Digital Signal Processing Reference
In-Depth Information
Fig. 2.5
Frequency planning
for inter-cell interference (the
entire band is used in all cells
for the central regions)
In Fig.
2.5
, mobile stations 11 and 12 are connected with base station 1, while
mobile stations 21 and 22 are connected with base station 2, and mobile stations 31
and 32 are connected with base station 3. It can be noticed that mobile stations 11,
21 and 31 are located at the intersection of 3 cells, while mobile stations 12, 22 and
32 are in the center of their respective cells. For the three mobile stations at the cell
edge, different sub-carriers are allocated to avoid the co-channel interference. For
the mobile stations near the base station, all the sub-carriers can be allocated and a
lower output power can be used, to achieve a high frequency reuse. In this example,
the frequency reuse factor is 3 for the cell edge and 1 for the inner part of the cell.
For the inner part of the cell, through the limitation of the transmission power, some
isolated islands are formed that do not interference each other. To conclude, the
benefits of using this flexible frequency planning with soft frequency reuse scheme
are:
•
High bitrate at the cell center;
•
Less interference at the cell edge, making easier the channel estimation, synchro-
nization, and cell selection;
•
Overall improved spectral efficiency of the network.
To achieve the benefits of soft frequency reuse, a tight coupling between fre-
quency allocation and output power is needed. This can be achieved by a thorough
network planning carried out during the roll out phase of the network. However,
with the trend towards smaller and smaller cells, a more distributed approach to-
wards power and frequency planning is needed. In recent years, Self-management
(Self-X) technologies that fully automate the tasks of managing (i.e. configuring,
monitoring, and optimizing) a cellular network are emerging as an important tool
in reducing costs for the service provider and will be a distinguishing feature of
