Hackaday Prize Entry: A Femtocell Repeater

For a Hackaday Prize entry, [TegwynTwmffat] is building a cell phone signal repeater. This sort of device is commercially available, but the options are either expensive or, as with some units available for $30 on DealExtreme, obviously noncompliant with RF regulations. This project intends to create a cost-effective, hackable device that works properly and conforms to the right regulations.

The core of this system is a LimeSDR transceiver. This is a board we’ve seen before, and it has a few interesting features. Basically, the core of the LimeSDR is a programmable RF transceiver with coverage from 100kHz to 3.8GHz. There’s also on-chip signal processing and USB 3.0 bandwidth to get the signals to and from a computer.

Right now, [TegwynTwmffat]’s focus is getting his LimeSDR up and working and figuring out how to set up a few radio blocks to do what is needed. There’s a great update to the project that showcases Pothos, and so far [Tegwyn] has a full-duplex repeater working. This is great work, and really showcases the capabilities of what software-defined radio can do.

9 thoughts on “Hackaday Prize Entry: A Femtocell Repeater

  1. I’m not sure that this is the wisest thing to do, especially if you don’t have an RF shielded test chamber with your own cellular base station to test with. You could potentially be clobbering a wide swath of spectrum and preventing other subscribers from making emergency calls on the cellular network.

    I realize that your intent is to make something that conforms to the correct regulations, but this prototype obviously doesn’t, and that’s not good!!!

    1. Some parts of the world, they’re straight plain illegal… even if you bought one from a reputable manufacturer, you don’t hold the license needed to operate it.

      I love the concept, but I think the legal issues will scupper this project.

      1. unless they stick a power amp section on this thing it won´t even reach the towerś noise floor.
        Recall parts of the modern LTE does collision handling to avoid already allocated public wifi bands.

        Technically it is illegal to operate more than 8 wifi devices even on ISM bands.
        Those IoT´urd projects are also illegal under FCC rules if actually in use.

        He should be using ISM band pass filers though, so the SDR doesn´t interfere with local equipment.

        1. You’d hope not… thing is, if he’s caught with it, there could be big trouble.

          This is where one has to very much be familiar with the local regulations (whether they be FCC, Ofcom, ACMA…etc). It does not take much power to go quite long distances. There used to be a 70cm amateur radio repeater up on Mt. Coot-tha here in Brisbane that I could hit with 50mW out of a rubber ducky antenna, and get in full quieting.

          Sure, LTE uses higher frequencies, but I know of people who regularly go up atop the hills and fire microwave signals at each-other. RF is an imprecise beast and will sometimes surprise you.

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  2. I don’t think using the LimeSDR is a good idea, unless his repeater also does things on the higher layers of the stack. Just plainly repeating the RF signal works, but you can only introduce very little delay, otherwise the TDMA and OFDMA resource allocation will totally fail.

    A repeater with and SDR would almost be a base station that simply proxies all requests to the real tower. Without having the network keys it is very limited in the form of resource allocation it can do, so running it without causing interference would be difficult.

    I also recommend connecting the repeater to the phone via coax cable and attenuators for testing. The repeater output is unlikely to cause interference, but a phone transmitting in the wrong place at full power easily will.

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