suppose, Carrier signal frequency = 2800KHz message signal frequency = 3KHz
Then you will get a signal that looks like this in the frequency plane.
Obviously this is not to scale, but you get the idea.
but what will happen if it is reversed ?
i.e Message signal frequency = 2800KHz Carrie signal frequency = 3KHz
please explain would happen here ?
Then you will get the following.
The positive and negative sides of the signal will almost completely overlap, and thus distort each other to the point that they likely cannot be recovered. Due to the +/- 3 kHz carrier frequency there will be 6 kHz that is non-overlapping (though they will still be distorted by the other signal's roll-off), but the rest will overlap.
To increase the energy of the signal we need to increase the frequency.
Signal power has nothing to do with increasing the signal frequency. I think that you are thinking of light photons, which do increase in energy as their frequency goes up. Communication signals, though they are (at a really, really, low-level) composed of photons, it is a non-issue because adding power just means adding more photons.
Anyway, suffice to say that you don't need to increase the frequency to increase the signal power. We increase the carrier frequency for the following reasons:
- Spectrum Availability. You send your signal where the FCC (or whatever governing body applies) says you can send it.
- Increased bandwidth. As the answer to your question demonstrated, to send wide-band signals you need higher carrier frequencies.
- Practicalities of antennas. It is difficult (impossible?) to make a good wide-band antenna. An antenna's bandwidth, though, is proportional to its center frequency, so increasing the carrier frequency makes it much easier to make good antennas with wide passbands.
- Channel characteristics. Different frequencies behave in different ways. Some frequencies get absorbed by rain and some resist that. Some frequencies bounce off the ionosphere and so can travel farther than "line of sight".