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The explosively increasing traffic demand in the fifth generation (5 G) mobile communication system has made the limited licensed spectrum quite competitive. Such status quo motivates the development of new radio (NR) based access to the unlicensed band, which is a promising solution for system performance enhancement. In this paper, we focus on the coexistence between the NR system and the WiFi system based on the duty cycle mechanism. Firstly, an optimization model is formulated to maximize the throughput of the NR system in the unlicensed band (NR-U) via the joint bandwidth and transmission opportunity allocation under the fairness requirement. Then, to solve this optimization problem, we convert it into a Markov decision process with continuous action space, based on which a deep deterministic policy gradient (DDPG) based coexistence algorithm is proposed. At last, simulation results show that …