# A Distributed RF Threat Sensing Architecture
## 3 Proposed Approach
### Sensor Expected Performance and Alarm Threshold Level  
Before testing the proposed RF sensor, we need to estimate the expected performance, taking into consideration system component losses, LNA and antenna gain, and the typical expected propagation losses in each frequency band. Based on the specifications of each component and measurements performed with an R&S FSH-8 Spectrum analyzer, the system losses are presented in Table 2.  
One also has to consider the system gain, where the LNA ZX60-83 LN-S+ provides a signal gain of 20.5 dB, with a noise figure of 1.5 at the highest frequency of sensor operation, and the directional Vivaldi wideband antenna, with a gain of 11.5-12 dBi at the frequencies of interest. The gain characteristics of the Vivaldi antenna are shown in Figure 4. \begin{table}
\begin{tabular}{l l} \hline \hline
**No** & **Component** & **Loss (dB)** \\ \hline
1 & Internal RF cables and connectors & \(-\)5.0 \\ \hline
2 & RF Coupler loss & \(-\)4.0 \\ \hline
3 & RF Filter loss & \(-\)3.0 \\ \hline  & TOTAL & \(-\)12.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: RF sensor system losses. Figure 3: RF sensor system calibration. As a result of the aforementioned gains and losses and data sheet specifications of the AD8318, Figure 5 presents the theoretical RF signal levels that can be present at various points of the receiver chain, including the gain of the Vivaldi antenna. The data sheet provides the various min and max RF levels at various frequencies across the 1 MHz to 8 GHz operating range, including values at 2.2 GHz and 5.8 GHz. For the purpose of this investigation, it is assumed that the 2.2 GHz figures provided in the data sheet, and the ones at 2.4 GHz where one of the two sensors will operate, will not differ significantly. One would also expect that the minimum detectable RF levels should be influenced by the overall noise figure of the individual RF receiver chain as a function of the gains and losses and the noise figure of the low-noise amplifier, as estimated by the Friis noise formula, where on this occasion the overall chain noise figure is not expected to exceed 3 dB. It also has to be noted that the min and max signal levels taken from the data sheet appearing in front of the log amplifiers refer to values in the \(\pm\)1 dB log conformance error region. If one considers an increased log conformance error vs. input power, as presented in the data sheet, lower RF signals than the ones presented in Figure 5 can be detected at the log amplifier input. In order to sense a malicious emitter and send an alarm, the recorded signal should exceed a predefined _Alarm Threshold Level_ (\(\mathit{ATL}_{(f/1)}\)) that is adequately higher than the average environmental noise of the monitored frequency band, \(f_{i}\), for time slot \(t\). This means that, in environments with dense utilization of wireless networks, any malicious interference below the average noise level will not be detected. Figure 4: Vivaldi directional antenna. Figure 5: Min and Max RF input levels. The \(ATL_{\text{(f/t)}}\) for each band depends on the signal-to-noise ratio (SNR) given by Equation (1), the jamming-to-noise ratio as per Formula (2), and each wireless technology jamming tolerance (\(J_{\text{tol}}\) measured in dBm) to handle such a jamming signal. \[\text{SNR(dB)}=20\log\biggl{(}\frac{S}{N}\biggr{)} \tag{1}\]  
\[\text{JNR(dB)}=20\log\biggl{(}\frac{S}{N+J_{s}}\biggr{)} \tag{2}\]  
From [PERSON]'s Equation, the channel capacity (CC) is given as:  
\[CC=B\log_{2}(1+\frac{S}{N}) \tag{3}\]  
where \(B\) is the bandwidth of the channel and \(S/N\) is the power ratio of the received signal and noise level. When a jammer is active in the environment, the noise level increases since the jamming interference is added to the environment noise that is already in the channel (Equation (1)), thus lowering the signal-to-noise ratio.