NARDA SRM-3006 Manual - page 4
AN_1106_SRM_LTE_TDD
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Subject to change
Figure 2. A protocol is also required for this, to ensure that the uplink and downlink in TDD alternate without conflicts.
Such a protocol is likely to be quite complex for a modern communication technology like LTE. It can be illustrated
simply as being like a conversation between two persons using a simple walkie-talkie link operating on one
frequency. When one person is talking, the other is listening, and vice versa. The two persons take turns to talk. If
they both try to talk at the same time, a conflict occurs and communication becomes unintelligible. If the first person
has more to say than the second, they would require more resources, just like the TDD downlink in Figure 1.
3 LTE-TDD uplink and downlink configurations
As explained in section 2, LTE-TDD is subject to a specific protocol. Among other things, this determines the
resources / number of timeslots that are to be allocated to the uplink and downlink within a given time period. This is
referred to as the uplink / downlink configuration of a TDD network. Seven configurations, numbered 0 to 6, are
defined for LTE-TDD, as shown in Figure 3. These allocate more resources to either the downlink or the uplink, as
appropriate.
Uplink / downlink-
configuration
Sub frame number
0 1 2 3 4 5 6 7 8 9
0
DL
SSF
UL
UL
UL
DL
SSF
UL
UL
UL
1
DL
SSF
UL
UL
DL
DL
SSF
UL
UL
DL
2
DL
SSF
UL
DL
DL
DL
SSF
UL
DL
DL
3
DL
SSF
UL
UL
UL
DL
DL
DL
DL
DL
4
DL
SSF
UL
UL
DL
DL
DL
DL
DL
DL
5
DL
SSF
UL
DL
DL
DL
DL
DL
DL
DL
6
DL
SSF
UL
UL
UL
DL
SSF
UL
UL
DL
However, as Figure 2 focuses on the LTE standard, the term used is sub frames rather than resources or timeslots. A
sub frame has a duration of one millisecond according to the specification. Ten sub frames make up a so-called radio
frame, which is the largest unit in the LTE frame structure. A sub frame can be used for either the downlink (DL) or
the uplink (UL), as shown in Figure 2. A sub frame can also function as a special sub frame (SSF). The special sub
frame has various functions: It contains pilot signals for the uplink and the downlink, and a large part of the special
sub frame is allocated as so-called guard time. As with many other communications systems, no signals are
transmitted during the guard time. This serves to provide immunity to the negative effects caused by multipath
propagation.
Figure 3: LTE-TDD uplink and downlink configurations