# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 3 Fundamentals of Channel Estimation
### Classification of Channel Estimation Schemes
#### 3.1.1 Overview of Pilot Based (Non-Blind) CE
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Reference** & **Aim of Research** & **CE Schemes Applied** & **Applicable Technique or Waveform** & **Approach** & **Obtained Results** \\ \hline \hline
[97] & Design of blind ML detection with single orthogonal space time block codes OFDM (OSTBC-OFDM blocks) & Semi-blind/blind CE based on use of minimal training sequence & Semi-blind/blind CE for OSTBC-OFDM & \(\bullet\) & Grouping of subchannels for reduction complexity detection schemes using low complexity cyclic ML scheme & The proposed scheme outperforms pilot-based LS and differential technique in terms SER \\ \hline \hline \multirow{2}{*}{[97]} & Design of blind ML detection with single orthogonal space time block codes OFDM (OSTBC-OFDM blocks) & Semi-blind/blind CE based on use of minimal training sequence & Semi-blind/blind CE for OSTBC-OFDM & \(\bullet\) & Use of limited pilots for ensuring a probability one identifying condition in semi-blind estimation & The proposed scheme achieves lower computational complexity in comparison to non-blind counterparts \\ \hline \hline \multirow{4}{*}{[98]} & Design of a low complexity semi-blind CE technique and recording scheme for two-way relay networks & Semi-blind CE based on MSE relay networks & Second order statistics based semi-blind CE for two-way relay networks & \(\bullet\) & The NMSE of the estimates is \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \hline \hline \multirow{4}{*}{[99]} & Derivation of a quasi-Newton based on quasi-Newton method & Semi-blind CE based on quasi-Newton method & data symbols CE method and sharing data and pilot symbols for estimating CSI using ML principle & \(\bullet\) & The proposed scheme offers significant MSE improvement by the classification of data symbols over conventional LS CE \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \cline{1-1} \cline{2-5}  & & & & \(\bullet\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._In recent times, NC-OFDM has been proposed for higher data rates transmission in cognitive radio context [100; 101]. This NC-OFDM technique employs the use of continuous/contiguous blocks of sub-carriers for the transmission of data symbols. In these systems, the sub-carriers been utilised by the PUs are deactivated by the secondary users (SUs) with the sole aim of avoiding license users (LUs) interference [102; 103]. Channel estimation is not well studied for NC-OFDM [103]; however, some CE algorithms have been proposed and implemented in the literature by many researchers to improve the performance of the NC-OFDM system. According to [104], it has been proven that equispaced and equippowered pilot tones yields the optimal pilot design for conventional OFDM but this does not hold true for an NC-OFDM system. A new pilot symbol design for CE in NC-OFDM based cognitive radio systems is proposed in [105]. In this proposed method, convex optimisation and cross entropy optimisation are adopted for designing of pilot symbols with the purpose of minimising the MSE of the channel estimate for frequency selective fading channels. In [100], a new LS CE algorithm is proposed for NC-OFDM systems in cognitive radio context. In this proposed scheme, LS CE is adopted and modified for obtaining the channel fading coefficient of sub-carriers that transmits the pilot symbols. Least square CE with IDFT based noise reduction is proposed to improve CE accuracy while pilot-block average based noised reduction (PBANR) is proposed to further improve the performance of CE for slow time-varying channels. Simulation results show that these proposed algorithms offer better performance than the conventional LS CE method.