# Hybrid FSK-FDM Scheme for Data Rate Enhancement in Dual-Function Radar and Communication
## 1 Introduction  
Spectrum-sharing techniques have received substantial attention from the research community to meet the high data rate demands of the users for 5G and beyond applications [1, 2, 3]. Modern technologies are prevalent today, such as smartphones, autonomous vehicles, wearable devices, etc. These technologies have made it easier for people to communicate, access information, and move around. They are changing the way we live, work, and interact with the world [4]. In response to the increased demand for the same spectrum, congestion has arisen. Due to this crisis, radar and communications systems, traditionally designed and developed independently, have emerged as a unified or compatible system [5]. A solution to the problem of two or more systems sharing the RF spectrumsimultaneously has been proposed in the literature known as spectrum sharing. Therefore, to cope with 4G and 5G services, frequency sharing has become an increasingly critical issue for communication devices [6]. This has encouraged researchers to explore more innovative ways to maximize the available spectrum. Due to its massive allocation of frequencies, the radar spectrum is a viable candidate for frequency sharing. Note that radar applications, nowadays, are not limited to monitoring purposes only. They are used for geophysical checking, weather monitoring and forecasting, air traffic surveillance, etc. [7]. Although the frequency spectrum (e.g., between 1 to 10 GHz) was originally divided between radars and communication system applications, a significant percentage of these frequencies are distributed among radars. Moreover, high frequencies, e.g., millimetre wave band frequencies, benefit communication system designs to achieve high data rates. Still, these are also used for radar systems as well for improved detection and tracking performances. More importantly, designing a hybrid radar and communication system in necessary to meet this new era's requirements. On the other hand, mutual interference issues may arise, concerning both military and civilian applications. However, due to rapid growth in the cellular sector, today's challenge is to maintain a quality service with higher data rates. For improved joint radar and communication (JRC) designs, all the aforementioned challenges need to be addressed, including identifying wireless broadband frequency bands. According to studies [8; 9; 10; 11], GSM systems (GPRS, EDGE) can interfere with UHF radars operating in the L band between 1 and 2 GHz; whereas, in the S-band, long-term evolution (LTE) and WiMax overlap with airport surveillance or air traffic control (ATC) radar with frequencies between 2 and 4 GHz. Moreover, WiMax and radar overlaps are also mentioned in [12; 13; 14]. It is also worth noting that millimetre waves, used for orthogonal frequency division multiplexing (OFDM), single carrier, and WLAN, ranging from 11 to 33 ft, used for indoor communication, overlap with high-resolution imaging radar systems. Similarly, the same OFDM-based wireless LAN (WLAN) commonly used for outdoor activities, with ranges between 100 m and 5 km, overlaps with weather radars operating between 2 and 4 GHz in the C band. In order to make wireless communication more efficient, efforts must be made to devise a way of sharing the spectrum to meet extra bandwidth needs. This will benefit us economically, politically, and socially in the near future [13]. Recently, dual-function radar and communication (DFRC) designs have been proposed to share the radar spectrum with communication designs [15]. Generally, the radar operations are performed primarily in a DFRC system, whereas the communication operations are secondary [4; 16; 17]. Note that the efficient collaboration of the radar and communication systems is required to provide diverse spectrum-sharing methods to meet the data rate requirements for the desired services [18]. Moreover, a few innovative designs and techniques, e.g., cognitive radios and radars [19; 20], may open new horizons in terms of usage and efficiency.