| Unmannaed Aerial Vehicles (UAVs) are powerful industrial Internet of Things (IoTs) equipment. They offer sensing, communications and data analysis in the air. Recently, deploying a swarm of UAVs is being considered as a promising scenario for future applications of IoTs with UAVs. In this context, communications is the key that unlocks the potential of building a UAV swarm. 3GPP TR 22.862, TS 22.261 and TR 36.777 have specified reliability performance for multiple UAVs communications in the 5G domain. This article, for the first time, overviews communications and networking challenges of UAV swarm with a focus on reliability. Recommendations for designing an ultra-reliable communications system for UAV swarm are presented with a focus on networking protocol stack and radio hardware. In particular, we developed EasySwarm, an open-source UAV swarming platform, which adopts a LoRa radio at the physical layer and low-latency channel access protocol at the MAC layer. Based on EasySwarm, separate real-life test-beds are built using LoRa, WiFi and LTE networks to evaluate communications performance at both local and operator networks. Communications reliability and polling delays are measured. Results show that the use of LoRa leads to better communications. In particular, with higher density and distance as compared to using WiFi. In addition, LTE networks provide the best communication performance for a UAV swarm, especially in terms of coverage. |