IEEE Communications Magazine - June 2017 - page 96

IEEE Communications Magazine • June 2017
0163-6804/17/$25.00 © 2017 IEEE
The 5G networks are envisioned to use
mmWave bands to provide gigabit-per-second
throughput. To extend the coverage of extreme
data rates provided by mmWave technologies, we
consider two-hop relaying based on D2D com-
munication in an integrated mmWave/sub-6 GHz
5G network. Compared to single-hop multi-cell
networks, two-hop D2D relaying in this network
will complicate the network management. Relay
selection and beam selection should be considered
together as relaying in mmWave bands would use
directional beamforming transmissions. MmWave/
sub-6 GHz multi-connectivity has to be managed,
and resources have to be allocated across fre-
quencies with disparate propagation conditions.
In this article, a hierarchical network control frame-
work is considered to address the relay and beam
selection, resource allocation, and interference
coordination problems. The sub-6 GHz band is
responsible for network control and for provid-
ing relatively reliable communications, while the
mmWave band provides high-throughput enhance-
ment. Opportunistic relay selection and mmWave
analog beamforming are used to limit the sig-
naling overhead. We evaluate mmWave/sub-6
GHz multi-connectivity with and without two-hop
relaying in urban outdoor scenarios for different
site deployment densities. MmWave/sub-6 GHz
multi-connectivity with relaying shows considerable
promise for reaching consistent user experience
with high end-to-end throughput in a cost-effective
network deployment.
The volume of mobile traffic and the number of
connected devices are predicted to increase sig-
nificantly in the fifth generation (5G) networks.
More spectrum, spectrum-efficient physical
layer techniques, and network densification are
key enablers to handle this growth. Consistent
user experience is regarded as a fundamental
5G requirement [1]. With current cellular tech-
nologies, users at the cell edge suffer from poor
service, even when complicated coordinated mul-
tipoint transmission technologies are applied [2].
Further densification of wireless networks using
millimeter-wave (mmWave) bands, combined
with massive multiple-input multiple-output and
beamforming techniques, provides a framework
to achieve throughput in the range of gigabits
per second. However, mmWave signals are more
vulnerable to blocking than sub-6 GHz signals. To
achieve both high capacity and consistent user
experience, mmWave infrastructure needs to be
densely deployed to increase line-of-sight (LOS)
probability, and to tackle the path loss and block-
age problems [3, 4]. It is estimated that an inter-
site distance (ISD) of 75–100 m is required for full
coverage in standalone mmWave deployments
[5]. Deploying dense sites increases capital and
operating expenditures (CAPEX and OPEX) for
operators, thus increasing cost for users. To this
end, extreme network densification for providing
full mmWave coverage may not be viable.
A reasonable way to introduce mmWave tech-
nology is to tightly integrate an mmWave network
with an existing sub-6 GHz network [6, 7]. How-
ever, in the integrated scenario, consistency of
user experience is jeopardized, as a large number
of users outside the mmWave coverage cannot
get high throughput. MmWave coverage can be
improved with relaying, by applying a multihop
cellular network (MCN) concept. In [8], relay
selection and interference management were
investigated in an interference-limited code-divi-
sion multiple access MCN. A time-division duplex
frame structure for integrating infrastructure relays
in mmWave with a 4G network was considered in
[9]. Recently, the potential benefits of deploying
mmWave relays in outdoor environments were
investigated in [10]. Deploying relays in mmWave
networks was shown to increase the coverage
probability and end-to-end (E2E) capacity.
In 5G, network controlled device-to-device
(D2D) communication is under consideration.
Accordingly, D2D relaying based on coopera-
tion between user equipments (UEs) can be used
for mmWave coverage extension and to tackle
inconsistent user experience. As the number of
UEs increases, the probability that a cell edge user
can find favorable mobile relays (e.g., LOS relays)
increases, and E2E performance can be boosted
by using D2D relaying transmission.
In this article, we investigate the downlink of a
5G network based on mmWave and sub-6 GHz
multi-connectivity. We consider a scenario where
Resource Allocation and Interference
Management for Opportunistic Relaying in
Integrated mmWave/sub-6 GHz 5G Networks
Junquan Deng, Olav Tirkkonen, Ragnar Freij-Hollanti, Tao Chen, and Navid Nikaein
5G N
The authors consider a
hierarchical network con-
trol framework to address
the relay and beam selec-
tion, resource allocation,
and interference coordi-
nation problems. They
evaluate mmWave/sub-6
GHz multi-connectivity
with and without two-hop
relaying in urban outdoor
scenarios for different site
deployment densities.
Junquan Deng, Olav Tirkkonen, Ragnar Freij-Hollanti are with Aalto University; Tao Chen VTT Technical Research Centre of Finland;
Navid Nikaein is with Eurecom, Biot Sophia-Antipolis.
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