“Tracking Areas” still exist in 5G, just as did in the 4G system. But when looking at the 5G NAS signalling there is a noticeable lack of the Tracking Area Update procedure. Does that mean that the 5G system does not support idle UE mobility? Seems far from probable…
During the 5G Initial Registration the “Registration Accept” message from the network carries network-specific parameters to the UE. Among them a “Registration Area”, a term that did not exist in the earlier 3GPP systems. A Registration Area is a set of Tracking Areas, specifically grouped for a particular UE. The actual concept of grouping Tracking Areas that way has already been defined as a feature in LTE (though without any particular term to denote it), but hasn’t been widely used by mobile operators. The selection of the Tracking Areas that comprise the UE’s Registration Area is meant to reflect the movements of the UE, taking into account its so-called Mobility Pattern. The system has therefore created a unique paging area for this particular mobile device. As long as the UE moves within the limits of its own paging region it will not need to perform any mobility updates to the 5G Core Network – which can mean a considerable saving on the signalling resources that otherwise would need to be used.
How to plan Registration Areas?
Of course, the cost of such a solution is that when the UE needs to be paged the paging message will be sent out in all the cells of the grouped Tracking Areas, which means an increased usage of the paging resources throughout a potentially big part of the network. The gain on saving on the signalling resources due to limiting mobility updates obviously has to outweigh the increased use of the paging resources. Such a setup makes a lot of sense for e.g. IoT devices that on the one hand are mobile, but on the other hand are almost never paged – like e.g. sensors or meters that report certain measured values or events, practically involved only in uplink data transfers, with basically no need for downlink communication.
The definition of the UE’s Registration Area should reflect its movements, which means the concept is mainly useful in case of devices that move on a fixed trajectory – like a sensor mounted on a tram or a city bus, for example. The device’s Mobility Patterns is known to the owner of the application, meaning it is the customer of the 3GPP network that is the best source of knowledge. How information from external Application Servers can be provided into the 3GPP networks is described in [link to my article “SCEF vs NEF”]. Otherwise, for the lack of externally provided information from the customers, the mobile operator can build Mobility Patterns by gathering statistics on a particular UE’s behaviour and, over time, build a relevant Registration Area.
Registration type Mobility Update
In this setup the UE will only perform a mobility update signalling procedure when it leaves its registration area – and that mobility procedure will be not be called a “Tracking Area Update”, the way it used to be in 4G. In the 5G system the UE shall, when re-selecting to a cell outside its current Registration Area, send a “Registration Request” message, with the Registration Type parameter set to ”Mobility Update”. It might turn out that UEs used for human communication (smartphones, tablets, laptops, etc) will predominantly have Registration Areas set to one Tracking Area, as the Mobility Patterns of people tend to be quite random, but devices moving along pre-defined routes will most probably end up with their personalized (or maybe “devicised”) Registration Areas.