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One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal ([[DS-CDMA]])
which is transmitted after it has been fed through an inverse FFT ([[Fast Fourier Transform]])
== Rationale ==
Wireless radio links suffer from frequency-selective channels.
If the signal on one subcarrier experiences an outage, it can
still be reconstructed from the energy received over other subcarriers.
== Downlink: MC-CDM ==
In the downlink (one base station transmitting to one or more terminals), MC-CDMA typically reduces to Multi-Carrier Code Division Multiplexing. All user signals can easily be synchronized, and all signals on one subcarrier experience the same radio channel properties.
In such case a preferred system implementation is to take N user bits (possibly but not necessarily for different destinations), to transform these using a Walsh [[Hadamard Transform]], followed by an I-FFT.
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== Variants ==
A number of alternative possibilities exist as to how this frequency ___domain spreading can take place, such as by using a long PN code and multiplying each data symbol, d<sub>i</sub>, on a subcarrier by a chip from the PN code, c<sub>i</sub>, or by using short PN codes and spreading each data symbol by an individual PN code — i.e. d<sub>i</sub> is multiplied by each c<sub>i</sub> and the resulting vector is placed on N<sub>freq</sub> subcarriers, where N<sub>freq</sub> is the PN code length.
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* [[OFDMA]], an alternative multiple access scheme for OFDM systems, where the signals of different users are separated in the [[frequency ___domain]] by allocating different sub-carriers to different users.
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