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I found it relatively easy to understand TDMA and FDMA but I just can't figure out CDMA and W-CDMA. For CDMA I think it is something like TDMA without time slots. At transmission side you can use whole bandwidth to send any signal from 0 to time "t". The one thing that secures you that signals wont interfere with each other is by "applying" to each signal different code (modulating that signal into that code). Receiver side only needs to know which code you used for which signal in order to "demodulate" it. I don't think I got it right, so any intuitive explanation would be helpful!

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  • \$\begingroup\$ In my intuition, it's the same way a boring old AM demodulator works by multiplying the incoming antenna signal with a sine wave and then averaging, except that it's not a sine wave that you're multiplying it with. \$\endgroup\$ – user253751 Jan 12 '18 at 1:08
  • \$\begingroup\$ @immibis I think I don't get the part "multiplying the incoming antenna signal with a sine wave and then averaging". Maybe I have holes in understanding demodulation... \$\endgroup\$ – Krushe Jan 12 '18 at 10:27
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The code is a frequency hopping, direct sequence or Spread Spectrum algorithm to make the signal look like random frequency Gaussian noise but at a faster symbol rate than the data which boosts the conversion from CNR to SNR and allows high traffic to share a wide bandwidth.

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It can be synchronous or asynchronous to all the other traffic.

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  • \$\begingroup\$ The key is that the pseudorandom codes of the various network users are all orthogonal to each other - see also Gold Code. So they interfere with each other only sometimes. And that Tb is huge compared to the code (chip) rate, so you might only get 700 out of 1024 chips above the noise floor, but that's enough. \$\endgroup\$ – Aaron D. Marasco Jan 12 '18 at 0:56
  • \$\begingroup\$ So basically that is some kind of "dynamic modulating"? You have your signal and you modulate it into pseudorandom code carrier signal which dynamically changes frequency of itself (changes frequency of modulated signal which contains pseudocode signal+your signal). Transmitter and receiver both know those pseudorandom code frequencies so during the time, receiver is „constantly“ unmodulating the signal from that random frequencies. Bear in mind that I imagine you are sending one signal at a time and at the receiver end only after unmodulating that signal fully you can send next one. \$\endgroup\$ – Krushe Jan 12 '18 at 10:22
  • \$\begingroup\$ @Part 2 If I make such pseudorandom code to change frequency (to hop from one to another frequency) theoretically every second, than what benefits I have when speed of signal hoping from one frequency is greater? \$\endgroup\$ – Krushe Jan 12 '18 at 10:42
  • \$\begingroup\$ It spreads the spectrum of each transmitter randomly in a coherent manner so they do not collide. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 12 '18 at 14:20
  • \$\begingroup\$ @TonyStewart.EEsince'75 I'm sorry could you be more specific, I'm bit an amateur. Maybe some explanation through example in simple fashion would help me "decode" what are you trying to say to me. Thanks! \$\endgroup\$ – Krushe Jan 12 '18 at 16:06

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