Analysis of PAPR and Pilot-Data Interference in DFRC-OTFS: Single vs Multiple Pilot Symbols

Orthogonal time-frequency space (OTFS) is a modulation scheme that transmits information over the delay-Doppler (DD) domain representation of a signal, widely used in pulseDoppler radar processing. In standard pulse-Doppler radar, the received discrete DD domain signal is given by the sum of delayed and Doppler-shifted copies of the DD domain representation of the transmitted signal, and the goal is to estimate the delays and Dopplers introduced by the scattering environment based on the knowledge of the transmitted DD signal. In contrast, OTFS places information symbols in the DD domain and aims to communicate this DD signal over the scattering environment (which is assumed known at the receiver via prior channel estimation). Dual function radar-communications (DFRC) OTFS involves placing both the radar signal and the information symbols in a single DD domain signal and achieving both, data transmission and DD estimation simultaneously. Thus, the radar DD signal has the interpretation of a pilot signal that is used to estimate the scattering channel. The pilot/radar signal governs the channel estimation accuracy, but it also affects the peak-to-average power ratio (PAPR) of the transmitted time-domain signal, and the bit error rate (BER) for decoding the data symbols due to the interference caused by pilots on the data symbols. The goal of this paper is to better understand the trade space involved in designing the pilot symbols in such a DFRC-OTFS signal. We derive the CramérRao bound (CRB) on DD estimation and use it as a reference for navigating this trade space. We show results where for a fixed estimation DD MSE performance based on the CRB, better PAPR and BER performance can be achieved at the expense of a larger pilot overhead. The framework used in this paper provides a general method for analyzing the impact of pilot symbols in DFRC-OTFS waveform design.