Power consumption in different RRC states

A few posts ago I looked at the current drain of 3G Vs 2G during a voice call. For this post I thought it would be interesting to quantify the current drain of a UE in different RRC states. This helps explain the need for fast dormancy, DRX optimisation and managing RRC state transitions from a UE battery perspective.

Looking at the graph (again obtained using Nokia's energy profiler app), we start off in RRC IDLE with the current drain at approx 60mA. The web browser is launched and the UE transitions to RRC DCH via the RRC Connection Setup procedure. A PDP context is requested and following the authentication procedure the UE is assigned an HSPA radio bearer. Current drain is approx 260mA. The actual web page requested was the Google homepage so the data transfer is completed very quickly. From there on the RRC inactivity timer on the RNC is initiated and after 5s the UE is transitioned to RRC FACH. The current drain is now approx 150mA. The reduction is due to the fact that in CELL_FACH the UE can switch off its transmitter. It still has to keep its receiver on however as the RNC can schedule a data transfer at any time.

On this network the timer to transition from FACH to URA_PCH is 30s, which is quite long. This clearly shows that sometimes operators can have netwotk settings that are not very battery friendly. If the UE used fast dormancy (the specific one in question did not), the UE could have released the RRC connection and moved back to IDLE. As it didn't, after the 30s of inactivity the UE is instructed to move to RRC PCH (URA_PCH on this network). As the DRX timer is configured at the same value as the IDLE DRX timer (on this specific network), the current drain is the same at approx 60mA.