Traditional LTE deployments, set the LTE carrier(s) at a higher priority than the 3G carrier(s) thus ensuring the UE will always camp on the LTE carrier when present. Any voice calls are handled by CSFB with an associated delay in call set-up and a few signalling issues as described in previous posts.
I have recently been thinking of the IE eutraDetection as broadcasted in SIB19 on 3G (shown above) and an alternative approach to CSFB came to mind..
First let's look at how the IE eutraDetection is described in the specifications. The applicable document is TS 25.331 and the description is given as:
Furthermore section 8.6.2.5 provides this additional information "If the IE E-UTRA detection is included in a received message and set to TRUE and the UE is in CELL_PCH, URA_PCH state or idle mode, the UE may detect the presence of a E-UTRA cell on a frequency with a priority lower than the current UTRA cell and report the information to the NAS."
Taking the above into consideration the alternative to CSFB would be to set the LTE carrier(s) at a lower priority thus ensuring the UE camps on the 3G carrier(s). By setting the IE E-UTRA detection to TRUE the UE will display the 4G icon on the screen as that is passed on to the NAS layer. This will ensure the customer is happy as for all he/she knows they are camped on the 4G network.
Any voice calls would be set up on the 3G carrier directly without the need for CSFB and avoiding any call set-up delays. If on the other hand the UE wanted to establish a data session the RNC would trigger a re-direct or handover to 4G. Once the UE would finish with the data transfer it would reselect back to 3G but still display the 4G icon as before. So in essence the opposite of CSFB can be created, which could be termed PSFB :)
One problem with this approach is that UEs, especially smartphones, are prone to many small bursts of data, so these could be handled on the 3G layer and use a traffic volume measurement report to trigger the redirect to 4G for larger data transfers. At the same time ISR (Inter-system Signalling Reduction) could be used to minimise the signalling when moving between the 3G and 4G networks.
So that is it, some small development needed on the RNC SW and PSFB could become reality..
Mark, although this would improve voice experience in LTE, I expect it will dramatically worsen the LTE experience especially the response time. That is actually one of huge advantages of LTE and customers perceive LTE networks as much faster just because their web pages are loading significantly faster than on 3G.
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