Peak-to-Average Power Ratio Reduction via Symbol Precoding in OTFS Modulation

Orthogonal time frequency space (OTFS) has recently attracted widespread attention for it leverages frequency dispersion as a source of diversity and mathematically unifies the classical multiple-access schemes. However, as a multi-carrier modulation in nature, OTFS is susceptible to the problem of high peak-to-average power ratio (PAPR), especially when the number of symbols is large in order to obtain a high Doppler resolution at the receiver. In this work, we recast the problem of PAPR reduction as constrained optimization of the precoding matrix. To efficiently solve the underlying nonconvex maximum-norm minimization problem, we propose an iterative algorithm based on block coordinate descent. Simulation results show that the proposed method can significantly mitigate the PAPR without unduly compromising the reliability of data transmission.

I. Introduction

AIR interface in the next-generation wireless networks is expected to offer higher flexibility and suit a broader variety of deployment environments for emerging mobile services. To satisfy the distinctly different performance requirements of diverse usage scenarios, specifically enhanced redesign of the physical-layer modulation technique is imperative. High mobility represents one particularly important yet challenging use case to support, as the demand for mobile services in vehicles and high-speed trains is growing rapidly, while the present mainstream waveforms such as orthogonal frequency division multiplexing (OFDM) exhibit severe performance degradation in highly dynamic channel conditions [1]. Orthogonal time frequency space (OTFS), a recently proposed two-dimensional (2D) multi-carrier modulation technique [2], is recognized as a promising candidate for future high-mobility integrated sensing and communication. [3],[4]. In contrast to OFDM, OTFS implements channel representation and symbol multiplexing in the transformed delay-Doppler domain (a.k.a. Zak domain) instead of time-frequency (TF) domain. By spreading the basis waveform over the entire TF plane, OTFS effectively converts the time-varying fading channel into an invariant flat fading equivalence and, thereby, achieves improved reliability in Doppler/multipath channel conditions [5]–[7].