Non-terrestrial networks (NTNs) have become appealing resolutions for seamless coverage in the next-generation wireless transmission, where a large number of Internet of Things (IoT) devices diversely distributed can be efficiently served. The explosively growing number of IoT devices brings a new challenge for massive connection. The long-distance wireless signal propagation in NTNs leads to severe path loss and large latency, where the accurate acquisition of channel state information (CSI) is another challenge, especially for fast-moving non-terrestrial base stations (NTBSs). Moreover, the scarcity of on-board resources of NTBSs is also a challenge for resource allocation. To this end, we investigate three key issues, where the existing schemes and emerging resolutions for these three key issues have been comprehensively presented. The first issue is to enable the massive connection by designing random access to establish the wireless link and multiple access to transmit data streams. The second issue is to accurately acquire CSI in various channel conditions by channel estimation and beam training, where orthogonal time frequency space modulation and dynamic codebooks are on focus. The third issue is to efficiently allocate the wireless resources, including power allocation, spectrum sharing, beam hopping, and beamforming. At the end of this article, some future research topics are identified.

翻译：非地面网络已成为下一代无线传输实现无缝覆盖的理想解决方案，可高效服务广泛分布的海量物联网设备。物联网设备数量的爆发式增长对大规模连接提出了全新挑战。NTN中的远距离无线信号传播会导致严重的路径损耗和较大延迟，而信道状态信息的精确获取是另一个挑战，尤其针对高速移动的非地面基站。此外，NTBS星载资源稀缺性也给资源分配带来难题。为此，本文研究了三个关键问题，全面阐述了现有方案和新兴解决方案。第一个问题是实现大规模连接，通过设计随机接入建立无线链路，并采用多址接入传输数据流。第二个问题是通过信道估计和波束训练在各种信道条件下精确获取CSI，其中正交时频空间调制和动态码本是研究重点。第三个问题是高效分配无线资源，包括功率分配、频谱共享、波束跳变和波束赋形。最后，本文指出了未来研究方向。