CSFB alternative using E-UTRA detection

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..

The issues with Wi-Fi offloading

Thinking about how my data consumption has changed over the past ten years, it is quite clear that it has increased at an enormous rate. Where ten years ago I was happy with some email and static web pages, today there is video content embedded everywhere, web pages are dynamic, flash based content, streaming, VoIP, attachments, the "Cloud", synching etc, etc. Either consciously or subconsciously this is true for most people and most people want to access all this on the go.

Mobile networks have obviously evolved over the last ten years to cater for this, and the 3GPP is continously working to improve the standard to allow for faster throughputs but more importantly more efficient networks.

Sometimes however either because site density is not adequate, spectrum is not enough, or networks are not configured properly, things come a grinding slow stop which is where "Wi-Fi offloading" comes into play.

As a quick search on Google will show, Wi-Fi offloading is presented as the solution to the problem (the cynic might say this mostly comes from Wi-Fi AP manufacturers) and quite a few operators have either partnered with Wi-Fi network operators or deployed their own networks, in the hope of offloading some traffic from the cellular network.

But does this really work? The biggest problem with Wi-Fi is obviously the fact that it uses shared spectrum. How big a problem is this? Well, a quick scan of the available Wi-Fi networks, as shown in the picture above, will quickly put things into perspective. Sitting at home I could pick out 17 access points of considerable strength, all fighting for the coveted non-overlaping channels. There were even two double bandwidth 802.11n APs spreading themselves over 40MHz. So QoS is obviously an issue here, as all of these uncontrolled access points can appear anywhere, anytime ready to interfere with your "offloading".

So even though Wi-Fi might be available, it is possible that the user experience on the cellular network is much better. This is something people in the industry are aware of and it was interesting to see that for a while even Apple were thinking of switching from Wi-Fi to cellular when things get bad. This screenshot below taken from iOS 6 beta shows this, but for reasons unknown it never made it into the official release (for now).

There are interference mitigation techniques of course, ranging from switching channels (not good if they are all congested) to using various smart antenna techniques (beamforming etc) to try to improve things. I have no personal experience of these, but of course these will come at a cost and with a few caveats.

Furthermore there is always the problem of seemless mobility between Wi-Fi and cellular which even though various solutions have been put forward for it, none have made it into the mainstream yet.

So it seems the only positive aspect of Wi-Fi offloading might be that it is free to use, but then of course that creates another problem for the operator as there is no return in investment.

This then is where the industry has started thinking about "small cells" and another story begins..