## 5 Experimental Results
### Preliminary Results  
#### 5.2.1 PER as a Function of the Data Rate  
Since the transmission power of IEEE 802.11 compliant transceivers is fixed by the regulatory authorities, the length of an IEEE 802.11 link (_i.e._, the transmission range of a node) depends only on the receiver sensitivity, which, in turns, is directly related to the selected data rate and modulation. The relationship between sensitivity and the allowed data rates of the IEEE 802.11 standard [31] is summarized in Table 1. Furthermore, the IEEE 802.11 standard does not provide any automatic rate adaptation mechanisms for devices operating in the _ad hoc_ mode. Therefore, the transmission rate must be set during the network initialization phase.  
For these reasons, it is interesting to preliminarily investigate the relationship between the transmission rate and the experimental Packet Error Rate (PER) observed in a direct IEEE 802.11 link--this investigation is expedient for performance investigation of the X-NETAD system. In this test, we consider two fixed smartphones, placed at a distance of \(15\) m: the first inside a car and the second inside a nearby building with large windows. For each value of the data rate, the first smartphone broadcasts \(100\) streams composed of \(1000\) User Data Protocol (UDP) packets with an application payload set at \(100\) bytes. In this setup, the packet losses are mainly caused by: (i) the multipath fading, due to the wireless propagation inside the room and the vehicle; and (ii) the shadowing due to the randommovement of persons and vehicle. We consider several transmission data rate values lying in the range, \([1,54]\) Mb/s. From the obtained experimental results, shown in Figure 7, it emerges that, as expected, lowering the data rate can help reduce the PER and extending the transmission range of X-NETAD nodes. However, we found that the minimum experimental PER is obtained when the data rate is set at \(11\) Mb/s, rather than at \(1\) Mb/s (_i.e._, at the lowest possible data rate). This behavior can be explained by the fact that when the data rate is set to \(11\) Mb/s, the modulation scheme switches to the Direct Sequence Spread Spectrum (DSSS), which is more robust in terms of PER, than the Orthogonal Frequency-Division Multiplexing (OFDM) used at lower rates. Further reduction in data rate does not lead to an additional PER reduction, since the transmission time becomes longer, increasing the likelihood of transmitting during a noise burst and, thus, resulting in a slight PER increase. According to the results obtained in our tests, in the remaining part of this work, we set the transmission rate to \(11\) Mb/s, in order to have a good trade-off between the transmission range and the PER.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data rate (Mb/s)** & **Scheme** & **Modulation** & **RX Sensitivity (dBm)** \\ \hline
1 & DSSS & BPSK & \(-89\) \\
6 & OFDM & BPSK & \(-82\) \\
9 & OFDM & BPSK & \(-81\) \\
12 & OFDM & QPSK & \(-79\) \\
18 & OFDM & QPSK & \(-77\) \\
24 & OFDM & 16-QAM & \(-74\) \\
36 & OFDM & 16-QAM & \(-70\) \\
48 & OFDM & 64-QAM & \(-66\) \\
54 & OFDM & 64-QAM & \(-65\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Relationship between modulation, data rate and receiver sensitivity defined by the IEEE 802.11 b/g standard (see [31]). DSSS, Direct Sequence Spread Spectrum; BPSK, Binary Phase-Shift Keying; OFDM, Orthogonal Frequency-Division Multiplexing; QPSK, Quadrature Phase-Shift Keying;QAM, Quadrature Amplitude modulation.  
Figure 7: Experimental Packet Error Rate (PER) as a function of the data rate, by considering \(P_{\rm t}=32\)\(mW\) and \(d=15\)\(m\).