# Chaos Based Frequency Modulation for Joint Monostatic and Bistatic Radar-Communication Systems
## 1 Introduction  
Due to an exponential increase in the utilization of communication devices and limitations on the electromagnetic spectrum, there is tremendous demand for radio frequency systems [1] to operate simultaneously without any mutual interference. In particular, radar is now sharing its allocated spectrum, while expectations are that it should perform optimally [2]. Consequently, the coexistence of communications and radar systems is necessary now more than ever.  
Joint radar-communication systems (RadComm) can be realized using two main approaches [3]. The first approach is using a single platform where hardware is reduced for simultaneous functionality. For instance, a sampling-based technology is utilized for radar and communication spectrum sharing [4], where the communications and radar systems transmit at separate spectral bands to avoid interference. Multiple-input multiple-output (MIMO) radar and communication systems are proposed in [5, 6]. It is noted that the functionality of these systems may be preferred by examining the environment [3, 7]. A transceiver architecture along with a dual-functional radar-communication (DFRC) system using a hybrid analog-digital beamforming technique in mm waveband is proposed in [8]. With many of these proposed techniques, the complexities associated with the transmitter hardware can be reduced along with the operational and functional costs [8]. One of the major drawbacks of this approach is the mutual interference caused by each system. Additional signal processing efforts are required to mitigate the interference [9, 10].  
The second approach is a more efficient way in which a single transmitter is used for dual radar-communication functioning. A single waveform in which the information is embedded into the radar transmission can be used for the dual purpose of communicating and radar sensing. Much of the research is focused on designing such a dual waveform. An excellent review of historical development and current state-of-the-art on joint radar-communication systems along with modulation schemes and systems are presented in [11]. Examples of these waveforms include intrapulse basis waveforms [12], orthogonal frequency division multiplexing (OFDM) [13], linear frequency modulated waveforms [14; 15], phase-modulated waveforms [16], random sequence encoding [17], embedding multiple phase-shift keying symbols onto linear frequency modulated waveforms [18], etc. An overview of dual-function radar-communications (DFRC) and various waveform schemes are noted in [19]. Many of the proposed waveforms are either radar-centric or communication-centric. They either have high sidelobes suffering high-resolution imaging capabilities, or they have high bit-error rates.  
Wide bandwidth waveforms are necessary to obtain finer resolution images to classify radar targets [20]. Similarly, wideband waveforms are necessary for communication systems to transmit data at higher rates [21]. The wideband waveforms are typically generated by modulating the frequency of a long pulse for a fixed signal-to-noise ratio. Alternatively, noise-like waveforms that are inherently wideband are used for simultaneous radar and communication systems [22]. Chaotic signals are a special case of noise-like waveforms that were demonstrated to have potentials for communications [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34], ranging [35; 36] and radar systems [37; 38; 39; 40; 41] individually. However, their advantages for dual-function radar communications are not fully explored.