Page 262 - From GMS to LTE

248 From GSM to LTE-Advanced Pro and 5G

mode of the transceiver unit and the maximum timing advance that can be encountered
in a cell. The timing advance influences the guard period because the more distant a
mobile device is from the center of the cell, the earlier it has to start its transmissions so
that they are synchronized with the transmissions of devices that are closer to the center
of the cell. The guard period has to be long enough to accommodate such earlier trans-
missions in relation to mobile devices closer to the cell center.
Two switching intervals between transmission and reception have been defined in the
specifications: 5 milliseconds and 10 milliseconds. These correspond to 5 and 10
subframes, respectively. After each interval, a guard period is inserted. Four TDD con-
figurations exist for the 5‐millisecond interval and three for the 10‐millisecond interval
to flexibly assign subframes to the uplink and the downlink direction. TDD configura-
tion 2, which can be found in networks today, assigns two subframes for the uplink
direction and six subframes for the downlink direction. The remaining two subframes
are special as the first part is used for downlink transmission, the middle part is used for
the guard period and the remainder of the subframe is used for uplink transmissions of
the next frame. This way, the 10‐millisecond timing of a radio frame, which comprises
10 subframes, is kept the same as in the FDD version, independent of the number of
OFDM symbols used for the guard period.
On a TD‐LTE carrier, System Information Block 1 contains the following two TDD
configuration parameters, which are specified in 3GPP TS 36.331:

SIB-1
TDD-Config
subframeAssignment = 2
specialSubframePatterns = 6

The subframe assignment parameter in this example from a live network is set to 2,
which implies the number of downlink, uplink and special subframes as described
above. The special subframe pattern parameter is set to 6 which means that nine slots
of that subframe are used for downlink transmissions, two slots for the uplink and three
slots are reserved for the guard period.
Owing to the different number of uplink and downlink subframes, it was necessary to
introduce a flexible resource assignment concept. In FDD‐LTE, where the number of
uplink and downlink subframes are identical because of the simultaneous transmission
and reception on two separate channels, an uplink transmission opportunity assigned
in a certain downlink subframe implicitly grants access to the uplink four subframes
later. As the number of uplink and downlink subframes is not necessarily the same in
TDD‐LTE, it is necessary to specify individual allocation schemes for each TDD
configuration.
A similar complication exists for the HARQ mechanism. Because of the non‐sym-
metric configuration of uplink and downlink subframes, some subframes have to con-
tain the ACK/NACK for transmissions in several subframes of the other direction. How
this is done again depends on the TDD configuration used.
In practice, a major disadvantage of TD‐LTE is the limited uplink capacity of a carrier.
In the example above only 20% of the channel is used for uplink transmissions. Even in
a 20 MHz carrier, this limits the theoretical maximum uplink speed to 15 Mbit/s. In
practice this value is even lower and has to be shared by all devices served by the cell.