## 4 Proposed MIMO Antenna for Body Contacts and Specific Absorption Rate  
Another important element to consider is the Specific Absorption Rate (SAR), which indicates how much power is absorbed by human tissues and should be considered when a MIMO antenna is assumed in a close proximity to the human body, such as in smartphones and wearable devices [74]. The SAR is the amount of energy absorbed by a unit mass of tissue. The American and European standards are the two most important. The first requires a SAR value of less than 2 W/kg for every 10 g of tissue, while the second demands a SAR value of less than 1.6 W/kg for every 1 g of tissue [76]. Figure 27 shows the MIMO antenna simulation setup on the arm and chest for simulation and computation of the SAR distribution owing to an 8 element MIMO antenna. At the whole working BW, computation was carried out for 1 g and 10 g tissues. The [PERSON] phantom body model was utilized in CST MWS. The skin, fat, muscle, and bone make up is of [PERSON]'s body model. Table 6 shows the average tissue characteristics for each of these bodily sections at 2.4 GHz [77]. Figure 27: The proposed 8 ports MIMO antenna on the [PERSON] voxel model: (a) chest and (b) arm. Figure 26: Transmission coefficient results of flexibility investigation for the proposed 8 ports UWB-MIMO (in degrees) when antenna 1 sends. Table 7 displays the full SAR values for both standards. In the close proximity of the human hand and chest models, the wearable UWB-MIMO is being investigated. The suggested antenna elements' input power at operational frequencies is expected to be 24 dBm for each element. Figures 28 and 29 depict simulated SAR values that are acceptable at the required frequencies. The antenna has a 5 mm proximity and a 65\({}^{\circ}\) inclination angle. Table 7 indicates that the SAR values on arm, depending on 1 g and 10 g, are within acceptable ranges (port 1 is active here). It is noticed that the SAR value exceeds the standard values by increasing in frequencies (higher frequencies). Under many exposure conditions for instances in eye exposure, specifically at higher frequencies, volume averaged SAR decreases [78]. Besides, it has been illustrated that the electromagnetic radiation is absorbed more with increases in frequency [79]. The comparisons between the proposed MIMO antenna and several different works are shown in Table 8. It shows that the proposed UWB-MIMO antenna offers improved performing assuming mutual coupling, gain, operating frequency bandwidth, radiation efficiency, and the ECC. The proposed flexible 8-ports MIMO antenna presents low mutual coupling among its ports, high gain and efficiency, and low ECC. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tissues/Properties** & \(\varepsilon_{r}\) & \(\sigma\) **(S/m)** & \(\rho\) **(Kg/m\({}^{3}\)) pls Define** \\  & & & \(\rho\) \\ \hline Skin & 38.01 & 1.464 & 1000 \\ \hline Fat & 5.28 & 0.105 & 1000 \\ \hline Muscle & 52.73 & 1.739 & 1000 \\ \hline Bone & 11.38 & 0.394 & 1000 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The average tissue characteristics of human body parts at 2.4 GHz [77]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Port 1/\(f_{r}\)** & **3.8 GHz** & **5.2 GHz** & **5.4 GHz** & **8 GHz** & **10 GHz** & **15 GHz** & **20 GHz** & **22 GHz** \\ \hline SAR Ig & 0.069 & 0.218 & 0.601 & 0.885 & 0.904 & 1.302 & 1.294 & 1.315 \\ \hline SAR 10g & 0.028 & 0.087 & 0.440 & 0.619 & 0.816 & 1.115 & 1.108 & 1.128 \\ \hline
**Port 1/\(f_{r}\)** & **25 GHz** & **28 GHz** & **30 GHz** & **32 GHz** & **35 GHz** & **38 GHz** & \\ \hline SAR 1g & 1.350 & 1.498 & 1.797 & 1.912 & 2.105 & 2.253 & \\ \hline SAR 10g & 1.133 & 1.180 & 1.562 & 1.802 & 2.001 & 2.126 & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: SAR values (W/kg) on arm at different frequencies.