Everyone is saying that Wi-Fi is more power-efficient than 3G on smartphones.

But, I don't think so, because two radios are turned on at the same time, the 3G radio is still active when enabling Wi-Fi

There is some reliable proof to support this?

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    Please add a few links for someone making the claim :) – nico Apr 4 '13 at 16:31
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    You don't HAVE to have your 3G turned on when WiFi is on. Obviously having both on is going to drain more power than WiFi alone. – DJClayworth Apr 4 '13 at 16:58
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    @DJClayworth: certain phones (mine for sure) turn off the 3G when they get a WiFi signal – nico Apr 4 '13 at 17:29
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    @DJClayworth: well, oversimplifying a bit, 3G is just 2G + 3G data connection. For normal voice call you just use 2G GSM. – vartec Apr 4 '13 at 18:28
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    The issue isn't dominated by the connection method when the connection is idle. It is the transmission of data that matters. On WiFi most non-voice data will go WiFi: the question that matters is whether transmitting that same data via 3G would use more power. "Power efficiency" is ambiguous if you don't define what the power is being used to do. A better question would be "which uses more power to transmit, say, 1MB of data?" – matt_black Apr 4 '13 at 20:26

Yes, WiFi is significantly more efficient, as the protocol is completely different. Once association with access point is established, WiFi only uses energy for actual transmission of data. On the other hand 3G remains in high-power state even after transmission is done.

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source: "Energy Consumption in Mobile Phones: A Measurement Study and Implications for Network Applications" Niranjan Balasubramanian, Aruna Balasubramanian, Arun Venkataramani, Department of Computer Science, University of Massachusetts Amherst

It's also worth mentioning, that WiFi has legal limit of power output of 200mW (0.2W), while 3G devices legal power limit is 2W.

  • I think the UMass Amherst link and graph provides the right answer. Most people notice that their battery drains faster with Wi-Fi enabled just because both radios are still on. My Verizon 4G phone drains really fast when I'm in 4G range even without WiFi enabled. With WiFi turned off and in 3G only range, I have the longest standby time. Another reason NOT to use WiFi when you are away from home or office is security. You do not want your phone wandering into any random WiFi networks that may be setup to do harm to your data. I turn off my WiFi radio when I leave the house or office for secur – mdSeuss Apr 4 '13 at 16:50
  • The reason for WiFI being more efficient in transferring data is not that it switches to a low power mode faster than a 3G connection does after transmission. That fact only speaks to tower search being very expensive. – bobobobo Apr 4 '13 at 20:24
  • @bobobobo: you're going into details why it stays in high-power state – vartec Apr 5 '13 at 8:41

Inverse square law. Just think about the distance a 3G connection has to travel to reach a cell phone tower (hundreds of feet away) vs how far it has to travel to the nearest router (not more than 10's of feet away). All of data communication between two transmission points is in the form of EM radiation.

Note that a TCP transmission download (standard type of confirmed data reception that web browsers use) is not all reception on the part of the phone. Every data packet received must be responded to with an ACKnowledgement packet back to the tower to indicate successful reception at the phone. So even when the phone is receiving, it is still actively "sending" (acknowledgement packets).

A much higher transmission power is needed to transmit data further distances. The inverse square law says that the effect of distance on a transmission link is squared.

  • This post does cite the inverse square law. Distance is the dominant factor that determines transmission cost, not protocol. If you don't understand the basic mathematics in the Wikipedia article, that isn't a reason to downvote. – bobobobo Apr 4 '13 at 20:22
  • A very simple detail you forgot: cellular radio towers have massive antennas and powerful amplifiers... besides, this is a theoretical speculative answer. – Sklivvz Apr 4 '13 at 20:36
  • Hence it would cost your phone more power to transmit to it. It is not theoretical or speculative. It is basic physics. – bobobobo Apr 4 '13 at 20:48
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    In my opinion, basic physics says the sensitivity of an antenna and the gain of the receiver is determinant in establishing the needed output power of the cell phone. As a side note, please remember that the answers here are not for a public of physics experts and answers should be targeted for non-specialists - not only in the sense of understanding them, but mostly in the sense of being able to judge their validity. I fully expect the community to be able to handle your answer on Physics but not here. – Sklivvz Apr 4 '13 at 20:51
  • By the way, a TCP transmission download is not all reception on the part of the phone. Every data packet must be responded to with an ACKnowledgement packet back to the tower to indicate successful reception at the phone. – bobobobo Apr 4 '13 at 20:52

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