# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 3 Fundamentals of Channel Estimation
### Channel Estimation for Filter Bank GFDM
The BER performance of the two proposed CE algorithms for GFDM system is analysed for various transceiver settings over AWGN channel. It is observed that GFDM systems using Tx-IC transmitter as well as MF receiver with Tx-IC CE offers relatively the same performance as GFDM system with Pilot-IC transmitter and zero forcing receivers with Pilot-IC CE. Since the two proposed CE schemes demonstrate similar performances, it is concluded in [126] that interference can be cancelled at either the transmitter or the receiver. In [125], the proposed CE scheme in [126] is compared with a novel CE scheme that is based on CS technology by the exploitation of approximate sparsity of CIR for accurate reconstruction of the channel state information (CSI) using few pilot symbols. The proposed novel OMP CS based iterative CE for GFDM in [125] is used for accurate estimation of the CIR so as to precisely reconstruct the interference caused by sub-carriers without added penalty. Simulation results demonstrate that the proposed OMP based CE scheme in [125] efficiently reduces the inherent ISI that is experienced at the receiver of the GFDM system without transmitting power penalty as compared to the existing Tx-IC method developed in [126]. In addition, when simulated using an MMSE receiver, the proposed OMP based CE scheme offers a performance that is almost equivalent to the CRB method proposed in [127]. As a result of the huge demand for the available scarce frequency spectrum, more devices are required to accommodate the improved features such as low-power consumption, low cost and computational complexity. The design improvements are considered in order to enhance the QoS delivery and QoE of network users. However, there is usually manufacturing imperfections that arise due to IQ (inphase (I) and quadrature (Q)) imbalance, which can occur either at the transmitter or receiver front-end. The imbalance consequently results in performance degradation of general wireless communication networks [124]. To reduce the undesired effects of IQ imbalance, training sequence design is investigated in [124]. The training sequence is used for estimating the IQ imbalance parameters on GFDM low complexity receivers over frequency selective channels. This shows an improvement in pilot overhead in comparison to the OFDM based estimation schemes. Moreover, the results demonstrate that the proposed scheme efficiently improves the BER performance of the GFDM based transceivers in comparison to uncompensated cases while providing reliable performances and robustness at extreme IQ imbalance environment. In [50], a novel CE method is proposed which demonstrates superior performance over conventional LS method when employing several pilot symbols per sub-carrier. The optimal choice of pilot design is analysed for GFDM systems where the performance of different pilot patterns such as block-type and rectangular grid is investigated. It is observed from simulation results that GFDM CE under rectangular grid pilot patterns attains better system performance in comparison to situations where pilots are distributed in different sub-carriers. It is also uncovered from this work [50] that GFDM demonstrates equivalent performance in comparison with OFDM systems when robust code rates (for instance, 1/3 code rate) are employed while the performance significantly improves at higher code rates (5/6 code rate) as a result of better interference rejection ratio in the GFDM system. To enhance system performances such as ultra-low latency, low OOB emission, robustness to interference and very high reliability for the emerging next-generation networks, a frequency domain IFPI scheme is developed in [51] for estimating the CIR of non-orthogonal multicarrier systems such as 2 \(\times\) 2 MIMO-GFDM system over multipath fading channel with Rayleigh distribution.