## 4 Key Enabling Techniques
### Millimeter-Wave Communications (mmWave)  
The majority of overwhelming communication networks are already operating in the microwave band, which makes it too scary. The millimeter-wave (mmWave), becomes an important facilitator of 5G networks as a result of the availability of abundant spectrum resources at the millimeter-wave band from 30 to 300 GHz promising high data rate [59]. It is a key research area for wireless power transfer [31], solution for high bandwidth vehicular communication [60] and unmanned aerial vehicles (UAVs) [61; 62], as it let huge antenna arrays to be arranged in lesser form-factor. Therefore, mmWave is considered to be the main candidate to achieve improved network capacity and data rate. According to the US Federal Communications Commission, various frequency spectrum's within the mmWave appear promising and suitable players for 5G networks and Beyond, as well as the local multi-point distribution service (LMDS) band ranging from 28 to 30 GHz, the non-licensed 60 GHz band, and 12.9 GHz located from 71-76 GHz, 81-86 GHz, and 92-95 GHz in the E-band [46]. Moreover, \begin{table}
\begin{tabular}{l l l} \hline \hline
**Technology** & **3 GPP RELEASE** & **Access Techniques** \\ \hline
2G system & Release 96 for 14.4 kbps user data & FDMA and TDMA \\  & Release 97 for GPRS & \\  & Release 98 for EDGE & \\
3G system & Release 99 for UMTS & CDMA \\ LTE (2 \(\times\) 2 MIMO) & Release 9 & OFDMA \\
4G LTE Advanced (4 \(\times\) 4 MIMO) & Release 10 & OFDMA and OFCDM \\
4G LTE-Advanced Pro & Release 13 and beyond & OFDMA and OFCDM \\ WiMAX & Release 9 & OFDMA \\
5G systems & Release 15 for Phase 1 & OMA for downlink, NOMA for uplink \\  & Release 16 for Phase 2 & \({}^{*}\) \\
5G-Advanced & Release 17 & CP-OFDM and DFT-S-OFDM \\  & Release 18 & TDD and FDD \\ \hline \hline \end{tabular}
\end{table} 
Table 3: An overview of different generations, 3 GPP Release’s and multiple access techniques. the mmWave is revolutionary because it has distinct propagation requirements such as: hardware constraints and atmospheric absorption as compared to microwaves. It is broadly acknowledged that the mmWave must be utilized together with a limited cell radius of less than 100 m to reduce high-rise path loss. High path loss in mmWave as compared to MW bands below 3 GHz is considered main challenge, give by:  
\[L_{FreeSpace(\mathrm{dB})}=32.4+20 log_{10}f+20 log_{10}R \tag{1}\]  
where \(L\) is free space path-loss measured in decibels (dB), \(f\) (GHz) the carrier frequency and the distance between transmitter and receiver is \(R\), measured in meters. This can be understand clearly that an extra path loss of approx. 23 dB & 31 dB when moving the operational frequency from 2-28 GHz and 70 GHz, respectively. As the non-LOS (reflected) signal is very weak, the mmWave can use highly directional antennas for line-of-site (LOS) communication. Another serious challenge of mmWave is signal attenuation at high frequency spectrum. This is very problematic as it limits signal propagation because the energy of mmWave is absorbed by water vapor and oxygen. The third challenge of mmWave signal is its penetrates with high loss, makes it very sensitive for the buildings [46]. However, due to the limited number of simultaneous connections at these extremely high frequencies, the mmWave channels are sporadic nature (scattered) in the angle/spatial domains. Hence, by using either hybrid analog-to-digital beamforming with a large scale antennas of coupling mmWave with massive MIMO and NOMA can avoid this restriction and can yield very high SE gain [54]. To the bottleneck of existing wireless bandwidth, enhancement of channel capacity, and SE a solution is proposed in [63]. There are various challenges of mmWave communications do exist, such as beam training and tracking, device association, directivity, sensitivity to blockage, and high propagation loss which are under research and need to address [64]. [PERSON] et al.