This is an architectural question regarding gnuradio(-companion) and since I am not sure how to tackle this problem in the first place I first describe what I want to achieve and then how I think I would to it.
Problem
I implement a special form of an RFID reader with an Ettus X310 SDR: The transmitter sends an OOK/AM modulated (PIE encoded) request, followed by a pure Sine wave. The RFID tag backscatters its response onto this sine wave using OOK/AM modulation in FM0 or "Miller subcarrier" coding (a form of a differential Manchester coding). I want to receive its response, translate it into bits (and form a PDU), buffer different responses in a FIFO and send them for further processing. The properties of the tag response are:
- It is asynchronuous. I do not know when the response is coming and if it does, when the proper sampling times are: I cannot simply filter, sample, decimate the signal and use a simple slicer because I do not know what the sample points are.
- The response comes into very small "bursts" (say, 100 bits). Hence I cannot afford performing timing recovery on bits and waste them (except I buffer the entire signal somehow which I do not think is the way to do it).
- The signal starts with a small preamble (UHF RFID Gen2 preamble) which is 6 bits (~8 bit transitions). This may not be enough for for time recovery but can be used to identify the start of a response somehow.
- It uses mentioned FM0 encoding, so I have a guaranteed transition every bit. For that reason, I do not have to sample them but could detect the transitions and convert them into bits. I would not need conventional clock recovery (e.g. M&M) either.
My Thoughts
"Ordinary" gnuradio preprocessing brings me to the received oversampled bits: Downconversion, filtering; possibly a slicer which uses a lowpass filter to subtract the mean value and a comparator (note that even this may be challenging because the lowpass filter may have a large settling time of few bits until it obtains the right mean value).
In order to detect the actual transmission, I do not think I have much choice other than a simple squelch that detects a higher signal level than the noise floor (is this true or is there a way to detect the transmission using the preamble only?)
Once the squelch block detects a transmission, I could use a differentiator (or similar) to get the edges. But my understanding of the transition between this "baseband land" and "bits/PDUs" ends: I would need a block that triggers asynchronously (rather than samples at fixed intervals). In an actual system, the edges from the described detector could act as clock input of a flip flop. However, I do not see which standard gnuradio block would allow me to do this.
Once in "bits land", the bits (or PDUs) would be processed at a much lower rate. However, two clock domains are crossed: the normal baseband sampling rate, an irregular rate by which the transitions are detected and the rate at which the bits are read. For that reason, I would be looking for a FIFO or shift register, in which the detected bits are shifted in at whichever edge transition rate they come in and read out at the regular bit rate on the other side.
Question
What is the correct architecture/approach to implement this in gnuradio?
I could imagine to implement this with my own blocks. But as much as possible I would like to use standard block, gnuradio-companion. I would like to resort to own blocks (in particular C++) only as last resort if either not possible otherwise or if it would really not be the right way to so it otherwise.
