Page 144 - From GMS to LTE

130 From GSM to LTE-Advanced Pro and 5G

Two subscribers of a cell A third subscriber would like to
one of them close communicate in the cell. This poses
to the cell edge sending a problem for the second subscriber
with its maximum possible as he can’t increase the power output
power level. to counter the additional interference.

Figure 3.9 Cell breathing.

To avoid this effect, the network constantly controls the signal‐to‐noise ratio of all
active users. By actively controlling the transmission power of each user the network is
aware of the impact an additional user would have on the overall situation of the cell.
Therefore, the network has the possibility to reject a new user to protect ongoing
sessions.
To preserve all ongoing connections and additionally allow a new user to enter the
system, it is also possible to use a different strategy. The goal of this strategy is to reduce
the interference to a level that allows all users, including the prospective new one, to
communicate. This can be done in a number of ways. One way is to assign longer
spreading codes to already established channels. As described in Section 3.3.2, it is pos-
sible for mobile devices to reduce their transmission power by using longer spreading
codes. This in turn reduces the interference for all other users. The disadvantage of
using longer spreading codes is of course a reduction of the maximum transmission
speed for some users. As not all connections might be impacted, again there are a num-
ber of possibilities for the selection process. Users could, for example, be assigned to
different user classes. Changing spreading factors could then be done only for users of
a lower user class who pay less for their subscription than others. It can also be imag-
ined that the network could start a congestion defense mechanism at a certain load
threshold before the system got into an overload situation. Once the threshold is
reached, the network could then, for example, assign only short spreading factors to
users with a higher priority subscription while the system load is above the threshold.
Besides cell breathing there are other interference scenarios. As already mentioned, it
is necessary to increase the transmission power if the spreading factor is decreased to
ensure a proper reception. Therefore, the maximum distance a user can be from the
center of the cell also depends on the spreading factor. If a user roams between two cells
it is possible that the current spreading factor would not allow data to be transferred as
reliably as before because of the interference encountered at the cell edge, whereas a
lower spreading factor would still allow a reliable data transfer. How this and similar
scenarios at cell edges are resolved depends on the vendor’s equipment and the