# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 2 Structure of Filter Bank Multi-Carrier Systems
### SC-FDMA  
The SC-FDMA scheme can also be referred to as the DFT-spread OFDM (DFT-S-OFDM) scheme [72; 75]. In addition, some text refer to this modulation and multiplexing scheme as linearly pre-coded OFDMA (LP-OFDMA). The structure of SC-FDMA is very similar to CP-OFDM except that an extra pre-coding stage is added in DFT-S-OFDM schemes before the IFFT block. The pre-coding block is used for limiting the PAPR of the transmitted signals. The pre-coding is implemented at the transmitter using an N-point DFT which is applied before the IFFT stage with the aim of mapping the transmitted signals into a number of adjacent-frequency sub-carriers. The reverse process of the above-described procedures is implemented at the receiver where the IDFT is specifically implemented after the FFT block. The structure of DFT-S-OFDM transceivers is depicted in Figure 8. Note that, if the pre-coding block were removed from Figure 8, the set-up could be viewed as a CP-OFDM.  
The upcoming 5G networks are expected to be allocated a larger bandwidth in order to boost the network data rate [76]. f-OFDM is a recently introduced waveform considered as a potential waveform for the upcoming xG (5G) networks [77; 78]. The structure of f-OFDM system is very similar to the conventional CP-OFDM system. The major difference is that, in f-OFDM systems, the assigned system bandwidth is split into subbands. The transmitting baseband signal on each subband is also filtered using a band-limited filter before the transmitting signals travel across the wireless multipath fading channel. The introduced subband based filtering is used for limiting inter-subband interference, which consequently eliminates sidelobe leakages by suppressing OOB emission. In the implementation of f-OFDM transceivers, two broad kinds of filters are considered: soft-truncated  
Figure 8: Structure of SC-FDMA modulation scheme.  
sinc filters and equiripple filters. The impulse response of an ideal low pass sinc filter is an infinitely long sinc function [76], where the long sinc function is adequately soft-truncated using different window functions that may either be Hann window or root-raised-cosine (RRC) window. Equiripple filters are desirable for attenuating inter-subband interference. These filters (equiripple) are designed using Remez exchange algorithm [79] for minimising the maximum error between the desired and actual frequency response [76]. The downlink transceiver structure of f-OFDM is shown in Figure 9. This structure is very similar to the structure of CP-OFDM systems except that subband filtering is introduced, which is not required for CP-OFDM systems. Table 1 shows the differences between f-OFDM and CP-OFDM systems.  
higher spectral efficient systems with improved OOB emission. FBMC-OQAM is well known to suit fragmented spectrum/cognitive radio systems while offering good localisation properties with high robustness against multipath fading. However, the system is inefficient for short burst transmissions as a result of long filter tail [47]. The transceiver structure and principles of operation of this waveform are elaborated in Section 4.1.