A Look at NTT DoCoMo’s 3G Network

What do I do on a short stopover in Japan? Right, I check out the 3G networks there. After all, NTT DoCoMo was the first network operator to launch UMTS (FOMA). Also, they have no GSM network to fall back to so they need to think a lot harder about 3G indoor coverage and ubiquitous coverage on trains, etc., so voice calls don't drop. In addition, if the 3G network works well in Tokyo with its high population density there's no excuse anywhere else. And indeed, the network worked very well during my stay and the setup was unlike anywhere else.

On the way from the airport small screen web surfing on my mobile worked just fine and I never lost the network. Also, I never lost the network indoors. In a shopping mall I always had a strong signal and only one cell, so it's likely there was dedicated indoor coverage and no repeater bringing in the signal from outside.

At first, it looked like DoCoMo had only one carrier on air in the Ginza district, as no inter-frequency measurements were configured by the network. But when transferring data, the UE was immediately put on another carrier frequency while it was in Cell-DCH state. I observed two additional carriers. Here, the UE was kept for around 10 seconds after the last piece of data was transferred, a value seen in many other networks as well.

When no more data is transferred, the mobile is returned to the initial carrier and kept in Cell-FACH state for around 90 seconds. Not very energy efficient and much longer values compared to those I've observed in many other networks which range from 10 to 30 seconds. (Hello Fast Dormancy!)

If still no data was transferred during that time, the network sets the connection in Cell-PCH state for around 2 minutes after which the connection is put into the idle state. Again quite a surprise for two reasons. Why a 90 seconds FACH phase when the networks puts the connection in Cell-PCH afterwards? And why only 2 minutes in this state afterwards? I am sure they have their reasons but I can't quite figure them out. Most (of the few) other networks that use Cell-PCH I have observed in Europe so far quickly put the mobile to the Cell-PCH state, for example after 5-10 seconds in Cell-DCH state and a further 5-10 seconds in Cell-FACH state. Afterwards, mobiles are kept there for long durations, 30 minutes not being uncommon. Some networks even use the URA-PCH state if a mobile moves between cells during that time to reduce the amount of required signaling. Quite a different strategy.

Ah yes, and then there are voice calls. I made a couple of calls and they were all handled on the basic carrier.

Softbank also has a 3G network in Japan and it would have been interesting to see what they are doing. Unfortunately, no roaming agreement was in place so my mobile would not roam on that network. Well, next time then…