Thinking over the design of the Mode-S/ADS-B receiver I have a possible
improvement. It changes the serial method of demodulation to a parallel
block method.
Mode-S/ADS-B is a smaller bandwidth signal in a wider bandwidth channel
of 8 MHz. There is a frequency tolerance of +/- 1 MHz. Desired signals
are from different sources and can overlap in time and frequency. There
are also undesired signals (ATCRBS) that share the same channel and have
a frequency tolerance of +/- 3 MHz. The incoming signal is oversampled
at eight or ten MSPS.
The recommended designs use serial processing and will only process one
data transmission at a time. If two data transmissions overlap then the
stronger one is selected and the weaker one is considered interfering.
In one recommended design there is mention of using megabytes of ROM
data to minimize the effect of the interfering signal on the selected
signal. The interfering signal is likely at a different frequency so
the ROM data is a lookup table based on the the effects of the
interfering signal on the sample values. This process is done after the
AM Detector. I assume a real ADS-B receiver has a single ADC that
digitizes the output of a hardware AM Detector which would limit the
possible methods.
The USRP output is a complex signal. If I use the recommended designs
then I am throwing information away by taking the magnitude of the
complex signal and recreating the information later using the ROM lookup
tables.
I want to experiment with implementing multiple detectors across the
channel. The detectors feed blocks of candidate samples to the
Mode-S/ADS-B decoder. It seems to me that SNR improves by narrowing the
detector bandwidth. The interference effect of multiple signals will be
reduced as well. This may require a bit of processing power to
implement. :)
73 Eric
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