Considering a non moving cell phone, not in a call, not receiving/sending a message, not connected to data transmission services: does such a cell phone emit a signal continuously? Or is it mostly "listening" the network signal and very rarely, emitting something (like "I'm still here")?

Note: I am not sure this is the good place to ask, but I didn't not find a better one in the whole SE community...

  • \$\begingroup\$ Yes - to the last question. \$\endgroup\$ – Tom Carpenter Jan 18 '17 at 23:02
  • \$\begingroup\$ Might be a useful read: telecomhall.com/… \$\endgroup\$ – Tom Carpenter Jan 18 '17 at 23:04
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    \$\begingroup\$ Yes. The mobile phone network needs to know which cell it's in so it can route incoming calls to it. \$\endgroup\$ – TonyM Jan 18 '17 at 23:55
  • \$\begingroup\$ This is why from at least a transmission perspective, texting is or was free. Could be and/or is buried in spare portions of the payload of the constant communication between the phone and the tower, granted it is not free when they have to extract it and find the target phone and then send it there. I want to say the size of a tweet had to do with something related to this. \$\endgroup\$ – old_timer Jan 19 '17 at 3:31

No, it does not transmit continuously. In fact, it may not even necessarily receive continuously. One major point for optimization in phones is power efficiency. The phone may have to transmit with quite a bit of power (on the order watts, unlike wi-fi which is usually less than 0.1 watt) so running that transmitter continuously is out of the question. Even when you are browsing the web or performing some other activity that uses data, the phone will turn the transmitter on only when it needs to send data to conserve battery life. The low noise amplifiers in the receiver also consume significant battery power, so turning those off when not required also saves power. Obviously, there will need to be occasional communication with the cell tower to manage hand-offs, calls, alerts, etc. but this can be done 'relatively' rarely - say, a few ms every 10 seconds to minutes.

When transmitting, the cell phone tower actually tells the phone how much power to transmit with. This serves two purposes - since the phone uses exactly as much transmit power required to reach the antenna, it will save battery power. Additionally, and more importantly, the signal from the phone at the tower's receive antenna will not drown out the signals from other phones that could be much farther away from the cell tower.

  • \$\begingroup\$ 1. If it does not "receive" continusously, or listen to the network "events", how could it know when there is an incoming call then? 2. would you know how often an "idle" cell phone (as described in my question) is emitting a signal? \$\endgroup\$ – Totor Jan 21 '17 at 0:06
  • \$\begingroup\$ 1. By periodically checking in with the tower on a timescale of ms to seconds, 2. Not sure offhand, would depend on the cell network generation and configuration, etc. This would be easy to check with a wideband antenna, power detector, and oscilloscope \$\endgroup\$ – alex.forencich Jan 21 '17 at 1:08
  • \$\begingroup\$ In 1., do you know if this checking implies a radio emission from the cell phone? \$\endgroup\$ – Totor Jan 23 '17 at 17:11

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