## 3 Spectrum Management Issues
### High-Spectrum Access
#### 3.4.5 Indoor Investigation  
The author of [196] has presented single- and multifrequency path loss models to identify the channel behavior at mmWave frequency bands, i.e., 28 and 73 GHz, in three typical indoor office layouts. All the measurements are conducted by using a 400 mega chips-per-second broadbandsliding correlator channel sounder. It has 800 MHz null-to-null RF bandwidth for 48 Tx-Rx location combinations. Additionally, co-and cross-polarized antenna configurations with LOS and NLOS environments are observed with antenna distance ranging between 3.9 and 45.9 m. Thus, a CI free space path loss model is implemented for single- and multifrequency systems because it provides simplicity and higher network performance without damaging accuracy in comparison with other path loss models, such as FI and ABG. However, the frequency band at 73 GHz is affected by scattering and other penetration losses due to a small wavelength. In [197], researchers have discussed error performance using different antenna polarization at a 60 GHz frequency band for the LOS environment. It is based on implementing an improvised IEEE 802.15.3c channel model to incorporate with polarization effects and extend it to calculate the error performance of mmWave communication systems that use circular polarization. The result has shown system performance in terms of ray experience and Bit Error Rate (BER) in comparison with linear polarization. The circular polarization in the LOS environment minimizes the multipath effect and, therefore, delivers high performance, specifically when the network operates at high throughput. In [198], the authors have presented mmWave propagation measurement at 28 and 73 GHz for an indoor office environment by using a correlator channel sounder and highly directional horn antennas. RMS delays spread with directional and omnidirectional path loss models; the figures show LOS and NLOS environments for co- and cross-polarized antenna configurations. The authors have concluded that the presented channel models can be utilized for mmWave wireless networks in an indoor environment for current 5G communication. However, an accurate and more vigorous antenna alignment is required for the LOS environment. The features of indoor propagation channel at four mmWave frequencies 28, 39, 60, and 73 GHz are investigated in [199]; propagation signal qualities measured based on building materials are also presented. To validate the behavior of these frequency bands, reliable ray-tracing software is implemented for LOS and NLOS conditions. Results have revealed that as the frequency increases, the received power and delay spread decreases. By contrast, a more robust power optimization method is in demand. Table 4 shows a summary of the above-discussed latest work for high-spectrum access. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Issues/Approach** & **Methodology/Technique** & **Advantages** & **Limitation/Future Work** & **References** \\ \hline \multirow{8}{*}{Outdoor investigation} & Channel characterization & & & \\  & performed at 26 GHz for tropical outdoor parking environment by utilizing Close-In Free Space (CI) and Floating Intercept (FI) path loss model & The CI model performance is higher than the FI model & More efficient model is needed for mobile users & [190] \\ \cline{1-1}  & MIMO channel characterization at 15 GHz for the outdoor scenario by using Okumura Hata and microcell model & & & \\ \cline{1-1}  & Propagation characteristics for 28 and 73 GHz for the outdoor environment by using CI and FI propagation model & Higher frequency spectrum delivers more data rate & Frequency Weighting (CIF) and Alpha-Beta-Gamma (ABG), can be used & [192] \\ \hline \multicolumn{5}{p{113.8 pt}}{Propagation characteristics comparisons of 28 and 73 GHz for the outdoor scenario using the ABG path loss model} & Better throughput and spectrum efficiency & Signal degrades when interference increases & [193] \\ \hline \hline \end{tabular}
\end{table}