When looking to refarm some spectrum for LTE (e.g. 1800MHz spectrum from GSM) the following simple approach will lead to optimal results.
Start by thinking of how much spectrum you would ideally refarm. This will typically be 20MHz. Assuming this was possible pick the centre frequency for this allocation. This will be your EARFCN. Then look at how much spectrum you can actually refarm. This will typically be less, as the traffic on the legacy RAT might not have reduced enough or frequency re-planning your whole legacy network will take time. Most operators go for 10MHz, but in some cases 5MHz is also used.
Deploy your network.
After some time has passed and more spectrum is available, keep the centre frequency the same and just expand the bandwidth. Some cells might be using 10MHz, some 15MHz or 20MHz but because the centre frequency has not changed, all mobility can be intra-frequency. No need for inter-frequency handovers, no need for additional neighbour planning, no need for measurement gaps, no need for additional SIBs being broadcasted. UEs will seamlesly reselect & handover taking into account the used bandwidth every time as this is broadcasted in the MIB which is read in idle mode and after every handover.
Although the above might sound like the obvious way of doing things, both EE in the UK (see here) and other LTE deployments (see here) don't follow this but rather offset their two bandwidth allocations leading to needless inter-frequency mobility.
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