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A related concept is the combination of a single carrier transmission with the single-carrier frequency-domain-equalization (SC-FDE) scheme. [10] The single carrier transmission, unlike SC-FDMA and OFDM, employs no IDFT or DFT at the transmitter, but introduces the cyclic prefix to transform the linear channel convolution into a circular one.
Frequency-division multiple access (FDMA) is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user.
An example of FDMA systems were the first-generation 1G cell-phone systems, where each phone call was assigned to a specific uplink frequency channel, and another downlink frequency channel. Each message signal (each phone call) is modulated on a specific carrier frequency .
At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency that serves to "carry" information. The carrier is much higher in frequency than the baseband signal. The carrier signal and the baseband signal are combined in a modulator circuit.
I will try to give the reasoning, and we need to edit the article one day. The explanation is suitable when we consider multiuser access as in OFDMA and SCFDMA. In OFDMA, each data symbol is carried by a single sub-carrier and single user is given multiple sub-carriers. In SC-FDMA, due to DFT the each data symbol is spread to multiple sub-carriers.
The OFDM diversity gain and resistance to frequency-selective fading may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDM symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable.
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