GNU Radio – CC1111 packets encoder/decoder blocks

Introduction

I recently worked with RF transmissions between CC1111-based devices (the chips that are supported by RFCat) and I was in the need to easily encode and decode my payloads using GNU Radio. GNU Radio already contains several packets encoder/decoder blocks, but none of them (if I’m right, and I hope that I am) deal with the same header length, whitening algorithm and/or checksum used by the CC1111 chips. In the CC1111 world, the header is one byte length, the checksum is CRC16 and the data whitening is performed using the following algorithm http://www.ti.com/lit/an/swra322/swra322.pdf.

Packet encoder

Here is a simple GRC flow-graph demonstrating how to encode and transmit CC1111 packets using your favorite SDR device:

As you can see, the Source block accepts a Source Queue argument defined like this:

Using this flow-graph, it is then easy to transmit new CC1111 packets from any other python scripts, just by tailing new payload into the “myqueue_in” message queue. Example:

#!/usr/bin/python

import sys
from gnuradio import gr
import grc_cc1111_hackrf_sender

rf = grc_cc1111_hackrf_sender.grc_cc1111_hackrf_sender()
rf.set_frequency(long(sys.argv[1]))
rf.start()

msg = gr.message_from_string("Hello from HackRF One")
rf.myqueue_in.insert_tail(msg)

sys.exit(0)

Packet Decoder

In the other sense, the following GRC flow-graph can be used to demodulate a 2FSK modulated signal (transmitted for example by RFCat), and to decode the CC1111 formated packets.

Here again, the decoder block accepts a message queue as argument (Target Message Queue = myqueue_out), that you can then access from your python script:

#!/usr/bin/python
import sys
from gnuradio import gr
import grc_cc1111_hackrf_receiver

freq = long(sys.argv[1])

rf = grc_cc1111_hackrf_receiver.grc_cc1111_hackrf_receiver()
rf.set_frequency_center(freq)
rf.start()

msg = rf.myqueue_out.delete_head().to_string()
print "[hackrf rcv] : '%s'" % msg

sys.exit(0)

Download and install

While these new blocks are still considered as beta, tests were performed between RFCat, RTL-SDR and  the HackRF One with good success rate. You will find multiple testing scripts in the repository, ready to handle RF transmissions between RFCat and GNU Radio (see “testing-scripts” folder).

Get the source from: https://github.com/funoverip/gr-cc1111

git clone https://github.com/funoverip/gr-cc1111.git
cd gr-cc1111/src/gr-cc1111/
mkdir build
cd build
cmake ../
make
sudo make install

Enjoy.
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