Precoded Non-Orthogonal Frequency Division Multiplexing with Subcarrier Index Modulation

In this paper, we propose a novel precoded non-orthogonal frequency division multiplexing (NOFDM) with sub-carrier index modulation (SIM) to increase spectral efficiency for multicarrier systems. The proposed NOFDM-SIM scheme is constituted by a filterbank multicarrier system under reduced subcarrier spacing, where precoded index modulated symbols are mapped onto the subcarriers. The detrimental effects of inter-carrier interference (ICI) resulting from the non-orthogonality between subcarriers are efficiently mitigated with the aid of eigenvalue-decomposition-based precoding. Our simulation results demonstrate that the proposed precoded NOFDM-SIM system exhibits a similar peak-to-average power ratio to OFDM and OFDM-SIM counterparts while achieving the bit error ratio limit of zero-ICI OFDM.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) [1] is a multicarrier transmission technique, widely employed in diverse industry standards, such as digital audio broadcasting, digital video broadcast, asymmetric digital subscriber line, powerline communications, 4G/5G mobile communication, IEEE 802.11 series. A key advantage of OFDM driving its widespread use is its ability to convert a broadband dispersive channel into multiple narrowband frequency-flat subchannels. However, the maximum spectral efficiency of OFDM is achievable when , where T is symbol interval and F is subcarrier spacing [2]. Since Gabor’s theory requires for time-frequency localization, the spectral efficiency of OFDM is compromised. Furthermore, OFDM suffers from high out-of-band (OOB) emissions. To combat the above-mentioned limitations of OFDM, several class of non-orthogonal frequency division multiplexing (NOFDM), including the filter bank multicarrier (FBMC) [3], generalized FDM (GFDM) [4], universal filtered multicarrier (UFMC) [5], spectrally-efficient FDM (SEFDM) [6], have been developed. In FBMC, by filtering each subcarrier using narrowband orthogonal shaping filters, the spreading of side-lobes, i.e., OOB emission, is reduced. Furthermore, owing to its lower sensitivity to carrier frequency offset relative to OFDM, the FBMC system typically performs better than OFDM in a high-mobility scenario. UFMC offers an intermediate choice between OFDM and FBMC, where in UFMC, each clustered set of subcarriers is filtered, which allows a shorter filter length than in FBMC. In SEFDM, subcarrier spacing in the frequency domain is set lower than that of OFDM to improve spectral efficiency. This benefit is achieved at the cost of detrimental inter-channel interference (ICI) effects in order to combat this limitation, the zero-forcing precoding [7] and the eigenvalue decomposition (EVD)-precoded diagonalization [8], [9] were developed. Also, in [8], [9], optimal power allocation employing the amplification factors formulated as the inverses of the eigenvalues was presented. Moreover, EVD-based SEFDM was extended to that supporting physical-layer security transmission [10].