IEEE Wireless Communications - April 2017 - page 124

IEEE Wireless Communications • April 2017
122
1536-1284/17/$25.00 © 2017 IEEE
A
bstract
A flexible multi-service 5G wide area (WA)
solution for time division duplex (TDD) operation
is outlined in this article. In particular, the associat-
ed frame design is in focus. Given the fundamen-
tal tradeoffs between capacity, coverage, latency,
and reliability, a flexible solution that allows opti-
mization on a per-link basis is proposed. The solu-
tion encompasses the possibility to schedule users
with different transmission time intervals to best
match their service requirements and radio condi-
tions. Due to the large downlink/uplink transmis-
sion power imbalance for each link, asymmetric
link operation is proposed, where users operate
with different minimum transmission times for
the two link directions. This is achieved by using
a highly flexible asynchronous hybrid automat-
ic repeat request (HARQ) scheme, as well as a
novel solution with in-resource control channel
signaling for the scheduling grants. Performance
results for the proposed 5G WA TDD solution
show clear benefits over current LTE, for example,
reduced latency and more scalable control over-
head to better support users with different QoS
requirements.
I
ntroduction
In this article we focus on the design of a 5G
multi-service air interface with wide area (WA)
coverage, using time division duplex (TDD). In this
context, WA coverage is achieved with high-pow-
er macrocells, deployed at relative low carrier
frequencies due to the more favorable radio prop-
agation properties at these bands. As an exam-
ple, roughly half of the available bands below 6
GHz that could be made available for future 5G
deployments is unpaired, that is, for TDD. 5G is
set to support a wide range of highly diverse ser-
vices. This includes enhanced mobile broadband
(MBB) with peak data rates of 10–20 Gb/s, offer-
ing spatial uniform availability of end-user data
rates of 100 Mb/s. Moreover, efficient expedi-
tion of MBB services should also support flexible
scheduling of smaller payloads, and thus requires
a scheduling framework that supports high
dynamic range of scheduled payload sizes. Fur-
thermore, machine type communication (MTC)
with massive machine communication (MMC)
and mission critical communication (MCC) are
other use cases. MCC is particularly challenging
as it requires both low latency and ultra reliable
communication. Among others, these service
requirements translate to the need for short trans-
mission time intervals for MCC, large bandwidth
for MBB, and low bandwidth operation for MMC
devices with low cost and energy consump-
tion requirements. For more information on 5G
requirements, we refer to [1, 2].
It is well known that there are fundamen-
tal tradeoffs between capacity, latency, reliabil-
ity, and coverage [3]. This basically means that
optimizing for one metric results in a loss for the
other metrics. As an example, this can be illustrat-
ed with the effective capacity, which expresses
the maximum source data arrival rate that a cer-
tain channel process can support, while fulfilling a
latency constraint [4]. With no latency constraints,
the effective capacity equals the Shannon capac-
ity, while it decreases asymptotically as stricter
latency constraints are enforced. From a system
design point of view, this tells us that we should
not optimize the air interface to, for example,
always fulfill strict latency requirements, as this will
incur a loss in capacity (spectral efficiency), and
vice versa. Instead, the focus of this study is on a
flexible system design that allows optimizing each
link in coherence with its service requirements.
Despite the relative short time that 5G has been
researched, the open literature already includes
an impressive number of 5G related studies.
Examples of 5G studies include: the METIS proj-
ect [5]; use of centralized network architectures
has been suggested in [6]; small cell optimized
TDD design for MBB has been proposed in [7];
and 5G cell densification in [8].
In our effort to design a flexible multi-service
5G WA TDD concept, we start by first identify-
ing the fundamental objectives and constraints in
the radio design. The tradeoffs between capacity,
coverage, and latency are studied, and solutions
aiming at making the best compromises between
these metrics are suggested. In particular, a con-
figurable 5G TDD frame structure for efficient
multiplexing of users is outlined, which comprises
cell-specific configurations, as well as flexibility for
user scheduling within the cells. Throughout the
study, we will use the Long Term Evolution (LTE)
Rel-12 solution with enhanced interference miti-
gation and traffic adaptation (eIMTA) as the refer-
ence for today’s 4G cellular [9]. In short, eIMTA
is the LTE TDD solution with adaptive adjust-
ment of downlink/uplink transmission patterns.
Although being a powerful concept, LTE eIMTA
K
laus
I. P
edersen
, G
ilberto
B
erardinelli
, F
rank
F
rederiksen
, P
reben
M
ogensen
A F
lexible
5G W
ide
A
rea
S
olution
for
TDD
with
A
symmetric
L
ink
O
peration
The authors are with
Nokia–Bell Labs.
Digital Object Identifier:
10.1109/MWC.2016.1500364WC
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