Using a LimeSDR as a Simple 4G Repeater

Over on YouTube user Goat Industries has uploaded a video that shows him successfully using his LimeSDR as a 4G repeater. More information about his project to build a cell phone signal repeater can be found on his hackaday.io page, and he describes the project as follows:

In more remote areas it is often not financially viable for the cell network operator to build extra base stations for a small number of people and their phones/modems etc. Fortunately, this is not the end of the road as we can, in theory, build our own base stations and even create our own cells.

There are currently available two groups of devices that already claim to do this, one of which is reassuringly expensive and the other is just plain illegal! This project aims to democratise the situation enabling cost effective, hackable devices to be built that not only work properly but also conform to the telecoms regulations.

In his video he shows the repeater running on his LimeSDR. For software he uses Pothos to create the receiver and LimeSuite to control the LimeSDR settings.

The LimeSDR is advertised as a full duplex RX/TX capable SDR with a 100 kHz – 3.8 GHz frequency range, 12-bit ADC and up to 80 MHz of bandwidth. Back in June 2016 they surpassed their $500k goal, raising over $800k on the crowdfunding site Crowdsupply, and today it’s now up to over $1.1 million. Most crowdfunding backers have now received their units in the mail, but some are still waiting. We paid $199 USD for an early bird unit, and currently a preorder unit costs $289 USD on Crowd Supply.

LimeSDR working as a simple 4G signal repeater

6 comments

  1. AD5NL

    I did a similar sketch in Pothos, based on some forum posts over on the MyriadRF discussion boards, where I was able to repeat an FM broadcast station at low power. Unfortunately I didn’t save my work or document it; I was just goofing off.

    I think a similar approach could be used to repeat NBFM or digital signals in the ham bands.

    Anyway LimeSDR is really interesting hardware, and it will be really great once there is a little bit better software support and documentation.

    • AD5NL

      As I understand it, one limitation of repeating (at least for simple analog stuff) is that the transmit frequency has to be in the same chunk of bandwidth the receiver is on.

      So for example it is trivial to repeat 90.5 FM on 101.7 FM. But you can’t put your output at 433MHz, because that is further away than the 60MHz receiver bandwidth will allow.

      I guess you could use another receiver (maybe an rtl-sdr.com dongle?) if you wanted to do cross-band repeating.

      • Jay Cox

        I think you might be a bit confused. The LMS7002M ic has two separate synthesizers, one for TX, one for RX, and as far as I can tell from briefly skimming the datasheet, both operate more or less independently while using a separate, common oscillator for a reference frequency. Unless you need phase coherent operations or something where you need to use one synthesizer for both RX and TX for “phase coherent operation in TDD mode” (i confess I don’t really know what that quote from the spec sheet means), you shouldn’t have any restrictions in TX or RX frequencies used, if I understand it correctly.

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