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IEEE Communications Magazine • November 2017
0163-6804/17/$25.00 © 2017 IEEE
AbstrAct
As a flexible and scalable architecture, hetero-
geneous cloud radio access networks (H-CRANs)
inject strong vigor into the green evolution of
current wireless networks. But the brutal truth is
that EE improves at the cost of other indices such
as SE, fairness, and delay. It is thus important to
investigate performance trade-offs for striking flex-
ible balances between energy-efficient transmis-
sion and excellent QoS guarantees under this new
architecture. In this article, we first propose some
potential techniques to energy-efficiently operate
H-CRANs by exploiting their features. We then
elaborate the initial ideas of modeling three fun-
damental trade-offs, namely EE-SE, EE-fairness, and
EE-delay trade-offs, when applying these green
techniques, and present open issues and challeng-
es for future investigation. These related results
are expected to shed light on green operation
of H-CRANs from adaptive resource allocation,
intelligent network control, and scalable network
planning.
IntroductIon
bAckground And MotIvAtIon
The dramatic increase in the number of smart-
phones and tablets with ubiquitous broadband
connectivity has triggered an explosive growth
in mobile data traffic [1]. Cisco forecasts that the
amount of global mobile data traffic will increase
7-fold from 2016 to 2021, the majority of which
are generated by energy-hungry applications such
as mobile video [1]. This is also referred to as
the well-known 1000× data challenge in cellu-
lar networks. Meanwhile, the number of devices
connected to the global mobile communication
networks will reach 100 billion in the future, and
that of mobile terminals will surpass 10 billion by
2020 [2].
Although unprecedented opportunities for
the development of wireless networks are creat-
ed by the massive traffic amount and connected
devices, a concomitant crux is that this growth
skyrockets the energy consumption (EC) and
greenhouse gas emissions in the meantime. From
statistical data, the information and communi-
cation technology (ICT) industry is responsible
for 2 percent of worldwide CO
2
emissions and
2–10 percent of global EC, of which more than
60 percent is directly attributed to radio access
networks (RANs) [3]. In this regard, 5G wireless
communication networks are anticipated to pro-
vide spectral efficiency (SE) and energy efficiency
(EE) growth by a factor of at least 10 and 10 times
longer battery life of connected devices [2].
concept of H-crAns
To meet the 1000× data challenge, heteroge-
neous networks (HetNets), composed of a diverse
set of small cells (e.g., microcells, picocells, and
femtocells) overlaying the conventional macro-
cells, have been introduced as one of the most
promising solutions [2]. However, the ubiquitous
deployment of HetNets is accompanied by the
following shackles:
• Severe interference: The spectrum reuse
among cells incurs severe mutual interfer-
ence, which may significantly reduce the
expected system SE and also decrease the
network EE.
• Unsatisfactory EE: The densely deployed
small cells lead to escalated EC and thus
reduced EE, and also increases capital expen-
ditures (CAPEX) and operational expendi-
tures (OPEX).
• No computing-enhanced coordination cen-
ters: There are no centralized units with
strong computing abilities to globally coor-
dinate multi-tier interference and execute
cross-RAN optimization, which dramatically
limits cooperative gains among cells.
• Inflexibility and unscalability: Fragmented
base stations (BSs) result in inflexible and
unscalable network control and operations,
thus leading to redundant network planning
and inconvenient network upgrade.
To overcome these challenges faced by Het-
Nets, cloud RANs (C-RANs), new centralized
cellular architectures armed with powerful cloud
computing and virtualization techniques, have
been put forward in parallel to coordinate inter-
ference and manage resources across cells and
RANs [4]. In C-RANs, a large number of low-cost
low-power remote radio heads (RRHs), connect-
ing to the baseband unit (BBU) pool through the
fronthaul links, are randomly deployed to enhance
the wireless capacity in hotspots. Consequently,
the combination of HetNets and C-RANs, known
Green Heterogeneous Cloud Radio Access
Networks: Potential Techniques,
Performance Trade-offs, and Challenges
Yuzhou Li, Tao Jiang, Kai Luo, and Shiwen Mao
green coMMunIcAtIons And coMputIng networks
The authors first propose
some potential tech-
niques to energy-efficient-
ly operate H-CRANs by
exploiting their features.
They then elaborate the
initial ideas of model-
ing three fundamental
trade-offs, namely EE-SE,
EE-fairness, and EE-delay
trade-offs, when applying
these green techniques,
and present open issues
and challenges for future
investigation.
Yuzhou Li, Tao Jiang, and Kai Luo are with Huazhong University of Science and Technology; Shiwen Mao is with Auburn University.
Digital Object Identifier:
10.1109/MCOM.2017.1600807