Page 255 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 241

First transmission
path

Direct line of
sight blocked Obstacle

Obstacle

Obstacle

eNodeB

UE
Second Obstacle
transmission
path
Figure 4.13 Simplified illustration of MIMO operation.

conditions, with very short distances between the transmitter and the receiver, can 64‐QAM
and MIMO be used simultaneously. As modulation, coding and the use of MIMO can
be changed every millisecond on a per‐device basis, the system can react very quickly to
changing radio conditions.
In some documents, the terms 2 × 2 MIMO or 4 × 4 MIMO are used to describe the
two or four transmitter chains and two or four receiver chains. In the LTE specifica-
tions, the term ‘rank’ is often used to describe the use of MIMO. Rank 1 transmission
mode signifies a single‐stream transmission while rank 2 signifies a two‐stream MIMO
transmission for an RB. The term is derived from the rank of the channel matrix that
describes the effects that the independent data streams have on each other when they
interfere with each other on the way from the transmitter to the receiver.
On a somewhat more abstract level, the 3GPP specifications use the term ‘layers’ to
describe the number of simultaneous data streams. MIMO is not used when only a
single layer is transmitted. A 2 × 2 MIMO requires two layers. Each transmission layer
uses its own reference symbols as described in Section 4.3.4. The corresponding sym-
bols on the other layer(s) have to be left empty. This slightly reduces the overall through-
put of MIMO as the number of symbols that can be used in an RB for the shared channel
is lower compared to that in single‐stream transmission.
Two different MIMO operation schemes can be used by the eNode‐B for downlink
transmissions. In the closed‐loop MIMO operation mode, a precoding matrix is applied
on the data streams before they are sent over the air to change the modulation of the
two signal paths in a favorable way to increase the overall throughput at the receiver
side. The mobile device can inform the eNode‐B as to which parameters in the precod-
ing matrix would give the best results. This is done by sending a Precoding Matrix
Indicator (PMI), a Rank Indicator (RI) and a CQI over the control channel. The RI