Cell Phone Signal Repeater / Booster / Femtocell Project

Partly based on the LimeSDR, this project has just won $1,000 in the Hackaday Prize 2017, round 1:

It’s a shame it’s not actually working properly yet, but I got quite a long way into it before I had to go off and plant potatoes.

This is an open invitation to anybody who wants to help finish the project - just register with Hackaday.io and press the ‘Ask to join project button’. There’s no time limit but I’d like to get it working before the competition ends if possible, either with the LimeSDR or hardware or both.


Just signed there…

Here doing something similar therefore like to support your project too.
My project is on hold at the moment because I am busy on other side
but hoping to make some progress soon.
My application is on Search and Rescue field when some one is lost somewhere
out of base station reach for example in the case of aviation or auto crash.

Any way good luck with your project…hope we can assist each other.

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Thank you @9a4db Djani.

Search and rescue sounds like an interesting project. I’m hoping to do some more development on the repeater project next week by populating PCBs for hardware tests.

Actually, something that is missing is a simple test on the LimeSDR to find cell phones on the 4G network and make screenshot videos of what they look like in the waterfall. I live in the middle of nowhere so only have my own phone to observe and I can’t be sure I’m not looking at the phone internal processors rather than the RF signal. Obviously I’m not talking about hacking phone here!

I joined hackaday but not your project. No time to help. I am not sure from looking at it what’s actually needed. What’s blocking you?

Congrats on winning the money. At least you recover some expenses.

p.s. I really dislike the look of the hackaday pages. No gui expert but it’s too cluttered for my eyes.

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I’m not sure why the LimeSDR setup’s not working and there are a lot of possible reasons. I can’t see anything wrong with the Lime setup in Limesuite and the FFT stuff looks entirely plausible. Possible problems include:

  1. Wrong Antennae
  2. Badly soldered duplexers
  3. Wrong band selected
  4. Inappropriate use of the LimeSDR ie it can’t actually do the intended job with NCO up/down convert.

I’m planning to work more on a ‘simple’ hardware based solution and thus rule out some of the above and in the meantime wait and see if other people can get anything out of the Lime. I presume Lime Microsystems themselves are working on, or have solved the same problem and I notice that they’re using 2 limeSDRs in their latest video which kind of makes sense as band 3 has too big a bandwidth for one SDR … OR … NCO up/down convert just does not work with 4G … I’m only guessing. No doubt we will all find out in due course.

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The Band 3 topic you bring up again. I recall that your band pass filters were for Band 20. So I’m confused about that. Pointers appreciated.

I would think that the best way to go is to use this and to test each part of your rig in isolation and/or discretely.

Maybe LimeSuiteGui is a bit complicated for basic testing. Even though I know it’s designed exactly for that.

Or maybe you’re past the need for this. Sorry if I’m not offering anything new here.
I guess I’m acting as a sounding board…

Do you have one of those inexpensive SDR dongles for tx/rx tests?

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I’ve got both band 3 and band 20 covered and yes I’m using a cheapo SDR as spectrum analyser.

I’ve just had a chat with a local RF EE and I’m happy to concentrate on creating the more basic hardware solution and just use the Lime as a very nice spectrum analyser … for now anyway.

psLM7002M could very well be the correct tool to use but kind of brings back memories of getting frustrated with Pothos for not handling duplex properly … or maybe it was just a clash of personalities? :slight_smile:

Sounding board help is appreciated :slight_smile:

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For some reason, up until very recently, I’ve found it incredibly difficult to actually find any 4G cell phones on band 20 of the RF microwave spectrum. It’s always been easy to see the base station transmitter signals but looking at the area in the spectrum where phones are supposed to be just came back completely and utterly blank! To say this was confusing is a massive understatement, but inspired by having the project get into the quarter finals of HAD 2017, I called upon the assistance of some neighbours to give me some technical advice and practical assistance.

Lured by the smell of coffee infused with chocolate mint, my actual next door neighbour came over and very generously wandered around the garden watching videos on YouTube via 4G until I was able to get a fix on her phone signal using the LimeSDR and GQRX software to work as a spectrum analyser:


Why has it taken me so long to get this result? It must be something to do with the relative positions of the transmitting phone and the receiving antenna connected to the Lime? If the two are very close together does the receiver get overpowered by the very strong phone signal?

I guess the next thing to do is boot up the LimeSDR once more and repeat the test using the Limesuite GUI instead of GQRX running my custom made .ini file. I just want to be able to see that phone on channel B for now.

Well you need proper instrumentation to be on the safe side… 2 years
ago I managed to purchase Agilent E7495A test set second hand all bells and whistles included
for bit more than 1 k USD shipped to Europe from Korea /South :wink:

Here some local GSM EU freq. set


Oh yes … I’d like one of those!

Latest update - I seem to be seeing my phone now on band 20 - I upgraded my iPhone to V7 and have now got constant 4G in my office, if only 1 bar of strength.

I play a film on my phone from Netflix and I run limesuite with it all set up for band 20 and, the good news, it does not jam my phone and the bad news, it does not give me extra bars of signal strength. I mess around with Rx and Tx gains and it seems to make no difference to my phone. You’d think it would jam my phone if it was set up wrong?

The display on the limesuite FFT looks very convincing and I can see the Base station transmit profile on channel A and my phone transmitting and being picked up on channel B.

Has anybody got any ideas? Are there any other adjustments like IQ corrections etc. that I should play about with? Thanks!

Since I ain’t no expert I can only tell you what I’d do. First thought that comes to mind is how does iPhone determine how many bars? The “bar” metric might not be that useful. I’d try a bandwidth metric. By now you should know that my advice is to be suspect and taken carefully. :no_mobile_phones:

But what’s your RF EE have to say? Perhaps you should have them on a retainer? A pint of whiskey maybe or some hot sauce.

@hTo137 This is a good point. Previously I used an iPhone hack to get the dBm level. Since then I’ve found an app that shows dBm:

My RF EE friend says that this project should be very simple, which is exactly what I thought when I started it! He advised not to use the LimeSDR but to go back to basic hardware, making sure that the amp components were very ‘linear’ for LTE use. I am trying hardware options as well, I’m not sure what his drinking/eating habits are yet, but rest assured there will be some kind of homemade product to reward him with.

There’s probably something very simple that’s wrong with my set up, but like diagnosing fuel starvation problems in diesel engines, just looking for leaking diesel is not the correct approach.

Upgrading my iPhone has helped a lot as it can actually receive 4G in my location - very useful! Much easier to spot it on the spectrograph:

I’m having a go at building some alternative duplex filters from scratch - they’re the most obvious candidate for troubleshooting at the moment - there seems to be quite a bit of noise generated and a lot of signal strength lost by using the current ones.

I agree to keep it simple is good. Is it possible to wait on making the duplexer? Can you (almost said “we”) create discrete tx and rx for testing? Am I wrong to think of the duplexer as a super cool way to have a single antenna to do rx and tx? That is what I gleaned from my cursory reading on the topic. But maybe I am misunderstanding what a duplexer means here, in this context (is there any other context???).

Have you been able to try a sdr dongle to test the signal strength?

You might also consider trying to engage the osmocom folks too. I have seen videos and such from them and they have some experience in cellular.

Sincerely, The sounding board

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@hTo137 … Actually, I’ve just finished a day of testing and I got one of my analogue RF amp circuits to add 15 dBm to my new iPhone! It might be that my old phone was faulty or the use of this antenna:

Or by just using one half of one of the duplexers as a simple band pass filter. Either way, I’ve got enough to go on now to finish the project using analogue components :slight_smile:

I used exactly the same duplex setup and antennae with the limeSDR in simple Rx to Tx mode this morning but unfortunately it just jammed my phone off 4G :frowning: . I then repeated yesterday’s test on analogue circuits and hey it worked again.
I’ve got a second cheapo SDR running on another computer monitoring Tx spectra and the signal from the limeSDR looked noticeably more noisy than the one coming from the analogue circuitry test :frowning: which is prob why my phone became jammed when using it. Tried recalibration a few times but no success. Maybe I’ve broken my LimeSDR?

Just to let you all know that I have not abandoned the idea of using a LimeSDR as a repeater! The project was set up to explore both digital and analogue solutions simultaneously and it’s just that the analogue ‘hardware’ solution is making more rapid progress ie it’s actually working! Ultimately I’d like to solder the main LimeSDR chip onto my own boards but that’s probably at least 12 months away, judging by my current progress. Note to self: Must practise my soldering skills.

Motivated by the chance of winning $130 million of Facebook sponsored Treasure I abandoned my potatoes and spent a few hours fiddling about with the calibration settings whilst running my 4G signal repeater program and …

  ...... I got it to work!

So now I can boost the signal from the base station to my phone. Not yet got it working in duplex mode but … Not worried!

Full details here:

And here:



what was the issue ? was it the Tx calibration getting trashed by the Rx calibration ?

@Kc7noa The Tx calibration had to be manually adjusted in Limesuite whilst monitoring the DC component of the transmitted signal on another computer running GQRX and a cheapo SDR. Upon zooming in on the DC signal I could see and hear the result of the manual changes and literally tune it by ear. Adjustments were made using the I and Q sliders in the DC component calibration panel.

Now to get the LimeSDR controlled by an Arduino Due.