GNURadio OFDM tutorial

Description

This tutorial illustrates the use of GNURadio OFDM blocks for data transfer between a pair of USRPs. The GNURadio flowgraphs used here were obtained from the workshop material provided for ​POWDER-RENEW Mobile and Wireless Week 2019. They are mainly based on the GRC examples available in the ​GNURadio repository. For a good understanding of the flowgraphs and the blocks, please refer to OFDM pages on GNURadio Wiki such as ​OFDM Basics and ​OFDM Synchronization

Prerequisites

In order to access the test bed, create a reservation and have it approved by the reservation service. Access to the resources is granted after the reservation is confirmed. Please follow the process shown on the COSMOS getting started page to get started.

Resources required

A pair of USRPs in COSMOS or ORBIT testbed.

• 2 USRP X310s (connected to node1-1,node1-2) in sb2.orbit-lab.org or
• 2 USRP 2974s (sdr2-s1-lg1,sdr1-md1) in sb1.cosmos-lab.org

Tutorial Setup

Follow the steps below to gain access to the sandbox 1 console and set up nodes with appropriate images.

1. If you don't have one already, sign up for a ​COSMOS account
2. Create a resource reservation on sandbox 1
3. Login into sandbox 1 console (console.sb1.cosmos-lab.org) with an SSH session. SSH session for COSMOS SB1 with Moba Xterm can be setup as shown, with remote host = console.sb1.cosmos-lab.org, and username = your COSMOS username. X11 forwarding is enabled to access GUI.

1. Make sure all the nodes and devices used in the experiment are turned off:

   omf tell -a offh -t system:topo:all  
   

2. Load gnuradio_ofdm.ndz on both the nodes

   omf load -i gnuradio_ofdm.ndz -t node1-1,node1-2
   

3. Turn the nodes on and check the status

   omf tell -a on -t node1-1,node1-2
   

   omf stat -t all
   

4. Open 2 terminal sessions and ssh into the nodes with -Y for X11 forwarding

   ssh root@node1-1 -Y
   ssh root@node1-2 -Y
   

Experiment Execution

Find the USRPs

1. As mentioned in the ​sb2.orbit page, the X310s have their port 1 directly connected to their respective nodes, and the IP addresses hardcoded to 192.168.40.2 as given in the ​X310 manual. To access the X310s Run eth_config_sb2_orbit.sh on both the nodes. It sets a static IP address 192.168.40.1 on eno2 interface.

2. Run uhd_find_devices and uhd_usrp_probe to detect the X310s and to make sure they have a compatible firmware installed.
3. In case of firmware mismatch, follow the directions displayed in the error message and use uhd_image_loader to update the firmware.

Run the GRC application

1. Start the OFDM receiver on node1-2

   gnuradio-companion  
   

root@sdr2-s1-lg1:~# gnuradio-companion  /root/gr-ettus/examples/rfnoc/rfnoc_fosphor.grc
<<< Welcome to GNU Radio Companion 3.7.14.0 >>>

Block paths:
        /usr/local/share/gnuradio/grc/blocks

Loading: "/root/gr-ettus/examples/rfnoc/rfnoc_fosphor.grc"
>>> Done

With X11 forwarding enabled, GRC should show up as below.

2. Run the OFDM transmitter on node1-1

3. Make sure the device arguments on both nodes are correctly set as shown below. Check the center frequencies and sample rates, and make sure they match. TX_OFDM.grc has a "File Source" block which reads the text file to be transmitted. RX_OFDM.grc stores the received data in a local file that is specified in the "File Sink" block.

4. Run the receive flowgraph followed by the transmit flowgraph.