/srsLTE

Modified version of the open source SDR LTE software suite from Software Radio Systems (SRS), allowing it to save the certain channel characteristics (CSI, RSSI, RSRP, ...).

Primary LanguageC++GNU Affero General Public License v3.0AGPL-3.0

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, ...)

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

  1. 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Ubuntu 18.04 has access to outdated UHD drivers.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
  1. 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
  1. 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

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

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

  1. 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
  1. 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.

  1. 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.

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.

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.