# Microwave Liquid Crystal Technology
## 4 Use Cases for LC-based Microwave Devices  
Modern communication systems are on the way to the 5 th generation (5G). The paradigm change includes three different main directions including increased data rate up to several gigabit per second, low-latency communication down to milliseconds, and finally, increased user density [12; 13; 14; 15]. While most of the functionality will be covered by improved network architecture including e.g., network slicing, there are new systems to be deployed at higher frequencies. As the ITU plans to standardize a new 60 GHz band at WRC'19, first providers demonstrated the potential at international sport events such as the last winter Olympiad in South Korea [16; 17]. Millimeter-wave (mmWave) communication at 60 GHz and higher frequencies is mainly limited by high free-space path loss and atmospheric impairments, compare Figure 10.  
A second trend in mobile communications is characterized by worldwide ubiquitous communication envisioned for a long time but brought to the real world by projects like Google's LOOM and OneWeb. These concepts summarized in the New Space Initiative use flying platforms such as balloons or low earth orbit (LEO) satellites to provide communication services. Independently from the final system concepts, high-gain antennas are required to establish a stable communication link from users' terminals to the network. For systems operated at \(\mathrm{K_{a}}\) (26.5 GHz to 40 GHz) or \(\mathrm{K_{u}}\) (12 GHz to 18 GHz) bands, high-gain antennas are required to bridge large distances between terminals, e.g., 600 km to the LEO satellite, which corresponds to approximately 180 dB of path loss at \(\mathrm{K_{a}}\)-band. For a 60 GHz system this free space path loss is already reached at a distance of 400 km. In practice, the reach is much lower due to mentioned atmospheric impairments such as absorption by oxygen (\(\approx 14.6\,\mathrm{dB/km}\)) and rain attenuation (\(\approx 5.8\,\mathrm{dB/km}\) for medium rain) for 60 GHz transmission, according to ITU recommendations for standard atmosphere [18; 19]. Figure 9: THz characterization results for the LC GT3-23001 from Merck KGaA, Darmstadt, Germany. As the transmitter output power is limited, the path loss must be compensated by the gain of the antenna. The gain \(g\) of an antenna and the beam width are connected by  
\[g=\eta\cdot D\approx\eta\cdot\frac{4\pi}{\mathrm{HPBW}_{\mathrm{az}}\cdot \mathrm{HPBW}_{\mathrm{el}}} \tag{13}\]  
with the antenna's efficiency \(\eta\) and the half-power beam width \(\mathrm{HPBW}_{\mathrm{az}}\) and \(\mathrm{HPBW}_{\mathrm{el}}\) in azimuth and elevation, respectively. Hence, to realize an antenna with enough gain (\(g>60\,\mathrm{dB}\)), the width of the beam gets with values of less than \(0.1^{\circ}\) very narrow. If you now want to establish a connection, the antennas must be aligned in a precise way. For the alignment of antennas, different methods can be used. Among the very common ones are mechanically steered antennas, where the antenna's axes are motorized. This results in heavy systems which additionally require high maintenance efforts. Beside this, electronically steerable antennas have been developed and explored in the last centuries. These are based on the array concept where many antenna elements are combined with respect to phase and amplitude to synthesize a steered antenna beam pattern. For high-frequency high-bandwidth applications as indicated by previous scenarios, analog beam-steering concepts are a low-cost alternative to fully digital systems. Figure 11 gives an overview on the most important implementations. LCs are the most promising candidates for applications above 15 GHz as their dielectric loss is superior compared to other technologies such as ferroelectrics [20; 21; 22; 23; 24; 25] and ferrites [26; 27; 28]. A direct competitor are micro-electro-mechanical systems (MEMS) [29; 30], which utilize mechanical changes on the micro scale to implement phase shifter components required for array antennas.