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Code Division Multiple Access scheme the transmitted symbols are multiplied by a sequence of +1 and -1s called psuedo random (PN) sequence.This increases the bandwidth needed since the pulse width of PN sequence is much smaller than the original data.The loss of bandwidth is partly compensated by allowing many users using there own PN sequence.The correlation of each user(i.e if we multiply and integrate PN sequence of one user by another's PN sequence) will be very less.But for the intended user this brings the original data since all +1 and -1s align.This multiplication and integration is done by the correlator.It multiplies and accumulates the incoming samples by the proper PN sequence.The effect of other user's interference is removed.
The PN sequences also have very low AutoCorrelation(ie if we multiply one sequence by a delayed replica of it).
The above property can be used to combat the effect of fading.
When there are multiple paths (This is made possible because we have increased the band widh by PN sequence) each path will fade independently.So we can have one correlator for each path to remove the effect of PN sequence.Since the chance of all the paths are under deep fade is less we can get strong signal from some paths.Thus we get multipath diversity.
Another use of expanding band width is the siganal can be hidden below thermal noise.At the receiver by knowing the proper PN sequence the symbols can be decoded.
Posted by crouse
In Code Division Multiple Access scheme the transmitted symbols are multiplied by a sequence of +1 and -1s called psuedo random (PN) sequence.This increases the bandwidth needed since the pulse width of PN sequence is much smaller than the original data.The loss of bandwidth is partly compensated by allowing many users using there own PN sequence.The correlation of each user(i.e if we multiply and integrate PN sequence of one user by another's PN sequence) will be very less.But for the intended user this brings the original data since all +1 and -1s align.This multiplication and integration is done by the correlator.It multiplies and accumulates the incoming samples by the proper PN sequence.The effect of other user's interference is removed.
The PN sequences also have very low AutoCorrelation(ie if we multiply one sequence by a delayed replica of it).
The above property can be used to combat the effect of fading.
When there are multiple paths (This is made possible because we have increased the band widh by PN sequence) each path will fade independently.So we can have one correlator for each path to remove the effect of PN sequence.Since the chance of all the paths are under deep fade is less we can get strong signal from some paths.Thus we get multipath diversity.
Another use of expanding band width is the siganal can be hidden below thermal noise.At the receiver by knowing the proper PN sequence the symbols can be decoded.
Posted by Mannai_Murali
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