# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 3 Fundamentals of Channel Estimation
### Channel Estimation for Filter Bank OFDM-OQAM  
CE is currently an active research area for FBMC-OQAM based transceivers [115; 116; 117], while different CE schemes have also been proposed in the literature for filter bank OFDM-OQAM systems. The CE schemes for filter bank OFDM-OQAM can be broadly classified as preamble based or scattered plot based CE. Accordingly, we classify the CE techniques adopted in the literature for filter bank OFDM-OQAM in Figure 13. Besides, we present a review of the CE techniques shown in Figure 13.  
In [118], pilot aided channel estimation schemes are implemented for estimating channel coefficients in OFDM-OQAM based communication system. A new preamble aided estimation scheme named modified interference approximation method (MIAM) is developed from the original interference approximation method (IAM) scheme for estimating channel conditions. The results are compared with existing methods such as the wide and narrow auxiliary pilot (AP) schemes. The proposed scheme (MIAM) is designed to solve the suffering of pilot symbol imaginary interference. However, from the results obtained, AP scheme achieves better estimation performance in terms of mean square error and bit error rate. In [119], a preamble-based CE scheme is proposed to improve the existing IAM estimation method. The IAM CE technique depends on the knowledge of pilot neighbourhood for the approximation of interference in order to improve CE performance. This technique (IAM) consists of IAM-R and IAM-C where R and C are real and imaginary pilot symbols, respectively. In the IAM-C method, the pilot symbols are imaginary (complex) where offset quadrature phase shift keying (OQPSK) is assumed. The proposed algorithm in [119] extends the IAM pilot-based algorithms (which have nulls at neighbouring time instants) and is built into a structure that consists of significantly larger pseudo-pilots. This new structure is called extended IAM-C (E-IAM-C). From this work, the proposed E-IAM-C algorithm provided a better estimation performance when compared to IAM-R and IAM-C. In [120], a short preamble-based CE algorithm is developed where the preamble consists of only one pilot FBMC symbol for highly frequency selective  
Figure 13: CE schemes for filter bank OFDM-OQAM. channels in OFDM-OQAM systems. Simulation results demonstrate the effectiveness of this algorithm in both frequency and time domain IAM-C CE methods for mildly and highly frequency selective channels. A novel pilot design scheme named composite pilot pair (CPP) which takes advantage of the filter's localisation properties for a simple CE method at receiver is proposed in [121]. It captures the major part of channel induced distortions. The results are compared with existing AP schemes in terms of mean square error (MSE) and un-coded BER. The proposed CCP demonstrates superior performance in comparison to the existing AP schemes. In [122], a novel preamble-based algorithm is proposed for narrowband per-subcarrier maximum likelihood (ML) CE. This new method is based on the ML channel estimator. The major extension in this method involves the assumption that only the training sequence transmitted in the observed sub-carrier is known while unknown (data) symbols are transmitted in two neighbouring sub-carriers that are immediately adjacent to themselves. This new CE method is improved using the expectation maximisation (EM) algorithm and it is designed to iteratively approach and improve the ML estimator's performance. The EM-ML channel estimator is known for providing an increase in spectral efficiency as only a few numbers of sub-carriers are filled with training symbols. In [115], a performance comparison between two models for the received sub-carrier signals is made. This includes a per-subcarrier broadband channel model as well as its narrowband equivalent. Three cases of interference embedded CE schemes are considered and applied to both the broadband and narrowband channel models.