## Communication protocol
### Mobile Network Implementation  
A small cell solution based on the 4G/5G protocol stack from Amarisoft was selected for the mobile network. The small cell is a radio front-end based on a Software Defined Radio (SDR), which operats with a powerful consumer computer. On this computer, the actual base station software (eNB/gNB) and the 4G/5G core network can be operated together. The base station is able to generate the radio signal in various standardized 4G/5G Frequency Ranges (FRs). The network architecture including the Core Network (CN) and the Internet gateway is shown in Figure 6.  
Figure 6: Overview of the mobile communications infrastructure.  
For outdoor LTE testing, active high-frequency components and antennas are necessary to ensure sufficient signal strength at the receiver. The utilized logarithmic-periodic antenna has an antenna gain of 11.5 dBi. The antenna diagram is directional and can be operated in the FR 790 MHz to 2700 MHz. The antennas were mounted on a 2 m mobile mast with a tilt of 10\({}^{\circ}\) to allow a better coverage for the UAVs. Transmit and receive antennas were mounted two meters apart and used different polarization to avoid interference from the transmit to the receive path. For the FR 2300 MHz to 2700 MHz, amplifier Kuhne Electronic KU PA 230270-18 is used. The output power of the amplifier at the 3 dB compression point is 44 dBm. In operation, the amplifier can be operated with a maximum input power of 7 dBm. To amplify the reception signal of the base station, a low-noise amplifier is required. The amplifier Kuhne electronic KU LNA BB 2227 with a gain of 25 dB was selected which operates in FR 2200 MHz to 2700 MHz.  
The LTE cell was operated in Frequency Division Duplex (FDD) mode. FDD enables Uplink (UL) and Downlink (DL) transmission at the same time, but over different frequency bands to avoid interference. Most LTE cells in rural areas of commercial operators in Germany are using this mode as well. For the DL from the LTE base station to the terminals, 5 MHz wide carriers were used. The UL from the terminals to the LTE base station was also in a 5 MHz wide carrier. The spacing of the LTE subcarriers is 15 kHz. This results in a raw symbol rate of 4.5 Msps for a system with 5 MHz bandwidth. The maximum data rate that can be achieved depends on the Signal-to-Noise Ratio (SNR) at the receiver and the resulting modulation of the signal at the transmitter. The exact LTE Modulation and Coding (MODCOD) format is thus selected depending on the prevailing conditions. LTE band 7 was used for the transmission itself. The equivalent isotropic radiated power in the DL was approximately 50 dBm. In addition, three Commercial Off-The-Shelf (COTS) UEs were utilized to connect the UTM-server and the GCS computers as shown in Figure 7. The two MMTs provide the air-to-ground LTE link for the UAVs. Each MMT is equipped with an omnidirectional 4 dBi antenna. An additional Internet gateway for the non-public network was enabled via a COTS LTE router with a SIM card.  
Since the NR cell operates in a different FR, different antennas and amplifiers are required for the NR scenario compared to LTE in band 7. Hence, a NSA mobile network is needed, in which the LTE and NR cells can be operated in parallel in different FRs. The main components for the 3.5 GHz FR are listed below:  
* Kathrein 80010922 sector antenna with 17 dBi gain
* LNA mini circuits ZX60-83 LN12+ with 20 dB gain  
Figure 7: The cooperative test environment.  
* MPA mini circuits ZHL-4240+ with 40 dB gain  
For the demonstration of the NR cell, the Huawei CPE NR Pro Router was integrated, one of the early NR devices since 2020. The Testbed allows NR access utilizing a 40 MHz Time Division Duplex (TDD) carrier in the FR 3700 MHz to 3800 MHz band for non-public networks in Germany. However, the focus of the evaluation is on LTE because this technology is more widespread in rural areas and is still preferred by network operators for cost reasons.