# Integrated Radar and Communications Waveform Design Based on Multi-Symbol OFDM
## 1 Introduction  
As two typical applications in modern radio technology, separately equipped radar and wireless communication systems suffer from problems such as heavy system weight, large size, high power consumption, and electromagnetic interference [1]. With the development of technology, these problems have become increasingly prominent and need to be solved urgently. In current technological development, radio-frequency front-end architectures in radar and communication technologies are becoming more and more similar, in particular, digital signal processing is gradually replacing the traditional hardware components. Therefore, an emerging and feasible trend is to integrate radar and wireless communication systems into one, i.e., to provide both radar and communication functions on a single hardware platform with a single waveform, and the resulting systems are known as \"RadCom\" [2], \"Joint Radar Communication (JRC)\" [3], \"Dual-Function Radar-Communication (DFRC)\" [4], \"Integrate Radar and Communications (IRC)\" [5], etc. In recent years, the research on IRC has attracted extensive attention, and it has very broad application prospects in military and civilian fields, such as intelligent transportation networks that need to establish communication links with other vehicles and perform active environmental sensing to ensure safety and radar sensor networks that perform information fusion to achieve efficient detection [2, 3, 4, 5, 6, 7]. The main challenge in IRC development is to find suitable waveforms that can be applied to radar sensing and communication transmission simultaneously. At present,the IRC waveform design is mainly developed in two directions, one is based on radar waveforms and the other is based on communication waveforms. Radar waveforms are designed to create waveforms with the optimum autocorrelation properties to guarantee a high dynamic range of measurements when correlation processing is applied in the receiver. Linear frequency modulated (LFM) pulse [2] is the most popular example of this requirement, and is often used to design IRC waveforms [8; 9]. However, this approach has a very significant disadvantage, i.e., the communication rate is very low, usually much lower than that achieved by traditional communication systems with the same bandwidth. Therefore, this approach is not optimal from the communications perspective. In order to achieve good communication performance, especially high communication rates, orthogonal frequency-division multiplexing (OFDM) in multicarrier modulation has attracted extensive attention in current communications systems [10; 11]. It is remarkable that OFDM is already one of the best candidates for IRC waveforms, owing to the advantages of flexible subcarrier modulation, easy implementation with fast Fourier transform (FFT), high spectral efficiency, and the availability of processing gain at the receiver [2]. Additionally, its ability to provide high performance in radar and communications is very promising. Therefore, this paper mainly focuses on OFDM-based IRC waveform design. However, some extremely challenging problems such as high peak-to-mean envelope power ratio (PMEPR) and undesirable autocorrelation properties or peak sidelobe level (PSLL) also exist in OFDM-based IRC waveform design. Since the OFDM signal is generated by the superposition of all subcarriers, it may have a high PMEPR. The high PMEPR will greatly reduce power amplifier (PA) efficiency or cause nonlinear distortion, thereby reducing signal energy and affecting radar detection capability or degrading communication bit error rate (BER) performance. Aiming at solving this problem, researchers have proposed various PMEPR reduction techniques, including the clipping techniques [12; 13], probabilistic techniques [14], code techniques [15; 16; 17], and tone reservation (TR) techniques [18; 19; 20; 21; 22; 23].