Hi guys,

I am trying to relate electromagnetic superposition to apply to radio waves (specifically cell phone signals). I understand the phenomena but I don't understand its effects.

Is this statement accurate?
Electromagnetic superpositon causes interference in waves of the same frequency. Because of this waves of identical frequencies cannot be transmitted in the same area, (the need for seperate transmission cells in a cellular network).

If this is incorrect what effects would superposition have on radio signals?

Thanks alot,

Bobby

That's really a tough one however very interesting.

Superposition Applications? How about Quantum Exploration of the Apha Centauri System for starters?

I have no idea what that means with respect to the concepts of atomic superposition. So, I'll assume you are referring to what happens when two waves of the same frequency meet, and why they interfere.

There are two types of electromagnetic interference. One is interference patterns caused by the creation of standing waves. That usually occurs when a wave encounters its reflection, especially in communication (although it happens with independently generated waves also). It communications, this is caused multipath interference. That is, the same signal took two different paths to get to the receiver, one longer than the other. You can see this as ghosts on TV sets, and this is the predominent interference you experience with FM radio.

The other kind is when two independent signals of comprable strength interfere in the receiver. They just plain "mess up the demodulator". Modern digital demodulation requires a certain Signal to Noise level in order to accurately demodulate and detect the data encoded in the signal.

So, two signals overlapping in a cellular system will cause signal to noise issues. One is seen as the signal, the other as noise. The good signal needs to be more that 24 dB or so (i'm guessing) above the noise signal.

So, again, I don't know what you meant by superposition. Maybe you can reask in terms of what I have said.

By a Fourier decomposition, you can compute multiple independent signals at different frequencies (particularly harmonics). Two signals on exactly the same frequency will not be seperated by such a method and you'd have to do some other idea to seperate them. For things like speech, that's okay since human discussion follows certain patterns you can analyse. If it was a random signal you want to record, then two random signals on the same frequency would combine to give a single random signal, only one data stream. Two random signals at different frequencies can always be seperated.

Hey thanks for the replies,

I am specifically talking about waves (not sure how atomic superposition got into this). What I understand is that when two waves of the same frequency meet in the same medium they sum together (superimpose) so that the amplitude of the resulting wave is either larger of smaller than before.

Would this be considered interference and cause a loss in signal?

Also you talked about mutlipath interference, would superposition be a factor in this concept as well?

Thanks again,

Bobby

Sorry about superposition. I work with RF a lot, and have never used that term I guess others do.

This site has a good explanation for sound waves. I think it applies nicely to EM waves also.

http://www.acoustics.salford.ac.uk/feschools/waves/super.htm

In terms of waves interfering, when waves intersect, the resultant is the sum of the two waves at any point. And, yes, they will be potentially interfereing with each other as I will explain below.

But it is important to note first that the interference is not due to the two waves in any way changing each other. The waves do not actually interact with each other in any way. They pass right through each other unaltered in any way. The interference only manifests itself if something is trying to detect the waves.

For example, assume a receiver with a perfect 1MHz bandwidth. Assume a tone in the middle of that bandwidth. The receiver can detect the power in the tone by various means. Assume a 2nd tone anywhere else in the receive bandwidth. That tone will add its power to the calculation, thereby effectively causing interference. (it is interfereing with an accurate measurement of the first signals power) If, however, the 2nd tone is outside the receive bandwidth, it is not detected, and so causes no interference.

But, if the receive bandwidth is increased, the 2nd tone can again interfere. So, in this concept of interference the presence of noise (tones, signals, whatever) in the recieve bandwith of a receiver can interfere with the detection of information in the desired signal.

The power ratio of the signals (desired to undesired) can be called the Signal To Noise Ratio (SNR). For a given signal to noise ratio, a digitally modulated signal will exhibit a Bit Error Rate (BER). So, a specific communication system may be specified as achieving a 10^-3 BER at a 12dB SNR. That means, that 1 bit in 1000 will be in error. (generally these specifications assume a Gaussian Noise Interferer). There are all sorts of other communications system specifications based on performance in the presence of adjacent frequency tones, etc.

Another completely different use of the word "interference" is interference patterns and standing waves caused when signals of EXACTLY the same frequency but different phases (delays) intersecting. Multipath is one form of this.

Calculating the impact of multipath waves on digital communications systems is a complex subject in itself. For a narrowband communications system, multipath can manifest itself as signal dropout points in areas where the waves are 180 degrees out of phase. (cancellation causes a signal dropout). For example a 2.4GHz cordless telephone with a single antenna will see dropouts every 6 inches or so. For longer delays and wider bandwidths these reflections manifest themselves as a filter, causing frequency dependent amplitude distortions. In order to protect agains these effects, systems can utilize antenna diversity --- that is they use more than one antenna, and either switch antennas or sum them together in interesting ways. There are other techniques also.

Multipath is why sometimes moving a small distance (inches) may end or cause a cellphone dropout. Or, an FM station on you car radio may go from totally bad to totally good just by rolling a few feet.

Multipath is characterized statististically by the mathematics of Raleigh and Rician fading.

Hope that helps

Since visible light is just another form of electromagnetic energy, I would think the same laws would apply and one might need to learn Bose-Einstein Statistics and Crystallography and perhaps Particle Physics to make sense of mixing 2 different signals using the same frequency without using TDM. I'm afraid when we mix things together it is not so easy to separate them afterwards nor is it a simple feat to get the broken tea cup back in order after it crashes onto the floor unless of course one is in posession of some sort of ''timemachine''. When we split the beam in the ''beamsplitter'', and then to further complicate ''matters' and ''energies'' by bouncing the beams off of ''mirrors'' which further obfuscates the electromagnetic waves, I'd expect nothing more than varous forms of quantumly entangled parametric downward energy conversions to form constructive and destructive interferences resulting in various upward conversions forming at the focal point of observation(s) which in itself would affect the resultant phenomenal effects. So all this leaves me wondering why it would not be possible to polarize andor place a spin on the radio waves. The radar folk I am sure have been working on this for some time. The Quantum Explorers of the Alpha Centauri System as well. I think the best we can do beyond space exploration and astronomy is to create quantum probes for deep space observations. Currently of course this would be a black op or pure science-fiction. Quantum Probes would permit us to see things beyond current limitations without distortions caused by gravity and matter.

What does THAT mean? Do you even know what MIXING means? We know EXACTLY what happens when two waves mix or when they superimpose. It is simple mathematics. And, what does TDM have to do wth anything? What are you rambling about? You may have a point, but you surely didn't state it.

re: what does TDM have to do with anything? What are you rambling about?

So sorry. Perhaps I was thinking out loud again in my sleep. Merging multiple signals without using some form of multiplexing, like time division multiplexing, the solution to the problem remains somewhat elusive. Mixing? Perhaps not in the electronic sense, no. I think if I needed to decipher a signal composed of two signals merged on the same carrier wave, I think it would be highly problematic to separate them.