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I have a small doubt in a one way wireless communication. If I am using a receiver chip to demodulate an FSK signal. How does the receiver understands, which frequency to listen to? For example, I have a central frequency of 2.4 GHz and my FSK operates at "2.4Ghz + X Hz" and "2.4Ghz - X Hz". How does the receiver now knows that it has to demodulate "2.4Ghz + X Hz" and not some "2.4Ghz + Y Hz" or "2.4Ghz + Z Hz"? Similar doubt in OOK modulation as for a "1" I can send 10 pulses or 50 pulses. How will the receiver know that it is representing "1" and not "11111"?

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With FSK the receiver locks on to the carrier frequency. You tune it in to that frequency, and a PLL locks on to it. As the carrier frequency changes so the PLL has to change to compensate. That change that it has to make is basically the demodulation and is output as the resultant signal for you to use.

The receiver will, of course, have a finite range in which the carrier frequency can vary - if it goes outside that range it won't detect it. It's basically the "channel width" and is why a frequency spectrum is normally split into a number of specific frequency channels or buckets. As long as your frequency stays within that channel it will be demodulated by the receiver.

For instance, normal FM radio has channels typically at 0.2MHz steps, which means a carrier has +/- 100kHz range it can deviate within and still be considered part of the channel. Of course it's good to have a bit of a gutter between them to separate channels, so typically only maybe 80% of the channel width is utilized to prevent interference between adjacent channels.

As for OOK, it's purely down to how you interpret it. The receiver itself just demodulates the signal into on and off, and it's up to you to time how long each is and decide if it's a 1 or a 0. Just like a UART, both ends have to agree on a baud rate, and stick to it.

There are of course ways of encoding a clock signal into the OOK signal, and that can be recovered at the receiver's end, such as Manchester Coding, 8b/10b, etc.

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