IEEE Wireless Communications - April 2017 - page 125

IEEE Wireless Communications • April 2017
123
does not fulfill all of the requirements of 5G, as
it was designed mainly for MBB use cases with
more modest data rates. As an example, the low-
est possible radio hybrid automatic repeat request
(HARQ) round trip time (RTT) for LTE eIMTA is
on the order of 9.8 ms–12.4 ms, depending on
the downlink/uplink configuration [10]. Our per-
formance analysis shows that the proposed 5G
WA TDD concept can achieve significantly lower
latency, and higher flexibility for scheduling of
users with extreme diverse service requirements.
The article is closed with concluding remarks and
outlook.
F
undamental
C
onstraints
U
plink
C
overage
and
B
andwidth
C
onstraints
Providing good coverage is obviously a priority
for a 5G WA design. Due to the lower user equip-
ment (UE) transmit power (as compared to the
base station, also known as eNB), the coverage is
typically determined by the uplink. For a TDD sys-
tem, the coverage is further challenged as UEs are
allowed to transmit only at certain time-intervals.
Coverage challenged UEs will need to transmit
for a certain minimum time-duration to allow the
receiving eNB to collect a sufficient amount of
energy to successfully decode the transmission. In
LTE link budget studies, it was, for example, found
that the physical uplink control channel (PUCCH)
transmitted during a 1 ms time-interval has a cov-
erage range of 1.4 km and 1 km for suburban and
dense urban non line of sight conditions (NLOS),
respectively [9]. These results are obtained for
four receive antennas at the eNB, assuming a
carrier frequency of 2 GHz. Reducing the trans-
mission time from 1 ms to 0.2 ms as assumed in
recent 5G (small cell) TDD concept studies [7], is
estimated to reduce the coverage range to ~ 300
meters. Hence, it is of paramount importance that
a new 5G WA TDD concept is designed with the
flexibility to allow configuration of (continuous)
uplink transmit opportunities to meet the desired
coverage target. For the downlink, the coverage
is obviously better due to the higher eNB transmit
power. It is therefore desirable to have support
for asymmetric link operation, where the transmis-
sion times of data and control can be set different-
ly for the two link directions on a per user basis,
depending on its coverage.
For MMC we also consider device cost and
energy constraints in our design. More specifi-
cally, we aim at designing a system that supports
concurrent operation of low bandwidth MMC
devices on wider bandwidth 5G carriers. We
consider support of low cost and energy efficient
MMC devices with a transceiver bandwidth of
no more than a couple of hundred kHz to a few
MHz for the downlink, and only a single anten-
na. For the uplink, even lower transmission band-
width is considered. Due to the lack of frequency
and space (i.e. antenna) diversity, additional time
diversity is desirable for MMC with relaxed laten-
cy requirements.
M
ulti
-C
ell
C
oordinated
TDD O
peration
It is well known from numerous studies that
dynamic TDD operation is feasible and attrac-
tive for small cell scenarios, allowing each cell
to autonomously decide the transmission direc-
tion depending on the needs within the cell (also
known as uncoordinated TDD). Dynamic TDD
operation is feasible for small cell scenarios due
to the balanced output power level from eNBs
and UEs, making it possible to manage cross-link
interference with advanced receiver interference
suppression techniques (e.g., [7, 8]). However, for
a WA setting, the eNB transmit power is typically
on the order of ~ 49 dBm, having antenna gain of
at least ~ 14 dBi, and thus resulting in an equiv-
alent isotropic radio power (EIRP) of ~ 63 dBm,
while the EIRP for the UE is typically only 23 dBm,
assuming a maximum transmit power of 23 dBm
and 0 dBi antenna gain. The large output power
imbalance ( ~ 40 dB in EIRP and ~ 26 dB without
antenna gains) between UEs and eNBs for a WA
scenario sets additional restrictions on the TDD
operation, since closely coupled cells will have
to coordinate the use of uplink/downlink trans-
mission patterns to avoid severe cross-link inter-
ference problems. This essentially calls for some
degree of multi-cell coordinated TDD operation
for WA scenarios. Thus, each cell does not have
the full freedom to determine if the cell resources
are used for uplink or downlink, as some align-
ment and coordination with other WA cells in the
vicinity is required. The use of massive MIMO and
advanced interference suppression techniques
can, however, help relax the requirements for
tight inter-cell coordination.
S
ystem
C
onstraints
Finally, there are system related constraints that
need to be considered. Among these, there need
to be regular downlink transmission resources
available for sending the broadcast channel with
the most essential system information, as well as
physical layer discovery signals. There also needs
to be resources available for uplink random
access (RA) [12]. Again, for large WA cells, the
required time-duration of the resources for RA
should approximately equal 1 ms for UEs at the
cell-edge to perform successful access. Moreover,
even for a cell with heavy downlink user plane
traffic, it is desirable to have frequent opportu-
nities for uplink transmission of various physi-
cal layer related control information. The latter
includes positive and negative acknowledgments
for HARQ, and various channel quality informa-
tion feedback that the eNB needs for link adap-
tation and scheduling purposes, as well as MIMO
adaptation. Thus, having long time periods with-
out uplink opportunities is undesirable.
S
ummary
of
the
M
ain
C
onstraints
The identified main constraints for a flexible
multi-service 5G WA TDD design are summa-
rized in Table 1. In line with the study in [11],
our hypothesis is that efficient scheduling of the
considered services requires the support for dif-
ferent transmission time intervals (TTIs). As a few
examples, users with tight latency constraints (e.g.
MCC) require short TTIs, while MMC users sched-
uled on a narrow bandwidth are most efficient-
ly served with longer TTIs. Moreover, users with
MBB traffic could also benefit from variable TTI
sizes. During the initial MBB data transmission
session, the end-user experienced performance
is primarily determined by the RTT due to the
Providing good coverage
is obviously a priority
for a 5G WA design.
Due to the lower
user equipment (UE)
transmit power (as
compared to the base
station, also known as
eNB), the coverage is
typically determined by
the uplink. For a TDD
system, the coverage
is further challenged as
UEs are only allowed
to transmit at certain
time-intervals only.
1...,115,116,117,118,119,120,121,122,123,124 126,127,128,129,130,131,132
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