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Cell Phone 4G LTE 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.

**** Please check out the 'Project Logs' for up to date progress ****

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 took me 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 was 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 = 100% - Both parts of the project are working!

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  • Cell Phone Behaviour - Town v. Country

    TegwynTwmffat07/10/2017 at 15:49 0 comments

    Today I decided to go on a religious pilgrimage to my local cell phone base station.

    After making the necessary offerings and sacrificing of a few chickens I turned on my RSSI test rig and began uploading live video to YouTube:Basically, I made about 1,000 measurements from an RSSI (Received Signal Strength Indicator), one every second, that I bought from ebay, based on the Analogue Devices AD8318. Datasheet HERE.

    The hypothesis was that my cell phone would transmit at lower power when close to the base station.

    With the antenna at the same 200mm from the phone, I drove out into the countryside and found a spot with really bad 4G coverage -120 Dbm compared to -71 Dbm in the town. I then repeated the RSSI test, storing all the data on a micro USB in the Arduino rig. And here's the result:There's a really obvious difference in behaviour between town and country. In the country my phone is transmitting at markedly higher power. I'm not sure if the graph is logarithmic or not - I'd have to look that up on the RSSI chip's datasheet.

    Looking at the green countryside graph, there's two obvious 'plateaux' right at the bottom where the phone is doing nothing. We don't get that on the orange town graph because there's loads of other phones nearby transmitting on the same 4G band.

    Why is this important? ........ Because it shows that either the base station can control the signal strength given out by each individual phone in it's catchment range by using a control signal ..... Or that the phone itself responds to low received signal strength by increasing it's transmit strength, guessing that it is a long way from the base station. In either case, the base station is probably going to get really annoyed if we boost our cell phone signal too much - no amount of sacrificed chickens is going to appease it's wrath! It's therefore really important to have individual gain controls on both the Rx and Tx in the cell phone booster.

  • Control LimeSDR Tx Gains using Arduino Due and Analogue Slider

    TegwynTwmffat07/03/2017 at 17:08 0 comments

    Please remember that this project has split into two - I'm exploring 2 possible solutions to the same problem, one using discrete components ie ICs and passives and the other a fully blown transceiver with built in FPGA - the LimeSDR.

    So here I'm looking at option 2 - Using the LimeSDR.

    The advantage of the Lime is that all the discrete components are located in one chip, the LimeLMS7002M, with an amazing array of registers that can be controlled using the SPI bus. As an example, register address 0x0100 can be set to a value of 0x7A94 to give a gains of 15 dB on the Tx pad. There's also a massive FPGA chip that has enough spare silicon inside to house a fairly powerful onboard Arduino (Work in progress) and even some basic signal processing.

    Here, the limeSDR is programmed with a special FPGA configuration file: FILE and, when 'normal' operation is required, is programmed back with this, somewhat out of date, file: FILE. The Arduino Due is programmed with this file: FILE.

    The main obstacle to success with this small step forward was getting the Arduino to replicate what I had previously achieved with the Limesuite software about a month ago. In theory, I should just be able to harvest all the register settings from Limesuite and copy and paste them into the Arduino program but ...... for a long while ...... No success. Eventually I narrowed it down to the fact that there is one particularly clever register that switches all subsequent register read/writes from one channel to the other ..... Confusing? Actually, the Lime LMS7002M datasheet explains it quite clearly and since I had already read the datasheet 5 times, it was no great surprise!

    LMS7002M_WR(0x0020, 0xFFFD);  //This register sets the following registers back to Read and Write Channel A only:

    Other than that there is a register to describe the DC correction, which is pretty essential:

    LMS7002M_WR(0x0204, 0xF020); // DC Correction.

    This code reads the slider value and spits out hex integers that the LMS7002M will recognise:

       int sliderValue = analogRead(A0);
       sliderValue = sliderValue/0x21;
       int gainValue = 0x7800 + sliderValue * 0x42;

    And here is the nice simple SPI write:

    // Write register value to LMS7002M via SPI
    void LMS7002M_WR(int addr, int val)
    {
      digitalWrite(LMS7_SS_Pin, LOW);
      SPI.transfer( ((addr >> 8) | 0x80) );
      SPI.transfer( (addr & 0xFF) );
      SPI.transfer( ((val >> 8) & 0xFF) );
      SPI.transfer( (val & 0xFF) );
      digitalWrite(LMS7_SS_Pin, HIGH);
    }

    Oh, there's also a spreadsheet HERE for converting the Limesuite register list to an Arduino 'register function list'. It does character analysis/manipulation to change everything in column 'A' to column 'F':Having a gain control slider on a 4G signal booster is pretty essential as the base station signal can vary in strength quite a bit depending on local weather conditions and, of course, we don't want too much gain or else we could get feedback between the local Rx and Tx antennae. Looking at some of the 'Black Box. signal boosters in the market place, I can't understand why they don't have a simple manual gain control so that systems can be tweaked to individual antennae setups. Actually, I think that some of them do 'appear' to have gain control, but actually it is fake!

    Anyway, If the isolation between antennae can be made really high by, for example, separating them by 100m, then the gain could be turned up and greater coverage could be obtained. But ..... 100m ..... That's a lot of cable and probably a substantial antenna installation.

    Especially for Ebrahim Bushehri, here's video of the Arduino Due controlling the LimeSDR Tx gain:

  • Analogue Repeater now assembled

    TegwynTwmffat06/16/2017 at 12:24 0 comments

    We now have a functional repeater with manual variable gain control and screen for displaying stuff. I think a nifty graph dislaying RSSI against time would be cool?

  • Testing Qorvo TQM879028 Variable gain amplifier

    TegwynTwmffat06/12/2017 at 19:46 0 comments

    After trying to use the Analog devices HMC742A VGA for a third time with no success, I went back to the search filters and specifically selected Qorvo as my preferred manufacturer, mostly because their technical support is truly fantastic.

    I'd seen their variable gain development board before, but was put off by the fact that it was tuned to 2150 MHz which is quite a bit above the 806 MHz that I required. I'm now pretty sure I can replace the necessary passives to correct it. Whether it was the fact that their dev board was very reasonably priced or just plain desperation, I do not know, but I'm very glad I bought it as I got it working fairly quickly. It even comes with a USB adapter daughter board and windows evaluation software - Total bargain!

    The really great thing about this gadget is that it has some real power - a massive 0.5 watts! It needed to be cascaded with my QPL9065 low noise amp, but that was ok. The results were very nice indeed. Now to get it hooked up to an Arduino via SPI and insert a RSSI unit to monitor the signal strength.

  • LimeSDR now controlled by Arduino via SPI

    TegwynTwmffat06/10/2017 at 17:15 0 comments

    At last I've managed to connect the LimeSDR to an Arduino Due. I was delayed by trying to mess about with JTAG adapters and only solved the problem by making a custom PCB to breakout the 0.05 inch pitch JTAG connector:

    The SPI connection has been tested and is working nicely. Now for some brain frazzling hex to binary register manipulation to load my repeater settings via the Arduino and then control it by the Arduino. Standalone repeater coming soooooooon!

  • RSSI test

    TegwynTwmffat06/10/2017 at 14:13 0 comments

    The RSSI was inserted after the LNA circuit and the antenna was moved in and out of a Faraday cage. The voltage recorded varied between 1.6 and 1.3 volts. I would have expected it to be about 2.1 volts inside the cage after looking at the datasheet for the RSSI.

  • Long term test of analogue circuits

    TegwynTwmffat06/08/2017 at 17:16 0 comments

      Since I actually need a 4G signal repeater in my office, I decided to run a more long term test of one of my analogue boards - a Low noise amplifier, the QPL9065.

      With my plate antenna positioned 1/2m away from my phone I was able to get a consistent 20 dBm increase in signal strength and a reliable 4G data connection. I felt so confident that it was going to work that I used it to run/assist an important conference call with some of the guys from BT, Facebook and Lime Microsystems. There's nothing quite like working under pressure!

      In the background of the photo above can be seen the LimeSDR with it's incredibly powerful LMS7002M chip. This chip contains all the necessary blocks that I need to complete this project but there are a few barriers that prevent me from using it namely:

      1. There's no breakout board for this chip alone unless I backstep to it's predecessor, the LMS6000.
      2. It requires a 6 layer PCB.
      3. KiCAD PCB design files are not available yet.
      4. Many functions are routed through the FPGA which prevents me from using them directly.
      5. Learning to use the FPGA is another steep learning curve.

  • LimeSDR now working as 4G signal booster

    TegwynTwmffat05/26/2017 at 15:17 0 comments

    Whilst waiting for my analogue boards to arrive I thought I should spend a bit of time fiddling about with the calibration settings whilst running my Limesuite program in an attempt to get a clean Rx to Tx signal ....... And, surprise, surprise .... It started to work!

  • Testing Limesuite calibration widgets using FM re-transmit

    TegwynTwmffat05/24/2017 at 18:48 0 comments

      Whilst waiting for my analogue VGA PCBs to arrive from Shenzeng I thought I'd have a go at the next logical step in trying to get the LimeSDR to work as a Femtocell.

      There's a lot of talk on the Lime Microsystems Myriad forum about calibration which got me thinking about why this gadget won't boost the band 20 4G signal when my DIY analogue circuits are now working and whilst the following video is not the most entertaining to watch, it does show that calibration settings can be adjusted manually.

      The workflow is as follows:

      1. Load up blocks into Pothos to build a simple FM receiver,
      2. tune it to my local radio station,
      3. load in blocks to transmit it to another frequency 1MHz higher,
      4. run the program in Pothos,
      5. stop the program,
      6. connect LimeSDR in Limesuite,
      7. change a couple of settings in the Limesuite tab to enable transmit once more,
      8. check there's a response on my second computer,
      9. fiddle about with the various calibration widgets in Limesuite, listening for audio improvement / degradation.

      Best watch the video really!

  • Must get the VGA chip working!

    TegwynTwmffat05/18/2017 at 08:46 0 comments

    Feeling motivated and inspired by the recent LNA and PA successes, I now must get the VGA working. The previous two proto boards were a bit of a mess so, since I needed some stepper motor controller boards made for another project, I thought that a nice tidy board would help me figure out how to use it.

    The device in question is the HMC742A VGA chip and after spending a bit more time studying the datasheet I feel 90% confident I can get it to work. Soldering the chip is a problem for me as the pitch is a tiny 0.5 mm and there's a massive GND paddle right in the middle of it. Fear not ..... I do have my

    https://hackaday.io/project/20260-manual-smt-pick-and-place-machine

    which is fully operational.

    Here's some of the board design pics - not the prettiest looking boards in the world:


View all 24 project logs

  • 1
    Step 1

    To build a basic band 20 cell phone repeater you will need:

    • LimeSDR development board x 1 (to do the work)
    • RTL SDR x 1 (for monitoring)
    • Duplexers CTS USD 020 x 1
    • Duplexer PCBs x 1
    • Plate antenna x 1
    • Stick antenna x 1
    • PCB ufl plugs x 6
    • Ufl cables x 100mm x 3
    • Computer x 2
    1. Solder the duplexer onto the PCB following the manufacture's recommended reflow heat v time profile.
    2. Solder on the ufl plugs.
    3. Connect duplexer to LimeSDR as per diagram below:
    4. Connect plate antenna to Tx1_1 on LimeSDR
    5. Using Limesuite, load this configuration file: 'Simple_806MHz_Repeat 03.ini' (see the files section) into the LimeSDR.
    6. Connect the RTL SDR to another computer and , using GQRX software, zoom right onto 806.00 MHz and you should see a small spike there. This is the DC component.
    7. In limesuite, open the TRF tab and adjust the Tx PAD part linearising gain and Tx PAD gain downwards to increase the gain. You should see a response on GQRX.
    8. In the limesuite 'calibrations' tab, adjust the DC corrections I and Q sliders until the DC spike is at a minimum.
    9. Check that the 4G band 20 signal is being amplified by using a cell phone that has previously been 'sniffed' ie you know it operates on 4G band 20. Carefully move the phone closer to the plate antenna until the signal starts to increase, monitoring the response on GQRX. If your phone operates on a different band, use the appropriate duplexer and retune the LimeSDR to the centre frequency of the base transmit frequency.
  • 2
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tooth_pick wrote 05/09/2017 at 04:46 point

Under some circumstances a simpler and lower tech method may work. Take a small satellite dish and point it at the cell tower, placing the cell phone right where the signal will be strongest. This will allow the cell phone to both receive a much stronger signal and to broadcast back a stronger signal. Then connect a bluetooth hands free headset; which will give you the relative freedom to stand and sit somewhere comfortable.  This solution obvious works best in a stationary environment and where line of sight is possible. Similar methods have been used with wifi to send signals over 100 miles.

  Are you sure? yes | no

TegwynTwmffat wrote 05/09/2017 at 08:36 point

Yes simple is often best. Currently, as I write this reply, my phone is 20 feet up a pole hoisted up in a waterproof box like a flag. The USB cable connects to my computer and I have perfect 4G communication. Making / receiving phone calls is a PITA!

  Are you sure? yes | no

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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