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How in CDMA mobiles receiver gets unique code to decode its respective signal?
This question says that a cdma "channel" has a publicly known code associated with it. Ok. But, how is this code (the pseudorandom number sequence) is synchronized between both receiver and transmitter ?
Having the code is not enough, because a out-of-"phase" code will have the same effect as having other code instead of the correct one.
How both correlators synchronize in order to both codes to match ?
The code is statistically random but it is not secret. In fact, it's chosen to not conflict with another code on the same frequency. Both ends know the code--it's part of the notion of "channel number". This code sequence stuff is common and you will find the same thing in GPS, and even modems. Sometimes it is called "scrambling".
Before the start of the unit step, the waveform will be a constant amplitude sine wave at the carrier frequency. After the unit step, if we assume the amplitude of the step is sufficient for 100% modulation, the waveform will be a constant amplitude sine wave at twice the amplitude that existed before the start of the unit step. A unit step amounts to a DC level so that it affects the amplitude of the carrier but not its shape.
Wolfram alpha visual depiction of what was described.
Best Answer
CDMA synchronisation is a deep topic.
A well designed CDMA code will have a noise-like autocorrelation, that is zero for all offsets other than the range +/- 1 chip, where the autocorrelation rises to 1.
So how do you find the code offset?
1) Brute force. There are codes that do this. You simply correlate, sequentially, with all possible offsets, and wait until you get a hit. This can take a long time.
2) Time assisted. The receiver knows about the transmitter's code offsets, and knows the exact time from another source (network sync, GPS, absolute with own reference, delta time since last fix, anyway somehow). This reduces the search time by many orders of magnitude, as there are only a few possible codes, and a restricted range of 'distance to base station time of flight' delays to to search over. If you are interested in IS-95 (Qualcomm) North American system http://users.ece.gatech.edu/yucel/is95.pdf it says in this presentation that it uses GPS to assist sync on the 15 bit M sequence spreading codes.
3) Composite code design. You can make a long code from several shorter sub-codes. This is used by the NASA deep space positioning system as an example. Each short code can be synchronised with a brute force search quite quickly. Then the long code position can be computed from the short positions, see 'chinese remainder theorem' for how this could work. This then gives a very tight estimate for where the long code is, and once acquired, it can stay in track.