The report came from wireless testing and measurement vendor Spirent Communications, which was acquired by Metrico Wireless in September. The company ran trials on a commercial VoLTE network in two U.S cities, and compared the power consumption of VoLTE against the power used by the same carrier’s CDMA systems.
The carrier was not named, but but it's pretty obvious, since only MetroPCS has a live VoLTE service. In addition, MetroPCS uses CDMA for its non-LTE service.
A 10 minute call used 680 milliwatts (mW) on CDMA an 1358 mW on VoLTE. Spirent estimated that on a full charge, and with all other data communications turned off, its test smartphone could deliver 502.6 minutes of talk time using CDMA, but only 251.8 minutes of talk time using VoLTE.
Right now, LTE handsets drop down to older technology to make a voice call. For example, on Verizon a handset drops to CDMA, while on AT&T it drops to GSM.
[That explains why Verizon and Sprint iPhone 5s cannot do both voice and data simultaneously. Apple would have had to add a separate antenna for that to work, and the company chose not to. Meanwhile, AT&T's iPhone 5 can do both voice and data, because its GSM service supports that functionality.]
Why is there such a difference? GSM and CDMA voice technologies have been optimized over the past two decadesto be energy efficient, according to Spirent Global Director of Insights Amit Malhotra. Remember, as well, that VoLTE, like VOIP, is really transmitting the voice over a data network. That is significantly different than a circuit-switched voice call.
The disadvantage in battery life of VoLTE compared to circuit-switched voice is driven by a few different factors. One is the more strenuous exercise of the device, including conversion of voice to packet data, transmission and receipt over the data network, and reconversion back to voice. Another is the use of less power-efficient components such as data modems versus voice transceivers.That's very bad news for carriers, but possibly worse news for consumers. Does anyone remember how poor the battery life was on the first generation of LTE handsets? We could see a rehash of those issues.
Naturally, things will get better, as they did with later editions of LTE handsets. In addition, things are already better for LTE handsets doing voice and data at the same time. Since they are using just one radio -- LTE -- when performing both tasks at once, a user will see slightly better battery life.