On the performance of hybrid carrier system with spectrum precoding based on WFRFT

Wang et al. EURASIP Journal on Wireless Communications and Networking (2017) 2017:102 DOI 10.1186/s13638-017-0890-7 RESEARCH Open Access On the performance of hybrid carrier system with spectrum precoding based on WFRFT Zhenduo Wang1,2 , Lin Mei1* , Xiaolu Wang1 , Xuejun Sha1 and Naitong Zhang1 Abstract High out-of-band (OOB) power emission has become the main shortcoming of multicarrier scheme in regard to the requirements and challenges of 5G. In this paper, two effective OOB power reduction methods, previously applied to traditional orthogonal frequency division multiplexing (OFDM) system, are proposed in hybrid carrier (HC) scheme based on weighted-type fractional Fourier transform (WFRFT). Simulation results demonstrate that, due to the flexible selection of WFRFT order in the HC system, combined bit error rate (BER) and peak-to-average power ratio (PAPR) performance advantages are gained without sidelobe impact and significant complexity increase in comparison with the OFDM scheme with spectrum precoding. In projection precoding, a smaller error vector magnitude of precoder Gχ is obtained in the proposed hybrid carrier scheme. In SVD precoding with an orthogonal decoding at the receiver, better BER performance could also be acquired at WFRFT order over the fading channels. The proposed two WFRFT-based structures are complementary, and their potential scenarios are given. In addition, applying the feature of projection precoding to channel estimation, a novel pilot structure based on WFRFT is posed to reduce the introduced error of spectrum precoding and finally helps to improve the BER performance. Keywords: Hybrid carrier (HC), Weighted-type fractional Fourier transform (WFRFT), Spectrum precoding, Out-of-band (OOB) power, Channel estimation 1 Introduction Recently, the efficient utilization of idle spectrum becomes the main means to solve the shortage of limited spectrum resources in modern wireless communication system. Cognitive radio and spectrum sensing [1–3] are usually the mainstream technologies to detect the free spectrum. In addition, reducing the unnecessary protection interval between sub-bands through suppressing the out-of-band (OOB) power of in-band data is another effective method to relieve the increasingly scarce spectrum resources. High OOB emission is always the shortcomings of traditional multicarrier schemes and becomes the opposed target of novel waveform candidates of the fifth generation (5G) with multi-antenna [4, 5]. For instance, traditional orthogonal frequency division multiplexing (OFDM) system suffers high OOB leakage *Correspondence: Communication Research Center, Harbin Institute of Technology, Harbin, China Full list of author information is available at the end of the article 1 due to slow tailing attenuation of sinc function and needs the protected subcarriers to inhibit OOB power radiation. Current 5G candidate waveforms such as generalized frequency division multiplexing (GFDM) and filterbankbased multicarrier (FBMC) in [6–8] focus more on the low OOB radiation through pulse shaping of the transmitted signal. Existing methods for reducing sidelobe of multicarrier scheme are proposed, involving time domain windowing, subcarrier weighting, insertion of cancelation carriers (CCs), and precoding. In [9], the used subcarriers are multiplied by subcarrier weights for sidelobe suppression based on an optimization algorithm with several optimization constraints. In [10], a multicarrier spectrum sculpting precoder is put forward with flexible choice of the notching frequencies and lower transmitter complexity than that in [9] but suffers bit error rate (BER) performance degradation due to introduced error. An orthogonal multiplexing is proposed in [11] to improve the BER performance according to the knowledge of a © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Wang et al. EURASIP Journal on Wireless Communications and Networking (2017) 2017:102 precoder at the transmitter. However, how to achieve the channel estimation at the receiver becomes a problem as a result of precoded data and pilot symbols. In [12], two kinds of structures of pilot carriers are proposed on the basis of the precoder in [10]. The practical design for out-of-band emission reduction and adjacent channel interference rejection in the OFDM system is put forward in [13]. N-continuous OFDM is proposed in [14] through precoding the information symbols and achieves obvious out-of-band power reduction with slight decline of reliability. A precoding scheme which focuses on suppressing out-of-subband emission of the DFT-based OFDM system is proposed in [15], whose BER performance is improved as a result of frequency diversity. In addition, there are also other technologies to shape the OFDM signals for sidelobe suppression. The method of inserting several CCs at each side of the OFDM spectrum is posed in [16] with a small degradation of system performance. In [17], the sidelobe of the licensed user in cognitive radio system is reduced through adding extended active interference cancelation signals. The optimal orthogonal precoding of DFT-based system is derived in [18] through accounting the leaked power as a matrix Frobenius norm minimization problem. In [19], an orthogonal projection matrix is given with balanced complexity and out-of-band power suppression performance. Without significant BER degradation, a prescribed mask is used to control the OOB emissions beneath a particular power level in [20] with more distortion on edge subcarriers. An overview of existing OOB reduction techniques is proposed in [21]. All in all, the OOB power radiation is always reduced accompanied by deteriorated BER performance. The high PAPR is another ubiquitous problem of multicarrier schemes including OFDM and other novel waveform candidates such as FBMC and GFDM. An overview of PAPR reduction techniques for OFDM signals has been given in [22], including clipping, coding, partial transmission sequence (PTS), selective mapping (SLM), nonlinear companding transforms, and tone reservation. However, few literatures focus on the PAPR reduction methods from the aspect of the carrier scheme. Moreover, joint PAPR reduction and sidelobe suppression is achieved in [23] through dynamically extending part of the constellation points on the secondary user subcarriers and adding several signal cancelation symbols on the primary user subcarriers. The optimal cancelation signal is obtained by solving a quadratically constrained quadratic program. A suppressing alignment approach is posed in [24] to achieve OOB power (...truncated)