# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 3 Fundamentals of Channel Estimation
### Classification of Channel Estimation Schemes
#### 3.1.3 Overview of Blind Channel Estimation  
The blind CE scheme requires many data and it is known for exploiting the statistical and mathematical properties of the transmitted data [92]. As shown in Figure 12, the blind CE techniques in the literature fall under two class: deterministic and statistical blind CE. Deterministic CE schemes offer better performance in comparison to statistical CE schemes at the expense of very high computational complexities which further increases with increase in the modulation constellation order. Blind CE schemes are known to be bandwidth efficient since they do not require training symbols in order to estimate the channel. However, they usually exhibit very high computational complexity.  
Several works in the literature adopt the blind CE techniques for different wireless communication technologies. A subspace based blind CE (SBCE) algorithm that is not affected by null subspace CE errors caused by noise and fading algorithm is proposed for CP-OFDM (ICBCE) schemes in [95]. The performance of this algorithm is compared with two conventional SBCE CE algorithms: RPBCE scheme and RMBCE scheme. It is known from the literature that the RPBCE scheme exhibits very high computational complexity. As such, the RMBCE scheme is designed to offer lower computational complexity in comparison to RPBCE schemes. Nevertheless, the performances of both schemes are similar. The ICBCE algorithm outperforms both conventional SBCE schemes (that is, RMBCE and RPBCE) in terms of MSE at low SNR values and it also offers lower computational complexity. Several OSTBC based MIMO-OFDM schemes have been reported in the literature and are designed to exploit the OSTBC structure together with the inter-sub-carrier relationships caused by the FIR channel model. These CE schemes are known for their very high computational complexities. Some works (such as the sub-channel grouping technique) have proposed different methods capable of reducing the computational complexities of existing OSTBC CE schemes.  
Most blind CE algorithms are usually applicable to a single block of OSTBC-OFDM data [97] and are only efficient in flat fading channels especially in MIMO-OFDM schemes. In [114], a blind CE scheme is proposed for OSTBC-OFDM schemes that take advantage of the structural propertiesof OSTBCs, where the processing across all sub-carriers are done coherently. This proposed scheme is efficient in time-varying channels and has no limitations on the number of transmit and receive antennas in comparison to other schemes. The algorithm proposed in [114] exploits a semi-definite relaxation (SDR) technique for converting the blind CE problem to a convex semi-definite programming (SDP) form. Substantial simulation performance is demonstrated in comparison with several conventional blind MIMO-OFDM based CE schemes in the paper [114]. It was mentioned in Section 2.4.2, that the addition of CP to CP-OFDM based transceivers for the purpose of combatting ISI and ICI, as well as the appending of training symbols for the purpose of CE results into significant degradation in system spectral efficiency. The performance of isotropic orthogonal transform (IOTA) based multicarrier transceiver is proposed as an alternative to CP-OFDM systems. This technique employs the use of well-designed pulse shaping with good frequency and time localisation properties for limiting system interference, as well as sustaining sub-carrier orthogonality in real field without the use of CP.