a quastion about CDMA and doppler shift

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sdahan112

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hi, i'm doing a comprfassion between ofdm and cdma.
doppler spread destroy the orthogonality between the sub carriers in OFDM and cause ICI.

but i cant figure if and how doppler spread effects CDMA (not MC CDMA but DS CDMA) i mean, every user spread its information with PN code on all the BW

can somone explein how and why is it effects\dosent effects CDMA?

thank you!
 

In the case of DS-CDMA you have a single signal and it is already spread over the entire frequency so if there is any small frequency shift, the signal integrator will be able to pick up the DS -CDMA signal without any issue, which is not the case for OFDM , MC-CDMA.
 

This is also related to the ambiguity function of a code. CDMA codes are usually chosen to have low correlation to other codes. Ambiguity also takes into account the equivalent effect of a frequency offset between codes. just as time shifts cause codes to be partially correlated to other codes, so does a frequency offset.
 

Doppler shift is harmful for OFDM because you need data bins to be placed exactly on their proper subcarriers for proper demodulation, this is not the case for CDMA, or any other single carrier scheme.
 

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