability and an extensive use of bandwidthintensive applications and services. According to our
results and taking into account current prognoses for
the future growth of access data rates, a remarkable improvement of energy efficiency due to a wide deployment of femto base stations can not be expected before
2016.
Acknowledgments
The work described in this paper was supported in
part by the project HOME-ICT funded by the Austrian
Fund for Climate and Energy and accomplished within
the framework of the program ”NEUE ENERGIEN
2020”.
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