The FACH Power Consumption Problem

In UMTS and HSPA, there are a number of different activity states on the air interface while data is exchanged with the network. During phases of high activity, the mobile device is usually put into dedicated state (Cell_DCH) and transmits/receives data on the high speed downlink shared channels and a dedicated uplink channel. During times of lower activity or to keep a physical connection open to resume data transfers quickly (e.g. the user clicks on a link after some time of inactivity) the network puts the connection into Cell_FACH (Forward Access Channel) state. While the FACH is quite slow, it reduces power consumption somewhat. However, not enough for all kinds of applications.

eMail Polling in 3G mode

While in Austria recently, I noticed that when using 3's UMTS network and Profilmail with a POP3 eMail polling interval of 5 minutes, my battery ran dry within 6 hours. Quite devastating and very short compared to GSM/GPRS/EDGE where the battery easily lasts a full day under the same conditions. With the help of Nokia's Energy Profiler I dwelled down to the bottom of the problem. It turns out that 3 leaves the air interface in DCH state for 20-25 seconds after the last data packet has been sent before putting it into the Cell_FACH state for 1 minute and 45 seconds. Afterwards, the air interface connection is put into Idle state. In Cell_DCH state, even if no data is transmitted, power consumption is around 1.5 watts. In Cell_FACH state, power consumption is still around 0.8 watts, while in idle state and backlight off, power consumption is "almost zero". Even if no eMail is sent/received, these values result in the radio being active for almost half the time of each 5 minute interval, resulting in an average power consumption "in the pocket" (i.e. backlight always off) of 0.5 watts on average. As the battery capacity is 4.4 Wh (that is watt hours), the result is that the battery is empty in just a couple of hours.

If noticed this behavior in 3G networks before but never in such an extreme. This is because most other 3 G networks I usually use have different activity timers. In most other networks, the Cell_DCH state is left after about 15 seconds and Cell_FACH after about 30-45 seconds. This of course decreases the browsing comfort because it often takes longer than 30-60 seconds to read a web page in which case the transition to from idle to Cell_DCH state takes longer than from Cell_FACH to Cell_DCH. On the other side, however, it increases the autonomy on a single battery charge.

eMail Polling in 2G mode

Polling eMails every 5 minutes while the mobile is locked to GPRS is much more efficient. Here, the mobile takes about 1.5 watts while communication is ongoing. However, power consumption goes down almost immediately after no data is sent or received. As a result the average power consumption is only 0.1 watts or only a fifth of the power consumption while in 3G mode.


Reducing the 3G timers to lower values is no option since it would have a negative impact on the users experience. Maybe the enhanced FACH, which is not yet implemented in devices and networks, will help somewhat in the future. When looking at the specifications, however, it looks like it mainly addresses capacity and not so much mobile device power consumption. So that remains to be seen. 

Another possibility is to switch from the POP3 pull approach to a push approach where the server starts communicating with the device only when a new eMail has been received or very infrequently to keep the TCP session open. Not sure how Blackberries receive their email, but it would be interesting to experiment a bit. IMAP push would be another option but unfortunately, Profimail does not support that extension.


An interesting case in which the 2G air interface is superior to 3G. How LTE and WiMAX fare in the same scenario is also in interesting question. LTE, for example, has a different air interface state model compared to 3G. Here, only active and idle state exist and active mode timers can be set by the network dynamically in a way to reduce the mobile's average radio activity time to almost the same values as when being in idle state. That should reduce power consumption somewhat if the base stations are clever enough to adapt the timers based on the traffic pattern observed. We shall see…

3 thoughts on “The FACH Power Consumption Problem”

  1. Interesting Analysis. From an operator point of view the trade off is between saving resources (such as use of Radio Base Stations HW consumption, SF Codes, RBS Power ecc) that leads to fast DCH to FACH switches; while on the other hand it’s important to reduce “unnecessary” channel switching (ex. DCH to FACH to DCH within few seconds) to improve user experience and reduce signalling messages.
    Battery power drain can be a problem but optimising a network it’s interesting to know what kind of device is used for PS traffic. If the majority of our user connect to the network with PC data cards, battery consumption becomes a lower priority matter… :-)Anyway the use of common channels is becoming an interesting topic and new timers are often tested and implemented, following the evolution of traffic patterns in our network

  2. Hi Martin, as always interesting post. I am aware of a number of devices with embedded e-mail clients which actually trigger the release of the RRC connection so they don’t have to go through the state transitions and waste battery. According to the specs the RRC connection cannot be released by the UE directly so what they have implemented is a RRC SIGNALLING CONNECTION RELEASE which triggers the release of the PS RAB (from the SGSN point of view). Once this occurs the RNC releases the RRC connection as well.

  3. With devices like Blackberries that are true push, I wonder if they would offer better battery life. RIM is starting to roll out 3G devices so we should be able to soon get data.

    As well, Apple offers true push now and enterprise (Microsoft) integration.

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