NARDA SRM-3006 Manual - page 3
AN_1106_SRM_LTE_TDD
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Subject to change
2 The LTE duplex modes
The duplex mode in cellular mobile telecommunications systems such as 2G, 3G and 4G refers to the way
communication between the handset and the base station takes place, that is, how the uplink and downlink are
separated from each other. In 2G as well as in 3G and most of the 4G frequency bands (see Table 1) a specific
frequency range is reserved for the uplink (i.e. the wireless signal from the handset to the base station). At the same
time, a different exclusive frequency range is used for the downlink (from base station to handset). The two
communications channels are thus separated by their frequencies, and this is therefore called frequency division
duplex mode.
LTE stipulates TDD bands as well as FDD bands. TDD means that the two communications channels are separated
from each other in time rather than in frequency. The uplink and downlink operate on the same frequency, but
alternate with each other in a strictly controlled manner. This is illustrated in Figure 1.
The diagram of FDD on the left of Figure 1 makes it clear that the uplink and downlink operate on two different
frequencies or bands, but can operate simultaneously. It is important to remember that this simultaneous operation is
not random, but follows a specific protocol depending on the mobile communications standard being used. It should
also be noted that some of the available timeslots can be empty, depending on the volume of data traffic (blank boxes
in the diagram).
TDD is shown on the right in Figure 1. This clearly shows that only one frequency band is used, and the uplink and
downlink signals must alternate with each other as a consequence. The uplink and downlink must never be allocated
the same resources (i.e. number of timeslots). On the contrary, the downlink is typically given more resources, as the
handset will generally download more data than it uploads. This is indicated by the larger number of blue DL boxes in
Downlink (DL)
Uplink (UL)
Figure 2: The difference between FDD (left) and TDD (right)
Time
Frequency
Frequency
Division Duplex
Time
Division Duplex