IEEE Wireless Communications - April 2017 - page 127

IEEE Wireless Communications • April 2017
125
that appears in the start of the block for User #1
contains a joint scheduling grant for the down-
link and uplink allocation to that user. The uplink
transmission for User #1 contains both data
(a.k.a. payload) as well as the corresponding
HARQ acknowledgment (ACK), or negative ACK
(NACK). The ACK/NACK for User #1 is multi-
plexed with uplink data. Similarly, the ACK/NACK
for the downlink data transmission to User #2 is
also sent in the uplink part of the block. In this
particular example, it is transmitted during the last
subframe of the block. Following the same spir-
it as used for LTE, several ACK/NACK’s can be
transmitted on the same set of uplink resources by
using different code signatures. From the perspec-
tive of users #1 and #2, the bi-directional TDD
block is said to be self-contained, as the downlink
data transmission and the corresponding uplink
ACK/NACK’s appear within the same block.
However, depending on the block configu-
ration (duration and switching point between
downlink and uplink) and the users’ coverage
requirement, it is not always possible to transmit
the users’ ACK/NACK in the same block as the
downlink data transmission. First of all, the UE
needs time to process the received downlink data
before it can send the corresponding ACK/NACK
in the uplink. This is referred to as the UE pro-
cessing time. For 5G, some terminal modem ven-
dors have reported that the UE processing time
can be as short as the guard period (i.e. less than
0.1 ms), while more conservative assumptions
for UE processing time is on the order of 0.3 ms
(e.g., [7, 11]). Second, as discussed earlier, a cer-
tain minimum uplink transmission time is required
for sending the ACK/NACK, depending on the
users’ uplink link budget conditions. Hence, users
in the close vicinity of their serving cell will need
only one subframe (or less) for the uplink ACK/
NACK transmission to have it reliably decoded at
the eNB, while users at the macro cell-edge need
longer transmissions. Referring to Fig. 1, only the
downlink data transmission for User #3 appears in
the block, while the corresponding uplink ACK/
NACK is postponed until a subsequent block
where there is sufficient uplink transmission time
available (assuming this user is coverage limited).
TDD R
adio
F
rame
C
onfiguration
An integer number of bi-directional TDD blocks
(and hence also an integer number of subframes)
forms the TDD radio frame, as illustrated in Fig. 2.
The bi-directional TDD blocks within each TDD
radio frame can have different configurations,
for example, different switching points between
downlink and uplink transmissions. Within each
cell, the TDD radio frame structure is periodically
repeated. As discussed earlier, downlink trans-
mission resources for system information broad-
cast appears at least once per TDD radio frame
at a predefined location (time-frequency) that is
known by the users in the cell. Similarly, the loca-
tions for the cell to transmit cell discovery signals
for mobility purposes are fixed within each TDD
radio frame. The same applies for uplink resourc-
es for random access [12]. The exact location and
design of common downlink signals for system
information broadcast, cell discovery, as well as
uplink random access, are outside the scope of
this article.
The TDD radio frame is assumed to be coor-
dinated between neighboring cells, such that the
same synchronized downlink and uplink switch-
ing pattern is used by the cells. Depending on
how fast cells in the same geographic area are
able to coordinate, the TDD radio frame con-
figuration can be semi-dynamically adjusted to
best match the time-variant offered traffic for the
two link directions. Hence, for a traditional dis-
tributed macrocellular network structure where
the base stations are inter-connected via a back-
haul, configuration of the TDD radio frame struc-
ture is only adjusted on a slow time-scale, using
self organizing network (SON) type of solutions
[13]. For centralized network architectures with
virtually zero-latency fronthaul connections [6],
adaptation of the TDD radio frame structure con-
figuration can be faster, exploiting more efficient
adaptation in coherence with the time-variant traf-
fic conditions. Notice furthermore that the TDD
radio frame structure can consist of bi-direction-
al TDD blocks of different lengths, for example,
a mixture of 1 ms and 4 ms blocks. The longer
4 ms block offers lower relative overhead from
the guard period, and options for using longer
TTI durations (e.g. improved time-diversity). The
4 ms block with relative long uplink transmission
time is also attractive for allocation of the random
access resources, and for serving uplink coverage
challenged users that require longer transmission
times for maintaining their uplink connectivity.
F
igure
1.
Basic bi-directional TDD block construct illustrating time-frequency
multiplexing of users.
Grant for DL data
Guard
T
UL
T
DL
T
subframe
T
subframe
T
subframe
T
subframe
T
subframe
ACK/NACK
for user #2
User #2
User #3
User #1
PRB
Time
Frequency
User #1
(payload + ACK/NACK)
One bi-directional TDD block
Grant for DL & UL data
F
igure
2.
TDD radio frame constructed of an integer number of bi-directional
TDD blocks.
Blocks can have
different UL/DL
configurations
System
broadcast
channel (BCH)
Cell discovery
channel
Random
access (RA)
resources
One TDD radio frame
(integer number of bi-directional TDD blocks)
1...,117,118,119,120,121,122,123,124,125,126 128,129,130,131,132
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