Iam not able to understand that why do we need a handover 2G-3G or 3G-2G??
Let me put my view:
We are now running a network 2G/2.5G.
And to meet out the requirements of high data rates we need the 3G network. OK! agreed
How to deploy a 3G network. Overlaying 3G n/w over the existing network(2G/2.5G)is an intelligent approach. hence both the signals wcdma and gsm will be available to the user with 3G handset. If the user is interested in a voice call the gsm path will be used or if he is interested in data(high speed one) call then wcdma path will be used. Now considering the case of mobility when a user on 3G call enters a cell where there is no 3G cell then ..what is the logic of handing over that call to the gsm cell. As the gsm cell will not be able to cater the 3G user needs.
however the reverse do make some sense in the terms that a 3G cell can cater a gsm user for voice as well as data needs. but again there also you have the option of underlayed gsm cell. Hence i don’t think that the need for handover between 2G and 3G networks is so important.