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Design of QAM-OFDM Radar-Communication Integrated Signal Based on Golay Complementary sequences

The integration of radar and communication is conducive to improving the utilization rate of hardware resources and spectrum. To address the issue that the communication information in orthogonal frequency division multiplexing (OFDM) integrated signal for radar and communication affects detection performance, an OFDM integrated signal for radar and communication based on precoding was proposed. The communication information of the integrated signal was pre-coded by using the Gray Complementary Sequences (GCS) with better correlation performance to improve the performance of the ambiguity function of the integrated signal and its detection ability. Simulation results show that GCS have better correlation performance compared to M and Gold sequences, and the OFDM integrated signal pre-coded by GCS has lower Peak Sidelobes Ratio (PSLR) and comparable radar performance to LFM.


Introduction
As the number of users and wireless devices increases, so does the demand for spectrum resources. Therefore, the integration and allocation of spectrum resources have become important hotspots in recent years. The combination of the radar system and communication system has gained widely attention for its equipment miniaturizing and high efficiency of the spectrum [1]. In addition, the demand is generated by 5G high-speed communication and high-performance radar, which promotes the rapid development of Dual-Functional Radar-Communication system (DFRC) [2]. Vehicles need to locate and transmit information to each other in the intelligent transportation system. The combat platform of the radar needs to detect targets and transmit information to the intelligence center [3]. Unmanned Aerial vehicles (UAV) need to perceive the environment and transmit environmental data to the target user [4].
There are two main ways to realize the integrated waveform design of DFRC. One is that the radar signal is modulated to carry communication information, and the other is that the communication signal is converted to pulse for detecting radar. OFDM integrated waveform is realized by using the second method, which is important for communication and radar detection [5]. Communication symbols are embedded into OFDM waveforms which are used in waveforms for delayed Doppler radar [6]. In [7], the Direct Sequence Spread Spectrum (DSSS) coding is used to form an integrated waveform. A design scheme for modulating Multicarrier Complementary Phase-Coded (MCPC) using OFDM was proposed in [8]. But, OFDM waveforms have problems of high autocorrelation sidelobe and Peak to Average Power Ratio (PAPR) which will cause serious distortion and poor performance of radar detection and tracking when passing the non-linear region of the high-power amplifier.

Ambiguity function
The output of matched filter represents the radar ambiguity function, which is widely used to analysis the target resolution of waveforms [9]. The integrated signal ambiguity function is defined as It can be obtained from equation (1) and It can be seen from equations (5) and (6), the correlation of communication symbols will affect the radar resolution of the integrated signal. The aperiodic auto-correlation and cross-correlation function of the modulation information is reduced in the OFDM signals, which decrease the maximum sidelobe of the ambiguity function between different symbols. GCS with excellent autocorrelation and crosscorrelation performances are introduced to pre-code communication data, it can improve the correlation of communication information and reduce the interference to radar resolution. Thus, an integrated signal with better radar performance can be obtained.

Communication symbol sequence characteristics
The aperiodic auto-correlation and cross-correlation functions of M, Gold, and GCS are in figure 3. In this simulation, a 31-bit pair is used to generate the GCS. The M sequences with series 5 and feedback  (0,0,1,0,1) (1,1,0,1,0) are used to generate the Gold sequences. The results show that GCS has excellent cross-correlation and as good auto-correlation as M sequences, which agree well with theoretical analysis.

Performance analysis of ambiguity function
In this simulation, the OFDM signal is modulated by 16QAM, the protection interval is one-quarter of the length of OFDM data, and the parameters are shown in table 1. Compared with LFM, the central peak of the modulated signal is narrower, indicating that the modulated signal has good radar performance in figure 4. Due to the excellent correlation characteristics of GCS, the OFDM signal modulated by GCS has narrower main lobes and lower sidelobes by analysis the zero-doppler cut and zero-delay cut in figure 5 and figure 6. Table 1 (13) Table 2 shows that the PSLR of the OFDM signal modulated by GCS is 8.43 − dB in the range dimension and 6.98 − dB in the velocity dimension, which is significantly lower than the OFDM signal modulated by other sequences. This indicates that the pre-coded OFDM integrated signal by using GCS with good correlation characteristics can improve the radar performance.

Conclusion
To improve the integration waveform performance of radar and communication, a method of precoding OFDM radar communication integrated signal by GCS is proposed in this paper. Firstly, the signal generation model is constructed by QAM symbol mapping, secondly, the ambiguity function of the signal is analyzed, and finally, the GCS is used to optimize the PSLR of the ambiguity function. The simulation shows that the GCS has excellent correlation performance, the PSLR of the improved