# Intrinsic Interference Cancellation Scheme for FBMC-OQAM Systems Based on Power Multiplexing
## 1 Introduction  
Multicarrier modulation is a crucial technique that enables high spectral efficiency and robustness to multipath fading. As one of typical multicarrier modulation techniques, orthogonal frequency division multiplexing (OFDM) has been widely used in 5G wireless communication [1]. However, OFDM has serious spectrum leakage, strict synchronization requirements and excessive dependence on cyclic prefixes, being not in line with future mobile communication requirements. As a promising candidate of OFDM, filter-bank multicarrier (FBMC) adopts a well-designed prototype filter to shape subcarriers, reducing spectral sidelobes significantly [2]. Additionally, a well-designed filter bank makes it less sensitive to the frequency offset, which relaxes synchronization requirements. Furthermore, FBMC needs no cyclic prefixes and has a higher spectral efficiency, making it more competitive in future communication systems. However, unlike the OFDM system, which satisfies the orthogonality in a complex domain, the orthogonality of FBMC is only satisfied in a real domain, resulting in intrinsic interference from FBMC data symbols to pilot symbols [3]. The intrinsic interference is connected with the filter bank adopted and data symbols transmitted but independent of channel states and environmental noise. Unfortunately, it degrades FBMC channel estimation performance seriously because of inaccurate pilot symbols [4]. As a result, the intrinsic interference must be eliminated clearly before channel estimation in the receiver. Many methods of intrinsic interference cancellation have been proposed, among which the pairs of pilots (POP) method is an effective solution. It assumes that two adjacent moments of the subcarrier have the same channel responses and thus writes out a set of equations about the channel responses at two adjacent pilot locations and then solvesthe equations to obtain correct channel estimation [5]. However, the estimation result is affected by the noise of pilot pairs. Based on the POP method, the authors in [6] utilized complex-valued symbols as pilot pairs to control the noise effect, whereas the authors in [7] solved it based on a preamble design, but they both led to a higher complexity. Another method named interference approximation method (IAM) adopted the preamble-based structure and estimated intrinsic interference by setting zeros or known symbols around the pilot symbol [8; 9]. However, IAM has a heavy pilot burden, which results in low spectral efficiency. An improved method in [10] did not require zeros between pilot symbols and data symbols by utilizing the distribution characteristic of demodulated offset quadrature amplitude modulation (OQAM) symbols, whereas the pilot overhead still needed two columns of OQAM symbols. An iteration method based on IAM did not set zeros around pilots but adopted the idea of an iteration to estimate unknown data symbols with known pilot symbols [11]. After several iterations, ideal estimates could be obtained with the price of a high computational complexity. Other improved iteration methods eliminated intrinsic interference more clearly, but the computation complexity became more unacceptable [12; 13; 14]. In summary, these methods estimate or eliminate intrinsic interference at the receiver. However, signals at the receiver have more uncertainty compared with signals at the transmitter, whereupon higher complexity is needed to achieve satisfactory results when eliminating intrinsic interference at the receiver. On the contrary, the interference cancellation method (ICM) eliminates intrinsic interference at the transmitter to reduce receiver burden [15; 16]. However, ICM adopts a preamble-based structure where pilot symbols concentrate in the front part of the FBMC symbol, leading to a limited application scenario. In this context, a scattered pilot-based structure is a better choice that inserts pilot symbols into the whole FBMC symbol separately, contributing to a widely used channel estimation.