Page 337 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 323

MIMO antennas is limited by the size of devices. Also, the numbers of sectors per cell
size is limited to 3 or 4, in practice, due to the increase in the overlap areas between the
sectors. Also, most network operators do not have more than 50–60 MHz of spectrum
that they can use for LTE. As a consequence, other means have to be found to further
improve overall‐network and single‐user throughput in the future to keep ahead of
rising bandwidth demands.
The first approach, already put into practice today, is to densify the macro network,
that is, to install additional macro base stations to reduce the coverage area of a cell site.
There are limitations to this approach as the number of suitable locations where macro
base stations with large antennas can be installed is decreasing. Further densification
therefore requires the use of much smaller base stations, referred to as ‘small cells’,
which cover only an area with a diameter of a few dozen meters at most. Such small cells
have a size similar to that of a typical Wi‐Fi access point at home. In waterproof and
ruggedized casing, such cells can be installed outdoors or indoors in heavily frequented
locations such as train stations, shopping malls and so on.
Although the price of such cells is significantly lower than that of a macro base station,
there are a number of other factors that influence the use of small cells. First, electrical
power needs to be available where the cell is located. Second, high‐speed network con-
nectivity must be available at the site, either over standard twisted pair copper cabling
or over an optical cable depending on the type of the small cell, as will be described
further below.
If small cells are used in a network, the homogeneous cell layout of macro cells is
complemented by the fully overlapping network coverage provided by small cells.
Therefore, such a network architecture is also referred to as a heterogeneous network
or as a HetNet.
Another difficulty to be overcome in heterogeneous networks is the interference
caused by the overlapping coverage of macro cells and small cells that would signifi-
cantly reduce the capacity increase that small cells could provide. Therefore, the LTE
specification offers a number of methods to reduce or avoid inter‐cell interference.
These methods are referred to as Inter‐cell Interference Coordination (ICIC)6.
The first ICIC scheme was already defined in the first LTE specification, 3GPP Release
8, and is described in Section 4.12.2. The idea behind what is referred to as FFR is to use
only some of the subcarriers at the cell edge by transmitting with a higher power on
them than on other subcarriers. Neighboring cells would do the same but for subcarri-
ers on different frequencies thus creating less interference. Naturally, a balance has to
be found between reduced interference on those subcarriers in cell‐edge scenarios and
the reduced number of subcarriers in such areas, which reduces the overall transmis-
sion speed for devices in such areas. In the center of such cells, all subcarriers can be
used for data transmission and hence the scheme does not reduce peak datarates close
to the cell. In effect, datarates close to the center of the cell may even benefit from FFR
as well because of the reduced neighbor cell interference.
While the basic ICIC FFR scheme may be beneficial in a pure macro network envi-
ronment, there are no benefits in a heterogeneous network environment where several
small cells are located in the coverage area of a single macro cell. In such a scenario, the
coverage area of the small cells fully overlaps with the coverage area of the macro cell
and thus there is no benefit from reducing the power of some of the subcarriers. This is
why in 3GPP Release 10, an additional ICIC scheme, referred to as eICIC was defined.