IEEE Wireless Communications - April 2017 - page 129

IEEE Wireless Communications • April 2017
127
rizes the experienced relative CCH scheduling
overhead for different scheduling options, and
different user experienced downlink SINR values.
The results in Fig. 4 are obtained by using the
same assumptions as in [11], where the required
resources for the in-resource CCH depends on
the user experienced SINR. It is observed that a
coverage challenged UE with a post-detection
SINR of –6dB will experience a CCH overhead of
~ 75 percent if scheduled with a TTI size of 0.2 ms
on a 2.5 MHz bandwidth. Scheduling such a user
with a TTI size of, for example, 2 ms (as possible
within the longer 4 ms TDD subframe), reduces
the CCH overhead by a factor of 10. The reduc-
tion of the CCH overhead translates to higher
spectral efficiency. Thus, users with delay tolerant
services (e.g. MBB and MMC) are most efficiently
served with longer TTIs to achieve higher spectral
efficiency. The users with a more favorable down-
link experienced SINR of 2dB generally experi-
ence lower CCH overhead, as less resources are
required for the in-resource CCH (i.e. less strong
coding). However, also for such users, there is a
clear reduction of the CCH overhead from using
longer TTIs. In fact, by using a TTI size of 2 ms for
such users, the relative CCH overhead is reduced
to less than 1 percent. In comparison, the phys-
ical layer CCH overhead for LTE is on the order
of 7 to 21 percent, depending on whether 1 to 3
symbols are configured for control per subframe
[10]. The proposed 5G solution is therefore supe-
rior to LTE, offering a more flexible and scalable
solution to efficiently adjust the tradeoffs between
latency and CCH overhead.
In summary, the tradeoffs between latency,
capacity, and coverage are visible from the results
in Table 2 and Fig. 4. A cell-edge user with –6 dB
SINR loses 66 percent in throughput from reduc-
ing the TTI size from 2 ms to 0.2 ms, but only
gains a factor of four in reduced user plane laten-
cy.
1
A user nearer the serving cell (with +2 dB
SINR), loses only 8 percent in throughput from
reducing the TTI size from 2 ms to 0.2 ms.
C
onclusion
In this article we have proposed a highly flexible
TDD solution for efficient multiplexing of users
with highly diverse service requirements in a wide
area (macro) type of environment. The proposed
TDD radio frame structure is composed of a
series of subframes that form bi-directional TDD
blocks with self-decodable physical layer control
and data channel elements. The TDD radio frame
configuration is coordinated between neighbor-
ing macro sites to avoid undesirable cross-link
interference. The solution allows operating each
link in coherence with its service constraints.
This includes scheduling of users with different
TTI sizes to efficiently control latency-capacity
tradeoffs, including asymmetric operation with
different uplink and downlink minimum trans-
mission times to meet coverage constraints. The
presented performance results show that short
latency can be achieved for users with good
coverage, while the coverage challenged users
tend to experience slightly higher latency. The
proposed scheme is superior to LTE eIMTA, both
in terms of the offered flexibility for multiplexing
users with diverse services requirements, and in
terms of achieved user plane latency. The most
important lesson learned in this study is, therefore,
the importance and benefit of designing 5G with
a highly flexible frame structure, offering efficient
tradeoffs between different optimization targets
to support users with highly diverse QoS require-
ments.
A
cknowledgment
Part of this work has been performed in the
framework of the Horizon 2020 project FAN-
TASTIC-5G (ICT-671660) receiving funds from
the European Union. The authors would like to
acknowledge the contributions of their colleagues
in the project, although the views expressed in
this contribution are those of the authors and do
not necessarily represent the project.
R
eferences
[1] ITU, “IMT Vision — Framework and Overall Objectives of
the Future Development of IMT for 2020 and Beyond,”
Document, Radiocommunication Study Groups, Feb. 2015.
[2] E. Dahlman
et al
., “5G Wireless Access: Requirements and
Realization,”
IEEE Commun. Mag. — Communications Stan-
dards Supplement
, vol. 52, no. 12, Dec. 2014, pp. 42–47.
F
igure
4.
Relative CCH scheduling overhead for different scheduling options
and user experienced quality, assuming a user scheduling bandwidth of
2.5MHz.
TTI size [ms]
0.2
10
0
10
-1
CCH overhead [%]
10
1
10
2
77%
1.0
15%
2.0
7.7%
0.2
9%
1.0
1.8%
2.0
0.9%
Downlink SINR=-6dB
Downlink SINR=2dB
T
able
2.
Average downlink one-way user plane latency budget. All latency val-
ues are in ms.
TDD radio
frame
configuration
DBDU (4x1 ms)
DDDD (4X1ms) + D (1X4ms)
UE coverage
position
Short
Medium
Large
Short
Extreme
eNB
processing
time
0.30
0.30
0.30
0.30
0.30
Frame
alignment
0.27
0.27
0.27
0.17
0.17
TTI duration
0.20
0.20
0.20
0.20
0.20
UE processing
0.30
0.30
0.30
0.30
0.30
HARQ retrans.
0.13 (0.1x1.3) 0.16 (0.1x1.6)
0.19 (0.1x1.9)
0.19 (0.1x1.9)
0.50 (0.1x5)
Total one way
delay
1.20
1.23
1.26
1.16
1.47
1
The reason for not gain-
ing a factor of ten when
decreasing the downlink TTI
size from 2 ms to 0.2 ms is
because a constant eNB and
UE processing time of 0.3
ms is assumed, as well as
the need for a relative long
uplink ACK/NACK transmis-
sion time for a cell-edge UE.
1...,119,120,121,122,123,124,125,126,127,128 130,131,132
Powered by FlippingBook