  Sixth-generation (6G) wireless communication systems, as stated in the
European 6G flagship project Hexa-X, are anticipated to feature the integration
of intelligence, communication, sensing, positioning, and computation. An
important aspect of this integration is integrated sensing and communication
(ISAC), in which the same waveform is used for both systems both sensing and
communication, to address the challenge of spectrum scarcity. Recently, the
orthogonal time frequency space (OTFS) waveform has been proposed to address
OFDM's limitations due to the high Doppler spread in some future wireless
communication systems. In this paper, we review existing OTFS waveforms for
ISAC systems and provide some insights into future research. Firstly, we
introduce the basic principles and a system model of OTFS and provide a
foundational understanding of this innovative technology's core concepts and
architecture. Subsequently, we present an overview of OTFS-based ISAC system
frameworks. We provide a comprehensive review of recent research developments
and the current state of the art in the field of OTFS-assisted ISAC systems to
gain a thorough understanding of the current landscape and advancements.
Furthermore, we perform a thorough comparison between OTFS-enabled ISAC
operations and traditional OFDM, highlighting the distinctive advantages of
OTFS, especially in high Doppler spread scenarios. Subsequently, we address the
primary challenges facing OTFS-based ISAC systems, identifying potential
limitations and drawbacks. Then, finally, we suggest future research
directions, aiming to inspire further innovation in the 6G wireless
communication landscape.
