Re: [Discuss-gnuradio] help QAM16 Encoding / Transmission

[Tom Rondeau]

On Wed, Jul 17, 2013 at 12:35 AM, tom sutherland <address@hidden> wrote:
> I want to QAM16 encode a band limited (0-3khz) signal on one computer. I
> want to connect this computer to another computer running a QAM16 Decoder
> program. The QAM16 encoded signal needs to be able to be sent over the audio
> channel(single channel) of the computer. How can take the complex output of
> the QAM16 module and "mix" it so that it can be sent over a single audio
> channel? Need also to demodulate it on the other side.
>
> Thanks...Tom

To maintain the phase information, you need to move the entire signal
from complex baseband (where it's centered at 0 and has information on
both sides) to some frequency that is purely in the positive
frequency. This probably means interpolating the signal first to give
you room to move it, then multiplying it with a complex sinusoid at
the new center frequency to do the actual frequency shift. Then take
the real part (complex_to_real block).

Regarding the upsampling and shift, if you're brave, you can try using
the pfb_synthesizer_ccf block with 3 channels. Put a null source into
channels 1 and 3 and your original signal into channel 2. That would
upsample the signal by a factor 3 and place your signal in the right
place (channel 0 spans 0 Hz, channel 1 is in the positive freq and
channel 2 is in the negative freq).

Probably easiest to start with an interpolating filter and doing the
frequency shift using a complex multiply, though.

Tom