Honored Contributor
Honored ContributorThe system has multiple channels, that all need to be decoded simultaneously.
They are simple modulations, IE: AM, OOK, FM, GFSK, FSK, and although it can be extended to QAM, etc, the system doesn't require this yet. The FFT is used to channelize the system. and the rest of the bit decoding is done in the frequency domain. There are some simple lowpass IIR filters used in the system, but the maximum clock rate is not as high as what would be required to do true FIR filters for 127 channels. (Either need lots of multipliers, or to run the multipliers extremely fast.) For the simplest case, AM/OOK, the output of the Real and Imaginary output of the FFT is going through a square root function to give magnitude. This is based through a low pass filter giving a slow DC average. This passes through a bit slicer giving 1's and 0's that then pass into a correlator looking for the sync word of the incoming packet. Since I'm decoding in the frequency domain, I'm not filtering in the typical sense. I have worked on systems in the past where a FFT is performed, bins are masked, then an IFFT is performed, but that was used for a system where they need the time domain back, but wanted to apply a complex comb filter. Pete
