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Cellular Architecture and Key Technologies for 5G Wireless
Communication Networks
Citation for published version:
Wang, C-X, Haider, F, Gao, X, You, X-H, Yang, Y, Yuan, D, Aggoune, HM, Haas, H, Fletcher, S &
Hepsaydir, E 2014, 'Cellular Architecture and Key Technologies for 5G Wireless Communication Networks',
IEEE Communications Magazine, vol. 52, no. 2, pp. 122-130.
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IEEE Communications Magazine • February 2014
122
0163-6804/14/$25.00 © 2014 IEEE
INTRODUCTION
The innovative and effective use of information
and communication technologies (ICT) is
becoming increasingly important to improve the
economy of the world [1]. Wireless communica-
tion networks are perhaps the most critical ele-
ment in the global ICT strategy, underpinning
many other industries. It is one of the fastest
growing and most dynamic sectors in the world.
The European Mobile Observatory (EMO)
reported that the mobile communication sector
had total revenue of €174 billion in 2010, there-
by bypassing the aerospace and pharmaceutical
sectors [2]. The development of wireless tech-
nologies has greatly improved people’s ability to
communicate and live in both business opera-
tions and social functions.
The phenomenal success of wireless mobile
communications is mirrored by a rapid pace of
technology innovation. From the second genera-
tion (2G) mobile communication system debuted
in 1991 to the 3G system first launched in 2001,
the wireless mobile network has transformed
from a pure telephony system to a network that
can transport rich multimedia contents. The 4G
wireless systems were designed to fulfill the
requirements of International Mobile Telecom-
munications-Advanced (IMT-A) using IP for all
services [3]. In 4G systems, an advanced radio
interface is used with orthogonal frequency-divi-
sion multiplexing (OFDM), multiple-input multi-
ple-output (MIMO), and link adaptation
technologies. 4G wireless networks can support
data rates of up to 1 Gb/s for low mobility, such
as nomadic/local wireless access, and up to 100
Mb/s for high mobility, such as mobile access.
Long-Term Evolution (LTE) and its extension,
LTE-Advanced systems, as practical 4G systems,
have recently been deployed or soon will be
deployed around the globe.
However, there is still a dramatic increase in
the number of users who subscribe to mobile
broadband systems every year. More and more
ABSTRACT
The fourth generation wireless communica-
tion systems have been deployed or are soon to
be deployed in many countries. However, with
an explosion of wireless mobile devices and ser-
vices, there are still some challenges that cannot
be accommodated even by 4G, such as the spec-
trum crisis and high energy consumption. Wire-
less system designers have been facing the
continuously increasing demand for high data
rates and mobility required by new wireless
applications and therefore have started research
on fifth generation wireless systems that are
expected to be deployed beyond 2020. In this
article, we propose a potential cellular architec-
ture that separates indoor and outdoor scenar-
ios, and discuss various promising technologies
for 5G wireless communication systems, such as
massive MIMO, energy-efficient communica-
tions, cognitive radio networks, and visible light
communications. Future challenges facing these
potential technologies are also discussed.
5G WIRELESS COMMUNICATION SYSTEMS:
PROSPECTS AND CHALLENGES
Cheng-Xiang Wang, Heriot-Watt University and University of Tabuk
Fourat Haider, Heriot-Watt University
Xiqi Gao and Xiao-Hu You, Southeast University
Yang Yang, ShanghaiTech University
Dongfeng Yuan, Shandong University
Hadi M. Aggoune, University of Tabuk
Harald Haas, University of Edinburgh
Simon Fletcher, NEC Telecom MODUS Ltd.
Erol Hepsaydir, Hutchison 3G UK
Cellular Architecture and Key
Technologies for 5G Wireless
Communication Networks
WANG_LAYOUT.qxp_Layout 1/30/14 1:29 PM Page 122
IEEE Communications Magazine • February 2014
123
people crave faster Internet access on the move,
trendier mobiles, and, in general, instant com-
munication with others or access to information.
More powerful smartphones and laptops are
becoming more popular nowadays, demanding
advanced multimedia capabilities. This has
resulted in an explosion of wireless mobile
devices and services. The EMO pointed out that
there has been a 92 percent growth in mobile
broadband per year since 2006 [2]. It has been
predicted by the Wireless World Research
Forum (WWRF) that 7 trillion wireless devices
will serve 7 billion people by 2017; that is, the
number of network-connected wireless devices
will reach 1000 times the world’s population [4].
As more and more devices go wireless, many
research challenges need to be addressed.
One of the most crucial challenges is the
physical scarcity of radio frequency (RF) spectra
allocated for cellular communications. Cellular
frequencies use ultra-high-frequency bands for
cellular phones, normally ranging from several
hundred megahertz to several gigahertz. These
frequency spectra have been used heavily, mak-
ing it difficult for operators to acquire more.
Another challenge is that the deployment of
advanced wireless technologies comes at the cost
of high energy consumption. The increase of
energy consumption in wireless communication
systems causes an increase of CO
2
emission indi-
rectly, which currently is considered as a major
threat for the environment. Moreover, it has
been reported by cellular operators that the
energy consumption of base stations (BSs) con-
tributes to over 70 percent of their electricity bill
[5]. In fact, energy-efficient communication was
not one of the initial requirements in 4G wire-
less systems, but it came up as an issue at a later
stage. Other challenges are, for example, aver-
age spectral efficiency, high data rate and high
mobility, seamless coverage, diverse quality of
service (QoS) requirements, and fragmented
user experience (incompatibility of different
wireless devices/interfaces and heterogeneous
networks), to mention only a few.
All the above issues are putting more pres-
sure on cellular service providers, who are facing
continuously increasing demand for higher data
rates, larger network capacity, higher spectral
efficiency, higher energy efficiency, and higher
mobility required by new wireless applications.
On the other hand, 4G networks have just about
reached the theoretical limit on the data rate
with current technologies and therefore are not
sufficient to accommodate the above challenges.
In this sense, we need groundbreaking wireless
technologies to solve the above problems caused
by trillions of wireless devices, and researchers
have already started to investigate beyond 4G
(B4G) or 5G wireless techniques. The project
UK-China Science Bridges: (B)4G Wireless Mobile
Communications (http://www.ukchinab4g. ac.uk/) is
perhaps one of the first projects in the world to
start B4G research, where some potential B4G
technologies were identified. Europe and China
have also initiated some 5G projects, such as
METIS 2020 (https://www.metis2020. com/) sup-
ported by EU and National 863 Key Project in
5G supported by the Ministry of Science and
Technology (MOST) in China. Nokia Siemens
Networks described how the underlying radio
access technologies can be developed further to
support up to 1000 times higher traffic volumes
compared to 2010 travel levels over the next 10
years [6]. Samsung demonstrated a wireless sys-
tem using millimeter (mm) wave technologies
with data rates faster than 1 Gb/s over 2 km [7].
What will the 5G network, which is expected
to be standardized around 2020, look like? It is
now too early to define this with any certainty.
However, it is widely agreed that compared to
the 4G network, the 5G network should achieve
1000 times the system capacity, 10 times the
spectral efficiency, energy efficiency and data
rate (i.e., peak data rate of 10 Gb/s for low
mobility and peak data rate of 1 Gb/s for high
mobility), and 25 times the average cell through-
put. The aim is to connect the entire world, and
achieve seamless and ubiquitous communica-
tions between anybody (people to people), any-
thing (people to machine, machine to machine),
wherever they are (anywhere), whenever they
need (anytime), by whatever electronic
devices/services/networks they wish (anyhow).
This means that 5G networks should be able to
support communications for some special sce-
narios not supported by 4G networks (e.g., for
high-speed train users). High-speed trains can
easily reach 350 up to 500 km/h, while 4G net-
works can only support communication scenarios
up to 250 km/h. In this article, we propose a
potential 5G cellular architecture and discuss
some promising technologies that can be
deployed to deliver the 5G requirements.
The remainder of this article is organized as
follows. We propose a potential 5G cellular
architecture. We describe some promising key
technologies that can be adopted in the 5G sys-
tem. Future challenges are highlighted. Finally,
conclusions are drawn.
A POTENTIAL 5G WIRELESS
CELLULAR ARCHITECTURE
To address the above challenges and meet the
5G system requirements, we need a dramatic
change in the design of cellular architecture. We
know that wireless users stay indoors for about
80 percent of time, while only stay ourdoors
about 20 percent of the time [8]. The current
conventional cellular architecture normally uses
an outdoor BS in the middle of a cell communi-
cating with mobile users, no matter whether they
stay indoors or outdoors. For indoor users com-
municating with the outdoor BS, the signals have
to go through building walls, and this causes very
high penetration loss, which significantly dam-
ages the data rate, spectral efficiency, and ener-
gy efficiency of wireless transmissions.
One of the key ideas of designing the 5G cel-
lular architecture is to separate outdoor and
indoor scenarios so that penetration loss through
building walls can somehow be avoided. This will
be assisted by distributed antenna system (DAS)
and massive MIMO technology [9], where geo-
graphically distributed antenna arrays with tens
or hundreds of antenna elements are deployed.
While most current MIMO systems utilize two
to four antennas, the goal of massive MIMO sys-
One of the key ideas
of designing the 5G
cellular architecture is
to separate outdoor
and indoor scenarios
so that penetration
loss through building
walls can be some-
how avoided. This
will be assisted by
distributed antenna
system (DAS) and
massive MIMO
technology.
WANG_LAYOUT.qxp_Layout 1/30/14 1:29 PM Page 123
tems is to exploit the potentially large capacity
gains that would arise in larger arrays of anten-
nas. Outdoor BSs will be equipped with large
antenna arrays with some antenna elements (also
large antenna arrays) distributed around the cell
and connected to the BS via optical fibers, bene-
fiting from both DAS and massive MIMO tech-
nologies. Outdoor mobile users are normally
equipped with limited numbers of antenna ele-
ments, but they can collaborate with each other
to form a virtual large antenna array, which
together with BS antenna arrays will construct
virtual massive MIMO links. Large antenna
arrays will also be installed outside of every
building to communicate with outdoor BSs or
distributed antenna elements of BSs, possibly
with line of sight (LoS) components. Large anten-
na arrays have cables connected to the wireless
access points inside the building communicating
with indoor users. This will certainly increase the
infrastructure cost in the short term while signifi-
cantly improving the cell average throughput,
spectral efficiency, energy efficiency, and data
rate of the cellular system in the long run.
Using such a cellular architecture, as indoor
users only need to communicate with indoor
wireless access points (not outdoor BSs) with
large antenna arrays installed outside build-
ings, many technologies can be utilized that are
suitable for short-range communications with
high data rates. Some examples include WiFi,
femtocell, ultra wideband (UWB), mm-wave
communications (3–300 GHz) [7], and visible
light communications (VLC) (400–490 THz)
[10]. It is worth mentioning that mm-wave and
VLC technologies use higher frequencies not
traditionally used for cellular communications.
These high-frequency waves do not penetrate
solid materials very well and can readily be
absorbed or scattered by gases, rain, and
foliage. Therefore, it is hard to use these waves
for outdoor and long distance applications.
However, with large bandwidths available, mm-
wave and VLC technologies can greatly
increase the transmission data rate for indoor
scenarios. To solve the spectrum scarcity prob-
lem, besides finding new spectrum not tradi-
tionally used for wireless services (e.g.,
mm-wave communications and VLC), we can
also try to improve the spectrum utilization of
existing radio spectra, for example, via cogni-
tive radio (CR) networks [11].
The 5G cellular architecture should also be a
heterogeneous one, with macrocells, microcells,
small cells, and relays. To accommodate high-
mobility users such as users in vehicles and high-
speed trains, we have proposed the mobile
femtocell (MFemtocell) concept [12], which
combines the concepts of mobile relay and fem-
tocell. MFemtocells are located inside vehicles
to communicate with users within the vehicle,
while large antenna arrays are located outside
the vehicle to communicate with outdoor BSs.
An MFemtocell and its associated users are all
viewed as a single unit to the BS. From the user
point of view, an MFemtocell is seen as a regu-
lar BS. This is very similar to the above idea of
separating indoor (inside the vehicle) and out-
door scenarios. It has been shown in [12] that
users using MFemtocells can enjoy high-data-
rate services with reduced signaling overhead.
The above proposed 5G heterogeneous cellular
architecture is illustrated in Fig. 1.
PROMISING KEY
5G WIRELESS TECHNOLOGIES
In this section, based on the above proposed
heterogeneous cellular architecture, we discuss
some promising key wireless technologies that
can enable 5G wireless networks to fulfill perfor-
mance requirements. The purpose of developing
these technologies is to enable a dramatic capac-
ity increase in the 5G network with efficient uti-
lization of all possible resources. Based on the
well-known Shannon theory, the total system
capacity C
sum
can be approximately expressed by
(1)
where B
i
is the bandwidth of the ith channel, P
i
is the signal power of the ith channel, and N
p
denotes the noise power. From Eq. 1, it is clear
that the total system capacity C
sum
is equivalent
to the sum capacity of all subchannels and het-
erogeneous networks. To increase C
sum
, we can
increase the network coverage (via heteroge-
neous networks with macrocells, microcells,
small cells, relays, MFemtocell [12], etc.), num-
ber of subchannels (via massive MIMO [9], spa-
tial modulation [SM] [13], cooperative MIMO,
DAS, interference management, etc.), bandwidth
(via CR networks [11], mm-wave communica-
tions, VLC [10], multi-standard systems, etc.),
and power (energy-efficient or green communi-
cations). In the following, we focus on some of
the key technologies.
MASSIVE MIMO
MIMO systems consist of multiple antennas at
both the transmitter and receiver. By adding
multiple antennas, a greater degree of freedom
(in addition to time and frequency dimensions)
in wireless channels can be offered to accom-
modate more information data. Hence, a signif-
icant performance improvement can be
obtained in terms of reliability, spectral effi-
ciency, and energy efficiency. In massive MIMO
systems, the transmitter and/or receiver are
equipped with a large number of antenna ele-
ments (typically tens or even hundreds). Note
that the transmit antennas can be co-located or
distributed (i.e., a DAS system) in different
applications. Also, the enormous number of
receive antennas can be possessed by one device
or distributed to many devices. Besides inherit-
ing the benefits of conventional MIMO sys-
tems, a massive MIMO system can also
significantly enhance both spectral efficiency
and energy efficiency [9]. Furthermore, in mas-
sive MIMO systems, the effects of noise and
fast fading vanish, and intracell interference
can be mitigated using simple linear precoding
and detection methods. By properly using multi-
user MIMO (MU-MIMO) in massive MIMO
systems, the medium access control (MAC)
layer design can be simplified by avoiding com-
∑∑
≈ +
⎛
⎝
⎜
⎞
⎠
⎟
CB
P
N
log 1
i
i
p
sum 2
ChannelsHetNets
IEEE Communications Magazine • February 2014
124
The 5G cellular archi-
tecture should also
be a heterogeneous
one, with macrocells,
microcells, small cells,
and relays. To
accommodate high-
mobility users such
as users in vehicles
and high-speed
trains, we have pro-
posed the mobile
femtocell concept,
which combines the
concepts of mobile
relay and femtocell.
WANG_LAYOUT.qxp_Layout 1/30/14 1:29 PM Page 124
IEEE Communications Magazine • February 2014
125
plicated scheduling algorithms [14]. With MU-
MIMO, the BS can send separate signals to
individual users using the same time-frequency
resource, as first pro. Consequently, these main
advantages enable the massive MIMO system
to be a promising candidate for 5G wireless
communication networks.
SPATIAL MODULATION
Spatial modulation, as first proposed by Haas et
al., is a novel MIMO technique that has been
proposed for low-complexity implementation of
MIMO systems without degrading system per-
formance [13]. Instead of simultaneously trans-
mitting multiple data streams from the available
antennas, SM encodes part of the data to be
transmitted onto the spatial position of each
transmit antenna in the antenna array. Thus,
the antenna array plays the role of a second (in
addition to the usual signal constellation dia-
gram) constellation diagram (the so-called spa-
tial constellation diagram), which can be used
to increase the data rate (spatial multiplexing)
with respect to single-antenna wireless systems.
Only one transmit antenna is active at any time,
while other antennas are idle. A block of infor-
mation bits is split into two sub-blocks of
log
2
(N
B
) and log
2
(M) bits, where N
B
and M are
the number of transmit antennas and the size
of the complex signal constellation diagram,
respectively. The first sub-block identifies the
active antenna from a set of transmit antennas,
while the second sub-block selects the symbol
from the signal constellation diagram that will
be sent from that active antenna. Therefore,
SM is a combination of space shift keying (SSK)
and amplitude/phase modulation. Figure 2
shows the SM constellation diagram with 4
transmit antennas (N
B
= 4) and quadrature
phase shift keying (QPSK) modulation (M = 4)
as an example. The receiver can then employ
optimal maximum likelihood (ML) detection to
decode the received signal.
Spatial modulation can mitigate three major
problems in conventional MIMO systems: inter-
channel interference, inter-antenna synchroniza-
tion, and multiple RF chains [13]. Moreover,
low-complexity receivers in SM systems can be
designed and configured for any number of
transmit and receive antennas, even for unbal-
anced MIMO systems. We have to point out that
the multiplexing gain in SM increases logarith-
mically with the increase in the number of trans-
mit antennas, while it increases linearly in
conventional MIMO systems. Therefore, the low
implementation complexity comes at the expense
of sacrificing some degrees of freedom. Most
research on SM focuses on the case of a single
receiver (i.e., single-user SM). Multi-user SM
can be considered as a new research direction to
be considered in 5G wireless communication sys-
tems.
COGNITIVE RADIO NETWORKS
The CR network is an innovative software
defined radio technique considered to be one of
the promising technologies to improve the uti-
lization of the congested RF spectrum [9].
Adopting CR is motivated by the fact that a
large portion of the radio spectrum is underuti-
lized most of the time. In CR networks, a sec-
ondary system can share spectrum bands with
the licensed primary system, either on an inter-
ference-free basis or on an interference-tolerant
basis [9]. The CR network should be aware of
the surrounding radio environment and regulate
its transmission accordingly. In interference-free
CR networks, CR users are allowed to borrow
spectrum resources only when licensed users do
not use them. A key to enabling interference-
Figure 1. A proposed 5G heterogeneous wireless cellular architecture.
CR network
Large MIMO
network
Mobile femtocell
network
LOS
channel
.
.
.
.
.
.
.
.
Internet
Internet
Internet
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..........
.
...
.
..
.
...
VLC
60 GHz
Gigabit Ethernet
WiFi
Femtocell
Core network
The 5G CR network
is an innovative soft-
ware defined radio
technique which has
been considered as
one of the promising
technologies to
improve the utiliza-
tion of the congest-
ed RF spectrum.
Adopting CR is moti-
vated by the fact
that a large portion
of the radio spec-
trum is underutilized
most of the time.
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