# Development and Testing of a Two-UAV Communication Relay System
## 3 Hardware Platform
### Communication Subsystem  
To construct the communication subsystem, a number of commonly used wireless communication technologies were considered as the possible candidate: IEEE 802.11n Wi-Fi, 3G/4G-LTE mobile network, and 433 MHz/915 MHz radio. Wi-Fi can provide a reliable, high-speed connection, but has a comparatively small range. Typical Wi-Fi routers have an effective range of fewer than 100 m, rendering them unsuitable for long-range outdoor missions. The applicability of the 3G/4G-LTE mobile network relies heavily on the strength of mobile phone signal, which varies greatly in different locations, and is not always available in remote areas where surveillance and rescue missions are likely to take place. A 433 MHz/915 MHz radio can be operated without a license in many parts of the world, and can be implemented with easily accessible, low-cost transceivers (for example, 3 DR Radio Telemetry as discussed below). A range of a few kilometers could be achieved. The radio can also be deployed at any desirable site without the need for supporting infrastructure. These features give 433 MHz/915 MHz radio advantages in various outdoor missions. The frequency options, 433 MHz or 915 MHz, can be selected to ensure compliance with local radio regulations. Based on the discussion above, to establish communication between the UAVs and GCS and to facilitate the communication relay system, 3 DR 915 MHz Radio Telemetry (3 DR Radio) [33] was selected as our radio transceiver hardware. 3 DR Radio, the built-in processor of which runs open-source SiK firmware [34], has the following useful features that enable communication relay:  
* Built-in time division multiplexing (TDM) support. * Supports frequency hopping spread spectrum (FHSS) among a configurable number of channels (configured to 50 channels in this study). Its frequency hopping sequence is user-configurable. Figure 4 illustrates the radio communication relay setup. Four 3 DR Radios were used, numbered T1-1, T1-2, T2-1, and T2-2. Table 1 gives the details of the configuration and installation of the telemetries. T1-1 and T1-2 carried communication between GCS and UAV \"Mom\" exclusively, whereas T2-1 and T2-2 carried communication between UAV \"Mom\" and UAV \"Son\". Figure 3: System hardware architecture. A combination of TDM and FHSS techniques was used to ensure that the presence of multiple radio telemetries did not lead to radio-frequency interference. By default, telemetries Tn-1 and Tn-2 (where \(n=1\), 2... ) formed one radio link between each another. On power-up, Tn-1 and Tn-2 first searched for synchronization signals from each another; these signals were used to synchronize the internal clocks of both telemetries. Once the clocks were synchronized, the telemetries then started a TDM process, in which Tn-1 and Tn-2 alternately transmitted data over radio, and listened to incoming data when not transmitting. The alternation of a transmission window occurred at a fixed frequency, and the time between consecutive alternations was referred to as the turnover time. The selection of the turnover time depends on the time required to transmit messages. On one hand, the turnover time should be long enough to allow an adequate number of messages to be transmitted in a single transmission window. On the other hand, the turnover time should not be too long, otherwise, the other telemetry would have to wait longer before it can transmit its own messages, causing delays in information update, and compromising the synchronization accuracy between telemetries. The value of the turnover time could be tuned based on the above-mentioned principles. In this research, the turnover time was set to 100 ms, which ensured stable operation of the communication relay function. Values ranging from 90 ms to 140 ms have been tested and could provide similar results. This process is illustrated in Figures 5 and 6. In addition to the TDM process, the transmission of telemetries Tn-1 and Tn-2 were frequency-hopped.