Demodulating GFSK - gnuradio

Question

I'm trying to demodulate a GFSK signal coming from an nRF24L01+ transceiver chip (hooked up to my Arduino). I've followed this guide so far:
https://www.bitcraze.io/2015/06/sniffing-crazyflies-radio-with-hackrf-blue/#comment-38046
..and managed to manually demodulate a package (the address and the message I sent 'martijn' are clearly recoverable):
https://drive.google.com/open?id=0B9CJ42CGPiF2TWoyelRmWldZcU0
However, now I want to receive packets and decode them as they come in. Someone already made a decoder for this job, but somehow it fails to find my nRF24 packets:
https://wiki.bitcraze.io/misc:hacks:hackrf
My Arduino code for sending the packets is as followed:
#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>
#include <RF24_config.h>
RF24 radio(9,10);
const uint64_t pipe = 0xe7e7e7e7e7;
char package[] = "martijn";
void setup() {
Serial.begin(9600);
radio.begin();
radio.setDataRate(RF24_1MBPS);
radio.setChannel(95);
radio.openWritingPipe(pipe);
radio.enableDynamicPayloads();
radio.setAutoAck(true);
radio.powerUp();
}
void loop() {
radio.write(&package, strlen(package));
delay(1);
}
Basically I just want to use GNU Radio Companion to obtain the nRF24 packets, and send their binary data into a file. I'm fine with writing my own decoder. However, I have no clue on how to get this binary data from the incoming signals.
(The comments at the bitcraze site are also mine)
I've be very happy if someone could help me (or even point me in the right direction). Thanks in advance!

Answer 1

After the Quadrature Demod you have to use a Clock recovery block. The M&M Clock Recovery of GNU Radio should do the job. This block will dramatically increase the performance of the decoding.
However you have to take care some parameters that this block requires. The most important is the 'omega'. 'Omega' roughly speaking corresponds to the number of samples per symbol. For example, if your GFSK baudrate is 9600 and your incoming signal from the hardware is 96000, each symbol corresponds to 10 samples. The omega can be any float number. Note however, that clock recovery does not work for large omega values. So try to keep the omega up to 8.0. To do that, either adjust properly the hardware sampling rate or do some resampling.
After the Clock Recover just use a 'Binary Slicer' block. This will convert the floats to bits of 0's and 1's. Using the Pack K bits block you can convert the bit stream into byte stream, that can easily saved to file with a 'File Sink'.
Here is a good step-by-step tutorial for an FSK receiver. GFSK adds only a Gaussian filter so the procedure is quite the same for both of them.