# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 1 Introduction
The scheme addresses the problem of bandwidth wastage encountered in FDMA schemes by the overlapping of the orthogonal sub-channels [3; 8]. The introduced CP in this scheme is used to offer robustness against the harmful effects of multipath fading and it is usually implemented based on inverse fast Fourier transform/fast Fourier transform (IFFT/FFT) [9]. Due to these improvements, the CP based OFDMA (CP-OFDMA) scheme is recommended as the main multiple access method for the fourth generation (4G) of mobile networks, which is designed based on the long-term evolution (LTE) standards for wireless broadband communication systems. Despite the improvements of the CP based orthogonal frequency division multiplexing (OFDM) technology, it is not considered as the most promising multiplexing scheme for the neXt generation (xG) of wireless networks which is the emerging fifth generation (5G) networks. It is proposedthat the emerging 5G wireless networks be designed using filter bank multi-carrier (FBMC) based transceivers. FBMC based schemes with offset quadrature amplitude modulation (FBMC-OQAM) provide an alternative modulation scheme that limits the shortcomings of CP-OFDM systems by the elimination of CP; hence, resulting in higher system spectral efficiency and provision of enhanced robustness to synchronisation errors [10]. Various FBMC systems have been considered as potential waveforms for the emerging 5G networks of wireless systems and beyond. These include filter bank orthogonal frequency division multiplexing with offset quadrature amplitude modulation (OFDM-OQAM) [11; 12], universal filtered multi-carrier (UFMC) [13; 14], bi-orthogonal frequency division multiplexing (BFDM) [15; 16] and generalised frequency division multiplexing (GFDM) [17; 18]. The concept of perfect reconstruction (PR) or near perfect reconstruction (NPR) is used to design the aforementioned FBMC waveforms where well-designed prototype filters are used to design the synthesis and analysis filter banks of these FBMC based transceivers. This concept either replaces or minimises the CP usage that is common to CP-OFDM of the 4G networks; thus, providing higher spectral efficiencies and data rate improvement. The transmitted symbols of a baseband signal are usually corrupted by noise as they propagate through the communication channel. They experience detrimental effects such as multipath propagation and fading which eventually makes it difficult for accurate detection of the transmitted symbols. For accurate detection of the transmitted symbols, pinpoint channel estimation (CE) is required [19; 20]. Channel estimation makes it possible for the receiver to approximate the channel impulse response (CIR) of the communication channel and to predict/understand the effects of the communication channel on the transmitted symbols. This is required for perfect reconstruction of the transmitted symbols. Several CE algorithms have been proposed in the literature for general wireless communication systems. This paper reviews and classifies the CE schemes in the literature. In particular, an overview of the CE schemes adopted in the literature for filter bank multi-carrier based systems is presented, where we classify the CE schemes used in filter bank OFDM-OQAM and GFDM-OQAM transceivers. Moreover, the paper reviews the performances of two linear CE schemes and three adaptive CE schemes for the two aforementioned FBMC-based waveforms assuming near perfect reconstruction (NPR) and non-perfect reconstruction (Non-PR) filter designs over slow and fast frequency selective Rayleigh fading channels. Note that, to efficiently review the performances of these CE schemes, we present a step-wise system description and implementation of the filter bank OFDM-OQAM and GFDM-OQAM FBMC-based transceiver. In addition, since the designs of these FBMC transceivers are based on filters, we firstly present a review of the filter design in Section 2. The contribution and relevance of this paper are as follows.