# Integrated Sensing and Communication Using Random Padded OTFS with Reduced Interferences
## 1 Introduction  
As 5G communication systems were delivered in 2019, today the scientific community is focused on next-generation systems, called 6G. Currently (July 2025), there is no official specification for 6G from the 3 GPP group, but there are a lot of publications on the vision of 6G systems--the most noticeable are [1, 2]. There are different opinions on some aspects of 6G. At the same time, there are some common points in most of the articles: increasing the scale of massive multiple input multiple output (mMIMO) technology, the usage of intelligent reflecting surfaces (IRS) [3], a more reliable network with more spectral efficiency, the use of artificial intelligence (AI) technologies [4], etc., but the most important of the common points for new waveform research are higher operating frequencies and the emergence of integrated sensing and communication (ISAC) [5, 6, 7, 8, 9, 10]. The increase in operating carrier frequency is the answer to the demand for an increasing data rate for potential 6G applications, for example, holographic video calls [2]. But this also brings new challenges. One of the main problems in physical layer processing is the Doppler effect. From a Doppler frequency shift point of view, the increase in the carrier frequency will further aggravate the situation, since the Doppler frequency shift is directly proportional to the carrier frequency. Orthogonal Frequency Division Multiplexing (OFDM) (de facto the standard waveform for most modern communication systems since the mid-1990s) assumes that the channel impulse response during the OFDM symbol does not change. This makes the usage of the OFDM waveform in a strong Doppler environment very problematic. For example, in 5G, special numerologies for frequency range 2 (FR2--above 24 GHz) [11] were introduced to make OFDM symbols shorter and to adapt OFDM for a strong Doppler environment [12]. But this also has its own drawback: more bandwidth is occupied for the same number of subcarriers. Moreover, the channel coherence time limits the length of the OFDM symbol, while on the other hand, the coherence bandwidth sets the spacing between subcarriers. In addition to this, considering that the bandwidth occupied by the signal is inversely proportional to its duration makes the situation even more contradictory. To better understand this, let us consider the following example. Let the number of OFDM subcarriers be constant. To operate in channels with strong Doppler effect (short coherence time), we want to shorten the duration of the OFDM symbol while keeping the number of subcarriers constant. This means increasing the spacing between them. However, on the other hand, increasing the sub-carrier spacing can lead to exceeding the coherence interval in the frequency domain. It should be noted that all OFDM symbols have a short cyclic prefix, not only to reduce transmission overhead, but also because the large spacing between subcarriers means a small mean delay time [13]. The Orthogonal Time Frequency Space (OTFS) waveform was designed to ensure reliable communication in a strong Doppler environment. OTFS is a two-dimensional modulation technique that modulates data in the Doopler-delay (DD) domain, not in the time-frequency (TF) domain like in OFDM. This new modulation is very attractive for ISAC systems, as in the case of OTFS, both parts of the system (sensing/radar and communication) use the DD domain to represent the data [14]. More details on OTFS can be found in [15; 16; 17; 18]. For the radar subsystem, there are two main approaches in (ISAC) system design, based on the estimation of the following: (1) the cross-ambiguity function (CAF) [19], and (2) the channel state information (CSI) [5]. The CAF-based processing practically does not depend on the waveform, and in the case of the OTFS, it could be the same as in the case of the OFDM [20].