In the overlay/underlay cell
design, two cells (micro or macro) are defined at the same Radio Base Station
(RBS) to transmit in the same direction, omnidirectional or sectored. The
overlay cell has a lower output power, smaller coverage and provides service to
Mobile Stations (MSs) close to the RBS. The underlay cell has higher power,
wider coverage and provides service to an MS farther away from the RBS.
The underlay cell contains
voice channels (analog or digital), control channels and a locating device. The
overlay cell contains only a group of voice channels (digital or analog) and
depends on the underlay cell to set up calls and perform handover. The
reverse-link RSS during call origination or page response determines whether
the overlay or the underlay cell is used.
Relationship between an overlay
and underlay cell.
The main advantage of using
overlay cells is to increase the traffic handling capacity without building new
sites. Overlay cells can be used to reduce problems in the transition areas
between small and large radius cells.
One of the disadvantages of the
overlay cell is based on the assumption that its originating traffic is very
close to the RBS. When the traffic is far away from the RBS, the overlay cell
will not be used.
The ideal places to position
overlay cells are on RBS sites, either in the center of or very close to major
roads and highways, especially where the traffic is forced to move slowly
during busy periods. They are most suitable at the edge of a suburban area. The
overlay can handle the suburban traffic and the underlay can be used primarily
for the rural area.