Hello! This is my first post on the site. I would like to ask a question if I may. I am trying to decode Mode S reply signals for my diploma thesis. The first thing I must get up and going is preamble detection. My teacher suggested that the analog processing before sampling would remove any DC component of the signal. He also suggested using a 16 MHz sampling frequency. Seeing as how the coding is Manchester, I was thinking of using a matched filter (time reversed and delayed version of the "1" bit symbol as an impulse response), checking the output values at 16 sample intervals and using an FSM to recognize the correct sequence: a "1" symbol would output 16, a "0" symbol would output -16 and a null symbol (meaning the spaces between pulses in the 2-3; 5-6; 6-7 and 7-8 us timeslots), which would mean checking for the following sequence of filter outputs: 16 16 0 -16 -16 0 0 0. Does anybody have a better idea on how to do this? I was thinking of using a correlator, but the problem is all those long sequences of "0" bits which ruin correlation (as anything multiplied by 0 would also yield 0). Thank you, Tudor.
Mode S preamble detection
Started by ●November 19, 2009
Reply by ●November 19, 20092009-11-19
On Nov 19, 6:26�am, "tudorelu87" <tudorel...@yahoo.com> wrote:> Hello! > > This is my first post on the site. > > I would like to ask a question if I may. I am trying to decode Mode S > reply signals for my diploma thesis. The first thing I must get up and > going is preamble detection. My teacher suggested that the analog > processing before sampling would remove any DC component of the signal. He > also suggested using a 16 MHz sampling frequency. > > Seeing as how the coding is Manchester, I was thinking of using a matched > filter (time reversed and delayed version of the "1" bit symbol as an > impulse response), checking the output values at 16 sample intervals and > using an FSM to recognize the correct sequence: a "1" symbol would output > 16, a "0" symbol would output -16 and a null symbol (meaning the spaces > between pulses in the 2-3; 5-6; 6-7 and 7-8 us timeslots), which would mean > checking for the following sequence of filter outputs: 16 16 0 -16 -16 0 0 > 0. > > Does anybody have a better idea on how to do this? > > I was thinking of using a correlator, but the problem is all those long > sequences of "0" bits which ruin correlation (as anything multiplied by 0 > would also yield 0). > > Thank you, > � � �Tudor.I doubt most people here know what "Mode S" means.... are you talking about aircraft ATC transponders? Mark
Reply by ●November 21, 20092009-11-21
>On Nov 19, 6:26=A0am, "tudorelu87" <tudorel...@yahoo.com> wrote: >> Hello! >> >> This is my first post on the site. >> >> I would like to ask a question if I may. I am trying to decode Mode S >> reply signals for my diploma thesis. The first thing I must get up and >> going is preamble detection. My teacher suggested that the analog >> processing before sampling would remove any DC component of the signal.H=>e >> also suggested using a 16 MHz sampling frequency. >> >> Seeing as how the coding is Manchester, I was thinking of using amatched>> filter (time reversed and delayed version of the "1" bit symbol as an >> impulse response), checking the output values at 16 sample intervalsand>> using an FSM to recognize the correct sequence: a "1" symbol wouldoutput>> 16, a "0" symbol would output -16 and a null symbol (meaning thespaces>> between pulses in the 2-3; 5-6; 6-7 and 7-8 us timeslots), which wouldme=>an >> checking for the following sequence of filter outputs: 16 16 0 -16 -160 =>0 >> 0. >> >> Does anybody have a better idea on how to do this? >> >> I was thinking of using a correlator, but the problem is all thoselong>> sequences of "0" bits which ruin correlation (as anything multiplied by0>> would also yield 0). >> >> Thank you, >> =A0 =A0 =A0Tudor. > >I doubt most people here know what "Mode S" means.... >are you talking about aircraft ATC transponders? > >Mark > > >Yes, exactly.
Reply by ●November 21, 20092009-11-21
On Nov 19, 6:26=A0am, "tudorelu87" <tudorel...@yahoo.com> wrote:> Hello! > > This is my first post on the site. > > I would like to ask a question if I may. I am trying to decode Mode S > reply signals for my diploma thesis. The first thing I must get up and > going is preamble detection. My teacher suggested that the analog > processing before sampling would remove any DC component of the signal. H=e> also suggested using a 16 MHz sampling frequency. > > Seeing as how the coding is Manchester, I was thinking of using a matched > filter (time reversed and delayed version of the "1" bit symbol as an > impulse response), checking the output values at 16 sample intervals and > using an FSM to recognize the correct sequence: a "1" symbol would output > 16, a "0" symbol would output -16 and a null symbol (meaning the spaces > between pulses in the 2-3; 5-6; 6-7 and 7-8 us timeslots), which would me=an> checking for the following sequence of filter outputs: 16 16 0 -16 -16 0 =0> 0. > > Does anybody have a better idea on how to do this? > > I was thinking of using a correlator, but the problem is all those long > sequences of "0" bits which ruin correlation (as anything multiplied by 0 > would also yield 0). > > Thank you, > =A0 =A0 =A0Tudor.Have you seen this? http://adsb.tc.faa.gov/WG3_Meetings/Meeting12/1090-WP-12-15.pdf