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Author Topic: Electromagnetic superposition application  (Read 17803 times)
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mikeyjames101
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on: November 13, 2007, 01:26:33 am » posted from:Ottawa,Ontario,Canada

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,

Mike
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Fu-Kwun Hwang
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Reply #1 on: November 13, 2007, 09:40:00 am » posted from:Taipei,T\'ai-pei,Taiwan

The transmission of cell phone signal is similar to transmission of radio signal. The only difference is the frequency range.
However, the frequency of the electromagnetic wave are much higher than sound wave. So sound wave are modulated with frequency (FM: Frequency modulation).

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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).
I do not think the above statement is correct.

You will find the following information from http://electronics.howstuffworks.com/cell-phone1.htm
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In a typical analog cell-phone system in the United States, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, like this:
       
Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells. The two purple cells can reuse the same frequencies.
Cell-phone Channels
A single cell in an analog cell-phone system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

  * A cell-phone carrier typically gets 832 radio frequencies to use in a city.
  * Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)

Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.

Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

  * The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure above, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.

  * The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.
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mikeyjames101
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Reply #2 on: November 14, 2007, 04:05:22 am »

Thanks for the reply Fu-Kwun Hwang

I am trying to come up with a physics concept to show why there is a need to seperate frequencies in cells?

Also what is the reason that there can only be 800 frequencies to use rather than infinite? What factors seperate one frequency from another (shouldn't there be an infinite number of frequencies in between)?

Thanks again,

Mike
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Fu-Kwun Hwang
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Reply #3 on: November 14, 2007, 08:20:35 am » posted from:Taipei,T'ai-pei,Taiwan

Do you have this kind of experience, if the frequency of two radio station was too close, sometime you will pick up signal from two both stations if you do not have a high quality radio.
You can select the frequency with RLC resonance circuit, how even there is a finite band width.
If you need a shorter band width, you need a higher quality device and the price will jump much higher if it is over the quality for normal circuit elements. That is the reason why there is a finite number of frequency available.
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mikeyjames101
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Reply #4 on: November 14, 2007, 12:13:30 pm »

Ya I thought so. Im trying to force something you would find in a introductory (university level) physics textbook to relate to wireless communication.
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forbidden18
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Reply #5 on: April 28, 2009, 12:20:59 pm »

i think it is correct in the statement that you say..thanks for information..


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FurnaceZer
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Reply #6 on: June 02, 2009, 07:54:17 pm » posted from:San Juan,Batangas,Philippines

I am experiencing the same problem like yours mikeyjames so I am thanking all the person who post some ideas about the concern problem of mikeyjames.,


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Bretjsarena
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Reply #7 on: September 01, 2011, 01:45:03 pm » posted from:Altoona,Pennsylvania,United States

Hey this is the great stuff even i am searching for it from too long!! I think this will help me a lot!!

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