## 1 Introduction
### Related Work  
In recent years, there has been increasing interest in developing ISAC systems, also known as joint radar and communication (JRC) systems. For instance, the study in [6] provides a survey of ISAC systems, emphasizing their potential to address spectrum scarcity and mitigate interference in wireless communications by integrating radar and communication functionalities. The study in [7] introduces a channel modeling approach for a heterogeneous vehicular ISAC system, where a multi-antenna base station communicates with a multi-antenna vehicular receiver while simultaneously sensing the environment. This study derives time-varying complex channel impulse responses for both sensing and communication channels, highlighting their correlation through shared clusters. Furthermore,the study in [8] discusses beamforming design for dual-functional radar-communication systems within 6G networks, focusing on scenarios involving imperfect channel state information. The authors propose an optimization framework to maximize radar output power while ensuring communication quality under probabilistic outage constraints. The study in [9] explores the innovative concept of non-orthogonal multiple access (NOMA) in aiding joint radar and multicast-unicast communication systems. This work proposes a framework where an MIMO dual-functional radar-communication base station utilizes the same spectrum to detect radar-centric users while transmitting mixed multicast-unicast messages to both R-users and communication-centric users. Current joint radar and communication systems may be classified into the following three categories [4; 5; 10]:  
* Communications centric design. In this class, radar sensing is an add-on to a communication system, where the design priority is on communications. Such a design exploits communication waveform to extract radar information through target echoes. * Radar-centric design. Conversely, such an approach aims at modulating or introducing information signaling in known radar waveforms. * Joint design and optimization. This class encompasses systems jointly designed from the start to offer a tunable trade-off between radar and communication performance. The main challenge in JRC development lies in finding suitable waveforms that can be simultaneously employed for information transmission and radar sensing [5]. The initial step in implementing ISAC systems involves identifying waveforms suitable for both wireless communications and radar applications. Waveform selection must align with radar requirements, particularly in terms of object range, velocity, and angle determination. Crucial criteria for all three aspects include dynamic range and resolution for effective object detection and separation, as well as robustness against interference and noise, especially in congested spectrum scenarios [4]. Presently, waveforms like FMCW (frequency-modulated continuous wave), pulsed FM, and other waveforms are being utilized for different radar applications [10]. For the communication function, the most critical parameters are data rate and bit error rate (BER). Like radar, the chosen waveform should be resilient to interference, noise, and multipath-induced distortion. Spread spectrum techniques, such as DSSS, enhance robustness by providing a processing gain at the receiver and reducing the power spectral density [2]. DSSS can be chosen as a basis for ISAC design due to two main characteristics: large bandwidth and pseudorandomness. These characteristics provide several advantages over traditional signals [11]. Specifically, (1) for radars, DSSS pseudorandom \(m\)-sequences are employed for discrete phase modulation, generating noise-like transmit signals with robust autocorrelation characteristics [12]. (2) DSSS signals effectively suppress harmful interferences caused by jamming, simultaneous users sharing the same communication channel, and self-interference due to multipath fading.