One of the major drawbacks of an OFDMA system is, that the transformation of a complex symbol mapped sequence (e.g. BPSK, QPSK, etc.) onto a small set of subcarriers produces time sequences that have high PAPR (Peak-to-Average Power Ratio). PAPR is the ratio between the maximum power and the averaged power This results in requirements for expensive transmission amplifiers and furthermore lead to high power consumption. Both effects are -particularly on terminal side unwanted.
It is thus a major design goal to limit this effect for the UL direction. In order to reduce the PAPR a variant of OFDMA is used. It is called SC-FDMA (Single Carrier Frequency Division Multiple Access). SC-FDMA works according to the following mechanism, described for the associated transmitter structure. SC-FDMA is the method of choice for EUTRAN in the uplink direction.
The data is mapped to complex symbols like in case of normal OFDM/OFDMA. But this time we interpret the resulting vector not as frequency domain signal, but as a “de-spread” or concentrated time signal. Thus before we can go to the mapping to subcarriers, we have to transform the sequence into a frequency domain signal. Thus a discrete Fourier transform is applied to the data vector. It gives us a vector of data symbol for each subcarrier to be used by the transmitter.
The next step is to map each transmitter symbol to one of the subcarriers of the system depending on which subcarrier was assigned to this transmitter. Obviously some subcarriers will remain free (0), that are the subcarriers for other transmitters. With this we go to the IFFT and do the normal OFDM processing.