IEEE Wireless Communications - April 2017 - page 126

IEEE Wireless Communications • April 2017
124
TCP slow start procedure (i.e. TCP flow control).
Therefore, it would be advantageous to first per-
form scheduling of the MBB TCP data with short
TTIs, followed by longer TTI sizes when reaching
steady state operation. Given the requirements in
Table 1, corresponding solutions are outlined in
the following section.
TDD F
rame
S
tructure
D
esign
S
ubframe
C
onstructs
The proposed solution is based on a series of
bi-directional TDD blocks that consists of an inte-
ger number of subframes. Given the WA cov-
erage requirements discussed in the previous
section, we consider a minimum block size of 1
ms, but also options of using longer blocks of,
for example, 4 ms duration. Each block is having
downlink transmission in the start, followed by a
short guard period, and uplink transmission. The
time resolution for the switching point between
downlink and uplink in a block is on subframe res-
olution, assuming a 0.2 ms subframe duration as
our default setting. Figure 1 shows a possible bi-di-
rectional TDD block configuration, where the first
three subframes are configured for downlink, and
the last two subframes are for uplink transmission
(minus the fraction that is punctured for guard).
Within each block, users are flexibly time-frequen-
cy multiplexed on subframe and physical resource
block (PRB) resolution in coherence with their
service requirements and radio conditions. The
per-user resource allocation is facilitated by
adopting the principle of in-resource control chan-
nel (CCH) signaling for physical layer scheduling
grants [11], as illustrated in Fig. 1. Among other
things, this allows scheduling users with variable
effective length transmission time intervals (TTI).
Referring to Fig. 1, User #3 is scheduled in the
downlink with an effective TTI size corresponding
to three subframes. User #1 is scheduled with an
effective TTI size of one subframe in the start of
the downlink part, while User #2 is scheduled on
the last subframe of the downlink part. Notice
that in the spirit of the in-resource CCH method,
the scheduling grant for users 1–3 appears in the
start of their downlink transmission. Those sched-
uling grants contain information on the physical
resources for the data transmission, as well as the
corresponding modulation and coding scheme,
HARQ, and MIMO transmission information (e.g.
if the user is scheduled with multiple streams). The
in-resource CCH is transmitted with quadrature
phase shift keying (QPSK), allowing a modest set
of different effective coding rates as also assumed
for the LTE physical dedicated control channel
(PDCCH) [10]. In the interest of UE complexity
to monitor for in-resource CCH transmissions, the
network can configure UEs to only search for such
scheduling grants with a certain time-frequency
resolution (see more details in [11]).
Notice from Fig. 1 that User #1 is scheduled
in both the downlink and uplink in the same
bi-directional TDD block. The in-resource CCH
T
able
1.
Summary of identified main constraints for a 5G WA TDD design.
Category
Related constraints
Service related
constraints
Mobile broadband
(MBB)
Flexibility to schedule MBB users with variable bandwidth and TTI sizes is of importance.
Large dynamic range of the user plane data payload sizes to be scheduled, ranging from
several tens of bytes (e.g., for application-layer control messages) to gigabytes for large data
file transmissions.
Massive machine
communication (MMC)
Support for low cost and energy efficient MTC devices that only operate on a narrow
bandwidth, but also support for wideband MTC devices. Typically moderate size payloads.
Mission critical
communication (MCC)
Support for low latency is essential, calling for short TTI sizes when needed. Ultra high
reliability for some MCC use cases. Typically moderate size payloads.
Uplink
coverage
constraints
• Users need a certain minimum continuous uplink transmission time to have reliable uplink reception at the eNB.
• Examples: with 0.2 ms transmission time, coverage is on the order of ~300 meters, while 1 ms transmission time
offers 1.4 km coverage.
Inter-cell TDD
coordination
• High-power macro cells in the same local area shall use coordinated TDD configurations of downlink/uplink
transmissions to avoid cross-link interference problems.
• The performance is at risk if cells in the same geographical area use different transmission directions at the same time.
• Use of massive MIMO and advanced interference suppression techniques can, however, help relax the requirements
for tight inter-cell coordination.
System
constraints
• Desirable to have frequent downlink and uplink transmission opportunities to have fast control loops, for example,
for HARQ ACK/NACK’s, channel state information, radio resource control.
• Known (semi-statically configured) downlink transmission opportunities for common cell control information,
for example, broadcast channel with system information and cell discovery signals for mobility purposes.
• Known (semi-statically configured) uplink transmission occurrences for random access (RA), and potentially also
uplink contention based data transmission access for MMC [12].
• Flexibility for configuration of different downlink and uplink transmission switching patterns.
Link
asymmetry
• Support link configurations where the downlink and uplink transmission time intervals can be either equal (symmetric) or
take different lengths (asymmetric).
• Asymmetric configurations with short transmission times for downlink, while using longer transmission times in the
uplink is relevant for uplink coverage reasons.
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.
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