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I have a small doubt. Suppose I transmit just 2 square pulse with frequency of 100 MHz over a wireless channel, will it spread in the time domain when received by a wireless receiver. In other words, a 100 MHz square pulse will have a High for 5ns, and then low for next 5ns and then again a High for 5ns. At the receiver, I receive the same pulse attenuated and which looks like a guassian distribution. But will the difference between the 2 peaks be still 5ns or it may change?

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Simon is right except there is one more assumption required. If the transmitter and receiver are moving toward or away from each other, then Doppler effects will cause the pulses to be closer together or farther apart in time.

One other kind of weird thing to consider. The channel between TX and RX could be time varying. The propagation speed of EM waves depends on the dielectric properties of the space in between. If a large dielectric solid material was somehow inserted between TX and RX after the first pulse, then the second pulse would be substantially delayed due to the slower EM propagation in the dielectric material. I don't think it is possible to do this in the space of 5ns, but since I don't know what your final application is, I thought I would mention it. Other things like humidity or rain or snow can have an effect also, but normally these are not high-speed processes.

I suspect multi-path will be your biggest problem. You will need to look into some kind of correlation detection scheme.

Also, you will have a hard time getting approval from FCC or similar agencies. In other words, have fun experimenting, but don't think you are going to market a product in the developed world based on broadband pulse transmission unless you are willing to go through a lot of trouble.

One last thing. The shape of your received pulse will depend somewhat on the channel bandwidth, which will depend on antenna bandwidth. The more bandwidth your antennas have, the closer the RX pulse will look to the TX pulse.

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Strictly speaking, the time between pulses will not change, at least as long as the nature of the air between the transmitter and receiver do not change appreciably during the interpulse interval. At 5 nsec between pulses, that's a pretty good bet. What can happen, though, is that the pulse shapes at the receiver can change. There are at least two reasons for this. Keep in mind that a square wave at 1 MHz will contain the harmonics of 1 MHz (2 MHz, 3 MHz, 4 MHz, etc) with differing amplitudes. The first effect is that the absorption rate of the atmosphere can and does vary with frequency. This will cause the square wave edges to become increasingly rounded with distance. The second effect is dispersion, and this will cause the harmonics to propagate with different speeds, again distorting the received waveform.

For short-range wireless signals, neither effect is important.

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  • \$\begingroup\$ Good answer. But it is 100 MHz and 5ns. Is there dispersion in atmospheric air? I thought it was pretty close to uniform propagation speed over a very wide range of frequencies. Although I know that absorption varies according to band. \$\endgroup\$
    – user57037
    Commented Apr 10, 2015 at 18:07
  • \$\begingroup\$ Like I said in the answer, "For short-range wireless signals, neither effect is important." \$\endgroup\$ Commented Apr 10, 2015 at 18:11
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If the transfer system is time invariant, the time between two events in the output is the same as the time between the events that caused them. A wireless transmission channel is time invariant, so that is given.

The receiver needs to accurately identify the events though. If the wireless channel has two paths of different length, the receiver sees an "echo", and needs to distinguish that from the signal received for the next pulse.

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    \$\begingroup\$ To say that a wireless channel is time invariant is a pretty broad generalization. \$\endgroup\$
    – Jotorious
    Commented Apr 10, 2015 at 17:21

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