Methods In The Wild To Return To LTE As Fast As Possible

LTE deployments are making steady progress and with good network coverage, network operators are keen to make sure devices are on LTE as much as possible instead of lingering around in 2G or 3G for too long. There are actually quite a number of mechanisms I've seen in various networks in the past so I thought I'd sum them up in a single post. Note that there are more mechanisms than described below but those are the one's I've seen in different networks while traveling.

Autonomous Reselection: This one's the easiest as the network only needs to signal LTE availability in 2G or 3G System Information messages (2G SIB 2quater and 3G SIB19). Mobile devices in idle state that have ended up in 2G or 3G for various reasons then periodically scan for LTE coverage in spectrum locations indicated in the SIBs and autonomously change back.

Release With Redirection: Instead of switching a mobile from active to a more power conserving state (e.g. from Cell-DCH to Cell-PCH), the network releases the connection with an RRC Connection Release message that contains redirect information to LTE. The mobile device then has a look if LTE is available straight away and leaves for greener pastures if LTE is found. For this to work the network operator must be reasonably sure that LTE coverage is actually present in the area where a release is made instead instructing the mobile device to use an active but less power hungry 3G state. Otherwise the mobile ends up in 3G idle state from which it takes more than one and a half seconds before data can flow again (for details see this post).

CSFB Release With Redirection: That's almost the same as above but part of a CS-Fallback procedure for voice calls. After the voice call ends the network releases all bearers and also includes redirect information. This is even done when a PS bearer is still established at the end of the call and IP packets are flowing.

GPRS to LTE Reselection During Data Transfer: This is a relatively new feature and allows the mobile to interrupt an ongoing 2G data transfer and switch back to LTE. Have a look here for the details.

UMTS to LTE Handover During Data Transfer: This functionality is unfortunately not yet widely deployed in networks, I haven't come across a single network that supported it. But from my point of view it's a very important piece of the puzzle as often I use a notebook and VPN that constantly keeps data flowing and once I end up in 3G due to running out of LTE coverage I'm stuck there until I run out of 3G coverage. At this point anything can happen, depending on how the network looks like once coverage is regained. I already wrote a post about this back in November 2014 so have a look in this post for the details.