# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 6 Findings and Possible Research Directions  
Channel estimation is an integral part of general wireless communication systems, most especially, with the emerging 5G technologies. The f-OFDM, filter bank OFDM-OQAM, BFDM, UFMC and GFDM-OQAM based transceivers are identified as suitable modulation schemes for the currently evolving xG of wireless networks. However, the varying nature of the communication channel has been a major challenge to the use of these aforementioned filter bank based transceivers. Different research works are currently ongoing in order to estimate the fading channel characteristics of these transceivers, thereby providing effective and reliable solutions to the use of FBMC based schemes. Extensive research  
Figure 23: Estimators performance comparison in terms of NMSE versus SNR. Figure 24: Estimators performance comparison in terms of NMSE versus SNR: NPR filter design. Figure 25: Estimators performance comparison in terms of NMSE versus SNR: Non-PR filter design. works have been carried out to estimate the channel fading features when adopting CP-OFDM and SC-FDMA transmission technologies as the underlying modulation schemes (including filter bank based OFDM-OQAM systems); nevertheless, little or no work has been done using GFDM-OQAM as the underlying modulation scheme in the FBMC system. In fact, very little research work has been carried out in this area in terms of developing novel algorithms for estimating the channel fading. GFDM-OQAM is considered to be one of the most promising candidate for the air interface of 5G networks. As such, CE for filter bank GFDM-OQAM (with particular reference to semi-blind and blind techniques) is still an open area for further research, in order to aid this modulation scheme to attain its full potential. Although this paper reviews the performances of filter bank OFDM and GFDM based schemes for two linear and three adaptive CE schemes assuming OFDM-OQAM and GFDM-OQAM as the underlying modulation, we emphasise that extensive CE research in this area can further improve its performance. Of particular interest, developing specific CE algorithms for these filtered based systems will be a plus. Likewise, developing specific CE schemes for BFDM-OQAM transceivers is still an open area of research. BFDM is also a potential candidate for the upcoming xG wireless networks because of its peculiar pulse shapes. However, as earlier mentioned, only a few works have analysed the performance of CE schemes for BFDM-OQAM. Thus more research works can be actualised in this area in order for BFDM schemes to attain its full potential. Moreover, CE for f-OFDM systems (which is considered as a potential waveform candidate of xG networks) is an open area for future research, as the performance of CE methods for this waveform is not well researched and documented in the literature. To ensure excellence signal transmission and receptions when using f-OFDM as the underlying network waveform while primarily considering user QoS and QoE enhancement, adequate CE techniques for this waveform is required. Additionally, as shown in the results presented in Section 5, we observe that the adaptive CE schemes perform better than the linear CE schemes for both the NPR and Non-PR conditions. This showcase the adaptive CE schemes as a better alternative to the linear CE schemes for FBMC systems, whereby researchers and practitioners in this field can easily make meaningful decisions based on their application requirements. However, as mentioned, the FBMC transceiver application requires specific CE algorithms for both waveform candidates considered in this paper, and other promising waveforms such as UFMC, BFDM, f-OFDM. More importantly, application specific CE algorithm is required for the FBMC transceiver because the studied CE schemes, both the adaptive and linear schemes, plummet when Non-PR condition is assumed. In reality, it is difficult to reconstruct a near perfect condition. This means we cannot eliminate error completely in real-time.