Page 256 - From GMS to LTE

242 From GSM to LTE-Advanced Pro and 5G

Table 4.5 LTE transmission modes.

Transmission mode Transmission scheme of the PDSCH

Mode 1 Single antenna port
Mode 2 Transmit diversity
Mode 3 Transmit diversity or large delay CDD
Mode 4 Transmit diversity or closed‐loop MIMO
Mode 5 Transmit diversity or multiuser MIMO
Mode 6 Transmit diversity or closed‐loop MIMO
Mode 7 Single antenna port 0 or port 5

informs the eNode‐B about the number of data streams that can be sent over the channel
from the receiver’s point of view. The CQI information is used by the eNode‐B to decide
which modulation (QPSK, 16‐QAM, 64‐QAM, 256‐QAM) and which coding rate, that
is, the ratio between user data bits and error detection bits in the data stream, should be
used for the transmission.
For fast‐moving users, it is difficult to adapt the precoding matrix quickly enough.
Such scenarios are thus better handled with open‐loop MIMO, for which only the RI
and the CQI are reported by the mobile device to the network.
And finally, multiple antennas can also be used for transmit and receive diversity. Here,
the same data stream is transmitted over several antennas with a different coding scheme
on each. This does not increase the transmission speed beyond what is possible with a
single stream but it helps the receiver to better decode the signal and, as a result, enhances
datarates beyond what would be possible with a single transmit antenna. In total, 12
different possibilities exist for using MIMO or transmit‐diversity subfeatures so that the
eNode‐B has a wide range of options to adapt to changing signal conditions.
In the uplink direction, only single‐stream transmission from the point of view of the
mobile device has been defined for LTE in 3GPP Release 8. In addition, methods have
been specified to enable the eNode‐B to instruct several mobile devices to transmit
within the same RB. The receiver then uses MIMO techniques to separate the individu-
ally transmitted signals. This is not visible to the mobile devices and is referred to as
multiuser MIMO. The eNode‐B is able to tell the data streams coming from different
mobile devices apart by instructing them to apply a different cyclic shift to their refer-
ence signals. In practice, however, multiuser MIMO is not used so far. In subsequent
versions of the specification, single‐user MIMO for the uplink has been specified as
well. Like multiuser MIMO, it is not used in practice so far.
Table 4.5 shows the different transmission modes specified in 3GPP Release 8, TS
36.213 [13] for the downlink shared channel. The decision as to which of these are
implemented and under which signal conditions they are used is left to the discretion of
the network vendor. On the mobile device side, however, all modes have to be supported.

4.3.10 HARQ and Other Retransmission Mechanisms
Despite adaptive modulation and coding schemes, it is always possible that some of the
transmitted data packets are not received correctly. In fact, it is even desirable that not
all packets are received correctly as this would indicate that the modulation and coding