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Theoretical assumption: I have multiple CDMA transmitters operating at same frequency and code placed 5 metres apart. My questions:

  1. Does the chances of my receiver receiving the data improve? That is to say does the snr of signal improve at receiver placed 40km away ?

  2. What is the signal strength at any point in space? is it vector sum of strength of both signals received from both transmitters?

  3. Can destructive interference happen at that point in space due to signal strength from multiple sources being out of phase at that point?

  4. Does the receiver frequency tracking loop need to shift from one transmitters signal to other , if at all any changeover between two signals from different sources does happen ?

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    $\begingroup$ Are they transmitting exactly the same thing? If so, it's effectively an antenna array. If not, it's just interference. $\endgroup$ – Phil Frost - W8II Aug 31 '17 at 13:07
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The CDMA transmitters will have independent frequency synthesizers/oscillators that will not be phase locked with one another. The receiver is therefore subjected to pseudo random phase angles from the various transmitters. The result will be largely destructive interference. The net result is receive performance under nearly all conditions will be substantially less compared to the deployment of a single CDMA transmitter.

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Cellphones that use this method are assigned a unique pseudo-random CDMA code to prevent just this sort of interference. Even when they share the same frequency they can communicate.

If you are using the same code and same frequency then they will definitely interfere with each other. Cellphone CDMA is designed to prevent this from being a problem, by specifically not having any two phones with the same code.

Any two transmitters on a single frequency will interfere in ways that are both constructive and destructive, so might find an arrangement that favors one transmitter over another.

In any case, to ensure the best possible data transfer you will get the best results if you deviate from the specifications in your question - eliminate one transmitter or make their codes different.

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