# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 2 Structure of Filter Bank Multi-Carrier Systems
### UFMC  
This is a potential waveform considered for xG networks. UFMC is designed to incorporate the advantages of FBMC systems as well as f-OFDM [80]. This is achieved by grouping the entire band of the OFDM system having N sub-carriers into subbands that are subsequently filtered [47; 80]. The complex modulated frequency domain symbols \(X(n,k)\) are eventually converted into time domain signals \(x(n,k)\) using \(N\)-point IDFT conversion as depicted in the synoptic UFMC transmitter structure of Figure 10. The resulting signals are filtered using a bandpass filter \(g(n,k)\) to produce the transmitting symbols (with optional CP insertion), which are summed up before been propagated across the wireless multipath fading channel. At the receiver, the transmitted signal is converted in the serial-to-parallel conversion block, filtered and transformed from the time domain to frequency domain, as illustrated in Figure 11. UFMC systems are designed to offer enhanced transmission latency and improved system spectral efficiency similar to FBMC-OQAM systems. These systems (UFMC) are well suited for short burst transmissions [47]. However, the large receiver DFT size of UFMC systems results into very high implementation complexity as compared to other considered waveforms of the emerging xG networks. These systems (UFMC) also suffer from drawbacks such as creation of interference due to partly overlapping subbands.  
Figure 10: Transmitter structure of UFMC system.  
GFDM  
GFDM is a flexible MCM scheme proposed for the air interface of the emerging next-generation 5G networks [37]. In GFDM schemes, independent blocks are made of a number of sub-carriers and subsymbols, which are modulated independently. Each sub-carrier of GFDM based transceivers is filtered using well-designed frequency and time domain circularly shifted prototype filters in order to curtail the destructive effects of OOB emission. This applied sub-carrier filtering may cause the GFDM sub-carriers to lose orthogonality, which may give rise to ICI or ISI. To keep the system overhead as small as possible, a short cyclic prefix is appended to an entire GFDM block in order to combat possible effects of ISI and ICI. GFDM along with other FBMC waveforms such as filter bank OFDM-OQAM and UFMC are regarded to offer suitable resistance against carrier frequency offsets (CFO). For these reasons, GFDM system is regarded as a block based non-orthogonal MCM scheme [61; 81]. Details of the principles of operation and system design of GFDM based transceiver is elaborated in Section 4.2. Table 2 presents a comparison of potential waveform candidates of upcoming 5G Networks.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specifications** & **CP-OFDM** & **SC-FDMA** & **f-OFDM** & **FBMC** & **UFMC** & **GFDM** \\ \hline Spectral efficiency & Low & Low & Low & High & High & High \\ Computational Complexity & Low & Low & Medium & High & High & High \\ OOB Emission & High & High & Low & Low & Low & Low \\ PAPR & High & Low & High & Medium & High & Medium \\ Latency & Medium & Medium & High & High & High & High \\ Flexibility & High & High & Medium & Medium & Medium & High \\ Robustness to CFO & Low & Low & Medium & High & High & High \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of waveform candidates of the emerging xG Networks.  
Figure 11: Receiver structure of UFMC system.