SC-FDMA Principles and Modulation

The OFDM transmission scheme shows robustness against multipath fading and is especially useful for mobile communication systems due to various reasons. An example is the complexity of the receiver where fairly simple and channel estimation/equalization is done in the frequency domain. Why is another transmission scheme selected for the LTE UL? A major disadvantage of OFDM systems is that the time domain signal which is amplified and transmitted shows a large dynamic range after modulating symbols on subcarriers and transformation to a time signal. This leads to a high Peak-to-Average Power Ratio (PARP) of the signal, which should be avoided for battery-powered handsets, which underlies a limit on the budget to justify the business case. The linear transmission of such a signal needs a highly complex RF amplifier for the handsets in order not to run into the nonlinear region of the transmitter. Additionally, the power consumption is larger compared to a transmitter running a smaller linear dynamic range. The PAPR even increases with a wider OFDM bandwidth for a larger number of modulated subcarriers. This results in another transmission scheme for the UL: Single Carrier FDMA (SC-FDMA).

Actually SC-FDMA is very similar to OFDMA but shows a better PAPR, leading to a longer battery lifetime and a cost-effective RF amplifier design. Figure 1 illustrates the components of a SC-FDMA transmitter system with its block entities. Highlighted in the figure are the main differences from a regular OFDM system. The main difference is the Discrete Fourier Transform (DFT) in the transmitter and the inverse DFT (iDFT) in the receiver, respectively. Thus, SC-FDMA is sometimes called DFT-spread-OFDM. Due to this difference, we can picture the information to be transmitted as modulated (bits mapped to two-dimensional QAM symbols with an I and Q component) to a time domain signal instead of modulating subcarriers of a frequency domain signal. The output of the DFT can be interpreted as a spectrum of the previously modulated data symbols. This spectrum has the characteristic of consecutive modulated subcarriers; it has therefore no scattered spectral distribution. Thus, it has the inherent behavior of localized RE usage as described in Section 1.

 
Figure 1: Block diagram of SC-FDMA transmitter with localized mapping to frequency resources

This localized spectrum is now mapped to the consecutive frequency REs which are specified in the UL scheduling grant, as only localized frequency resource assignments are allowed with UL transmission, which refers to the intrinsic signal characteristic of DFT-spread-OFDM. The rest of the spectrum for the full system bandwidth is filled with zeros. This zero patched spectrum is fed to an iFFT unit transforming a full system-wide spectrum "back" to the time domain for transmission. Figure 1 shows this mapping of the DFT symbols to the full iFFT width by adding zeros to frequency positions at the block in the center. The zero patched frequency areas are not used by this user and could be assigned to other users transmitting in the same time slot.

As for the future, a current research item is to overcome the lack of a high PAPR of OFDM signals in such a way that a digital reverse distorted signal is added to the signal which is to be transmitted. The nonlinear distortion is known for a given RF amplifier. Therefore, it is possible to pre-calculate the distortion applied to the transmitted signal. This distortion is inverted and joined to the signal to such a degree that the RF amplifier distortion eliminates the inverted signal again.