More HSPA+: Enhanced Cell-FACH

HSPA+ is about more than just higher data rates, it is also about enhancing the radio interface to allow more devices to simultaneously connect to the network in a more power efficient way. I’ve described most of those features in various blog entries in the past but it seems I have missed one feature: Enhanced Cell-FACH.

One of the challenges of always on Internet connectivity is that mobile devices or PCs running instant messaging applications, Voice over IP prgrams, push eMail and other connected programs are anything but silent even while these applications are just running in the background. Even if just one of those applications is running, the device transmits and receives several IP packets per minute to keep the connection to the servers on the Internet alive. This means that in most cases, the radio link to mobile devices is not in idle state for most of the time.

As keeping the mobile in a fully connected state while only little data is transfered is quite wasteful in terms of bandwidth and battery capacity. UMTS networks therefore usually set device into the so called Cell-FACH state, once they detect that there is only little activity. In this state, the device uses the random access channel to transmit IP packets in uplink and the Forward Access Channel (FACH) in downlink to receive IP packets.

This method is quite efficient for the mobile, since no power control is performed on those channels. Hence, there is no radio layer signaling overhead in this state, which leaves more air interface capacity for other devices and also saves battery capacity. For the network, however, managing more than a few mobiles per cell on the FACH is not as efficient, since the channel was never designed to function as an always on data pipe for a high number of devices.

This is where the Enhanced Cell-FACH extension comes in. Once mobiles support this feature and they are set into Cell-FACH state, their data packets are sent on a Highspeed Downlink Shared Channel (HS-DSCH) instead of the Forward Access Channel. This improves the efficiency of downlink transmissions and also speeds up a state transmission into dedicated state once more packets are transferred again. An application note by Rhode and Schwarz goes into the details in Chapter 6.

What puzzles me a bit at this point is two things:

  • When will the feature become available?
  • In Cell-FACH state, the mobile is identified via the Cell-Radio Network Temporary ID (C-RNTI). In theory, this is a 16 bit value, i.e. up to 65536 mobiles per cell could be in Cell-FACH state simultaneously. Strangely enough, most networks only seem to increase this value up to 0xFF (256) before the being reset back to 0. Anyone got any idea why?