# On Vulnerability of Selected IoT Systems to Radio Jamming--A Proposal of Deployment Practices
## 7 Calculations of the Interference Margin Based on CNIR (PER) Measurements  
In this section a simple procedure will be demonstrated for translating results of the measured CNIR values (shown in Figure 6) into the interference margin \(M_{int}\), understood as a safety offset in the energetic link budget that secures reception from jamming. First, let us take Equations (1) and (2) for the basic free-space loss that relates the pathloss in \(L_{bf}\) incurred by an electromagnetic wave as it propagates in the unobstructed [PERSON]'s zone, that corresponds to a purely Line-Of-Sight (LOS) situation [32]. In this scenario, \(L_{bf}\) is only dependent of the distance \(d\), the signal center frequency \(f\) and the receiving antenna isotropic gain \(G_{i}\) (the receiver being here the BS antenna, as indicated in Figure 1). The LOS assumption is highly probable because the malevolent party is interested in attaining the highest possible jamming effect on the BS under EM cyberattack. \[L_{bf}=d^{2}f^{2}\Big{(}\frac{4\pi}{c}\Big{)}^{2}\cdot{G_{i}}^{-1}\left[ \frac{W}{W}\right] \tag{1}\]  
\[L_{bf}=32.45+20\log(d_{km})+20\log(f_{MHz})-G_{i}\ [dB] \tag{2}\]  
Each of the sensing devices connected to the BS operates within a perimeter defined by the cell range, marked as \(CR\) in Figure 7. For a generic IoT network it was defined in [32] as 866 m i.e., half of the Inter-Site Distance (ISD) equal to 1732 m. Provided that the jammer transmits at a center frequency 868 MHz with the maximum permissible power of 14 dBm, the interference power \(I\) delivered to the IoT BS will therefore equal to: 14 dBm - \(L_{bf}\) (= 91.2 dB) + \(Gi\) (= 8 dBi) = \(-\)69.2 dBm, a rather high value as for the received radio signal power. This means that the lowest desired signal power \(C\) in the Weightless slowest transmission mode (i.e., 0.625 kb/s) can be up to 15 dB lower (see Figure 6), i.e., equal to \(-\)84.2 dBm, in order to sustain communication at PER = 50%. Figure 6: Collected measurement results of CNIR threshold measurements for achieving PER = 50% for Weightless operation modes and SigFox. As was already mentioned in Section 3, an Interference Margin (\(M_{int}\)) will be used as an indicator of a \"safety offset\", expressed in decibels, separating the useful signal from the interference power (CNIR), at which a system will still be able to operate with appropriate level of PER (here: 50%). The interference margin is considered as completely wasted as it approaches 0 dB. Once \(M_{int}\) has dropped below 0 dB, interference will inevitably cause PER to exceed 50%, as stems from the measurements carried out in the anechoic chamber, presented in Section 6.1. The step-by-step procedure for calculating \(M_{int}\) can be retrieved by following Equations (3) through (7), where I stands for interference power, \(d_{min}\) stands for the minimum jammer separation from the BS such that its jamming signal pathloss equals \(L_{max}\) at its highest. In the experiment a verification was provided for how \(M_{int}\) changed as the jammer moved toward the BS at 10% steps relative to the initial position, thus making the interference more critical with every step. The movement began at \(CR\) equal to 866 m away from BS, as the farthest point on the coverage brim, corresponding to the normalized distance \(d/CR\) of 1.0 in Figure 8 and Table 3. As could be expected, at this distance \(M_{int}\) was the highest and positive both systems, regardless of their transmit mode (defined by a bit rate). As the jammer was approaching the BS site (decreasing \(d/CR\) down to 0.1), \(M_{int}\) fell to c.a. 10 dB for the two slowest Weightless modes (i.e., 0.625 kb/s, 1.25 kb/s), whereas for the modes 6.25-10 kb/s it fell below 2 dB. For the two fastest Weightless modes (i.e., 50 kb/s, 100 kb/s), however, \(M_{int}\) dropped below \(-10\) dB. In SigFox, in turn, positive \(M_{int}\) was maintained all the way from \(d/CR\) of 1.0 through 0.1, although falling to 1.8 dB at the normalized distance of 0.3.