## 1 Introduction
[25] explores different properties of signals available for radar sensing and aims to combine the ISAC technology with the cellular network by optimizing the multi-dimensional resource scheduling. The joint state estimation problem for unmanned aerial vehicle (UAV)-aided 6G communications is considered in [26], and the channel characteristics of the spatial channel are analyzed. Ref. [27] optimizes the ISAC waveform by filling the empty subcarriers of a 5G base station working in downlink mode and optimizing the transmission power of some of the communication subcarriers. However, the communication waveform is random in nature. And the number of empty subcarriers in the communication depends on the number of users served by the networks. The data transmitted by the users has burst and discontinuity. Ref. [28] proposes a novel ISAC scheme which constructs three adjacent base stations as a cooperative sensing system. But this scheme would reduce the duration in downlink mode by one-third, which would significantly reduce the communication data rate. Therefore, how to reuse the hardware of existing widely deployed 5G base stations to achieve ISAC while minimizing the impact on the existing communication data rate is an important problem. It can be seen from the above literature that the waveform design of ISAC is usually uniform. In fact, it cannot realize uniform sensing in a 5G NR communication system because the waveform design of ISAC in a 5G NR communication system needs to satisfy the constraints of the communication protocol which contains uplink and common channel. However, a nonuniform sensing waveform leads to nonuniform sampling. And the uniform signal processing method cannot work. Its ambiguity function has some big side-lobes which will affect its estimation performance. And the FFT method can work no longer under the nonuniform distribution. On the other hand, the millimeter wave band in 5G greatly reduces the time of each communication symbol [29; 30]. Hence, utilizing a small number of symbols in one frame for sensing can greatly alleviate the problem of low efficiency. In this paper, we design a TD-ISAC sensing waveform based on a 5G communication base station with its millimeter wave band, which realizes sensing by reusing the hardware and OFDM waveform of an existing 5G NR communication base station. That is, we realize ISAC by reusing hardware resources. And the designed sensing waform reuses the millimeter wave of the 5G NR communication with OFDM waveform. That is, we realize ISAC by reusing communication spectrum and waveform. Furthermore, to avoid the interference between sensing and communication, we choose the time-division mode to design the waveform. And we allocate one OFDM symbol for sensing in one time slot at most to deduce the impact on communication efficiency. Then, we design two dual-PRF waveforms under the constraints of communication protocol, and the designed waveform is nonuniform adaptive to the real communication scenarios. To verify that the designed waveform has good adaptability to different signal processing methods, we realize the parameter estimation by two types of methods: the discrete Fourier transformation-based method and the compressed sensing-based method. Finally, we verify the effectiveness of the designed waveform in vehicle sensing applications by simulation and real traffic experiments.