4G RACH optimisation through SON

The scope of LTE SON (Self Optimising Network) is vast and although reading through the standard might make one think that images like the one above are a thing of the past, what vendors offer and operators deploy today is fairly limited. The one that is very obvious in use is ANR (Automatic Neighbour Relations) and true to its name it has made neighbour planning a thing of the past in LTE deployments.

I was recently looking through some air interface logs of one UK operator and I was pleasantly surprised to see one more SON feature in use, namely RACH reporting. RACH performance historically has relied on drive testing to quantify, as a failed procedure is not recorded by the network. With RACH reporting the UE (if supported) can be requested to report how many preambles it used to access the network and if it encountered any contention. In a simple solution this information could simply be recorded statistically for an operator to look at, or for a full SON solution the requested preamble power could be adjusted (either way depending if UEs are reporting too many preambles or too few) or more RACH signatures could be assigned if contention is widely reported (typically the signature pool is statically split between contention based and contention free usage) or some other relevant parameter based change could be automatically triggered.

The procedure itself starts with the UE reporting its support as shown below (click to expand).

The network can then request from the UE to report on the result of the RACH procedure through the UE information request procedure as shown below.

Finally the UE reports the result in the UE Information Response message. In this particular example, two preambles had to be sent as the result of the first one was met with contention.

A simple solution, to an age old mobile network problem. Quite good..