IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 1
What Will 5G Be?
Jeffrey G. Andrews, Fellow, IEEE, Stefano Buzzi, Senior Member, IEEE, Wan Choi, Senior Member, IEEE,
Stephen Hanly, Member, IEEE, Angel Lozano, Fellow, IEEE, Anthony C.K. Soong, Fellow, IEEE,
Jianzhong Charlie Zhang, Senior Member, IEEE
Abstract—What will 5G be? What it will not be is an in-
cremental advance on 4G. The previous four generations of
cellular technology have each been a major paradigm shift
that has broken backwards compatibility. And indeed, 5G will
need to be a paradigm shift that includes very high carrier
frequencies with massive bandwidths, extreme base station and
device densities and unprecedented numbers of antennas. But
unlike the previous four generations, it will also be highly
integrative: tying any new 5G air interface and spectrum together
with LTE and WiFi to provide universal high-rate coverage and
a seamless user experience. To support this, the core network
will also have to reach unprecedented levels of ﬂexibility and
intelligence, spectrum regulation will need to be rethought and
improved, and energy and cost efﬁciencies will become even more
critical considerations. This paper discusses all of these topics,
identifying key challenges for future research and preliminary
5G standardization activities, while providing a comprehensive
overview of the current literature, and in particular of the papers
appearing in this special issue.
A. The Road to 5G
In just the past year, preliminary interest and discussions
about a possible 5G standard have evolved into a full-ﬂedged
conversation that has captured the attention and imagination
of researchers and engineers around the world. As the long-
term evolution (LTE) system embodying 4G has now been
deployed and is reaching maturity, where only incremental
improvements and small amounts of new spectrum can be
expected, it is natural for researchers to ponder “what’s next?”
. However, this is not a mere intellectual exercise. Thanks
largely the annual visual network index (VNI) reports released
by Cisco, we have quantitative evidence that the wireless
data explosion is real and will continue. Driven largely by
smartphones, tablets, and video streaming, the most recent
(Feb. 2014) VNI report  and forecast makes plain that
an incremental approach will not come close to meeting the
demands that networks will face by 2020.
In just a decade, the amount of IP data handled by wireless
networks will have increased by well over a factor of 100:
from under 3 exabytes in 2010 to over 190 exabytes by 2018,
on pace to exceed 500 exabytes by 2020. This deluge of data
J. G. Andrews (firstname.lastname@example.org) is with the University of Texas
at Austin, USA.
S. Buzzi (email@example.com) is with University of Cassino and Southern
Lazio, Italy, and with CNIT, Italy.
W. Choi (firstname.lastname@example.org) is with Korea Advanced Institute of Science
and Technology (KAIST), Daejeon, South Korea.
S. Hanly (email@example.com) is with Macquarie University, Sydney,
A. Lozano (firstname.lastname@example.org) is with Universitat Pompeu Fabra
(UPF), Barcelona, Spain.
A. C. K. Soong (email@example.com) is with Huawei Technologies,
Plano, Texas, USA.
J. C. Zhang (firstname.lastname@example.org) is with Samsung Electronics,
Richardson, Texas, USA.
Article last revised: May 14, 2014
has been driven chieﬂy by video thus far, but new unforeseen
applications can reasonably be expected to materialize by
2020. In addition to the sheer volume of data, the number of
devices and the data rates will continue to grow exponentially.
The number of devices could reach the tens or even hundreds
of billions by the time 5G comes to fruition, due to many new
applications beyond personal communications –. It is our
duty as engineers to meet these intense demands via innovative
new technologies that are smart and efﬁcient yet grounded in
reality. Academia is engaging in large collaborative projects
such as METIS  and 5GNOW , while the industry
is driving preliminary 5G standardization activities (cf. Sec.
IV-B). To further strengthen these activities, the public-private
partnership for 5G infrastructure recently constituted in Europe
will funnel massive amounts of funds into related research .
This article is an attempt to summarize and overview many
of these exciting developments, including the papers in this
special issue. In addition to the highly visible demand for
ever more network capacity, there are a number of other
factors that make 5G interesting, including the potentially
disruptive move to millimeter wave (mmWave) spectrum, new
market-driven ways of allocating and re-allocating bandwidth,
a major ongoing virtualization in the core network that might
progressively spread to the edges, the possibility of an “Internet
of Things” comprised of billions of miscellaneous devices,
and the increasing integration of past and current cellular and
WiFi standards to provide an ubiquitous high-rate, low-latency
experience for network users.
This editorial commences with our view of the “big three”
5G technologies: ultra-densiﬁcation, mmWave, and massive
multiple-input multiple-output (MIMO). Then, we consider
important issues concerning the basic transmission waveform,
the increasing virtualization of the network infrastructure, and
the need for greatly increased energy efﬁciency. Finally, we
provide a comprehensive discussion of the equally important
regulatory and standardization issues that will need to be
addressed for 5G, with a particular focus on needed innovation
in spectrum regulation.
B. Engineering Requirements for 5G
In order to more concretely understand the engineering chal-
lenges facing 5G, and to plan to meet them, it is necessary to
ﬁrst identify the requirements for a 5G system. The following
items are requirements in each key dimension, but it should be
stressed that not all of these need to be satisﬁed simultaneously.
Different applications will place different requirements on
the performance, and peak requirements that will need to
be satisﬁed in certain conﬁgurations are mentioned below.
For example, very-high-rate applications such as streaming
high-deﬁnition video may have relaxed latency and reliability
requirements compared to driverless cars or public safety
applications, where latency and reliability are paramount but
lower data rates can be tolerated.
arXiv:1405.2957v1 [cs.IT] 12 May 2014
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 2
1) Data Rate: The need to support the mobile data trafﬁc
explosion is unquestionably the main driver behind 5G. Data
rate can be measured in several different ways, and there will
be a 5G goal target for each such metric:
a) Aggregate data rate refers to the total amount of data
the network can serve, characterized in units of bits/s/area. The
general consensus is that this quantity will need to increase by
roughly 1000x from 4G to 5G.
b) Edge rate, or 5% rate, is the worst data rate that a
user can reasonably expect to receive when in range of the
network, and so is an important metric and has a concrete
engineering meaning. Goals for the 5G edge rate range from
100 Mbps (easily enough to support high-deﬁnition streaming)
to as much as 1 Gbps. Meeting 100 Mbps for 95% of users will
be extraordinarily challenging, even with major technological
advances. This requires about a 100x advance since current
4G systems have a typical 5% rate of about 1 Mbps, although
the precise number varies quite widely depending on the load,
cell size, and other factors.
c) Peak rate is the best-case data rate that a user can hope
to achieve under any conceivable network conﬁguration. The
peak rate is a marketing number, devoid of much meaning to
engineers, but in any case it will likely be in the range of tens
Meeting the requirements in (a)-(b), which are about 1000x
and 100x current 4G technology, respectively, are the main
focus of this paper.
2) Latency: Current 4G roundtrip latencies are on the order
of about 15 ms, and are based on the 1 ms subframe time
with necessary overheads for resource allocation and access.
Although this latency is sufﬁcient for most current services,
anticipated 5G applications include two-way gaming, novel
cloud-based technologies such as those that may be touch-
screen activated (the “tactile Internet” ), and virtual and
enhanced reality (e.g., Google glass or other wearable comput-
ing devices). As a result, 5G will need to be able to support
a roundtrip latency of about 1 ms, an order of magnitude
faster than 4G. In addition to shrinking down the subframe
structure, such severe latency constraints may have important
implications on design choices at several layers of the protocol
stack and the core network (cf. Sect. III).
3) Energy and Cost: As we move to 5G, costs and energy
consumption will, ideally, decrease, but at least they should
not increase on a per-link basis. Since the per-link data rates
being offered will be increasing by about 100x, this means
that the Joules per bit and cost per bit will need to fall by at
least 100x. In this article, we do not address energy and cost
in a quantitative fashion, but we are intentionally advocating
technological solutions that promise reasonable cost and power
scaling. For example, mmWave spectrum should be 10-100x
cheaper per Hz than the 3G and 4G spectrum below 3 GHz.
Similarly, small cells should be 10-100x cheaper and more
power efﬁcient than macrocells. A major cost consideration
for 5G, even more so than in 4G due to the new BS densities
and increased bandwidth, is the backhaul from the network
edges into the core. We address backhaul and other economic
considerations in Section IV-C. As for energy efﬁciency, we
address this more substantially in Section III-C.
C. Device Types and Quantities.
5G will need to be able to efﬁciently support a much larger
and more diverse set of devices. With the expected rise of
machine-to-machine communication, a single macrocell may
need to support 10,000 or more low-rate devices, along with its
traditional high-rate mobile users. This will require wholesale
changes to the control plane and network management relative
to 4G, whose overhead channels and state machines are not
designed for such a diverse and large subscriber base.
II. KE Y TECHNOLOGIES TO GET TO 1000XDATA RATE
Of the requirements outlined in Sect. I-B, certainly the one
that gets the most attention is the need for radically higher data
rates across the board. Our view is that the required 1000x will,
for the most part, be achieved through combined gains in three
a) Extreme densiﬁcation and ofﬂoading to improve the area
spectral efﬁciency. Put differently, more active nodes per
unit area and Hz.
b) Increased bandwidth, primarily by moving towards and
into mmWave spectrum but also by making better use
of WiFi’s unlicensed spectrum in the 5 GHz band.
Altogether, more Hz.
c) Increased spectral efﬁciency, primarily through advances
in MIMO, to support more bits/s/Hz per node.
The combination of more nodes per unit area and Hz, more
Hz, and more bits/s/Hz per node, will compound into many
more bits/s per unit area. Other ideas not in the above cate-
gories, e.g., interference management through BS cooperation
– may also contribute improvements, but the lion’s
share of the surge in capacity should come from ideas in the
above categories. In the remainder of this section, these are
distilled in some detail.
A. Extreme Densiﬁcation and Ofﬂoading
A straightforward but extremely effective way to increase
the network capacity is to make the cells smaller. This ap-
proach has been demonstrated over several cellular generations
, . The ﬁrst such generation, in the early 1980s, had
cell sizes on the order of hundreds of square kms. Since then,
those sizes have been progressively shrinking and by now they
are often fractions of a square km in urban areas. In Japan,
for instance, the spacing between BSs can be as small as two
hundred meters, giving a coverage area well under a tenth of a
square km. Networks are now rapidly evolving  to include
nested small cells such as picocells (range under 100 meters)
and femtocells (WiFi-like range) , as well as distributed
antenna systems  that are functionally similar to picocells
from a capacity and coverage standpoint but have all their
baseband processing at a central site and share cell IDs.
Cell shrinking has numerous beneﬁts, the most important
being the reuse of spectrum across a geographic area and
the ensuing reduction in the number of users competing for
resources at each BS. Contrary to widespread belief, as long
as power-law pathloss models hold the signal-to-interference
ratio (SIR) is preserved as the network densiﬁes .1Thus,
in principle, cells can shrunk almost indeﬁnitely without a
sacriﬁce in SIR, until nearly every BS serves a single user
(or is idle). This allows each BS to devote its resources, as
well as its backhaul connection, to an ever-smaller number of
As the densiﬁcation becomes extreme, some challenges
1The power-law pathloss model ceases to apply in the near ﬁeld, very close
to the transmitter .
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 3
•Preserving the expected cell-splitting gains as each BS
becomes more lightly loaded, particularly low-power
•Determining appropriate associations between users and
BSs across multiple radio access technologies (RATs),
which is crucial for optimizing the edge rate.
•Supporting mobility through such a highly heteroge-
•Affording the rising costs of installation, maintenance
We next brieﬂy discuss these challenges, particularly in view
of the other technologies raised in this article.
1) Base Station Densiﬁcation Gains: We deﬁne the BS
densiﬁcation gain ρ > 0as the effective increase in data rate
relative to the increase in network density, which is a proxy
here for cost. Speciﬁcally, if we achieve a data rate R1(could
be any measure thereof, e.g., edge rate or aggregate) when the
BS density is λ1BSs/km2and then we consider a higher BS
density λ2, with corresponding rate R2, then the corresponding
densiﬁcation gain is
For example, if the network density is doubled, and the
aggregate data rate also doubles, then the densiﬁcation gain is
ρ= 1: the increase in BS density has an exactly proportional
payoff in terms of achieved rates in this case.
In an interference-limited network with full buffers, the
signal-to-interference-plus-noise ratio (SINR) is essentially
equal to the SIR and, because the SIR distribution remains
approximately constant as the network densiﬁes, the best case
scenario is ρ≈1. In reality, buffers are not always full, and
small cells tend to become more lightly loaded than macrocells
as the network densiﬁes. Therefore, the SINR usually increases
with density: in a noise-limited network by increasing the
received signal power, and in interference-limited networks
because the lightly loaded small cells generate less interfer-
ence (while still providing an option for connectivity) .
Nevertheless, at microwave frequencies the gain in SINR is not
enough to keep up with the decrease in small cell utilization,
and thus ρ < 1. In an extreme case, consider λ1and R1held
ﬁxed with λ2→ ∞. In this asymptotic setting, the small cells
compete for a ﬁnite pool of UEs, becoming ever more lightly
loaded, and thus ρ→0.
Empirically and theoretically, we also observe that ρim-
proves and can approach 1with macro-BS muting (eICIC in
3GPP) vs. the macrocells transmitting all the time and thus
interfering with the small cells all the time. This observation
is relevant because the result is not obvious given that the
macrocells are the network bottleneck.
An intriguing aspect of mmWave frequencies is that den-
siﬁcation gains ρ1may be possible. This is because, as
discussed in Section II-B, at these frequencies communication
is largely noise-limited and increasing the density not only
splits the cell resources and lightens the load, but it may
increase the SINR dramatically. As a striking example of this,
it was recently showed that, under a plausible urban grid-based
deployment, increasing the BS count in a given area from 36
to 96—which decreased the inter-BS distance from 170 meters
down to 85 meters—increased the 5% cell-edge rate from 24.5
Mbps up to 1396 Mbps, giving 
24.5·96 = 21.3.(2)
Fig. 1: User association in a multi-RAT network over many frequency
bands is complex. In this simpliﬁed scenario, a mobile user in turn
associates with different BSs based on a tradeoff between the gain to
that BS and the trafﬁc load (congestion) that it is experiencing.
While conceding that this massive densiﬁcation gain corre-
sponds to a particular setup and model, it is nevertheless
In general, quantifying and optimizing the densiﬁcation
gains in a wide variety of deployment scenarios and network
models is a key area for continued small cell research.
2) Multi-RAT Association: Networks will continue to be-
come increasingly heterogeneous as we move towards 5G.
A key feature therein will be increased integration between
different RATs, with a typical 5G-enabled device having radios
capable of supporting not only a potentially new 5G standard
(e.g., at mmWave frequencies), but also 3G, numerous releases
of 4G LTE including possibly LTE-Unlicensed , several
types of WiFi, and perhaps direct device-to-device (D2D)
communication, all across a great many spectral bands. Hence,
determining which standard(s) and spectrum to utilize and
which BS(s) or users to associate with will be a truly complex
task for the network .
Determining the optimal user association can be a massive
combinatorial optimization problem that depends on the SINR
from every user to every BS, the instantaneous load at each BS,
the choices of other users in the network, and possibly other
constraints such as the requirement to utilize the same BS and
standard in both uplink and downlink (to facilitate functioning
control channels for resource allocation and feedback) ,
. Therefore, simpliﬁed procedures must be adopted ,
an example of which appears in this special issue . Even
a simple, seemingly highly suboptimal association approach
based on aggressive but static biasing towards small cells and
blanking about half of the macrocell transmissions has been
shown to increase edge rates by as much as 500% .
The joint problem of user association and resource allocation
in two-tier heterogeneous networks (HetNets), with adaptive
tuning of the biasing and blanking in each cell, is considered
in , , –. An interesting model of hotspot trafﬁc
is considered in – where it is shown that, under
various network utility metrics, the optimal cell association
is determined by rate ratio bias, rather than power level bias.
It will be interesting to extend these models to more general
scenarios, including more than two tiers. A dynamic model of
cell range expansion is considered in , where trafﬁc arrives
as a Poisson process in time and the feasible arrival rates, for
which a stabilizing scheduling policy exists, are characterized.
User association and load balancing in a HetNet, with massive
MIMO at the BSs, is considered in . The problem of
determining the optimal associations when there are multiple
RATS, operating at different frequencies and using different
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 4
protocols, has not yet received much attention. However, an
interesting game theoretic approach is taken in  to the RAT-
selection problem, where convergence to Nash equilibria and
the Pareto-efﬁciency of these equilibria are studied. A related
paper in this special issue  explores the interaction between
cellular operators and WiFi network owners.
Adding mmWave into the picture adds signiﬁcant additional
complexity, since even the notion of a cell boundary is blurry
at mmWave frequencies given the strong impact of blockages,
which often result in nearby BSs being bypassed in favor of
farther ones that are unblocked (cf. Fig. 2). On the positive
side, interference is much less important in mmWave (cf.
Section II-B) and thus the need for blanking is reduced.
In summary, there is a great deal of scope for modeling,
analyzing and optimizing BS-user associations in 5G.
3) Mobility Support: Clearly, the continued network densi-
ﬁcation and increased heterogeneity poses challenges for the
support of mobility. Although a hefty share of data is served
to stationary indoor users, the support of mobility and always-
on connectivity is arguably the single most important feature
of cellular networks relative to WiFi. Because modeling and
analyzing the effect of mobility on network performance is
difﬁcult, we expect to see somewhat ad hoc solutions such as in
LTE Rel-11  where user-speciﬁc virtual cells are deﬁned to
distinguish the physical cell from a broader area where the user
can roam without the need for handoff, communicating with
any BS or subset of BSs in that area. Or in mmWave, restricting
highly mobile users to macrocells and microwave frequencies,
thereby forcing them to tolerate lower rates. Handoffs will be
particularly challenging at mmWave frequencies since transmit
and receive beams must be aligned to communicate. Indeed,
the entire paradigm of a handoff initiated and managed at
layer 3 by the core network will likely not exist in 5G;
instead, handoffs may be opportunistic, based on mmWave
beam alignments, or indistinguishable from PHY/MAC inter-
ference management techniques whereby users communicate
with multiple coordinated BSs, as exempliﬁed by  in this
4) Cost: Evolving to ever-smaller cells requires ever-
smaller, lower-power and cheaper BSs, and there is no fun-
damental reason a BS needs to be more expensive than a
user device or a WiFi node . Nevertheless, obtaining
permits, ensuring fast and reliable backhaul connections, and
paying large monthly site rental fees for operator-controlled
small-cell placements have proven a major hindrance to the
growth of picocell, distributed antennas, and other enterprise-
quality small cell deployments. Of these, only the backhaul
is primarily a technical challenge. Regulatory reforms and
infrastructure sharing (cf. Section IV-C) may help address the
Turning to end-user-deployed femtocells and WiFi access
points, these are certainly much more cost-effective both from
a capital and operating expense perspective . However,
major concerns exist here too. These include the coordination
and management of the network to provide enterprise-grade
service, which given the scale of the deployments requires
automated self-organization . A further challenge is that
these end-user deployments utilize the end-user’s backhaul
connection and access point, both of which the end-user has
a vested interest in not sharing, and in some countries a legal
requirement not to. Anecdotally, all readers of this article
are familiar with the scenario where a dozen WiFi access
points are within range, but all are secured and inaccessible.
From an engineering perspective, this closed-access status
Fig. 2: Calculated mmWave BS associations with real building
locations . The shaded regions correspond to association with the
BS centered at that shade. Blocking, LOS vs. non-LOS propagation,
and beam directionality render our usual notion of cell boundaries
quo is highly inefﬁcient and the cost for 5G would be
greatly reduced in an open-access paradigm for small cells.
One preliminary but successful example is Fon, which as
of press time boasts over 13 million shared WiFi access points.
5G and all networks beyond it will be extremely dense
and heterogeneous, which introduces many new challenges for
network modeling, analysis, design and optimization. We fur-
ther discuss some of the nonobvious intersections of extreme
densiﬁcation with mmWave and massive MIMO, respectively,
in the next two sections. Before proceeding, however, we
brieﬂy mention that besides cell shrinking a second approach
to densiﬁcation exists in the form of direct D2D communica-
tion. This allows users in close proximity to establish direct
communication, replacing two long hops via the BS with a
single shorter hop. Provided there is sufﬁcient spatial locality
in the wireless trafﬁc, this can bring about reduced power
consumption and/or higher data rates, and a diminished latency
–. Reference  in this special issue proposes a novel
way of scheduling concurrent D2D transmissions so as to
densify while offering interference protection guarantees.
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 5
B. Millimeter Wave
Terrestrial wireless communication systems have largely
restricted their operation to the relatively slim range of mi-
crowave frequencies that extends from several hundred MHz
to a few GHz and corresponds to wavelengths in the range
of a few centimeters up to about a meter. By now though,
this spectral band—often called “beachfront spectrum”—has
become nearly fully occupied, in particular at peak times and
in peak markets. Regardless of the efﬁcacy of densiﬁcation
and ofﬂoading, much more bandwidth is needed , .
Although beachfront bandwidth allocations can be made
signiﬁcantly more efﬁcient by modernizing regulatory and
allocation procedures, as discussed in Section IV-A, to put
large amounts of new bandwidth into play there is only one
way to go: up in frequency. Fortunately, vast amounts of
relatively idle spectrum do exist in the mmWave range of 30–
300 GHz, where wavelengths are 1–10 mm. There are also
several GHz of plausible spectrum in the 20–30 GHz range.
The main reason that mmWave spectrum lies idle is that,
until recently, it had been deemed unsuitable for mobile com-
munications because of rather hostile propagation qualities,
including strong pathloss, atmospheric and rain absorption, low
diffraction around obstacles and penetration through objects,
and, further, because of strong phase noise and exorbitant
equipment costs. The dominant perception had therefore been
that such frequencies, and in particular the large unlicensed
band around 60 GHz , were suitable mainly for very-short-
range transmission –. Thus, the focus had been on
WiFi (with the WiGiG standard in the 60-GHz band) and also
on ﬁxed-wireless applications in the 28, 38, 71–76 and 81–
86 GHz. However, semiconductors are maturing, their costs
and power consumption rapidly falling—largely thanks to the
progress of the aforementioned short-range standards—and
the other obstacles related to propagation are now considered
increasingly surmountable given time and focused effort –
1) Propagation Issues: Concerning mmWave propagation
for 5G cellular communication, the main issues under investi-
Pathloss. If the electrical size of the antennas (i.e., their
size measured by the wavelength λ=c/fcwhere fcis the
carrier frequency) is kept constant, as the frequency increases
the antennas shrink and their effective aperture scales with
4π; then, the free-space pathloss between a transmit and a
receive antenna grows with f2
c. Thus, increasing fcby an
order of magnitude, say from 3 to 30 GHz, adds 20 dB
of power loss regardless of the transmit-receive distance.
However, if the antenna aperture at one end of the link is
kept constant as the frequency increases, then the free-space
pathloss remains unchanged. Further, if both the transmit and
receive antenna apertures are held constant, then the free-space
pathloss actually diminishes with f2
c: a power gain that would
help counter the higher noise ﬂoor associated with broader
Although preserving the electrical size of the antennas is
desirable for a number of reasons, maintaining at the same
time the aperture is possible utilizing arrays, which aggregate
the individual antenna apertures: as the antennas shrink with
frequency, progressively more of them must be added in the
original area. The main challenge becomes cophasing these
antennas so that they steer and/or collect energy productively.
This challenge becomes more pronounced when the channel
changes rapidly, for example due to mobility and the higher
Doppler shifts at mmWave frequencies or due to rapid alter-
ations in the physical orientation of the devices.
Blocking. MmWave signals exhibit reduced diffraction and
a more specular propagation than their microwave counter-
parts, and hence they are much more susceptible to blockages.
This results in a nearly bimodal channel depending on the
presence or absence of Line-of-Sight (LoS). According to re-
cent measurements , , as the transmit-receive distance
grows the pathloss accrues close to the free-space value of 20
dB/decade under LoS propagation, but drops to 40 dB/decade
plus an additional blocking loss of 15–40 dB otherwise.
Because of the sensitivity to blockages, a given link can rapidly
transition from usable to unusable and, unlike small-scale
fading, large-scale obstructions cannot be circumvented with
standard small-scale diversity countermeasures. New channel
models capturing these effects are much needed, and in fact
currently being developed , ,  and applied to
system-level analysis , – and simulation studies
such as  and  in this special issue.
Atmospheric and rain absorption. The absorption due to
air and rain is noticeable, especially the 15 dB/km oxygen
absorption within the 60-GHz band (which is in fact why this
band is unlicensed), but it is inconsequential for the urban
cellular deployments currently envisioned ,  where
BS spacings might be on the order of 200 m. In fact, such
absorption is beneﬁcial since it further attenuates background
interference from more distant BSs, effectively increasing the
isolation of each cell.
The main conclusion is that the propagation losses for
mmWave frequencies are surmountable, but require large an-
tenna arrays to steer the beam energy and collect it coher-
ently. While physically feasible, the notion of narrow-beam
communication is new to cellular communications and poses
difﬁculties, which we next discuss.
2) Large arrays, narrow beams: Building a cellular system
out of narrow and focused beams is highly nontrivial and
changes many traditional aspects of cellular system design.
MmWave beams are highly directional, almost like ﬂashlights,
which completely changes the interference behavior as well as
the sensitivity to misaligned beams. The interference adopts an
on/off behavior where most beams do not interfere, but strong
interference does occur intermittently. Overall, interference is
de-emphasized and mmWave cellular links may often be noise-
limited, which is a major reversal from 4G. Indeed, even the
notion of a “cell” is likely to be very different in a mmWave
system since, rather than distance, blocking is often the ﬁrst-
order effect on the received signal power. This is illustrated in
Link acquisition. A key challenge for narrow beams is
the difﬁculty in establishing associations between users and
BSs, both for initial access and for handoff. To ﬁnd each
other, a user and a BS may need to scan lots of angular
positions where a narrow beam could possibly be found, or
deploy extremely large coding/spreading gains over a wider
beam that is successively narrowed in a multistage acquisition
procedure. Developing solutions to this problem, particularly
in the context of high mobility, is an important research
Leveraging the legacy 4G network. A concurrent utiliza-
tion of microwave and mmWave frequencies could go a long
way towards overcoming some of the above hurdles. An inter-
esting proposal in that respect is the notion of “phantom cells”
(relabeled “soft cells” within 3GPP) , where mmWave
frequencies would be employed for payload data transmission
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 6
Data & signalling
Fig. 3: MmWave-enabled network with phantom cells.
from small-cell BSs while the control plane would operate
at microwave frequencies from macro BSs (cf. Fig. 3). This
would ensure stable and reliable control connections, based
on which blazing fast data transmissions could be arranged
over short-range mmWave links . Sporadic interruptions
of these mmWave links would then be far less consequential,
as control links would remain in place and lost data could be
recovered through retransmissions.
Novel transceiver architectures needed. Despite the
progress made in WiFi mmWave systems, nontrivial hardware
issues remain, and in some cases will directly affect how the
communication aspects are designed. Chief among these is the
still-exorbitant power consumption of particularly the analog-
to-digital (A/D) but also the digital-to-analog (D/A) converters
needed for large bandwidths. A main consequence is that,
although large antenna arrays and high receiver sensitivities
are needed to deal with the pathloss, having customary fully
digital beamformers for each antenna appears to be unfeasible.
More likely are structures based on old-fashioned analog phase
shifters or, perhaps, hybrid structures where groups of antennas
share a single A/D and D/A –. On the ﬂip side, offering
some relief from these difﬁculties, the channels are sparser and
thus the acquisition of channel-state information is facilitated;
in particular, channel estimation and beamforming techniques
exploiting sparsity in the framework of compressed sensing
are being explored , .
C. Massive MIMO
Stemming from research that blossomed in the late 1990s
, , MIMO communication was introduced into WiFi
systems around 2006 and into 3G cellular shortly thereafter.
In essence, MIMO embodies the spatial dimension of the
communication that arises once a multiplicity of antennas are
available at base stations and mobile units. If the entries of
the channel matrix that ensues exhibit—by virtue of spac-
ing, cross-polarization and/or angular disposition—sufﬁcient
statistical independence, multiple spatial dimensions become
available for signaling and the spectral efﬁciency multiplies
accordingly , .
In single-user MIMO (SU-MIMO), the dimensions are lim-
ited by the number of antennas that can be accommodated on
a portable device. However, by having each BS communicate
with several users concurrently, the multiuser version of MIMO
(MU-MIMO) can effectively pull together the antennas at
those users and overcome this bottleneck. Then, the signaling
dimensions are given by the smallest between the aggregate
number of antennas at those users and the number of antennas
at the BS.
Furthermore, in what is now known as coordinated multi-
point (CoMP) transmission/reception, multiple BSs can coop-
erate and act as a single effective MIMO transceiver thereby
turning some of the interference in the system into useful
signals; this concept in fact underpins many of the approaches
to interference and mobility management mentioned earlier in
Well-established by the time LTE was developed, MIMO
was a native ingredient thereof with two-to-four antennas per
mobile unit and as many as eight per base station sector, and
it appeared that, because of form factors and other apparent
limitations, such was the extent to which MIMO could be
leveraged. Marzetta was instrumental in articulating a vision
in which the number of antennas increased by more than an
order of magnitude, ﬁrst in a 2007 presentation  with the
details formalized in a landmark paper . The proposal was
to equip BSs with a number of antennas much larger than the
number of active users per time-frequency signaling resource,
and given that under reasonable time-frequency selectivities
accurate channel estimation can be conducted for at most some
tens of users per resource, this condition puts the number of
antennas per base station into the hundreds. This bold idea,
initially termed “large-scale antenna systems” but now more
popularly known as “massive MIMO”, offers enticing beneﬁts:
•Enormous enhancements in spectral efﬁciency with-
out the need for increased BS densiﬁcation, with the
possibility—as is always the case—of trading some of
those enhancements off for power efﬁciency improve-
ments , .
•Smoothed out channel responses because of the vast
spatial diversity, which brings about the favorable action
of the law of large numbers. In essence, all small-scale
randomness abates as the number of channel observa-
•Simple transmit/receive structures because of the quasi-
orthogonal nature of the channels between each BS
and the set of active users sharing the same signaling
resource. For a given number of active users, such
orthogonality sharpens as the number of BS antennas
grows and simple linear transceivers, even plain single-
user beamforming, perform close-to-optimally.
The promise of these beneﬁts has elevated massive MIMO
to a central position in preliminary discussions about 5G ,
with a foreseen role of providing a high-capacity umbrella
of ubiquitous coverage in support of underlying tiers of small
cells. However, for massive MIMO to become a reality, several
major challenges must ﬁrst be overcome, and the remainder
of this section is devoted to their dissection. For very recent
contributions on these and other aspects, the reader is referred
to a companion special issue on massive MIMO . The
present special issue contains further new contributions, men-
tioned throughout the discussion that follows, plus reference
 dealing with the massiﬁcation of MIMO multicasting ,
1) Pilot Contamination and Overhead Reduction: Pilot
transmissions can be made orthogonal among same-cell users,
to facilitate cleaner channel estimates , , but must
be reused across cells—for otherwise all available resources
would end up consumed by pilots. This inevitably causes
interference among pilots in different cells and hence puts a
ﬂoor on the quality of the channel estimates. This interference,
so-called “pilot contamination,” does not vanish as the number
of BS antennas grows large, and so is the one impairment
that remains asymptotically. However, pilot contamination is
a relatively secondary factor for all but colossal numbers
of antennas . Furthermore, various methods to reduce
and even eliminate pilot contamination via low-intensity BS
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 7
TABLE I: FD-MIMO system-level downlink simulation results at 2.5
GHz. Half-wavelength antenna spacings in both the horizontal and
vertical dimensions at the BSs, 2 antennas per user, 30% overhead.
The baseline is SU-MIMO with 4 antennas per BS and the FD-MIMO
results (average and edge data rates) are for MU-MIMO with 16 and
64 antennas, respectively corresponding to 4×4and 8×8planar
arrays per BS sector.
SU-MIMO FD-MIMO 16 FD-MIMO 64
Aggregate Data Rate (b/s/Hz/cell) 2.32 3.28 6.37
Edge Data Rate (b/s/Hz) 0.063 0.1 0.4
coordination have already been formulated , . Still,
a careful design of the pilot structures is required to avoid an
explosion in overhead. The ideas being considered to reign in
pilot overheads include exploiting spatial correlations, so as
to share pilot symbols among antennas, and also segregating
the pilots into classes (e.g., channel strength gauging for link
adaptation v. data detection) such that each class can be
transmitted at the necessary rate, and no faster.
2) Architectural Challenges: A more serious challenge to
the realization of the massive MIMO vision has to do with
its architecture. The vision requires radically different BS
structures where, in lieu of a few high-power ampliﬁers feeding
a handful of sector antennas, we would have a myriad of
tiny antennas fed by correspondingly low-power ampliﬁers;
most likely each antenna would have to be integrated with
its own ampliﬁer. Scalability, antenna correlations and mutual
couplings, and cost, are some of the issues that must be
sorted out. At the same time, opportunities arise for innovative
topologies such as conformal arrays along rooftops or on
building facades, and we next dwell on a speciﬁc topological
aspect in which innovation is taking place.
Within this special issue,  explores alternative and
highly innovative antenna designs based on the utilization of
an electromagnetic lens-focusing antenna.
3) Full-Dimension MIMO and Elevation Beamforming:
Existing BSs mostly feature linear horizontal arrays, which
in tower structures can only accommodate a limited number
of antennas, due to form factors, and which only exploit the
azimuth angle dimension. By adopting planar 2D arrays similar
to Fig. 3 and further exploiting the elevation angle, so-called
full-dimension MIMO (FD-MIMO) can house many more
antennas with the same form factor . As a side beneﬁt,
tailored vertical beams increase the signal power and reduce
interference to users in neighboring cells. Some preliminary
cell average and edge data rates obtained from Samsung’s
network simulator are listed in Table I where, with numbers of
antennas still modest for what massive MIMO is envisioned
to be, multiple-fold improvements are observed.
4) Channel Models: Parallel to the architectural issues run
those related to channel models, which to be sound require
extensive ﬁeld measurements. Antenna correlations and cou-
plings for massive arrays with relevant topologies must be
determined, and a proper modeling of their impact must be
established; in particular, the degree of actual channel orthog-
onalization in the face of such nonidealities must be veriﬁed.
And, for FD-MIMO, besides azimuth, the modeling needs to
incorporate elevation –, which is a dimension on
which far less data exists concerning power spectra and angle
spreads. A 3D channel modelling study currently under way
within 3GPP is expected to shed light on these various issues
. References ,  in this special issue also deal
with this subject.
5) Coexistence with Small Cells: As mentioned earlier,
massive MIMO BSs would most likely have to coexist with
tiers of small cells, which would not be equipped with massive
MIMO due to their smaller form factor. Although the simplest
alternative is to segregate the corresponding transmissions in
frequency, the large number of excess antennas at massive
MIMO BSs may offer the opportunity of spatial nulling
and interference avoidance with relative simplicity and little
penalty. To conﬁrm the feasibility of this idea, put forth in
 and further developed in  within this special issue,
comprehensive channel models are again needed.
As networks become dense and more trafﬁc is ofﬂoaded to
small cells, the number of active users per cell will diminish
and the need for massive MIMO may decrease. Aspects such
as cost and backhaul will ultimately determine the balance
between these complementary ideas.
6) Coexistence with mmWave: As discussed in Sec. II-B,
mmWave communication requires many antennas for beam-
steering. The antennas are much smaller at these frequen-
cies and thus very large numbers thereof can conceivably ﬁt
into portable devices, and these antennas can indeed provide
beamforming power gain but also MIMO opportunities as
considered in  within this special issue. Any application
of massive MIMO at mmWave frequencies would have to ﬁnd
the correct balance between power gain/interference reduction
III. DESIGN ISS UE S FO R 5G
In addition to supporting 1000x higher throughput, 5G
cellular networks must decrease latencies, lower energy con-
sumption, lower costs, and support many low-rate connections.
In this section, we discuss important ongoing research areas
that support these requirements. We begin with the most
fundamental aspect of the physical layer—the waveform—and
then consider the evolution of cloud-based and virtualized net-
work architectures, latency and control signaling, and energy
A. The Waveform: Signaling and Multiple Access
The signaling and multiple access formats, i.e., the wave-
form design, have changed signiﬁcantly at each cellular gen-
eration and to a large extent they have been each generation’s
deﬁning technical feature. They have also often been the
subject of ﬁerce intellectual and industrial disputes, which
have played out in the wider media. The 1G approach, based
on analog frequency modulation with FDMA, transformed
into a digital format for 2G and, although it employed both
FDMA and TDMA for multiple access, was generally known
as “TDMA” due to the novelty of time-multiplexing. Mean-
while, a niche spread spectrum/CDMA standard that was
developed by Qualcomm to compete for 2G  became
the dominant approach to all global 3G standards. Once the
limitations of CDMA for high-speed data became inescapable,
there was a discreet but unmistakable retreat back towards
TDMA, with minimal spectrum spreading retained and with
the important addition of channel-aware scheduling .
Due to the increasing signal bandwidths needed to support
data applications, orthogonal frequency-division multiplexing
(OFDM) was unanimously adopted for 4G in conjunction with
scheduled FDMA/TDMA as the virtues of orthogonality were
viewed with renewed appreciation.
In light of this history, it is natural to ponder the possibility
that the transition to 5G could involve yet another major
change in the signaling and multiple access formats.
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 8
1) OFDM and OFDMA: The Default Approach: OFDM has
become the dominant signaling format for high-speed wireless
communication, forming the basis of all current WiFi standards
and of LTE, and further of wireline technologies such as digital
subscriber lines, digital TV, and commercial radio. Its qualities
•A natural way to cope with frequency selectivity.
•Computationally efﬁcient implementation via FFT/IFFT
blocks and simple frequency-domain equalizers.
•An excellent pairing for MIMO, since OFDM allows for
the spatial interference from multiantenna transmission
to be dealt with at a subcarrier level, without the added
complication of intersymbol interference.
From a multiple access vantage point, OFDM invites dy-
namic ﬁne-grained resource allocation schemes in the digital
domain, and the term OFDMA is employed to denote orthog-
onal multiple access at a subcarrier level. In combination with
TDMA, this parcels the time-frequency grid into small units
known as resource blocks that can be easily discriminated
through digital ﬁltering . Being able to do frequency
and time slot allocation digitally also enables more adaptive
and sophisticated interference management techniques such
as fractional frequency reuse or spectrum partitions between
small cells and macrocells. Finally, given its near-universal
adoption, industry has by now a great deal of experience
with its implementation, and tricky aspects of OFDM such
as frequency offset correction and synchronization have been
2) Drawbacks of OFDM: Given this impressive list of
qualities, and the large amount of inertia in its favor, OFDM is
the unquestionable frontrunner for 5G. However, some weak
points do exist that could possibly become more pronounced
in 5G networks.
First, the peak-to-average-power ratio (PAPR) is higher in
OFDM than in other formats since the envelope samples are
nearly Gaussian due to the summation of uncorrelated inputs in
the IFFT. Although a Gaussian signal distribution is capacity-
achieving under an average power constraint , in the
face of an actual power ampliﬁer a high PAPR sets up an
unattractive tradeoff between the linearity of the transmitted
signal and the cost of the ampliﬁer. This problem can be largely
overcome by precoding the OFDM signals at the cost of a more
involved equalization process at the receiver and a slight power
penalty; indeed, this is already being done in the LTE uplink
Second, OFDM’s spectral efﬁciency is satisfactory, but could
perhaps be further improved upon if the requirements of strict
orthogonality were relaxed and if the cyclic preﬁxes (CPs)
that prevent interblock interference were smaller or discarded.
The paper  in this special issue, instead, proposes the
use of a novel OFDMA-based modulation scheme named fre-
quency and quadrature amplitude modulation (FQAM), which
is shown to improve the downlink throughput for cell-edge
Perhaps the main source of concerns, or at least of open
questions, is the applicability of OFDM to mmWave spectrum
given the enormous bandwidths therein and the difﬁculty of
developing efﬁcient power ampliﬁers at those frequencies. For
example, a paper in this special issue proposes a single-carrier
signaling with null cyclic preﬁx as an alternative to OFDM at
mmWave frequencies .
3) Potential Alternatives to OFDM: To address OFDM’s
weaknesses, we now overview some alternative approaches
being actively investigated. Most of these, however, can be
considered incremental departures from OFDM rather than
the step-function changes that took place in previous cellular
Time-frequency packing. Time-frequency packing 
and faster-than-Nyquist signaling – have been re-
cently proposed to circumvent the limitations of strict orthog-
onality and CP. In contrast to OFDM, where the product of
the symbol interval and the subcarrier spacing equals 1, in
faster-than-Nyquist signaling products smaller than 1 can be
accommodated and spectral efﬁciency improvements on the
order of 25% have been claimed.
Nonorthogonal signals. There is a growing interest in
multicarrier formats, such as ﬁlterbank multicarrier , that
are natively nonorthogonal and thus do not require prior syn-
chronization of distributed transmitters. A new format termed
universal ﬁltered multiCarrier (UFMC) has been proposed
whereby, starting with an OFDM signal, ﬁltering is performed
on groups of adjacent subcarriers with the aim of reducing
sidelobe levels and intercarrier interference resulting from poor
time/frequency synchronization , .
Filterbank multicarrier. To address the drawbacks of rect-
angular time windowing in OFDM, namely the need for large
guard bands,  shows that the use of ﬁlterbank multicarrier
permits a robust estimation of very large propagation delays
and of arbitrarily high carrier frequency offsets, whereas
OFDM would have required a very long CP to attain the same
Generalized frequency division multiplexing. GFDM is
a multicarrier technique that adopts a shortened CP through
the tail biting technique and is particularly well suited for
noncontiguous frequency bands , , which makes it
attractive for spectrum sharing where frequency-domain holes
may have to be adaptively ﬁlled.
Single carrier. Single-carrier transmission has also been
attracting renewed interest, chieﬂy due to the development
of low-complexity nonlinear equalizers implemented in the
frequency domain . This may be of particular interest
for mmWave as discussed in this same special issue .
Tunable OFDM. We conclude with our own opinion that
OFDM could be well adapted to different 5G requirements
by allowing some of its parameters to be tunable, rather than
designed for essentially the worst-case multipath delay spread.
In particular, given the increasingly software-deﬁned nature of
radios, the FFT block size, the subcarrier spacing and the CP
length could change with the channel conditions: in scenarios
with small delay spreads—notably dense urban/small cells and
mmWave channels—the subcarrier spacing could grow and the
FFT size and the CP could be signiﬁcantly shortened to lower
the latency, the PAPR, the CP’s power and bandwidth penalty,
and the computational complexity; in channels with longer
delay spreads, that could revert to narrower subcarriers, longer
FFT blocks, and a longer CP.
B. Cloud-based Networking
Although this special issue is mainly focused on the air
interface, for the sake of completeness we brieﬂy touch on
the exciting changes taking place at the network level. In
that respect, the most relevant event is the movement of data
to the cloud so that it can be accessed from anywhere and
via a variety of platforms. This fundamentally redeﬁnes the
endpoints and the time frame for which network services
are provisioned. It requires that the network be much more
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 9
0Frequency−packed OFDM with frequency spacing set to 0.8/T
Fig. 4: Frequency-domain magnitude responses of some adjacent
waveforms for OFDM, frequency-packed OFDM, and ﬁltered OFDM.
The two signaling formats alternative to OFDM trade subcarrier
orthogonality for either better spectral efﬁciency (frequency-packed
OFDM) or lower out-of-band emissions (ﬁltered OFDM).
nimble, ﬂexible and scalable. As such, two technology trends
will become paramount in the future: network function vir-
tualization (NFV) and software deﬁned networking (SDN).
Together, these trends represent the biggest advance in mobile
communication networking in the last 20 years, bound to
fundamentally change the way network services are provided.
Although the move towards virtualization is thus far taking
place only within the core network, this trend might eventually
expand towards the edges. In fact, the term cloud-RAN is
already being utilized, but for now largely to refer to schemes
whereby multiple BSs are allowed to cooperate . If and
when the Bss themselves become virtualized—down to the
MAc and PHY—-this term will be thoroughly justiﬁed .
1) Network Function Virtualization: NFV enables network
functions that were traditionally tied to hardware appliances
to run on cloud computing infrastructure in a data center. It
should be noted that this does not imply that the NFV infras-
tructure will be equivalent to commercial cloud or enterprise
cloud. What is expected is that there will be a high degree of
reuse of what commercial cloud offers.
It is natural to expect that some requirements of mobile
networks such as the separation of the data plane, control
plane and management plane, will not be feasible within
the commercial cloud. Nevertheless, the separation of the
network functions from the hardware infrastructure will be the
cornerstone of future architectures. The key beneﬁt will be
the ability to elastically support network functional demands.
Furthermore, this new architecture will allow for signiﬁcant
nimbleness through the creation of virtual networks and of
new types of network services . A detailed description of
the NFV architecture is beyond the scope of this paper, and
interested readers can consult – and the references
As virtualization of the communication network gains trac-
tion in the industry, an old concept, dating back to the 1990s,
will emerge: the provision of user-controlled management in
network elements. Advances in computing technology have
reached a level where this vision can become a reality, with
the ensuring architecture having recently been termed software
deﬁned networking (SDN).
2) Software Deﬁned Networking: SDN is an architectural
framework for creating intelligent programmable networks.
Speciﬁcally, it is deﬁned as an architecture where the control
and data planes are decoupled, network intelligence and state
are logically centralized, and the underlying network infras-
tructure is abstracted from the application .
The key ingredients of SDN are an open interface be-
tween the entities in the control and data planes, as well as
programmability of the network entities by external applica-
tions. The main beneﬁts of this architecture are the logical
decoupling of the network intelligence to separate software-
based controllers, exposing the network capabilities through
an application program interface, and enabling the application
to request and manipulate services provided by the network
From a wireless core network point of view, NFV and SDN
should be viewed as tools for provisioning the next generation
of core networks with many issues still open in terms of
scalability, migration from current structures, management and
automation, and security.
C. Energy efﬁciency
As speciﬁed in our stated requirements for 5G, the energy
efﬁciency of the communication chain—typically measured
in either Joules/bit or bits/Joule—will need to improve by
about the same amount as the data rate just to maintain the
power consumption. And by more if such consumption is to be
reduces. This implies a several-order-of-magnitude increase in
energy efﬁciency, which is extremely challenging. Unsurpris-
ingly, in recent years there has been a surge of interest in the
topic of energy efﬁcient communications, as can be seen from
the number of recent special issues, conferences and research
projects devoted to “green communications” –. In
addition to laudable environmental concerns, it is simply not
viable from a logistical, cost or battery-technology point of
view to continually increase power consumption.
Due to the rapidly increasing network density (cf. Sect.
II-A), the access network consumes the largest share of the
energy . Research has focused on the following areas.
1) Resource allocation: The literature is rich in contribu-
tions dealing with the design of resource allocation strategies
aimed at the optimization of the system energy efﬁciency
–; the common message of these papers is that, by
accepting a moderate reduction in the data rates that could
otherwise be achieved, large energy savings can be attained.
Within this special issue,  introduces an energy-efﬁcient
coordinated beamforming design for HetNets.
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 10
2) Network Planning: Energy-efﬁcient network planning
strategies include techniques for minimizing the number of
BSs for a coverage target  and the design of adaptive
BS sleep/wake algorithms for energy savings –. The
underlying philosophy of these papers is that, since networks
have been designed to meet peak-hour trafﬁc, energy can be
saved by (partially) switching off BSs when they have no active
users or simply very low trafﬁc. Of course, there are different
degrees of hibernation available for a BS2and attention must
be paid in order to avoid unpleasant coverage holes; this is
usually accomplished through an increase of the transmitted
power from nearby BSs.
3) Renewable energy: Another intriguing possibility is that
of BSs powered by renewable energy sources such as solar
power . This is of urgent interest in developing countries
lacking a reliable and ubiquitous power grid, but it is also
intriguing more broadly as it allows “drop and play” small
cell deployment (if wireless backhaul is available) rather than
“plug and play”. A recent paper showed that in a dense HetNet,
plausible per-BS trafﬁc loads can actually be served solely
by energy harvesting BSs . A more relaxed scenario
is considered in , where the resource allocation makes
efﬁcient use of both renewable and traditional energy sources.
4) Hardware solutions: Finally, much of the power
consumption issues will be dealt with by hardware engineers,
with recent work in low-loss antennas, antenna muting, and
adaptive sectorization according to trafﬁc requirements (see,
In summary, energy efﬁciency will be a major research
theme for 5G, spanning many of the other topics in this article:
•True cloud-RAN could provide an additional opportunity
for energy efﬁciency since the centralization of the
baseband processing might save energy , especially
if advances on green data centers are leveraged .
•The tradeoff between having many small cells or fewer
macrocells given their very different power consump-
tions is also of considerable interest .
•A complete characterization of the energy consumed
by the circuitry needed for massive MIMO is currently
•MmWave energy efﬁciency will be particularly crucial
given the unprecedented bandwidths .
IV. SPE CT RUM, REGULATION AND STANDARDIZATION
FO R 5G
Departing from strictly technical issues, we now turn our
attention to the crucial intersections that 5G technologies will
encounter with public policy, industry standardization, and
A. Spectrum Policy and Allocation
As discussed in Section II-B, the beachfront microwave
spectrum is already saturated in peak markets at peak times
while large amounts of idle spectrum do exist in the mmWave
realm. Due to the different propagation characteristics, and
recalling the concept of phantom cells, future systems will
need to integrate a broad range of frequencies: low frequencies
for wide coverage, mobility support, and control, and high
frequencies for small cells. This will require new approaches
2As an example, a BS serving few users may choose to operate on a reduced
set of subcarriers, or it may switch off some of its sectors.
to spectrum policy and allocation methods. Topics such as
massive MIMO and small cells, which address the efﬁcient
use of spectrum, must also be considered important issues
in spectrum policy. Needless to say, spectrum allocation and
policy is an essential topic for 5G, so this section considers the
pros and cons of different approaches to spectrum regulation
in that context.
1) Exclusive Licenses: The traditional approach to spectrum
policy is for the regulator to award an exclusive license to a
particular band for a particular purpose, subject to limitations
(e.g., power levels or geographic coverage). Exclusive access
gives full interference management control to the licensee
and provides an incentive for investments in infrastructure,
allowing for quality-of-service guarantees. Downsides include
high entry barriers because of elevated sunk costs, both in the
spectrum itself and in infrastructure, and that such allocations
are inherently inefﬁcient since they occur over very long time
scales—typically decades—and thus the spectrum is rarely
allocated to the party able to make the best economic use of
To address these inefﬁciencies, market-based approaches
have been propounded . Attempting to implement this
idea, spectrum auctions have been conducted recently to refarm
spectrum, a process whereby long-held commercial radio and
TV allocations are moved to different (smaller) bands releasing
precious spectrum for wireless communications; a prime exam-
ple of this is the so-called “digital dividend” auctions arising
from the digitization of radio and TV. However, there are
claims that spectrum markets have thus far not been successful
in providing efﬁcient allocations because such markets are not
sufﬁciently ﬂuid due to the high cost of the infrastructure .
According to these claims, spectrum and infrastructure cannot
be easily decoupled.
2) Unlicensed Spectrum: At the other extreme, regulators
can designate a band to be “open access”, meaning that there is
no spectrum license and thus users can share the band provided
their devices are certiﬁed (by class licenses). Examples are
the industrial, scientiﬁc and medical (ISM) bands, which are
utilized by many devices including microwave ovens, medical
devices, sensor networks, cordless phones and especially by
WiFi. With open access, barriers to entry are much lower and
there is enhanced competition and innovation, as the incredible
success of WiFi and other ISM-band applications makes plain.
The downside of open access is potentially unmanageable
interference, no quality-of-service guarantees, and, possibly,
the “tragedy of the commons,” where no one achieves a desired
outcome. Still, it is useful to consider the possibility of open
access for bands utilized in small cells as future networks
may involve multiple players and lower entry barriers may be
needed to secure the emergence of small-cell infrastructures.
Although interference is indeed a signiﬁcant problem in
current open access networks, it is interesting to note that
cellular operators nevertheless rely heavily on WiFi ofﬂoading:
currently about half of all cellular data trafﬁc is proactively
ofﬂoaded through unlicensed spectrum . WiFi hotspots are
nothing but small cells that spatially reuse ISM frequencies. At
mmWave frequencies, the main issue is signal strength rather
than interference, and it is therefore plausible that mmWave
bands be unlicensed, or at a minimum several licensees will
share a given band under certain new regulations. This question
is of pressing interest for 5G.
3) Spectrum Sharing: Options do exist halfway between ex-
clusive licenses and open access, such as the opportunistic use
of TV white space. While the potential of reusing this spectrum
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 11
is enticing, it is not crystal clear that reliable communication
services can be delivered that way. Alternatively, Authorized
Shared Access  and Licensed Shared Access  are
regulatory frameworks that allow spectrum sharing by a lim-
ited number of parties each having a license under carefully
speciﬁed conditions. Users agree on how the spectrum is to
be shared, seeking interference protection from each other,
thereby increasing the predictability and reliability of their
4) Market-Based Approaches to Spectrum Allocation: Given
the advantages of exclusive licenses for ensuring quality of
service, it is likely that most beachfront spectrum will continue
to be allocated that way. Nevertheless, better utilization could
likely be obtained if spectrum markets could become more
ﬂuid . To that end, liberal licenses do not, in principle,
preclude trading and reallocation on a fast time scale, rendering
spectrum allocations much more dynamic. Close attention must
be paid to the deﬁnition of spectrum assets, which have a space
as well as a time scale, and the smaller the scales, the more
ﬂuid the market .
In small cells, trafﬁc is much more volatile than in macro-
cells and operators may ﬁnd it beneﬁcial to enter into sharing
arrangements for both spectrum and infrastructure. Dynamic
spectrum markets may emerge, managed by brokers, allowing
licenses to spectrum assets to be bought and sold—or leased—
on time scales of hours, minutes or even ms . Along
these lines, an interesting possibility is for a decoupling of
infrastructure, spectrum and services . In particular, there
may be a separation between spectrum owners and operators.
Various entities may own and/or share a network of BSs,
and buy and sell spectrum assets from spectrum owners,
via brokers. These network owners may offer capacity to
operators, which in turn would serve the end customers with
performance guarantees. All of this, however, would require
very adaptable and frequency agile radios.
We conclude this discussion by noting that ofﬂoading onto
unlicensed spectrum such as TV whitespace or mmWave
bands could have unexpected results. In particular, adding an
unlicensed shared band to an environment where a set of
operators have exclusive bands can lead to an overall decrease
in the total welfare (Braess’ paradox) . This is because
operators might have an incentive to ofﬂoad trafﬁc even when
this runs counter to the overall social welfare, deﬁned as the
total proﬁt of the operators and the utilities of the users, minus
the costs. An operator might have an incentive to increase
prices so that some trafﬁc is diverted to the unlicensed band,
where the cost of interference is shared with other operators,
and this price increase more than offsets the operator’s beneﬁts.
Further, while unlicensed spectrum generally lowers barriers
to entry and increases competition, the opposite could occur
and in some circumstances a single monopoly operator could
emerge  within the unlicensed bands.
B. Regulation and Standardization
1) 5G Standardization Status: Several regional forums and
projects have been established to shape the 5G vision and to
study its key enabling technologies , –. For ex-
ample, the aforementioned EU project METIS has already re-
leased documents on scenarios and requirements , .
Meanwhile, 5G has been increasingly referred to as “IMT-
2020” in many industry forums and international telecommu-
nications union (ITU) working groups  with the goal,
as the name suggests, of beginning commercial deployments
To explore 5G user requirements and to elaborate a standards
agenda to be driven by them, the ETSI held a future mobile
summit  in Nov. 2013. The summit concluded, in line
with the thesis of this paper, that an evolution of LTE may
not be sufﬁcient to meet the anticipated 5G requirements.
That conclusion notwithstanding, 5G standardization has not
yet formally started within 3GPP, which is currently ﬁnalizing
LTE Rel-12 (the third release for the LTE-Advanced family of
4G standards). The timing of 5G standardization has not even
been agreed upon, although it is not expected to start until
later Rel-14 or Rel-15, likely around 2016–2017. However,
many ongoing and proposed study items for Rel-12 are already
closely related to 5G candidate technologies covered in this
paper (e.g., massive MIMO) and thus, in that sense, the seeds
of 5G are being planted in 3GPP. Whether an entirely new
standards body will emerge for 5G as envisioned in this paper
is unclear; the ongoing success of 3GPP relative to its erstwhile
competitors (3GPP2 and the WiMAX Forum) certainly gives
it an advantage, although a name change to 5GPP would seem
to be a minimal step.
2) 5G Spectrum Standardization: Spectrum standardization
and harmonization efforts for 5G have begun within the ITU.
Studies are under way on the feasibility of bands above
6 GHz , including technical aspects such as channel mod-
elling, semiconductor readiness, coverage, mobility support,
potential deployment scenarios and coexistence with existing
To be available for 5G, mmWave spectrum has to be
repurposed by national regulators for mobile applications and
agreement must be reached in ITU world radiocommunication
conferences (WRC) on the global bands for mmWave commu-
nications. These processes tend to be tedious and lengthy, and
there are many hurdles to clear before the spectrum can indeed
be available. On the ITU side, WRC-18 is shaping up as the
time and venue to agree on mmWave spectrum allocations for
In addition to the ITU, many national regulators have also
started their own studies on mmWave spectrum for mobile
communications. In the USA, the technological advisory coun-
cil of the federal communications committee (FCC) has carried
out extensive investigations on mmWave technology in the
last few years and it is possible that FCC will issue a notice
of inquiry in 2014, which is always the ﬁrst step in FCC’s
rulemaking process for allocation of any new frequency bands.
As discussed above, it is also unclear how such bands will
be allocated or even how they should be allocated, and the
technical community should actively engage the FCC to make
sure they are allocated in a manner conducive to meeting
5G requirements. Historically, other national regulators have
tended to follow the FCC’s lead on spectrum policy.
C. Economic Considerations
The economic costs involved in moving to 5G are substan-
tial. Even if spectrum costs can be greatly reduced through
the approaches discussed above, it is still a major challenge
for carriers to densify their networks to the extent needed to
meet our stated 5G requirements. Two major challenges are
that BS sites are currently expensive to rent, and so is the
backhaul needed to connect them to the core network.
1) Infrastructure Sharing: One possible new business model
could be based on infrastructure sharing, where the owners of
IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 12
infrastructure and the operators are different. There are several
ways in which infrastructure could be shared.
Passive sharing. The passive elements of a network include
the sites (physical space, rooftops, towers, masts and pylons),
the backhaul connection, power supplies, and air-conditioning.
Operators could cover larger geographical areas at a lower cost
and with less power consumption if they shared sites, and this
might be of particular importance in dense 5G networks .
Regulation could be required to force major operators to share
their sites and improve competition.
Active sharing. Active infrastructure sharing would involve
antennas, BSs, radio access networks and even core networks.
BS and/or radio access network sharing may be particularly
attractive when rolling out small-cell networks . This
type of sharing could lead to collusion, with anticompetitive
agreements on prices and services . Regulations are
required to prevent such collusion, but on the positive side
are the economies of scale.
Mobile virtual network operators. A small cell may be
operated by a mobile virtual network operator that does not
own any spectrum but has entered into an agreement with
another operator to gain access to its spectrum within the small
cell. The small cell may provide coverage to an enterprise or
business such that, when a user leaves the enterprise, it roams
onto the other operator’s network.
Ofﬂoading. Roaming is traditionally used to increase cover-
age in scenarios when service providers’ geographical reaches
are limited. However, in 5G, and as discussed above, trafﬁc
may be ofﬂoaded for a different reason: spatial and temporal
demand ﬂuctuations. Such ﬂuctuations will be greater in small-
cell networks. Recent papers consider the incentive for invest-
ment under various revenue-sharing contracts , . It
is shown in  that sharing increases investment, and the
incentive is greater if the owner of the infrastructure gets the
larger fraction of the revenue when overﬂow trafﬁc is carried.
A bargaining approach for data ofﬂoading from a cellular
network onto a collection of WiFi or femtocell networks is
considered in  in this special issue.
2) Backhaul: A major consideration that has been consid-
ered in several places throughout the paper is backhaul, which
will be more challenging to provide for hyper-dense ultra-fast
networks. However, we ﬁnd optimism in three directions.
•Fiber deployments worldwide continue to mature and
reach farther and farther into urban corridors.
•Wireless backhaul solutions are improving by leaps and
bounds, with considerable startup activity driving inno-
vation and competition. Further, mmWave frequencies
could be utilized for much of the small-cell backhauling
due to their ambivalence to interference. This may in
fact be the ﬁrst serious deployment of non-LoS mmWave
with massive beamforming gains given that the backhaul
connection is quite static and outdoors-to-outdoors, and
thus more amenable to precise beam alignment.
•Backhaul optimization is becoming a pressing concern,
given its new status as a performance-limiting factor,
and this is addressed in ,  in this special
issue. The problem of jointly optimizing resources in
the radio network and across the backhaul is considered
in . Compression techniques for uplink cloud-
RAN are developed in . Another approach is the
proactive caching of high bandwidth content like popular
It is an exciting time in the wireless industry and for
wireless research at large. Daunting new requirements for 5G
are already unleashing a ﬂurry of creative thinking and a sense
of urgency in bringing innovative new technologies into reality.
Even just two years ago, a mmWave cellular system was
considered something of a fantasy; now it is almost considered
an inevitability. As this article has highlighted, it is a long
road ahead to truly disruptive 5G networks. Many technical
challenges remain spanning all layers of the protocol stack
and their implementation, as well as many intersections with
regulatory, policy, and business considerations. We hope that
this article and those in this special issue will help to move us
forward along this road.
The authors thank Arunabha Ghosh (AT&T Labs), Robert
W. Heath Jr. (UT Austin), and Federico Boccardi (Vodaphone)
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IEEE JSAC SPECIAL ISSUE ON 5G WIRELESS COMMUNICATION SYSTEMS 17
Jeffrey G. Andrews [S’98, M’02, SM’06, F’13]
received the B.S. in Engineering with High Dis-
tinction from Harvey Mudd College, and the M.S.
and Ph.D. in Electrical Engineering from Stanford
University. He is the Cullen Trust Endowed Professor
(#1) of ECE at the University of Texas at Austin,
Editor-in-Chief of the IEEE Transactions on Wireless
Communications, and Technical Program Co-Chair
of IEEE Globecom 2014. He developed CDMA sys-
tems at Qualcomm from 1995-97, and has consulted
for entities including Verizon, the WiMAX Forum,
Intel, Microsoft, Apple, Samsung, Clearwire, Sprint, and NASA. He is a
member of the Technical Advisory Boards of Accelera and Fastback Networks,
and co-author of the books Fundamentals of WiMAX and Fundamentals of
Dr. Andrews received the National Science Foundation CAREER award in
2007 and has been co-author of nine best paper award recipients: ICC 2013,
Globecom 2006 and 2009, Asilomar 2008, the 2010 IEEE Communications
Society Best Tutorial Paper Award, the 2011 IEEE Heinrich Hertz Prize, the
2014 EURASIP Best Paper Award, the 2014 IEEE Stephen O. Rice Prize, and
the 2014 IEEE Leonard G. Abraham Prize. He is an elected member of the
Board of Governors of the IEEE Information Theory Society.
Stefano Buzzi [M’98, SM’07] is currently an As-
sociate Professor at the University of Cassino and
Lazio Meridionale, Italy. He received his Ph.D. de-
gree in Electronic Engineering and Computer Sci-
ence from the University of Naples ”Federico II”
in 1999, and he has had short-term visiting ap-
pointments at the Dept. of Electrical Engineering,
Princeton University, in 1999, 2000, 2001 and 2006.
His research and study interest lie in the wide area
of statistical signal processing and resource alloca-
tion for communications, with emphasis on wireless
communications; he is author/co-author of more than 50 journal papers and 90
conference papers; Dr. Buzzi was awarded by the Associazione Elettrotecnica
ed Elettronica Italiana (AEI) the ”G. Oglietti” scholarship in 1996, and was
the recipient of a NATO/CNR advanced fellowship in 1999 and of three CNR
short-term mobility grants. He is a former Associate Editor for the IEEE
Communications Letters, and the IEEE Signal Processing Letters.
Wan Choi [M’06, SM’12] received the B.Sc. and
M.Sc. degrees from the School of Electrical En-
gineering and Computer Science (EECS), Seoul
National University (SNU), Seoul, Korea, in 1996
and 1998, respectively, and the Ph.D. degree in the
Department of Electrical and Computer Engineering
at the University of Texas at Austin in 2006. He is
currently an Associate Professor of the Department
of Electrical Engineering, Korea Advance Institute of
Science and Technology (KAIST), Daejeon, Korea.
From 1998 to 2003, he was a Senior Member of
the Technical Staff of the R&D Division of KT Freetel, Korea, where he
researched 3G CDMA systems.
Dr. Choi is the recipient of IEEE Vehicular Technology Society Jack
Neubauer Memorial Award in 2002. He also received the IEEE Vehicular
Technology Society Dan Noble Fellowship Award in 2006 and the IEEE Com-
munication Society Asia Paciﬁc Young Researcher Award in 2007. He serves
as Associate Editor for the IEEE Transactions on Wireless Communications,
for the IEEE Transactions on Vehicular Technology, and for IEEE Wireless
Stephen V. Hanly [M’98] received a B.Sc. (Hons)
and M.Sc. from the University of Western Australia,
and the Ph.D. degree in mathematics in 1994 from
Cambridge University, UK. From 1993 to 1995, he
was a Post-doctoral member of technical staff at
AT&T Bell Laboratories. From 1996-2009 he was at
the University of Melbourne, and from 2010-2011
he was at the National University of Singapore. He
now holds the CSIRO-Macquarie University Chair in
Wireless Communications at Macquarie University,
Sydney, Australia. He has been an Associate Editor
for IEEE Transactions on Wireless Communications, Guest Editor for IEEE
Journal on Selected Areas in Communications, and Guest Editor for the
Eurasip Journal on Wireless Communications and Networking. In 2005 he was
the technical co-chair for the IEEE International Symposium on Information
Theory held in Adelaide, Australia. He was a co-recipient of the best paper
award at the IEEE Infocom 1998 conference, and the 2001 Joint IEEE
Communications Society and IEEE Information Theory Society best paper
Angel Lozano [S’90, M’99, SM’01, F’14] received
the M.Sc. and Ph.D. degrees in Electrical Engineer-
ing from Stanford University in 1994 and 1998,
respectively. He is a Professor and the Vice-Rector
for Research at Universitat Pompeu Fabra (UPF) in
Barcelona, Spain. He was with Bell Labs (Lucent
Technologies, now Alcatel-Lucent) between 1999
and 2008, and served as Adj. Associate Professor
at Columbia University between 2005 and 2008.
Prof. Lozano is an Associate Editor for the IEEE
Transactions on Information Theory, the Chair of the
IEEE Communication Theory Technical Committee, and an elected member
to the Board of Governors of the IEEE Communications Society. His papers
have received two awards: ISSSTA 2006 and the 2009 IEEE Stephen O. Rice
Anthony C. K. Soong [S’88, M’91, SM’02, F’14]
received the B.Sc. degree in animal physiology and
physics from the University of Calgary, and the B.Sc.
degree in electrical engineering, the M.Sc. degree
in biomedical physics and Ph.D. degree in electrical
and computer engineering from the University of Al-
berta. He is currently the chief scientist for wireless
research and standards at Huawei Technologies Co.
Ltd, in the US. He serves as the vice-chair for 3GPP2
TSG-C WG3. Prior to joining Huawei, he was with
the systems group for Ericsson Inc and Qualcomm
Inc. Dr. Soong has published numerous scientiﬁc papers and has over 80
patents granted or pending. He was the corecipient of the 2013 IEEE Signal
Processing Society Best Paper Award.
Jianzhong (Charlie) Zhang [S’96, M’02, SM’09] is
currently senior director and head of Wireless Com-
munications Lab with Samsung Research America
at Dallas, where he leads technology development,
prototyping and standardization for Beyond 4G and
5G wireless systems. From Aug 2009 to Aug 2013,
he served as the Vice Chairman of the 3GPP RAN1
working group and led development of LTE and
LTE-Advanced technologies such as 3D channel
modeling, UL-MIMO and CoMP, Carrier Aggrega-
tion for TD-LTE, etc. Before joining Samsung, he
was with Motorola from 2006 to 2007 working on 3GPP HSPA standards, and
with Nokia Research Center from 2001 to 2006 working on IEEE 802.16e
(WiMAX) standard and EDGE/CDMA receiver algorithms. He received his
Ph.D. degree from University of Wisconsin, Madison.