After the low-earth orbit (LEO) satellite Internet has gone through the two stages of competing with the terrestrial network and supplementing the terrestrial network, it has begun to enter the third stage of constructing the satellite-ground integrated network with the terrestrial network to provide seamless global coverage. 5G New Radio (NR) is one of the core enabling technologies of the third stage of satellite Internet. This paper focuses on how to make full use of the power and bandwidth resources on the LEO satellite by using adaptive transmission scheme to maximize the throughput of the user downlink based on 5G NR. To solve the problem that the ultra-long propagation delay, outdated channel state information (CSI) and dynamic multi-scenario of LEO satellite will lead to the high implementation cost and greatly reduced performance when applied the 5G adaptive transmission scheme to LEO satellites, we optimized the adaptive transmission scheme of 5G NR based LEO satellite from multiple dimensions such as adaptive transmission process, signal to noise ratio (SNR) prediction and modulation and coding scheme (MCS) adaptive switching strategy. The simulation results show that compared with the fixed threshold switching strategy based adaptive transmission scheme, the proposed scheme can improve the average throughput of the system by 26.6% under the dynamic multi-scenario environment served by the LEO satellite.

低地球轨道（LEO）卫星互联网在经历了与地面网络竞争和地面网络补充两个阶段后，开始进入与地面网络共建星地一体化网络的第三阶段。提供无缝的全球覆盖。 5G新空口（NR）是卫星互联网第三阶段的核心使能技术之一。本文重点研究如何利用自适应传输方案充分利用LEO卫星上的功率和带宽资源，最大化基于5G NR的用户下行链路吞吐量。针对低轨卫星超长传播时延、过时的信道状态信息（CSI）和动态多场景等导致5G自适应传输方案应用于低轨卫星时实施成本高、性能大幅下降的问题，我们从自适应传输过程、信噪比（SNR）预测和调制编码方案（MCS）自适应切换策略等多个维度优化了基于5G NR的LEO卫星的自适应传输方案。仿真结果表明，与基于固定阈值切换策略的自适应传输方案相比，该方案在LEO卫星服务的动态多场景环境下，可使系统平均吞吐量提高26.6%。