Overlaid Underlaid Subcells in GSM

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−Traffic Capacity of a cellular network can be increased by either adding more frequencies or reducing the frequency reuse distance.
−One approach is to apply a second frequency re-use pattern with a tighter frequency reuse (Overlay) on the existing pattern.
−These cells should be restricted in size, so shorter reuse distance can be accomplished without causing Co-channel/Adjacent channel interference.
−They are termed Overlaid (OL) Subcells, whereas the original cells will be called Underlaid (UL) Subcells.
−Now by having more frequencies per cell, then Network capacity is increased
-The fundamental idea behind the OL/UL subcells is to let the traffic close to the site to be moved to the OL subcell, while traffic close to the cell border to be moved to the UL subcell.
−In that way of treading the traffic, the frequencies in the OL subcell can have tighter frequency reuse.
-Using the OL/UL concept we can solve the case as follows:






-f4 will be used in the OL subcell and it will be restricted to serve in a small area only near to the site so interference from the neighbor cell will be minimized and a good C/I can be enjoyed.
To maintain the service area of the OL subcell restricted to a certain region we have three thresholds we can play with:
A.Path Loss Threshold
B.Timing Advance Threshold
C.Distance to Cell Border Threshold
−With the ordinary OL/UL subcells, the MS near the cell will camp on the overlaid subcell but even if the OL subcell got high utilized there is no way to push traffic to the UL subcell.
−Using Subcell Load Distribution (SCLD) Concept, we can configure the cell to use the OL as the preferred subcell initially and when traffic on the OL increased beyond certain load, any extra traffic will be offloaded to the UL subcell.





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