Precoded Faster-Than-Nyquist Signaling Using Optimal Power Allocation for OTFS

A precoded orthogonal time frequency space (OTFS) modulation scheme relying on faster-than-Nyquist (FTN) transmission over doubly selective fading channels is proposed, which enhances the spectral efficiency and improves the Doppler resilience. We derive the input-output relationship of the FTN signaling in the delay-Doppler domain. Eigenvalue decomposition (EVD) is used for eliminating both the effects of inter-symbol interference and correlated additive noise encountered in the delay-Doppler domain to enable efficient symbol-by-symbol demodulation. Furthermore, the power allocation coefficients of individual frames are optimized for maximizing the mutual information under the constraint of the derived total transmit power. Our performance results demonstrate that the proposed FTN-based OTFS scheme can enhance the information rate while achieving a comparable BER performance to that of its conventional Nyquist-based OTFS counterpart that employs the same root-raised-cosine shaping filter.

I. Introduction

Traditional wireless technologies, such as orthogonal frequency-division multiplexing (OFDM), typically struggle to cope with high Doppler shifts in high-mobility scenarios. In order to overcome this limitation, orthogonal time frequency space (OTFS) modulation [1] was proposed, which modulates data symbols in the delay-Doppler (DD) domain and allows a sparse and quasi-static channel representation. Several studies [2], [3], [4] considered the OFDM-based OTFS (OFDM-OTFS) architecture, where a cyclic prefix (CP) is inserted in each frame, while employing an ideal rectangular pulse shaping filter that satisfies bi-orthogonality in the time and frequency domains [1]. By contrast, the input-output relationship of OTFS using a non-rectangular pulse shaping filter was derived in [5].