Hi,
The main usage for underlaid and overlaid,The traffic capacity of a cellular network can be increased by adding more frequencies or reducing the frequency re-use distance. One approach is to apply a second frequency re-use pattern, using the existing sites, with a tighter frequency reuse as overlay on the existing pattern. These cells should be restricted in size, making shorter re-use distance possible without causing excessive co-channel or adjacent channel interference. They are termed overlaid (OL) subcells, whereas the original cells, now with the OL subcells associated, are termed underlaid (UL) subcells. By having more frequencies per cell, the network capacity is increased.The fundamental idea with OL/UL subcells is that traffic close to the site is moved to the OL subcell, while the traffic closer to the cell border is moved to the UL subcell. By controlling the traffic in this way, the frequencies in the OL subcell can have a tighter reuse.