LimeSDR GSM/LTE with physical attachment


Please bear with me, I’m not an RF guy but I’m trying to satisfy my curiosity before I bring up a potential solution for something.

We have done a lot of development of systems that use cellular connections for data retrieval and settings changes. We are looking into doing some changes to the way our system works and testing cellular has always been a bit troublesome because of where we are and our building.

We are looking to update a product to use the SIMCOM 7XXX series modems, it occurred to me having recently gotten into SDR that there may be a more convenient way to test things.

What I want to do is connect the SIMCOM modem to the LimeSDR via cable, this I believe will allow me to use attenuators to simulate various situations, also by using an emulated GSM network, I can test different configurations of network (NAT, dynamic IP, static IP) etc which is something that would be extremely convenient, especially as it will all happen locally without the actual involvement of the internet or a real GSM network.

So, it’s obvious to me that I can use the SIMCOM module with an antenna plugged in, and with 2 antennas plugged into the SDR, one for TX and one for RX. All good so far.

But how do I achieve what I want to do with a physically connected “emulated” link using cable, where one end has one connection and the other has 2! Now, I’m thinking of various solutions, mixers, duplexes etc, but I really don’t know.

Can anybody give me an idea of what I need to achieve this and how I go about it?

I do actually have a colleague who is an RF engineer, but I’d rather have some clue as to what is possible/needed before I make any suggestions.


You need a duplexer connected to LimeSDR Tx and Rx ports, with the common/antenna port connected to the GSM modem. Probably an attenuator also, but not sure whether this would be best between the duplexer and modem, or the duplexer and SDR Rx port. Maybe @zack can advise.

Might also be worth a look at the osmo-gsm-tester for automating testing.

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Perfect, this is what I figured I need but thought it best to ask the experts. Thank you so much for your response.

I’ll take a look at the osmo-gsm-tester as well.

He will definitely want the attenuator, between the duplexer and the Lime - the modem will be putting out enough power to smoke the Lime’s front end. Make sure the attenuator can handle about half a watt of power, and has at least 20dB attenuation.

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Right, was about to reply also to say need to confirm the appropriate spec and placing for the attenuator before cabling up. Not sure whether a smaller one might be best in the downlink (SDR -> modem) path also.

Thank you guys. I’ve passed on all the information to my collegaue saying what we need and what we are trying to achieve here.

I don’t deal with RF stuff myself, although we are involved in RF stuff (radar) but at much higher frequencies, I deal with signal processing software, so my knowledge starts at IF! RF to me is some sort of magic dark art.

We do actually have a lot of RF test equipment, but again, I don’t get involved in that partly because I work remotely so never touch it directly and GSM/LTE is not a technology we are primarily engaged in, it’s something that is an add on to our equipment as a means to get data in and out of the equipment remotely.

I really appreciate you taking the time to reply.

Semi-OT: what sort of environment do you do you SDR work in? Do you use a standard framework or did you roll your own? And who’s RF test gear do you use?

We don’t actually use SDR, this is an aside for a specific situation that has arisen that could make development of some cellular stuff easier and more robust.

Have use Tektronix signal analysers for 24Ghz and use down converters for higher frequencies. We also have specialised equipment as well for testing.

Thanks - I do RF test equipment and SDR professionally, so I was curious if you were using anything I was responsible for.