Constant Envelope OTFS Waveform Design and Performance Analysis for Mobile ISAC Systems

Integrated Sensing and Communication (ISAC) technology leverages the incorporation of sensing capabilities into communication systems, meeting the demands for intelligent applications in sixth-generation (6G). As a result, it has garnered increasing attention in recent research and development. Orthogonal Time Frequency Space (OTFS) modulation is a promising high-mobility ISAC modulation technique that has also resulted in the emergence of various waveform design methods with excellent performance. However, existing OTFS-based waveform designs primarily focus on reducing PAPR, without making the reduction to zero decibels the main research objective. In this paper, we put forward a Constant Envelope (CE) OTFS waveform design approach. We first oversample the symbol sequence after QAM modulation. Through the OTFS transformation and phase modulation, we then obtain a CE-OTFS signal with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0~\mathbf {dB}$</tex-math></inline-formula> PAPR, resolving the high PAPR issue associated with traditional OTFS signals. Furthermore, the closed-form expressions of PAPR, Bit Error Rate (BER), and Ambiguity Function (AF) for the proposed CE-OTFS ISAC waveform are derived, confirming the practicality of the CE-OTFS waveform for ISAC uses. The numerical and simulate results show that the designed CE-OTFS waveform with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0~\mathbf {dB}$</tex-math></inline-formula> PAPR outperforms current ISAC waveforms including OFDM, OTFS, CE-OFDM in terms of sensing and communication performance.