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When I disable "3G or EDGE" (cellular data) connection from my smart phone's settings, I can still make a phone call.

Do phone calls and cellular data use different antennas and frequencies in a mobile phone? Or it is just an inner arrangement?

I mean do the cellular voice and data networks use different antennas?

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The difference between GSM @ 1900 Mhz and UMTS @ 2100 Mhz is a lot smaller than the difference between GSM @ 1900 Mhz and GSM @ 450 Mhz ! –  MSalters Jun 25 '12 at 7:43
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3 Answers 3

Your connection to the cellular network, no matter the "G", always uses the same antenna. You can see this in a teardown of whatever phone you have. For instance, here is a teardown of my phone:

HTC Thunderbolt Teardown

In the last photo, you can see that the LTE and EVDO antenna is one and the same. This also handles your 2G connections as well even though they all operate on different frequencies:

Cellular Frequencies

What you are doing when you disable your data, is shutting off the modem in the chip that talks on a particular frequency. Each cell phone has several modems. For 2G, 3G, 4G, Bluetooth, etc. Since the "Gs" are similar in the way they operate, they can use the same antenna. Bluetooth and WiFi operate very differently and require different antennas as can be seen in the teardown.

2G connections provide voice and text and, since it's the oldest, has the widest coverage. 3G provides voice (GSM only, see below) and data on the same frequency, and, in some model phones, at the same time. 4G provides data only and voice is either done by falling back to a different communications model (frequency/"G") or via voice over IP. So when a 3G or 4G signal weakens in a particular area, there is usually a 2G signal to fall back onto but it will only provide voice and text.

There are some interesting notes in the Wikipedia article on 4G if you search for the word "voice". Such as:

EV-DO is not designed for voice, and requires a fallback to 1xRTT when a voice call is placed or received.

And:

Since the 2.5G GPRS system, cellular systems have provided dual infrastructures: packet switched nodes for data services, and circuit switched nodes for voice calls. In 4G systems, the circuit-switched infrastructure is abandoned and only a packet-switched network is provided, while 2.5G and 3G systems require both packet-switched and circuit-switched network nodes, i.e. two infrastructures in parallel. This means that in 4G, traditional voice calls are replaced by IP telephony.

Another thing to note, is that while voice requires less bandwidth, it requires a continuous connection. Whereas data can be packetized. A good discussion of how voice and data share the network (when both are available) is available in the following article on Using data versus voice in an emergency

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thanks for the great explanations! actually I am given a very complicated task. I am not sharp in telecom but I am trying to find a way to boost cellular data strength for a mobile phone. Maybe a very complicated issue but do you have any broad idea about it? I was thinking to design an antenna and then amplify the cellular data signals with an amplifier and transmit them again. I dunno where to start this research. Many Thanks! I think you are really expert in the field. –  cmd1024 Jun 24 '12 at 23:18
    
I wouldn't call myself an expert in the field. I just happen to live in an area with crappy cell reception. So I've done a lot of research into how to get a better signal. I use a femtocell which plugs into my broadband connection and acts as a mini tower in my house: en.wikipedia.org/wiki/Femtocell But you're pretty much on the right track with getting/designing a larger antenna and repeating the signal. There are commercial devices that may be of interest to you: repeaterstore.com –  embedded.kyle Jun 24 '12 at 23:30
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No. The cellular voice and data go over the same channel and therefor use the same antenna. Often the up and down links are at different frequencies, so different antennas (or at least different performance characteristics) are used. But the division of labour has nothing to do with Data vs Voice.

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Ok but why then when 3G connection weakens the cellular voice doesn't get effected. It seems like data and voice are modulated and send over different channels. Are you sure that they use the same channel on a fixed carrier frequency and same antennas? –  cmd1024 Jun 24 '12 at 22:13
    
Yes, they use the same antenna. Voice requires much lower bandwidth than 3G data, so the 3G will fail first. –  Rory Alsop Jun 24 '12 at 22:36
    
i got your point. thanks! so the larger the bandwidth the more fragile is the signal if I understood you. –  cmd1024 Jun 24 '12 at 23:20
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Actually, most phones support several different frequency bands which they may use for different carriers or different parts of a given carrier's network or different services from that carrier.

An obvious demonstration of this is that the two main US GSM carriers - AT&T and Tmobile have tended to use different frequency bands for their highest speed data offerings. Often, phones sold through a carrier will only support the bands actually used by that carrier. This can mean that even if unlocked, a phone on the "wrong" network is only able to make voice calls and used low-rate data, without access to the high-rate data service of the "foreign" network. And it's not just a US problem - there was a case recently where a deceptive advertising claim was leveled against a certain tablet because it's "4g" capability was incompatible with the networks in a particular country where it was allegedly being sold as a "4g" device.

Given that the difference in frequencies can be up to a factor of two, it's not inconceivable that different antennas - or at least different antenna modes/matching networks would be used for the different bands, though cost and size pressures would point towards minimizing components (designing antennas to match disparate frequency bands has a long history). On the cell-site end, there would probably be even more likelihood of having distinct components.

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