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Cell Phone Signal Repeater / Booster / Femtocell

An outside pole mounted aerial picks up RF signals which are then filtered, amplified and re-transmitted through a second inside aerial.

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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.
Searching the Interweb, I found very little useful information on how to do this project and so have had to delve into the dark world of RF (Radio Frequency) and try and understand how exactly our cell phones work. Apart from the theory, there are also many practical obstacles such as the extremely small size of components.

Here is a basic diagram of how the device 'should' work:The band pass filters (BPF) only allow the desired frequency ranges to go into the amps and so makes them much more efficient, otherwise they would be trying to amplify all the local radio stations etc. The amps may or may not need to be cascaded and there may or may not need to be other building blocks in the set up.

Fortunately, at the moment at least, there is no complicated coding as the main variable gain amp can be controlled by using 6 digital pins on an Arduino in conjunction with a simple truth table. This will be explained a bit later on when I have actually got it to work (hopefully).

There are 2 'received signal strength indicator' blocks (RSSIs) which send analogue signals to the Arduino telling it how good or bad the signals on the two aerials are. If the external signal is good for some reason, for example if it's just started to rain, then the amp is turned down so as not to send very strong signals to the cell base station or to the phone. Simply turning up the gain to full can have serious side effects such as locking up the phone or, yes, locking up the cell base station, in which case you're in serious trouble!

There is a certain amount of risk in the project in that for one, it may just not work at all or two, that it may be too simplistic. Personally, I prefer option two and I'm fully prepared to go down a completely different route if necessary. If the project fails then as long as I know why it failed then I have learnt something!

In parallel with this 'build my own hardware' option, I am working on using a pre-built system, namely the LimeSDR transceiver. The main problem with this option is that, coming from a place of almost no previous experience of RF, this gadget currently seems incredibly complicated and it could well take several months just to learn how to use it. Undaunted by the challenge, I am inspired by the guys at Lime Microsystems in Guildford, UK ( http://www.limemicro.com/ ) who have been incredibly helpful. The key to success here seems to be to create software models in a system called 'Pothos' which is an excellent learning tool as it explains a lot of stuff really well and outputs a really useful log of what parameters are being changed in real time in the device. Limemicro has also helped me link an Arduino to their LimeSDR via SPI which enables, in theory, an Arduino Due to control the transceiver or at the very least, upload a predetermined set of register values.

Current chances of project success = 75%

Band_20_Repeater_02.ini

Finally I have produced a LimeSuite .ini for the LimeSDR that actually 'seems' to work! Probably needs some fine tuning.

ini - 16.57 kB - 04/18/2017 at 13:08

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Simple_806MHz_Bandwidth_30_Repeat_03.ini

Here I attempted to re-transmit received frequencies at centre frequency 806MHz with a bandwidth of 30MHz. (Band 20 base Tx LTE). Although I got a nice Rx spectrum in the LimeSuite FFT, my second SDR rig failed to notice successful transmission :(

ini - 16.57 kB - 04/18/2017 at 09:32

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Simple 806MHz repeat 01.pothos

A simple repeater sketch was set up in Pothos and 'Scooped' up in Limesuite and a second SDR was used to prove that it worked.

pothos - 101.45 kB - 04/16/2017 at 18:07

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Simple_806MHz_Repeat 01.ini

A simple repeater sketch was set up in Pothos and 'Scooped' up in Limesuite and a second SDR was used to prove that it worked.

ini - 16.57 kB - 04/16/2017 at 18:04

Download

pothos - 182.29 kB - 04/16/2017 at 16:29

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FM retransmit 04.ini

https://youtu.be/Xh1dsejrtdY

ini - 16.57 kB - 04/16/2017 at 16:24

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duplexer-04.gif

Another duplexer test, this time with a stronger signal. This scenario showed a better signal to noise ratio with nice clean lines.

Graphics Interchange Format - 7.49 MB - 04/14/2017 at 18:04

Preview Download

USD020A Datasheet.pdf

Band 20 LTE duplexer

Adobe Portable Document Format - 458.22 kB - 04/14/2017 at 15:18

Preview Download

Test MIMO 02.pothos

Testing the MIMO capabilities of the LimeSDR - Channels 1 and 2 simultaneous receive. Revised sample rates and clock speed. Adjusting frequencies on channel B continues to only affect channel A .... FAIL!

pothos - 174.97 kB - 04/10/2017 at 08:51

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Test MIMO 01.pothos

Testing the MIMO capabilities of the LimeSDR - Channels 1 and 2 simultaneous receive.

pothos - 137.46 kB - 04/09/2017 at 15:27

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View all 14 files

  • At last a promising .ini file for the LimeSDR!

    TegwynTwmffat04/18/2017 at 12:41 0 comments

    Thanks to Zack (again!) at Lime Microsystems, I seem to have managed to produce a functioning repeater in full duplex mode. It does not actually help with 4g on my own phone yet as it probably needs some fine tuning.

    Here's the file:

    https://cdn.hackaday.io/files/20500877072000/Band_20_Repeater_01.ini

  • LimeSDR test: Limesuite was used to 'scoop up' settings generated by Pothos

    TegwynTwmffat04/16/2017 at 16:27 0 comments

    I used Pothos to create a block diagram for the LimeSDR that receives a local FM station and re-transmits it 1Mhz higher at very low power.
    Limesuite was then used to 'scoop up' the settings generated by Pothos with the idea of closing Pothos and recreating the whole test using Limesuite alone.
    The whole test worked nicely!

    I then tried a similar thing in the LTE band 20 at 809 MHz, just repeating the whole base station Tx spectrum, using my trusted USD020 duplexers ..... And that worked too! I've been trying to do this for about 5 days, so why does it suddenly work now? I think the answer is that the Limesuite needs to be a 'blank canvas' before trying to scoop from Pothos, otherwise settings 'left over' from previous trials mess things up.

    Files used:

    https://cdn.hackaday.io/files/20500877072000/Simple_806MHz_Repeat 01.ini

    https://cdn.hackaday.io/files/20500877072000/Simple 806MHz repeat 01.pothos

    https://cdn.hackaday.io/files/20500877072000/FM retransmit 04.ini

    https://cdn.hackaday.io/files/20500877072000/FM Re-Transmit 04.pothos

  • Noise and Insertion Loss using Band 20 Duplexer

    TegwynTwmffat04/14/2017 at 15:11 0 comments

    The animation below shows really clearly the noise created and the loss of signal incurred when the CTS USD020 duplexer is used. For comparison, when the duplexer, which is basically a pair of band filters, is used I ramped up the LNA (low noise amplifier) by 4 dB. This is not necessarily bad as the advantages of having a reasonably good band pass filter are enormous.

  • Testing MIMO capabilies of LimeSDR

    TegwynTwmffat04/09/2017 at 15:31 0 comments

    Not sure what's going on here. Adjusting one channel seems to affect the other channel and I wouldn't expect that to happen! If anybody can check this out, the file is in the files section: Testing MIMO 01.pothos.

  • Successful Transmit on FM using LimeSDR and Pothos Software

    TegwynTwmffat04/05/2017 at 17:03 0 comments

    At last I seem to be getting somewhere with the LimeSDR transceiver. Yesterday I started using Pothos to create an FM radio and today I was able to re-transmit one of my local radio stations to another frequency just before my local telecoms regulator discovered what I was doing.

    This is an important step towards producing a proper 4G signal repeater - there's no point in diving in right at the deep end so a simple transmit is a good step forwards.

    The Pothos file for this project is here on this Hackaday page under the 'Files' section.

    Please could someone tell me why I was getting double transmission either side of the selected frequency? This can be seen on the video below:

  • Learning the LimeSDR Tranciever.

    TegwynTwmffat04/04/2017 at 09:32 0 comments

    Whilst waiting for new antenna cables to arrive, I'll be programming this LimeSDR transceiver to pick up an FM radio station as a learning exercise. I did try using this device as a Femtocell but failed due to the incredible complexity of this gadget and my incredible lack of expertise! There are plans afoot to connect the SDR to an Arduino and carry on learning how to use it properly.

  • Some small success

    TegwynTwmffat04/04/2017 at 09:15 0 comments

    At last, I managed to get the 800 MHz frequencies boosted using some cascaded basic power amps and one side of one of the duplex filter:

    Tried to replicate it today but one of the antenna cables failed :(

  • Building and testing microwave duplex band pass filters

    TegwynTwmffat03/25/2017 at 14:45 0 comments

    I decided to try and be a bit more logical in my approach to this project and start building the gadget from the beginning - the band pass filters. Why do we need them? They're to filter out all those FM radio stations etc and provide (hopefully) a nice clean break between the incoming signals and the outgoing ones, which operate on well defined frequency bands eg band 20 base stations transmit on frequencies between 791 and 821 MHz.Our cell phones, if they're actually using band 20, transmit on 832 to 862 MHz and we don't really want each of our amps trying to process both sets of frequencies at the same time .... And certainly not FM radio stations!

    Ok, less of the theory and more of the mechanics. Looking at the actual filter units, the USD020 s from CTS, it looks like they've been attacked by some kind of cyber-kinetic squirrel:

    And looking more closely we can see that they've actually been deliberately gouged out with a precision machine:I contacted CTS, worried that somebody had sabotaged my filters, but they reassured me that this was in fact fine tuning at the manufacturing plant - FANTASTIC!

    So, check, re-check, quadruple check the soldering guide for these beasts and place them on their mini PCBs and into the toaster oven for breakfast:

    The filters are heavy blocks of metals so my thermometer probe is going to lag behind the actual readings by about 30 seconds or so by my estimates. In any case, the maximum temperature was not exceeded and it all working out fine - thank God, as these components are not cheap!

    Now for testing on the LimeSDR:

    I paused the FTT waterfall halfway through and changed the filter connections from Rx to Tx and we can see nice clean separation between the two sets of frequencies - wonderful - those cyber-kinetic squirrels did a really good job! Now to solder up the rest of them and test them as well.

  • Building blocks at the initial stages of the project

    TegwynTwmffat03/21/2017 at 14:03 0 comments

    Here are some boards for putting together a basic 4G femtocell.

    It would be very tempting to try and design and build a PCB with all the desired features from the very beginning and, in an apparent contradiction, there is actually a final design in progress as well. So what I have done is to split the project up into building blocks, mostly on separate PCBs or using PCBs with different blocks on different sides of the board. Confusing? - Yes, I am very possibly confused about what I am doing as it seems that I am trying to do the initial and the final stages simultaneously! There is however some logic to this, as l will try and explain.

    Here below are the building blocks for stage 1 (There are already some more consolidated building block PCBs for stage 2, but I'm trying to ignore them at the moment ):

    The top board has a couple of RF amp circuits and a power supply. The left hand RF circuit has a basic amp with no gain control which can be used in series with the other amp or on it's own. The big difference is that it's easy to solder. The middle circuit is another amp but it has digital variable gain control. I had the chip soldered in by a specialist company so that at the very least I had one possible error eradicated. I will try and solder subsequent boards myself and will use this board as a testing reference to compare and check my own soldering with a multimeter or oscilloscope. The two amp circuits were taken directly from the datasheets for the main component so all the passives (capacitors, inductors and resisters) were given by those sheets.

    The middle board has an enormous band pass filter (BPF) pad on it which is factory tuned for high accuracy. It also has a couple of tiny 'cell phone' BPFs professionally soldered on, just to test the difference between the two options, one being big and expensive and the second being ridiculously small and 1/4 of the price.

    The bottom board, which is actually the reverse of the middle board, has pads for home made filter circuits with one tunable inductor. This circuit is here just to prove to myself that we can't design and build our own BPFs because of the poor tolerances of the available components (mostly +-5%). It might even work!

    The next log will look at another board that is already sitting on my table ready to be soldered up and that has some of the above features combined. Maybe stage 2 will work straight off, but if it doesn't, then I've got stage one as a good fallback. I give stage 1 about 70% chance of success and stage 2 about 50% chance.

    One of the problems with working with PCBs like this is that the turn around time for manufacture is 10 days, including postage from China. For this reason it's good to try and be 10 days ahead of myself. Another problem is that some of the components are very hard to get hold of. The large BPFs are made in China for a USA company who only sell through one UK company so the supply chain takes about 3 weeks to navigate.

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TegwynTwmffat wrote 04/14/2017 at 15:30 point

If you take the phone outside it will try to receive from the base station and will also try and receive from the antenna inside the building. If the system is working properly it should be able to do this simultaneously, I think.

Receiving from base station and from the inside antenna is all done on the same frequency.

Transmitting from the phone and from the inside antenna is done on another close-by frequency and a good quality duplexer will provide a very sharp separation between the two frequencies mentioned above (Rx and Tx).

This answer above is slightly simplified because the 2 frequencies mentioned are actually located in a vast bundle of frequencies that constitute the LTE band itself.

  Are you sure? yes | no

hTo137 wrote 04/12/2017 at 02:45 point

I had to research what a duplexer really is.... The essence is that it allows Rx/Tx with one antenna. Probably obvious to most but it's something I didn't know.

Since the inside-the-dwelling band is different than the outside-the-dwelling band what happens if you take the phone outside? Will it talk to the cell tower or try to talk to the outside antenna? Or try to talk to the inside antenna?

  Are you sure? yes | no

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