HOW IS SC-FDMA SIGNAL GENERATED IN LTE? HOW IS UPLINK SIGNAL GENERATED?

LTE uplink transmission is based on Discrete Fourier Transform (DFT) – spread OFDM (DFTS-OFDM). It is also often referred to as Single-Carrier FDMA (SC-FDMA).

Figure above shows the basic structure of DFTS-OFDM modulation with a size-M DFT being applied to the block of M modulation symbols. The output of the DFT is then mapped to selective inputs of a size-N Inverse Fast Fourier Transform (IFFT). The DFT size M,(i.e., the number of consecutive non-zero inputs to the IFFT), determines the instantaneous bandwidth of the transmitted signal, while the frequency mapping determines the position of the transmitted signal within the overall available uplink spectrum. Finally, a cyclic prefix is inserted.

DFTS- OFDM can also be seen as conventional OFDM transmission, combined with DFT-based pre-coding. Thus, one can very well speak about a “sub-carrier spacing” also in case of DFTS-OFDM. Furthermore, similar to OFDM, the DFTS-OFDM physical resource can be seen as a time-frequency grid with the additional constraint that a resource assigned to a mobile terminal must always consist of a set of consecutive sub-carriers in order to keep the single-carrier property (with low peak to average ratio) of the uplink transmission.