Page 254 - From GMS to LTE

240 From GSM to LTE-Advanced Pro and 5G

the meantime, it is not yet widely used for this technology because of backward compatibility
issues and the necessity to upgrade the hardware of already‐installed UMTS base stations.
With LTE, however, MIMO has been rolled out with the first network installations.
The basic idea behind MIMO techniques is to send several independent data streams
over the same air interface channel simultaneously. In 3GPP Release 8, the use of two or
four simultaneous streams is specified. In practice, up to two data streams are used
today. MIMO is only used for the shared channel and only to transmit those RBs
assigned to users who experience very good signal conditions. For other channels, only
a single‐stream operation with a robust modulation and coding is used as the eNode‐B
has to ensure that the data transmitted over those channels can reach all mobile devices
independent of their location and current signal conditions.
Transmitting simultaneous data streams over the same channel is possible only if the
streams remain largely independent of each other on the way from the transmitter to
the receiver. This can be achieved if two basic requirements are met.
On the transmitter side, two or four independent hardware transmit chains are
required to create the simultaneous data streams. In addition, each data stream requires
its own antenna. For two streams, two antennas are required. In practice, this is done
within a single antenna casing by having one internal antenna that transmits a vertically
polarized signal while the other antenna is positioned in such a way as to transmit its
signal with a horizontal polarization. It should be noted at this point that polarized
signals are already used today in other radio technologies such as UMTS to create diver-
sity, that is, to improve the reception of a single signal stream.
A MIMO receiver also requires two or four antennas and two or four independent
reception chains. For small mobile devices such as smartphones, this is challenging
because of their limited size. For other mobile devices, such as notebooks or LTE home
routers, antennas for MIMO operation with good performance are much easier to
design and integrate. Here, antennas do not have to be printed on the circuit board but
can, for example, be placed around the screen or through the casing of the device. The
matter is further complicated because each radio interface has to support more than
one frequency band and possibly other radio technologies such as GSM, UMTS and
CDMA, which have their own frequencies and bandwidths.
The second requirement that has to be fulfilled for MIMO transmission is that the
signals have to remain as independent as possible on the transmission path between the
transmitter and the receiver. This can be achieved, for example, as shown in Figure 4.13,
if the simultaneous transmissions reach the mobile device via several independent
paths. This is possible even in environments where no direct line of sight exists between
the transmitter and the receiver. Figure 4.13 is a simplification, however, as in most
environments, the simultaneous transmissions interfere with each other to some
degree, which reduces the achievable speeds. In theory, using two independent trans-
mission paths can double the achievable throughput and four independent transmission
paths can quadruple the throughput. In practice, however, throughput gains will be
lower because of the signals interfering with each other. Once the interference gets too
strong, the modulation scheme has to be lowered, that is, instead of using 64‐QAM (or
256‐QAM in special cases) and MIMO together, the modulation is reduced to 16‐QAM.
Whether it is more favorable to use only one stream with 64‐QAM or two streams with
16‐QAM and MIMO depends on the characteristics of the channel, and it is the eNode‐
B’s task to make a proper decision on how to transmit the data. Only in very ideal