How to change frequencies on a cellphone?

Discussion in 'General Electronics Chat' started by Moleculor, May 29, 2011.

  1. Moleculor

    Thread Starter New Member

    May 29, 2011
    Ok. I saw this forum in an article (lifehacker? gizmodo?), wandered over, and realized I might be able to ask a question here that's been bugging me for a few months.

    A preface: I'm not an electronics expert. Hell, I'm not even a beginner. Closest I've ever gotten to understanding electronics is being able to put computer parts together to build a PC. I *think* I know the basic functionality of a few basic parts (resistors, capacitors), but pretty much any terminology is going to be something I'm going to have to Google feverishly just to understand. (Hell, I was reading your rules, and I had to spend two or three minutes Googling "LED to mains" just to figure out why it was against your rules to talk about. And I'm still not entirely certain why. Fire hazard is my guess.)

    Anyhow... this is going to require a bit of background explanation.

    Cellphones. Specifically GSM (as opposed to CDMA) cellphones, in this case. There is, so far as I understand it, two (or maybe six to ten) radios in a cellphone.

    The first kind of radio is the actual GSM radio (operates on bands that are described as 850/900/1800/1900, and I'm pretty sure those are MHz). Most phones come "quad-band" like that, as different carriers in different locations have rights (owned or rented) to specifically only one or so of those four bands, and those GSM bands are the ones that the phone uses to actually "talk" for phone conversations. For example, in America, 850 and 1900 are the used frequencies.

    Almost all phones come with the full set of four frequencies, and I think it's a very rare situation (Brazil only, I think?) where any other frequencies are used for standard phone operation (1900/2100 in Brazil, I think).

    None of that matters, since all phones (that I'm looking at) come in quad-band GSM style that's compatible almost anywhere I'm likely to go.

    2G data networks also operate on GSM bands and frequencies. However, 2G is slow. A bit like a dial-up modem.

    Let me be clear: Everything above relating to GSM is pretty much unrelated to what I'm trying to ask, and I only included so I can be specific about it NOT being what I'm asking about. I've asked this question elsewhere, and it took a couple days for people to figure out I wasn't talking about GSM, so I'm being clear in advance. GSM is not what I'm talking about.

    However, their data networks (specifically what's described as "3G") operate over something called "UMTS" or sometimes "CDMA2K".

    This (apparently) requires a separate radio in the phone. A second one, in addition to the GSM radio. I don't know why. All I know is that phones seem to come with a few particular sets of frequencies they can talk on:

    900/1700/2100 MHz
    1700/2100 MHz
    900/2100 MHz
    850/1900 MHz

    are common sets of frequencies.

    Note, that out of those combinations of frequencies, there isn't a single combination that includes all four possibilities: 850/900/1700/2100. No phone I've ever looked at supports all four, and no one I know has ever been able to find a phone that supports all four.

    Specifically, either a phone has 850, or it has 2100, but never both, and never all four.

    This is a problem, because AT&T is only compatible with 850/1900, and T-Mobile is only compatible with 1700/2100.

    Which means that if you buy a cellphone, your 3G service will ONLY work on one carrier, and not the other. Ever. (And despite what you've heard about "unlocked" cellphones, "unlocking" only applies to the GSM radio. So it doesn't "unlock" 3G service for you so you can use it on the competition's carrier.)

    I'm fairly certain ("common" sense, talking) that this isn't due to technical limitations. I've been lead to believe that it's actually more to do with cellphone companies having the legal backing to where they need to approve every cellphone sold for the USA, and they'd never approve of a device made for the competition. So, for example, AT&T will only approve of a cellphone that supports radios that talk on 850/1900. So any phone that's built like that won't on T-Mobile's 3G networks. Same is true for the reverse.

    Now, to my questions (finally).

    First, what the <snip>? Anyone know, or have backstory on this crap? Is this design simply greed, or is there more to it than that?

    Second, is there anything at all that I can do?

    I'd like to avoid slapping down $700 for an unlocked GSM phone that I still can only use 3G on one single network, and never any other network. It's like signing a contract to stay with that network for the life of the phone, even without the contract. And yes, before you ask, I am going to be aiming for an unlocked cellphone to use for service with no contract. The phone I have currently is exactly that, with an AT&T subsidiary providing service with no contract. It's just also a nine year old phone, and I'd like to upgrade without signing my soul away.

    Is there a way for me to retune the radio? Install a second one, with a switch to flip between them? Some other solution I'm not thinking of? I'm definitely not looking for a walkthrough (just yet), so it's probably best not to spend a few hours coming up with diagrams, etc, as I don't even have the phone I'm going to upgrade to yet, but I would like to know my options ahead of time.
    Last edited by a moderator: May 29, 2011
  2. #12


    Nov 30, 2010
    That is a very well explained question. If anyone knows the answer they will have no trouble figuring out what you want. Now you wait and see if anybody here has the information. Unfortunately, it isn't me.

    Good luck.

    ps, The real reason I posted an "I Don't Know" answer is that your question is very long and my short reply might encourage someone to read the whole question instead of being put off by the size of it.
    Last edited: May 29, 2011
    Moleculor likes this.
  3. Moleculor

    Thread Starter New Member

    May 29, 2011

    If no one knows that particular question, does anyone know whom I'd ask?
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
    For what it's worth, lots of people would like to do the same thing so if it's possible, the talk of it would be all over the net.

    These guys know the answer, but do they have a way for you to ask and will they answer honestly?
  5. DumboFixer

    Active Member

    Feb 10, 2009
    A mobile phone is designed/manufactured with a particular market in mind, ie EMEA, APAC, LTA etc. Within that they'll also be aiming the phone at particular operators. It is these constraints that will determine what frequencies are designed in (along with whether the phone is 2G, 3G or 2G/3G and with 4G on its way).

    The frequency shown is not the exact frequency the phone works at otherwise every phone would be worknig at one of a few frequencies.

    It is possible to tune the transceiver within the phone (I've done it many times) but this tunes the transceiver to the antenna in the phone.

    Unlocking a phone does not apply to the "GSM radio". The unlocking changes a set of security codes which are checked against the sim card (country code and operator code), changing them from a particular operator etc to a "I don't care" setting. This then means you can put a sim from another operator in and it will work - I used to write the sim lock files for the US market :)

    I think you'll find that WCDMA and GSM actually operate on different techniques. You could, conceiveably, put in a second transceiver to suit the other system but it's not that straightforward. yuo'd also need to tell the OS that you're using a different system - the Protocol messaging between the phone and the base station do differ.
  6. cjdelphi

    New Member

    Mar 26, 2009
    ^^ in other words, your computer has to say hello to the other computer, depending on protocols, if you changed the frequency, the most you'd get from the other end is 'hello' and your end will send something the server has no clue about and will terminate the connection.

    Having said that, my E72 (nothing special there, getting on too) supports E72-2 Quad-band EGSM 850/900/1800/1900, WCDMA 850/2100 HSDPA


    Not far off.. but i've seen phones which support a lot more than this phone does...
  7. Moleculor

    Thread Starter New Member

    May 29, 2011

    Unless I misunderstand what you're trying to say here, you're just repeating what I've already explained, and this has nothing to do with my question. I spent the first two-thirds of my very long post explaining that I understood exactly that, and that I wasn't trying to get GSM to talk to WCDMA. (And I wasn't trying to change the GSM radio at all.)

    Unless I misunderstand what you're trying to say. Which is entirely possible.

    I'd assume that this is trying to say that the WCDMA implementation between two different carriers is in a completely different protocol but then

    A) the quote is immediately after the quoted GSM line above, and seems to be referencing the difference between GSM and WCDMA, not the differences between WCDMA on one set of frequencies and WCDMA on another set of frequencies

    and B) Isn't the point of having a protocol like WCDMA to have interoperability? It'd be like having a computer that could only use HTML sent via one specific ISP, and not another.
  8. DumboFixer

    Active Member

    Feb 10, 2009
    No, the 3G protocol is (well, it should be) the same for every operator, regardless of location. I can take my 3G mobile phone and, with a suitable sim card for the location/operator, use it anywhere in the World where there's a 3G operator. Have a look here

    What I was trying to get across is that there is a difference between the GSM and WCDMA protocols (layers 1-3)
  9. Moleculor

    Thread Starter New Member

    May 29, 2011
    No, you can't. Not if we're talking about WCDMA-based 3G technologies. (Remember, I stated earlier that I am not talking about GSM. At all.)

    Please, go back and re-read my original post.

    Or, to put it another way:

    A phone designed to work on AT&T's 3G WCDMA network will not work on T-Mobile's 3G WCDMA network, and vice-versa.

    This limitation is (so far as I can tell) exclusively due to hardware-based hobbles (locked/limited frequencies) imposed by regional carriers on what frequencies the WCDMA radio talks on. You change those frequencies, and you remove any hardware based limitations on what data networks you can use.

    There MAY be software limitations (proprietary alterations to the WCDMA protocol), but A) That wouldn't make much sense and B) Those can be solved with a software solution if they exist, but a software solution can't happen until the hardware solution is found.

    This... SOUNDS like exactly what I'm asking for, but you sorta just said "Yeah, I've done it!" without explaining how I'd do it, or how I'd learn to do it. Assuming that in this case you were specifically talking about the WCDMA radio, and not the GSM one, as it seems I'm still having problems explaining that I'm not talking about GSM, despite saying that I'm not talking about GSM.
  10. DumboFixer

    Active Member

    Feb 10, 2009
    I disagree.

    Really ? Again, I disagree as I've done it.

    I would go into our test suite which is a vibration damped, anechoic chamber/faraday cage. A 30 min RF sniff test would then be carried out to ensure that there was no stray RF energy being received. The UE would then be plugged in to its test receptacle and the chamber sealed. I would then sit myself, outside of the chamber, in front of a PC which was connected to a number of very expensive test boxes and signal generators. I would then run a number of tests and tweak the tunings contained in the protected memory in the UE.
  11. Moleculor

    Thread Starter New Member

    May 29, 2011
    Ok. Every bit of evidence I've read says that T-Mobile and AT&T operate on different UMTS/WCDMA/CDMA2k frequencies.

    For example, this. Or this. Or this. Or this. Or this. Or this. Or this. Or this. Or note how on this list of frequency bands, AT&T communicates using the 824 - 849MHz band for upload, and T-Mobile communicates on the 1710–1755MHz band for upload. 869 - 894MHz vs 2110–2155MHz for download.

    While I admit that I'm not an expert, my understanding of the basic physics of the universe suggest that a radio talking on one frequency will not be heard by a radio listening on another frequency.

    As I have plenty of evidence supporting my understanding of how the two data networks are incompatible, I'd love to see some evidence from you supporting your "disagreement" with my understanding.

    Remember, I'm not talking about GSM, or EDGE. I'm exclusively talking about WCDMA/UMTS.

    Unless I'm mistaken, that would make you one very unique individual.

    Are you sure you're not confusing EDGE-based "3G" with WCDMA based 3G technology? You did seem to be confused about the fact that I wasn't talking about GSM, earlier, so I could understand you being confused between EDGE and WCDMA. The wireless terminology is muddled, primarily due to marketing ploys by various wireless carriers.

    Well, <snip>.

    What would the feasibility be of installing a second tranceiver, and switching between the two using a hardware switch? Obviously at this point we're talking major hardware modification, even to the externals of the phone, but I'm trying to look at all options.
    Last edited by a moderator: Jun 16, 2011
  12. loosewire

    AAC Fanatic!

    Apr 25, 2008
    When you buy a phone with all the gigits,your service bill you for the different
    freq.s that Is Involved and you are locked In to pay for It.So the phone Is one of
    the most Important Inventions ever. So when you slide your finger across the face
    of the phone you are changing freq's,Its not magic like you would want to think.
  13. sansam

    New Member

    Jun 16, 2011
    Thanks for the useful info's...