# Channel Estimation Based on IOTA Filter in OFDM/OQPSK and OFDM/OQAM Systems
## 2 Related Work  
Recently, many experiments and theoretical analyses of OFDM-IOTA systems have been proposed. A new pilot and preamble structure for the channel estimation appropriate for the OFDM/OQAM system was proposed. A limitation of the paper was that it was implemented in an uncoded OFDM system [4]. Although a comparison of cyclic prefix (CP)-OFDM with OFDM-IOTA under a typical system was proposed, the disadvantage of the system was the existence of frequency offset error at the receiver [5]. A hardware architecture of the pulse shaping filter used in multicarrier systems was proposed in [6]. IOTA pulse shaping filters were used to reduce hardware overhead in multicarrier systems based on faster-than-Nyquist (FTN) signaling. The IOTA filter has an overhead of 28% in silicon area compared with the FTN iterative decoder. The disadvantage is that the high complexity of the hardware mapped architecture. A multiple-input multiple-output (MIMO)-IOTA system was proposed in [7], and the MIMO-IOTA system achieved better performance compared with an OFDM system at low-to-medium signal-to-noise ratios (SNRs). Raised cosine, IOTA, Hermite, and Gaussian wavelets were proposed in [8]. The BER results of the IOTA method were better than those of the square function method and slightly worse than the Hermite function method. As the SNR increased, the Hermite filter showed the best BER performance compared with IOTA and square filters. The disadvantage of the Hermite filter is that it only concentrates on ICI suppression and does not consider the effect of different types of multipath in the simulation results. The capacity of OFDM/OQAM systems with IOTA filters was evaluated through information theoretical analysis in [9]. The promising results motivate further research on the utilization of the intrinsic interference in order to explore the potential to attain maximum performance gain in future OFDM/OQAM systems. OFDM/OQAM systems showed significant spectral efficiency while compared with CP-OFDM systems.  
In [10], the BER performance of OFDM interleave division multiple access (IDMA) with wavelet families, such as Haar, Daubechies, and Symlets, was compared over an AWGN channel. The Daubechies wavelet has the best BER performance under 16-quadrature amplitude modulation (16-QAM). The circular 16-QAM constellation has been derived and applied in Fourier- and wavelet-based OFDM systems [11]. The circular scheme showed slightly better BER performance than the square scheme. The reason is that the power consumption of the circular constellation signals is less than that of the square scheme. To maximize the system capacity, the filter-bank-based multicarrier (FBMC) scheme provides higher capacity performance compared with the OFDM system [12]. In contrast to the OFDM system, the FBMC waveform has been demonstrated to be less sensitive to timing error between the different cells, due to the better frequency localization of the prototype filter. A new OFDM system based on a discrete cosine harmonic wavelet transform (DCHWT) for binary phase shift keying (BPSK) and QPSK signals was proposed in [13]. The proposed DCHWT-OFDM system provided superior performance in terms of BER and peak-to-average power ratio (PAPR) compared with the discrete cosine transform (DCT)-OFDM, discrete Fourier transform (DFT)-OFDM, and Haar WT-OFDM systems. However, the DCHWT-OFDM and Haar WT-OFDM systems may not have significant BER improvements under 16-QAM and 64-QAM signal constellation.