With the industry now thinking about multimode/unified GSM, UMTS, LTE base stations (see this post from back in 2008) it is likely that as we go forward there will be little incentive to switch-off UMTS because of LTE becoming more prevalent. But just for the sake of the argument: What would have to happen before UMTS could be switched off? The following things come to mind:
- Voice: LTE is rolled out without an operator based voice solution. So one way or another, and there are quite a few, the voice question has to be answered. No matter whether its VoLTE, VoLGA, CS fallback, dual active radio in the phone, over the top internet based voice solution, something must fall into place first as a mainstream solution for simple voice calls. And, it needs to be at least as good as today's circuit switched voice in coverage, call continuity for moving subscribers, in-house coverage, etc.
- Coverage: LTE coverage must be at least as good as 3G coverage today. For voice it must be far better than 3G coverage today.
- LTE Devices: Smartphones not having an LTE chip inside must have become a minority in the network.
Quite tough requirements. But then, it's not a digital on/off decision. UMTS can be gradually phased out once necessary. Future multimode base stations are likely to be very flexible and bandwidth released from one air interface technology can be moved via configuration changes to another air interface protocol.