srsLTE

Fork of the srsLTE software slightly modified to extract the channel estimate (i.e. the Channel State Information (CSI)), as well as other features (RSSI, RSRQ, ...)

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How to install

Tried with Ubuntu 18.04 and CMake 3.5.1

1. Disable CPU frequency scaling:

for f in /sys/devices/system/cpu/cpu[0-9]*/cpufreq/scaling_governor ; do
	echo performance > $f
done

Not disabling CPU frequency scaling might lead to performance issues

2. Install USRP Hardward Driver (UHD):

Ubuntu 18.04 has access to outdated UHD drivers. Hence we need to use the official PPA by Ettus Research (the company making USRP).

sudo apt-get remove -y uhd
sudo apt-get remove libuhd-dev libuhd003 uhd-host -y
sudo apt-add-repository --remove "deb http://files.ettus.com/binaries/uhd/repo/uhd/ubuntu/trusty trusty main"
sudo add-apt-repository ppa:ettusresearch/uhd -y
sudo apt-get update
sudo apt-get -y --allow-unauthenticated install python python-tk libboost-all-dev libusb-1.0-0-dev
sudo apt-get -y --allow-unauthenticated install libuhd-dev libuhd003 uhd-host

3. Install the required packages for this fork of srsLTE:

sudo apt-get install cmake libfftw3-dev libmbedtls-dev libboost-program-options-dev libconfig++-dev libsctp-dev

4. Download, build and install this fork of srsLTE:

git clone https://github.com/arthurgassner/srsLTE.git
cd srsLTE-modified
mkdir build
cd build
cmake ../
make
make test
sudo make install
sudo ldconfig

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How to setup

1. Edit the configuration file governing the connection to eNodeBs (i.e. srsue/ue.conf) so that you can connect to the eNodeB you wish to connect to

Typically, you just have to change the field dl_earfcn

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How to run

1. Connect the Software-Defined Radio (in our case the USRP B200mini) by USB 3.0 to the computer running the srsLTE-modified software

Using USB 2.0 might cause speed issues, so don't do that

2. Connect the PC/SC reader (in our case the HID OMNIKEY 3121) by USB to the computer running the srsLTE-modified software

• To install a smartcard on Linux, simply run sudo apt install pcscd (follow this if you run into issues)
• To make sure that the PC/SC reader is recognized, run pcsc_scan. To install it, run sudo apt install pcsc-tools

3. Plug in the SIM card (with PIN deactivated) in the PC/SC reader

It has to be a SIM card allowing you to connect to the eNodeB that you want to connect to. If you're unsure of whether you can connect to the expected eNodeB, first try with a phone.

4. Go to srsue/ and run

   sudo srsue ue.conf
   

The first time running the above command, you might be prompted to run sudo /usr/lib/uhd/utils/uhd_images_downloader.py to download packages required to connect to the USRP Software-Defined Radio.

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Explanation

• The ce array (located in srsLTE/lib/src/phy/ch_estimation/chest_dl.c) should contain the channel estimate (i.e. the CSI).

  • The function srslte_chest_dl_estimate_cfg(...) is called by the function estimate_pdcch_pcfich(...) (located in lib/src/phy/ue/ue_dl.c). According to Innovations, Telesystem. ”LTE in a Nutshell.” White paper (2010). pp.11-12, channel estimation is performed in the PDCCH (Physical Downlink Control Channel), which supports the idea that ce contains the channel estimate.

  • The ce array is a 3D array of complex floats of dimension (##ports) x (##Rx antennas) x (#subcarriers).

  • It seems that ce contains the channel estimates for all subcarriers, not just the ones carrying Cell-Specific Reference Signals (CRS). The channel estimate of non-CRS-carrying subcarriers is estimated through interpolation of the channel estimates of CRS-carrying subcarriers. This is supported by the comment at the head of the chest_dl.c file:

  Estimates the channel in the resource elements transmitting references and interpolates for the rest of the resource grid

  • The ce array can be saved to a file by modifying the estimate_pdcch_pcfich(...) function. This function is called every ms. Below is a plot of its amplitude over each subcarrier. The shape and overall aspect is in concordance with what can be seen in CSI-related papers for WiFi.

  

Other features (RSSI, RSRQ, SNR, ...)

• We can exploit the function estimate_pdcch_pcfich(...) (located in lib/src/phy/ue/ue_dl.c). It calls srslte_chest_dl_estimate_cfg(...) and populate a srslte_chest_dl_res_t struct. This struct contains the RSSI, RSRQ, RSRP, SNR, CFO for the currently connected eNodeB. We can save the content of that struct from there. Below is a picture of those measurements through time.

  

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Common issues

• Running make when building srsLTE-modified might give you the following error:

[ 52%] Linking CXX executable pss_usrp
//usr/lib/x86_64-linux-gnu/libSM.so.6: undefined reference to `uuid_unparse_lower@UUID_1.0'
//usr/lib/x86_64-linux-gnu/libSM.so.6: undefined reference to `uuid_generate@UUID_1.0'
collect2: error: ld returned 1 exit status
lib/src/phy/sync/test/CMakeFiles/pss_usrp.dir/build.make:102: recipe for target 'lib/src/phy/sync/test/pss_usrp' failed
make[2]: *** [lib/src/phy/sync/test/pss_usrp] Error 1
CMakeFiles/Makefile2:3830: recipe for target 'lib/src/phy/sync/test/CMakeFiles/pss_usrp.dir/all' failed
make[1]: *** [lib/src/phy/sync/test/CMakeFiles/pss_usrp.dir/all] Error 2
Makefile:160: recipe for target 'all' failed
make: *** [all] Error 2

To fix it, follow these instructions.