What is Single Carrier FDMA (SC FDMA)?

Today multi-carrier transmission and OFDM are means to allow for very high overall transmission bandwidth while still being robust to signal corruption due to radio-channel frequency selectivity. However, a drawback of OFDM modulation, as well as any kind of multi-carrier transmission, is the large variations in the instantaneous power of the transmitted signal. Such power variations imply a reduced power-amplifier efficiency and higher power-amplifier cost. This is especially critical for the uplink, due to the high importance of low mobile-terminal power consumption and cost. Several methods have been proposed on how to reduce the large power variations of an OFDM signal. However, most of these methods have limitations in terms of to what extent the power variations can be reduced. Furthermore, most of the methods also imply a significant computational complexity and/or a reduced link performance. Thus, there is an interest to consider also wider-band single-carrier transmission as an alternative to multi-carrier transmission, especially for the uplink, i.e. for mobile-terminal transmission. One of such single-carrier transmission scheme can be implemented using DFT-spread OFDM

DFT-spread OFDM
DFT-spread OFDM (DFTS-OFDM) is a transmission scheme that can combine the desired properties for uplink transmission i.e. :
• Small variations in the instantaneous power of the transmitted signal (‘single carrier’ property).
• Possibility for low-complexity high-quality equalization in the frequency domain.
• Possibility for FDMA with flexible bandwidth assignment.
Due to these properties, DFTS-OFDM has been selected as the uplink transmission scheme for LTE, which is the long-term 3G evolution.

Basic principles
The basic principle of DFTS-OFDM transmission is illustrated in Figure 1. Similar to OFDM Modulation, DFTS-OFDM relies on block-based signal generation.

 

DFTS-OFDM signal generation

Fig-1 DFTS-OFDM signal generation

 

In case of DFTS-OFDM, a block of M modulation symbols from some modulation alphabet, e.g. QPSK or 16QAM, is first applied to a size-M DFT. The output of the DFT is then applied to consecutive inputs of a size-N inverse DFT where N >M and where the unused inputs of the IDFT are set to zero. Typically, the inverse- DFT size N is selected as N =2n for some integer n to allow for the IDFT to be implemented by means of computationally efficient radix-2 IFFT. Also similar to OFDM, a cyclic prefix is preferable inserted for each transmitted block , the presence of a cyclic prefix allows for straightforward low-complexity frequency-domain equalization at the receiver side.

If the DFT size M would equal the IDFT size N, the cascaded DFT and IDFT blocks of Figure would completely cancel out each other. However, if M is smaller than N and the remaining inputs to the IDFT are set to zero, the output of the IDFT will be a signal with ‘single-carrier’ properties, i.e. a signal with low power variations, and with a bandwidth that depends on M. More specifically, assuming a sampling rate fs at the output of the IDFT, the nominal bandwidth of the transmitted signal will be BW =M/N * fs. Thus, by varying the block size M the instantaneous bandwidth of the transmitted signal can be varied, allowing for flexible-bandwidth assignment. Furthermore, by shifting the IDFT inputs to which the DFT outputs are mapped, the transmitted signal can be shifted in the frequency domain.

The main benefit of DFTS-OFDM, compared to a multi-carrier transmission scheme such as OFDM, is reduced variations in the instantaneous transmit power, implying the possibility for increased power-amplifier efficiency. This benefit of DFTS-OFDM is illustrated in Figure, which illustrates the distribution of the Peak-to-Average-power Ratio (PAR) for DFTS-OFDM and conventional OFDM.

The PAR is defined as the peak power within one IDFT block (one OFDM symbol) normalized by the average signal power. It should be noted that the PAR distribution is not the same as the distribution of the instantaneous transmit power as illustrated in Figure 2. Historically, PAR distributions have often been used to illustrate the power variations of OFDM.

 

PAR distribution for OFDM and DFTS-OFDM, respectively.

 

 

Fig-2 PAR distribution for OFDM and DFTS-OFDM, respectively. Solid curve: QPSK. Dashed curve: 16QAM.

As can be seen in Figure 2, the PAR is significantly lower for DFTS-OFDM, compared to OFDM. In case of 16QAM modulation, the PAR of DFTS-OFDM increases. On the other hand, in case of OFDM the PAR distribution is more or less independent of the modulation scheme. The reason is that, as the transmitted OFDM signal is the sum of a large number of independently modulated subcarriers, the instantaneous power has an approximately exponential distribution, regardless of the modulation scheme applied to the different subcarriers.

A better measure of the impact on the required power-amplifier back-off and the corresponding impact on the power-amplifier efficiency is given by the so-called cubic metric .The cubic metric is a measure of the amount of additional back off needed for a certain signal wave form, relative to the back-off needed for some reference wave form. As can be seen from Figure2, the cubic metric (given to the right of the graph) follows the same trend as the PAR. However, the differences in cubic metric are somewhat smaller than the corresponding differences in PAR.