PathSim

PathSim is a program that can be used to simulate radio propagation paths using a soundcard and/or wave files as the source and destination.
The main screen is fairly descriptive of the signal processing paths. Three HF paths are available though usually only two are used.
Each simulation "session" can be titled and saved to a file for future retrieval. A set of canned simulations are included in this zip file and are somewhat standard test conditions.
The real input signal can come from either the soundcard or a wave file. It is first bandpass filtered and converted to an I/Q complex signal using a Hilbert transform(incorporated in the bandpass filter). Three paths are then created with two of them having a variable time delay.
Each Path has a Spreading frequency selection and a frequency offset parameter. The spreading is performed by low pass filtering a complex AWGN signal to the desired bandwidth and then multiplying by the incoming complex signal.
Each path also has a frequency offset function which is just a complex NCO that is multiplied by the incoming signal.
Just the I or real term of the three paths are summed together to create a real output.
Basically heres how the S/N ratio is achieved. The AWGN source is filtered by a 3KHz LP filter to bandwidth limit the noise. The input signals rms "voltage" is measured and averaged over a second or so.
There are two gain/attenuator blocks that are varied to achieve the desired S/N ratio in dB. The noise and signal are then added to produce the final output signal. A small fft and realtime output plot are available to monitor output signals. The time plot turns red if you are overdriving the 16 bit soundcard/wavefiles.