###### Abstract
Like other surveillance systems [12], both technologies share the spectrum at 1030 and 1090 MHz for this purpose. With 400,000 forecasted drones by 2050 ([13] p. 4) this part of the spectrum becomes a limited resource [14]. Furthermore, certified hardware is often heavy, bulky and costly and therefore less suitable for the integration into small UAVs. Hence, different approaches have been proposed to imitate the functionalities of ADS-B with the help of alternative, more lightweight point-to-point communication technologies like Automatic Packet Reporting System (APRS) [15], Bluetooth, LoRa or Wi-Fi [16]. Another promising approach is cellular networks. They provide high capacity and bandwidth, large coverage, low latency, reliability, inherent navigation, availability and beyond line-of-sight operation. For instance, the Fifth Generation Mobile Networks (5G) supports data rates of up to 10 Gbits/s and provides latencies of about 1 ms [17]. However, there are some challenges that need to be carefully investigated in the design process, like coexistence with ground mobile users, handovers and the tilt of base stations. To solve these problems, UAVs are an essential part in the standardization of 5G for the safe operation of UAS, and there has been a lot of activities in the 3 rd generation partnership project (3 GPP) to ensure reliable 5G New Radio (NR) connectivity for drones, e.g., Release 15 and Release 17 [18]. Unlike in manned aviation, there is no established UAS Traffic Management (UTM) system in Europe yet. The literature contains different approaches for the safe and efficient integration of UAS into national airspaces [19; 20; 21; 22]. Within the framework of the SESAR Joint Undertaking, the EU is pursuing the concept of U-Space [23], which proposes several services that should lead to automated, inter-operable and sustainable UTM solutions. In this context, the research and development of the different services is distributed in four successive phases U1-U4, with each development phase increasing the level of autonomy and connectivity of the UAS. According to the current state of research report, most of the previous work focused on the services of phases U1 and U2, whereas few services of U3 and none of U4 have been studied [24] by now. Among the least studied services were collaborative interface with ATC (7 %) and tactical conflict resolution (3 %). The literature also points out that end-to-end latency (UAV-to-UAV) is a critical metric in the UTM context, however, data from real life experiments is still missing ([16] p. 94). From the work done so far, cellular networks will represent an essential component in future UTM systems. At the same time, it must be assumed that the transponder method in combination with ADS-B will be the primary technology for collision avoidance in manned aviation for the near future. In the following, this article introduces a Multi-Mode-Transceiver (MMT) that combines current Mode S transponder technology including ADS-B In/Out with a cellular interface to enhance UAS visibility and connectivity in mixed Air Traffic Management (ATM) and UTM environments. Embedded in a prototypical UTM system, the capabilities of the MMT as well as a novel collaborative interface to ATC are tested under real life conditions. The selected use case represents a UAV transit through a controlled airspace with simulated and real traffic. Furthermore, this work provides the following contributions:  
* Experimental implementation of a cooperative UTM system based on a cellular Long Term Evolution (LTE) network utilizing the Message Queuing Telemetry Transport (MQTT) protocol [25]
* Measurements of message trip times in the UTM network under real life conditions
* Evaluation of a collaborative interface with ATC using the MMT
* Evaluation of a power-reserve-based tactical collision avoidance method utilizing cooperative traffic information data