Re: building a ccsds transceiver

[Christophe Seguinot]

Hi

Fisrt I should says, cascading blocks without knowing what each block does should normally don't work.

Personally I never used these ones but they are numerous inconsistencies.

about decoding

• Your sample rate (input of Lime Sink and output of Lime source) is 32 kHz so the GMSK signal is 32 kHz bandwidth
• GMSK help states that "The output is a stream of bits packed 1 bit per byte (the LSB)" so don't convert to float, but just use a  unpack block to extract bits.
  
• CCSD27 decode block use -1 +1 input for 0 -1 bits so you have to convert bits to +1-1 float using a math operation such as input=(2*bit-1.0)
  

I suspect that your received signal is shifted in frequency.  You won't receive anything as soon as the frequency shift is larger than 32 kHz

Suggested action:

• May be you can try modulation + demodulation without hardware, and then with a channel model
  
• look at the received spectrum: is there any signal ?
  
• increased the received bandwith= sampling_rate at LimeSDR source . 32khz*oversampling_factor with oversampling_factor being 5 or 10
• do you see any signal in the spectrum?
  
• if yes add a Xlating filter to shift ad=nd center the received signal
  
• and then you should be able to demodulate it
• However, I don't know if the GMSK demod perform carrier recovery or synchronisation. You should have this feature in your flowgraph for the demod to work.
  

Hope this helps

Regards

On 31/01/2021 17:02, kaihua cheng wrote:

hello , i am trying to build a ccsds transceiver. i got this from someone "Decode CCSDS 27 expects soft symbols, while GMSK demod puts out hard symbols. It would be advisable to use a demodulator that puts out soft symbols, but if you want to keep GMSK demod, then you must convert from char to float". 

but after i have connected everything together, the decoding does not work. can someone help me and tell me what i  should do?