In this paper, a downlink non-orthogonal multiple access (NOMA) system with partial channel state information (CSI) in mobile scenarios is considered. In this case, the channel gain of each user will vary as its position changes, which has an influence on spectrum efficiency of conventional NOMA. An analytical framework is developed to evaluate the impact of estimated position deviation in terms of decoding order error probability and average sum rate performance. To further improve the system performance, a novel NOMA scheme based on position filtering is proposed. Monte Carlo simulation is also provided to demonstrate the improvement of spectrum efficiency compared with conventional NOMA and OMA schemes. And the results verify the accuracy of proposed analytical framework.