One of the major challenges facing 3G network planners is how to deploy the network in a way that can coexist with the existing 2G networks. However, co-location on the same site presents a set of challenges to network planning and performance, and hence potential revenues. In many cases, co-site problems must be accepted and the network co-planning aimed to minimise the degradation. A site which was deemed suitable at installation time may deteriorate due to environmental effects and remedial action may be required to restore the site's coverage. Band planning has a major impact on the likelihood of co-site problems. In Europe, the Band I used for UMTS has been chosen such that the risk of adjacent channel rejection and Intermodulation Distortion (IMD) problems caused by the GSM900 and GSM1800 networks is minimised. However, another effect, blocking, can still occur. In the USA and other parts of the world, UMTS network planners are not so fortunate. Due to fragmented spectrum and the co-existence of a number of different radio network technologies, UMTS bands must be slotted in wherever there is available spectrum.
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