Explain about the Underlay-Overlay Arrangement.

Underlay-Overlay Arrangement:

In actual cellular systems cell grids are seldom uniform because of varying traffic conditions in different areas and cell-site locations.

Overlaid Cells: 

To permit the two groups to reuse the channels in two different cell-reuse patterns of the same size, an “underlaid” small cell is sometimes established at the same cell site as the large cell (see Fig. 10a). The “doughnut” (large) and “hole” (small) cells are treated as two different cells. They are usually considered as “neighbouring cells.”

The use of either an omnidirectional antenna at one site to create two sub ring areas or three directional antennas to create six subareas is illustrated in Fig. 10b. As seen in Fig.10, a set of frequencies used in an overlay area will differ from a set of frequencies used in an underlay area in order to avoid adjacent-channel and cochannel interference.

The channels assigned to one combiner—say, 16 channels—can be used for overlay, and another combiner can be used for underlay.

Implementation:

The antenna of a set-up channel is usually omnidirectional. When an incoming call is received by the set-up channel and its signal strength is higher than a level L, the underlaid cell is assigned; otherwise, the overlaid cell is assigned. The handoffs are implemented between the underlaid and overlaid cells. In order to avoid the unnecessary handoffs, we may choose two levels L1 and L2 and L1 > L2 as shown in Fig. 10(c). When a mobile signal is higher than a level L1 the call is handed off to the underlaid cell. When a signal is lower than a level L2 the call is handed off to the overlaid cell. The channels assigned in the underlaid cell have more protection against cochannel interference.