## 1 Introduction
OTFS multiplexes information symbols over two-dimensional orthogonal basis functions in the delay-Doppler domain to combat the dynamics of the time-varying multipath channels [25]. As OTFS still converts symbols to the time-frequency domain, it can be expected to have a similar spectrum to OFDM, which means that OTFS improves communication performance under high-mobility channels while retaining the high-precision measurement capability of OFDM. Therefore, the application of OTFS modulation to the LEO satellite constellation is expected to provide better broadband communication and PNT services. In recent years, OTFS has been investigated for use in LEO satellite constellations with the expectation of providing PNT services [26]. A recent study preliminarily explored the performance of OTFS as a LEO navigation signal; the results show that the theoretical delay and Doppler measurement accuracy of OTFS are better than BPSK, but no receiver was developed to verify the conclusions [27]. In another work, a red and blue frequency shift discriminator for OTFS modulation was designed to enable high-precision Doppler measurements with LEO signals [28]. An OTFS-based communication and navigation-integrated signal superimposes the AltBOC-modulated navigation message with quadrature amplitude-modulated communication data and then the symbols are transmitted via OTFS modulation, but the signal still utilizes time-domain PRN sequences for delay measurements rather than OTFS itself [29]. Some other studies have focused on pilot-based communication channel estimation [30; 31; 32], but these works cannot be directly applied to navigation. Currently, LEO satellite navigation signals face a challenge, i.e., neither traditional navigation modulation nor OFDM reduces acquisition complexity; at least to the best of our knowledge, no studies have shown that they have an advantage in this regard. It is well known that the acquisition of navigation signals is a three-dimensional search process of the satellite, the code phase, and the Doppler frequency. For a specific satellite, the acquisition complexity mainly depends on the loss introduced by the Doppler frequency offset in the signal cross-correlation, and the larger this loss, the longer the frequency search time required [7]. It can be anticipated that the Doppler loss for a pulsed signal is less than that for a continuous signal for the same power, because the former has a shorter coherent integration time, even though they have equal signal periods. The signal of the Locata system can be operated in a pulse amplitude modulation (PAM) with different duty cycles, power output, and pseudorandom noise (PRN) codes; PAM is commonly used to reduce interference and increase the working range, i.e., to overcome the \"near-far\" effect [33; 34]. Numerous experiments have demonstrated that the Locata signals satisfy most scenarios where the GNSS is not available [35]. If the pulsed signals are applied to LEO satellites, it is expected that the correlation loss caused by Doppler frequency offset can be mitigated to reduce the acquisition complexity of the signal. Inspired by OTFS modulation and pulsed signals, we propose Pulse-OTFS for LEO satellites that achieves low acquisition complexity and high-precision measurements, as well as better compatibility and anti-multipath performance. We make four contributions in this paper:  
1. A novel signal named Pulse-OTFS is proposed in this paper, which has the advantages of both high-precision measurement and fast acquisition. We exploit the property of OTFS modulation to naturally convert a continuous signal into a pulsed signal by arranging multiple identical PRN code sequences in the delay-Doppler domain, which results in the generation of time-domain Pulse-OTFS signals with different duty cycles. 2.