suppose if we send AM modulated message signal through space how the receiver will be knowing that we applied AM modulation basically how it will be knowing that which modulation is applied for our modulated signal

  • $\begingroup$ Please review the answers you have received and see if any of them need a tick. The answers seem good. Did they answer your question? $\endgroup$
    – Peter K.
    May 23 '17 at 11:32

Communications systems are always designed under the assumption that both emitter and receiver know what "language" they will be speaking to each other.

AM modulation comms are standardized in frequency, channel width for example. So each receiver is materially designed to demodulate AM signals with the appropriate hardware.

When people start trying to find which modulation was applied to a signal its generally involving electronic warfare or reverse engineering.

  • $\begingroup$ HI @MaximGi it means that if suppose i am sending MODE2 uplinks from the aircraft which is following D8PSK modulation it can not be dencoded by the GROUND STATION recievers which are not using D8PSK decoding technique.iS it Right? $\endgroup$
    – rajee
    Mar 7 '16 at 12:56
  • $\begingroup$ Would you speak chinese to a machine designed to speak french ? Nope. You have to speak french, or design another machine which speaks chinese. However, the ground station is still receiving the signals, physically speaking, but cannot decode them because it's just not designed to. $\endgroup$
    – MaximGi
    Mar 7 '16 at 13:04
  • $\begingroup$ @rajee, as an addition to the answer: a receiver can be designed to understand multiple modulation and coding schemes. Usually the link has a control subchannel so that Tx/Rx are always in agreement about what modulation is being used. There are also some techniques to do automatic modulation recognition, mainly with military applications. $\endgroup$
    – MBaz
    Mar 7 '16 at 14:37

This is a signal intelligence version of a randomness test (plus possibly a Turing test).

A receiver (of many simple modulation schemes) can "demodulate" any signal, but the result might just be noise.

Given a slice of spectrum, one can try analyzing it to estimate the likelihood that it is random or perhaps varied by some non-random process. Given that one finds some non-random modulation(s), one might try estimating whether that modulation was produced by a physical process or holds much more information than can be produced by any known non-intelligent mechanism, thus indicating a signal. But a smarter spy might try to make their modulation look like random noise rather than patterned or intelligent to try and fool you. Game On.


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