Overlay and Underlay Cells

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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.

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