thanks PIX for your answer.
I have the constraint not to touch my 2G sites.
For urban areas, i made some calculations, and i achieve the same distance as you suggest, 3-4 km for 2G coverage.For the 3G coverage, it’s about the same. so, i have to use the same frequencies at about 8 kms.
but for rural, with the propagation models, i face some difficulties, because, the 2G coverage is about 30 km. so i need to use the same frequencies on the 2G network at about 40 kms.
i just wanted to know if some of you guys have made some studies about it.