|From:||Eric A. Cottrell|
|Subject:||[Discuss-gnuradio] Improving Mode-S/ADS-B demodulation using parallel detectors and blocks of samples|
|Date:||Sat, 14 Jun 2008 12:11:16 -0400|
|User-agent:||Thunderbird 220.127.116.11 (X11/20080421)|
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. :)
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