# A Perspective on Terahertz Next-Generation Wireless Communications
## 4 Space-Based Terahertz Systems  
Terahertz research has expanded recently to more dedicated efforts in extreme-environment systems such as satellite and space-based electronics [95; 96; 97; 98]. For space-based applications terahertz has several advantages over conventional RF communications: (1) higher data rate due to wider channel bandwidth, (2) improved security from narrower beam widths and higher attenuation in the atmosphere, and (3) compact size/volume (e.g., smaller antenna size for same directivity) [99], which may also reduce launching costs. In comparison to laser-based communication systems, terahertz systems can also have a much more reliable link establishment between terrestrial stations and satellites under varying whether conditions [100; 101]. Similar to most other performance-driven applications, space-based terahertz communications typically require maximized performance metrics [102; 103]. Specifications such as antenna gain, component bandwidth, linearity, noise, data rate, and reliability are critically important in space applications. However, the same arguments could be made for terrestrial wireless systems, albeit with perhaps slightly less rigor. Therefore, space-based terahertz systems become an excellent development platform even for terrestrial wireless links. Indeed, given the relatively immature state of terahertz technology (compared to RF), one might even expect space-based applications to be the harbinger of future terrestrial ones. For this reason, they become an important topic in any discussion of future terahertz wireless systems. We first consider size and weight and power (SWaP). The physical volume/weight of a satellite or a cubeSat are strictly fixed, and the entire terahertz communication system must meet these dimensional requirements. At the same time, low-power operation is also mandatory in space, due to limited availability of solar or radioisotope-based power, especially for deep-space exploration. For that reason, CMOS-based terahertz has again proven attractive [104]. Jet Propulsion Laboratory (JPL) has been the innovation leader in space-mission terahertz technology [104], and they have certainly not dismissed CMOS. However, research indicates that CMOS offers substantial SWaP benefits in only selected areas, such as frequency synthesizers [104]. For terahertz transmitters, CMOS simply does not compete well with InP and GaN, nor receivers, where the low noise figures of GaAs or InP dominate. Nevertheless, research continues to push this envelope, especially as low-cost cubeSats become increasingly important. It is therefore conceivable that CMOS-infused terahertz technologies that are suitable for space would likewise meet SWaP requirements of terrestrial systems (e.g., mobile handsets). While channel interference is not generally the first concern of any present-day terahertz system (many such bands are not even yet allocated), terahertz waves are used for various space-based geophysical sensing platforms (e.g., radiometers). The widespread use of terahertz for wireless communication then poses a threat to disturb the measurements from these systems. It has been anticipated that such a threat could be anticipated and compensated by careful consideration of wireless communication systems in early design phase [105]. Similar studies would translate to interference between terrestrial links, although such work would also be covered by general channel studies. Satellite-to-satellite wireless communications has long been recognized as a niche market for terahertz waves since there is so little intervening atmosphere to attenuate the waves. In addition, relatively small antennas with good directivity may be employed due to the short terahertz wavelengths. With better directivity comes much greater potential for beam-pointing errors and harmful vibration effects, both of which may arise from satellite motion. Such research has begun and showed that both beam-pointing and vibration can significantly affect signal-to-noise ratio, and thus achievable data rates [106].