# Hybrid FSK-FDM Scheme for Data Rate Enhancement in Dual-Function Radar and Communication
## 1 Introduction
Note that radar and communication design sharing can be achieved initially in terms of time and frequency. In time-based sharing, a strobe switch allocates slots to radar and communication receivers [21]. Contrary to the aforementioned method, the spectrum-sharing-based approach provides opportunities simultaneously, leading to an emerging research domain named communication and radar spectrum sharing (CRSS) or integrated sensing and communication (ICAS). Two broad categories can be identified within this research field, i.e., joint radar and communication coexistence (JRCC) and dual-function radar and communication (DFRC). In JRCC, the radar and communication designs use their own transmitting hardware and only share the available frequency spectrum. A radar system of this type is also called an opportunistic system, in which the radar holds the primary position while the communication takes on secondary roles. In addition to coexistence, cooperation, and co-design, JRCC can be divided into three subcategories [22]. Coexistence setups mitigate interference without exchanging information. In contrast, the primary objective of a cooperation setup is to explicitly share information with the beneficiaries, such as radars and communication systems [23]. On the other hand, the DFRC design uses a suitable transmitter that simultaneously facilitates both the radar and communication system in terms of hardware and frequency spectrum, where the system performance is improved by using shared knowledge. More importantly, the spectrum in the DFRC design can be shared with the wireless communication system by three well-known methods, i.e., (a) radar waveform, (b) communication waveform, and (c) multi-beam methods [1]. As radar is a primary operation, the information bits are generally embedded into radar-based waveforms [16]. However, all these techniques require further investigation to improve performance, including waveform diversity with reduced hardware costs for efficient spectrum utilization [24]. The embedding of the information bits into the radar-based waveforms using time-modulated arrays is presented in [25], where the information is encoded in the side lobe levels (SLLs) of the radiated radar beam pattern. Next, the information is decoded at the communication receiver by computing the received power level. Although this SLL-based design is easy to implement, the data rate is significantly low. To make an improved and secure transmission, amplitude shift keying (ASK) has been proposed for information embedding towards communication receivers located in the side lobe regions of the radar beam [26]. The main disadvantage of the ASK-based modulation is its poor performance and bit rate deterioration if both the radar and communication receivers reside at the main lobe of the radar beam pattern. Fortunately, a phase shift keying (PSK)-based technique has been proposed to overcome this issue [25]. The PSK-based technique performs well for the communication receiver located in the main lobe region of the radar beam. The authors of [27] claimed that the PSK-based method is more secure than the ASK method because interference can disintegrate the SLL compared to the phases of the waveform. Unfortunately, the overall communication system performance drastically deteriorates when amplitude modulation-based bit embedding acts in the main lobe region and phase modulation acts in the side lobe regions, respectively. This motivates us to find a method suitable for both the main lobe and side lobe-based communication directions. Furthermore, the QAM-based information embedding was discussed in [28], where SLL and waveform diversity were efficiently controlled, claiming performance efficiency compared to the existing ASK- and PSK-based techniques. For more information on existing information embedding techniques utilizing radar-based waveforms, communication-based waveforms, and sub-beam sharing techniques, readers may refer to [1] for more details.