Load Sharing

As described in the preceding discussions, inter-cell interference plays a
role in the capacity of a given cell. In both the uplink and downlink, higher
interference from nearby cells means a lower capacity and possible smaller
footprint in the cell of interest. Conversely, lower interference from nearby
cells means that the cell of interest can have higher capacity or larger footprint.
This means that one cell can effectively “borrow” capacity from one or
more nearby cells that is less loaded.

Consider Figure 12-8 for example. Some subscribers move from Cell A to
Cell B. Thus, Cell A becomes less loaded and Cell B becomes more loaded.
If Cell B were already heavily loaded, then the existing inter-cell interference
could have meant that it might not have been possible to accommodate
more users in Cell B. However, the fact that Cell A now has fewer users
means that it is generating less inter-cell interference in Cell B. Thus, it
may well be possible to accommodate the additional load on Cell B. This
example shows that the capacity of a cell is not static and it varies with the
load on nearby cells.

The foregoing discussions regarding uplink and downlink capacity and
their effect on coverage emphasize the fact that coverage and capacity are
interrelated. Because we need to develop an RF design that supports both
coverage and capacity requirements and because capacity affects coverage,
the development of the RF design is an iterative process.We start with an
initial coverage-based design, and we check that design against the
expected demand.We then modify the design to allow for additional capacity
where needed. As the implementation phase proceeds, we may find that
we need to deal with other constraints, such as the inability to acquire a cell site in the ideal location or drive test results that do not match expectations.
In such cases, we will need to change the design to account for different
cell site locations, different correction factors, and so on. Several
iterations of design may be required until we converge to a point where we
can provide both the coverage and capacity required.