## 5 Link Budget Estimation for IoT Indoor Scenario  
The proposed antenna achieved omnidirectional radiation with CP and an average gain of 4.75 dBi. From this achieved gain and radiation pattern, the designed antenna is suitable for IoT applications in indoor scenarios. The transceiver performance of the designed antenna between different IoT devices such as mobile, fridge, and washing machines is studied in this section. The propagation of millimeter-wave signals experiences drastic attenuation for the first meter of distance [26]. Therefore, it becomes essential to estimate the losses for indoor scenarios. In our analysis, a carpet area of 75.39 m\({}^{2}\) was considered. The area was partitioned into two rooms connected by a corridor path, as demonstrated in Figure 15. In calculating the link margin, the reflection from the wall of a 3.6 m height, reflection from objects, and path loss exponents were considered. For simplicity, the rooftop effect was neglected in the analysis. The TRX1, TRX2, and TRX3 were a mobile phone, fridge, and washing machine. The TRX2 and TRX3 were assumed to be stationary, whereas TRX1 moved from point A to B, B to C, and then to D (relative distance (Rel. Dis.) was considered instead of the actual distance between devices), as depicted in Figure 15a. The TRX1, TRX2, and TRX3 antennas were assumed to be in the XY, YZ, and XY planes, respectively. With this setup, the transmission and reception power between the antennas were well studied for circular polarization behavior. The TRX1, TRX2, and TRX3 were at heights of 1 m, 1.5 m, and 0.85 m, respectively, as shown in Figure 15b. The analysis was carried out for two cases: (i) the communication between TRX1 and TRX2,  
Figure 14: Simulated and measured radiation patterns. (**a**) XZ plane at 28 GHz, (**b**) YZ plane at 28 GHz, (**c**) XZ plane at 38 GHz, and (**d**) YZ plane at 38 GHz. where the antennas were in a cross-polarization setup and (ii) the communication between TRX1 and TRX 3, where the antennas were in co-polarization. The simulation was performed using the HFSS savant tool. The estimated received power at TRX2 and TRX3 was calculated using Equation (6) [27]:  
\[P_{TRXn}(\mathrm{dB})=P_{TRX1}(\mathrm{dB})+G_{TRX1}(\mathrm{dB})+G_{TRXn}( \mathrm{dB})-P_{Loss}\ (\mathrm{dB}) \tag{6}\]  
where, \(n\) = 2 and 3, and \(G_{TRX1}\) and \(G_{TRXn}\) are the antenna gains of TRX1, 2, and 3, which were considered based on the measurement results of the antenna. \(P_{Loss}\) is the path loss at 28 GHz. The effects of the line-of-sight (LOS) and non-line-of-sight (NLOS) conditions were also studied in estimating the link margin. When TRX1 moved from point A to B and then to mid-B to C, it had an LOS condition with TRX2; beyond this point, they both underwent an NLOS scenario. The same applied to TRX1 and TRX3; they had an LOS from  
Figure 15: (a) 2D and (b) 3D simulation environment representation for link margin estimation. point D to C until mid-C to D and later entered into the NLOS condition. Considering an ideal binary phase-shift keying (BPSK) modulation, the actual required power (\(RP_{TRXn}\)) at TRX2 and TRX3 was calculated using Equation (7) [27]:  
\[RP_{TRXn}=\frac{E_{b}}{N_{0}}+KT+B_{r}\ (\mathrm{dB}) \tag{7}\]  
where \(\frac{E_{b}}{N_{0}}\) is the symbol-to-noise ratio in the case of BPSK, \(K\) is Boltzmann's constant, \(T\) is the temperature in Kelvin, and \(B_{r}\) is the varied increased bit rate. Figure 16a,b present the computed received signal between TRX1 and TRX2 (Case I) and between TRX1 and TRX3 (Case II). The received signal strength for Case II was higher than for Case I due to co-polarization. In Case I, the received signal strength was relatively weak due to cross-polarization. However, the proposed CP antenna did better even in this scenario. The link margin estimation results for Case I in Figure 17 indicated that TRX1 and TRX2 had good LOS conditions from point A to B until mid-B to C, after which a significant drop in received power was observed. Consequently, a drop in the link margin also took place.