## 3 Design of Mobile Unit (MU)
### Radio Subsystem Design  
The wireless communication in the proposed system is based on the license-free 869 MHz band. This band was chosen because, in our case (Europe), it allows 0.5 W (27 dBm) transmit power (for band occupancy below 10% [21; 22]). The radio subsystem design starts with a standard link power budget estimation:  
\[P_{r}=P_{t}+G_{t}+G_{r}-L, \tag{1}\]  
where:  
\(P_{r}\)--power received via MU [dBm];  
\(P_{1}\)--power transmitted via BS [dBm];  
\(G_{1}\)--BS antenna gain [dB];  
\(G_{2}\)--MU antenna gain [dB];  
\(L\)--communication path loss [dB].  
The power loss in the transmission lines between transceiver and antenna (both MU and BS) is neglected because they are directly connected (or by a short cable length). We start with the basic case of free space loss [23; 24]:  
\[L=20\text{log}(d)+2-\text{log}(f)+32, \tag{2}\]  
where:  
\(d\)--communication distance [km];  
\(f\)--operating frequency [MHz].  
Additional correction \(\Delta L\) [dB] is added using the _[PERSON]'s foliage model_ (i.e., the influence of trees, bushes, etc.) [25]:  
\[\Delta L=\begin{cases}\begin{array}{ll}0.063f^{0.284}d_{f}&0<d_{f}\leq 14\\ 0.188f^{0.284}d_{f}^{0.588}&14<d_{f}\leq 400^{\prime}\end{array}\end{cases} \tag{3}\]  
where \(d_{f}\) [m] is the maximal foliage depth, and \(f\) [MHz] is the operating frequency.  
Alternatively, an _Egli propagation model_[26] takes into account the transmitter and receiver antenna heights:  
\[L=-20\text{log}\left(\frac{h_{1}h_{2}}{d^{2}}\right)-20\text{log}\left(\frac{ 40}{f}\right)+122 \tag{4}\]  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Assumption** & **Value** & **Unit** \\ \hline System capacity (number of MUs) & 200 & \\ Maximal positioning time for 50 MUs & 1 & [s] \\ Minimal communication distance & 2 (1) & [km] \\ Minimal battery life-time & 10 & [h] \\ \hline \hline \end{tabular} (1) two cases are considered: case 1—communication distance up to 2 km; case 2—communication distance below 1 km.  
\end{table}
Table 2: Basic system assumptions for all cases.  
where:  
\(L\)--communication path loss [dB];  
\(h_{1}\)--height of the BS antenna above ground [m];  
\(h_{2}\)--height of the MU antenna above ground [m].  
The calculated values for our cases are shown in Table 3. The cases differ in terms of specific airport location and internal structure (e.g., maximum communication distances), RF modules used, and scenarios (portable MU vs. vehicle-mounted).  
The calculated transmitter power \(P_{t}\) and receiver sensitivity values are sufficient for many specific RF modules equipped with on-board power amplifiers. The initial test for case 1 has demonstrated reliable communication at a data rate of 9.6 kbps at a distance of up to 1 km for a radio module 1 (rmod_1, FPX3 RF) and a radio module 2 (rmod_2, ARF-54 RF).  
In case 2, a reference design with a radio transceiver and power amplifier (radio module 3, mod_3) from Texas Instruments was used as a low-cost prototype solution. The data sheet confirms that the required receiver sensitivity should be sufficient for 1.2, 4.8, and 38.4 kbps data rates. The tests performed proved reliable communication at a 1.2 kbps rate and a distance of up to 0.5 km in a medium-density foliage environment (mainly bushes). Open-air measurements extended this range to 1 km.