Hi everybody, I am working on a satellite receiver system for AX.25 packets modulated in BPSK RRC filtered(NRZI encoding and G3RUH scrambling). There is no error correction, and data rates are in the range of 1200 to 9600 bits per second.
I am using the FLL Band-Edge and Costas Loop blocks for carrier phase and frequency recovery, and the Symbol Sync block for timing/clock recovery.
How should I set the parameters of these blocks? I am particularly focusing on the Loop Bandwidth values that is central to all of these blocks, I know that increasing the bandwidth will result in faster locking times and a wider offset bandwidth, while adding noise, but I don't exactly know how to determine the value.
The packets have a preamble, consisting of AX.25 flags (01111110) sent one after another. The preamble length is 100 milliseconds in all data rates.
I understand that the Loop Bandwidth value should be dependent on the sample rate, and that it is presumably tuning the loop filter's bandwidth, although I don't know what the relation is between the Loop Bandwidth and the actual bandwidth of the Loop filter, as well as how I should tune/calculate the optimal value.
I also need help setting the Symbol Sync parameters, what TED should I use? What should the TED Gain and Damping Factor be?
Lastly, I understand I can use the Correlation Estimator block to assist in initial lock by correlating against the preamble, but I don't know how to use it.
From experimental results (working with the satellite while its in the other room, SNR is at least 20 dB, optimal conditions and also loopback tests with a LimeSDR and an attenuator to simulate the space channel's SNR ~15 dB), we get fairly good results with our values (I will include the flowgraph). We don't have a problem locking at 9600 bits per second, but at 1200 bits per second we are unable to lock onto the first packet, and our lock bandwidth goes down considerably.
To summarize, my main questions are:- How to set the Loop Bandwidth value, can I relate it to the sample rate and required lock speed (in my case less then 100 milliseconds). Should the value in the FLL Band-Edge and Costas Loop be set differently?
- How to set other synchronization-related values, like the TED, TED Gain, Damping Factor, Maximum Deviation etc...
- How can I use the Correlation Estimator?
- Any other suggestions on how to optimize the flowgraph?
I appreciate the help!
Regards,
Ido.
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bpsk_ax25_v2.grc