[Discuss-gnuradio] Simulate transmission

[irene159]

Hello,

I would like to simulate a transmission in order to test the configuration
of the receiver's Costas loop. 
I'd like to mix the modulated signal to IF, add some noise and convert back
to baseband for demodulation. The frequency value configured for channel
selection at reception is to be slightly different from the real frequency
used. Do you think this model is representative?  

Here is how the simulation part is coded :

        # Transmission channel simulation (cc)
        
        # Mix to IF
        lo = gr.sig_source_c(sample_rate, gr.GR_SIN_WAVE, if_freq, 1.0, 0)
        mixer = gr.multiply_cc()
        fg.connect(lo, (mixer, 1))
        # Take real part as transmit
        ctof = gr.complex_to_float()
        
        # Simulate noise in the channel
        noise = gr.noise_source_f(gr.GR_GAUSSIAN, noise_mag)
        air_noise = gr.add_ff()
        fg.connect(noise, (air_noise, 1))
        
        # Mix to baseband / channel selection
        chan_taps = gr.firdes.low_pass(1.0, sample_rate, lp_cutoff, lp_tw,
gr.firdes.WIN_HAMMING)
        chan_filter = gr.freq_xlating_fir_filter_fcf(1, chan_taps, if_freq,
sample_rate)            


and it is connected as follows : 

        fg.connect(..., objMod, mixer, ctof, air_noise, chan_filter, objDemod, 
...)

where objMod = dqpsk.dqpsk_mod() and objDemod = dqpsk.dqpsk_demod().

The code works without the transmission simulation part (fg.connect(...,
objMod, objDemod, ...)).

Once I implement the simulation, data received is corrupted. Is something
wrong in the code?

Regards,
Irene



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