In the Overlaid/Underlaid technology, all frequency carriers in the cell are divided into two parts, of which the TRX power of some frequency carriers is lowered, then two Overlaid/Underlaid with different coverage appear, as shown in the below figure. The frequency carriers used by the inner circle can be planned in a more close mode than outer circle frequency carriers due to its small coverage.
This technology is quite simple and requires no special software and hardware, just modify parameters. It should be noticed that there is less traffic on original edge area of the cell and handover may happen in the cell, inner circle and outer circle handover is required for corresponding cell.
Overlaid Underlaid Frequency Configuration
As shown in the figure below, the idea of Overlaid/Underlaid is to divide BTS frequencies into two parts (or two layers), one layer is called “REGULAR layer”, the other layer is called “SUPER layer”.
“REGULAR layer” has longer frequency reuse distance with loose frequency reuse mode; “SUPER layer” has shorter frequency reuse distance with close reuse mode.
Supposing there are 51 frequency carriers, 15 of them are used by BCCH in 4*3 reuse mode, and each cell is allocated with one carrier. REGULAR layer uses 24 frequency carriers in the same 4*3 reuse mode, and each cell is allocated with 2 frequency carriers. SUPER layer uses 12 frequency carriers in 2*3 reuse mode, each cell is allocated with 2 frequency carriers.
Thus, each cell is allocated with 5 frequency carriers in total, the largest site mode is S5/5/5 when using Overlaid/Underlaid technology. If 4*3 reuse mode is used alone, the largest site mode can only be S4/4/4.
Attention should be paid to the traffic distribution of the overlaid circle and underlaid circle to make sure there is no congestion in the underlaid circle.