Cellular networks are notoriously difficult to design and operate; in particular, defining the optimal location of the base stations and fine tuning their configuration parameters is very challenging. For this reason, government agencies (such as the FCC for the US) usually sell or rent, for example by auction, each operator a frequency band for its exclusive usage in a given country or region. Only a small part of the whole spectrum is allocated as a shared spectrum , in which networks function in the same (unlicensed) frequency band.