## 6 System Evaluation
### Xbee 868 MHz Radio Module
#### 6.2.2 Transmission and Mobility Tests  
The goal of the speed tests is to measure the amount of data that two radio modules are capable of exchanging when one of them is mobile. The same scenario as presented in Section 6.1.3 has been used. Table 5 and Figure 15 show the results obtained at low velocity (80, 100 and 120 km/h). Data for higher speed values has been extrapolated. These results should be taken with caution and require verification in real tests.  
Based on linear extrapolation using 0 dbi antennas, a single transmitter would be able to transmit about 5 100-byte packets to a train moving at 320 km/h. Thus, if we assume an average of 5 nodes in range with each other communicating with the train and knowing the physical medium for communication is shared, we cannot ensure that all nodes can send information to each train. It is therefore necessary to use additional mechanisms to increase the effective bandwidth of the radios that send information to the train. Possible solutions to this problem are the use of multiple radio receivers on the train, for example one at the front and one at the back, the use of more powerful antennas or the use of more sophisticated channel access methods such as TDMA (Time Division Multiple Access).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Transmitter**} & \multicolumn{2}{c}{**Receiver**} & \multirow{2}{*}{**Distance**} \\ \hline
**Antenna Gain** & **Height** & **Antenna Gain** & **Height** & \\ \hline Small (0 dBi) & Floor & Small (0 dBi) & Floor & 110 m \\ Small (0 dBi) & Floor & Small (0 dBi) & 2 m & 150 m \\ Small (0 dBi) & 2 m & Small (0 dBi) & 2 m & 300 m \\ Big (4.5 dBi) & Floor & Big (4.5 dBi) & 2 m & 200 m \\ Big (4.5 dBi) & 2 m & Big (4.5 dBi) & 2 m & 600 m \\ \hline \hline \end{tabular}
\end{table}
Table 4: Maximum communication range of Xbee 868 MHz.  
Figure 14: Fresnel zone.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Small Antenna (0 dBi)** & **Big Antenna (4.5 dBi)** \\ \hline
**Km/h** & Packets & Bytes & Packets & Bytes \\
**80** & 47 & 4700 & 154 & 15,400 \\
**100** & 44 & 4400 & 145 & 14,500 \\
**120** & 40 & 4000 & 131 & 13,100 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Transmission and mobility tests with different antennas (868 MHz).  
In order to study the possible interferences caused by the use of multiple antennas in the same area (as in a real environment), an additional test was carried out. We added a second fixed emitter. This test was performed with a constant speed of 100 km/h, with both 2 and 3 emitters. Tables 6 and 7 summarize the results.  
If multiple transmitting antennas are in range, the amount of data that can be transmitted with each of them is relatively low. This behavior is to be expected since a shared medium is being used and nodes cannot transmit simultaneously. However, the amount of information that can be sent by each radio module is not the same, depending as it does on the time at which each antenna starts sending or coming into range, which is something that cannot be controlled. For example, in some cases where we have used two emitters we noticed that one of them is capable of transmitting much more information than the other one.