Search for concentric cells, we already discussed this many times years ago.
Concentric (or Single-BCCH) vs Dual-BCCH (1 cell 900 + 1 cell 1800)
– the 180 cell does not require a BCCH (easier frequency planning)
– no SDCCH on 1800
– easier neighbour planning (only one cell per sector)
– GPRS may be forced to be on 900 only (E/// provides way to place PDCH on 1800 inner cell)
– 900 layer handles all the signalling, all the idle mobility, all the SDCCH traffic. But 900 has low radio quality, so overall you may be decreasing your QOS, especially the call setup and the GPRS TBF qos.
Concentric cells are a great way to achieve G1800 refarming, in order to make room for L1800 (10MHz, 15MHz or even 20MHz, using only the LTE guardband for allocating few GSM channels on the side)