# A Perspective on Terahertz Next-Generation Wireless Communications
## 4 Space-Based Terahertz Systems
Again, these are important issues that will also appear in terrestrial or ground-to-space links, which will compel further necessary research. Finally, we mention the hostile environmental conditions in space. Space-based communication electronics should have protection circuitry and layers to reduce performance degradation associated with exposure to long-term and dynamic space radiation. In addition, extremely cold, hot, and/or wide temperature ranges significantly change the system performance due to variations in transconductance, linearity, and high-frequency parameters of active devices. Regarding radiation effects, one of the key damage mechanisms is total ionizing dose (TID), which leads to long-term degradation of device performance such as leakage increase and gain/bandwidth reduction [107]. Changes in transistor parameters include threshold voltage, off-state current, noise, transconductance, and parasitic components. In addition, resulting degradations in high-frequency metrics such as scattering parameters will affect circuit and system performance. Radiation effects in SiGe-based devices serve as good representative examples since they can reach terahertz operational speeds (\(f_{\text{MAX}}>700\) GHz). One study result is shown in Figure 3, which shows orders of magnitude increase in base currents for a 90 nm SiGe HBT (heterojunction bipolar transistor) as a function of total dose. While this is a microwave device, it is clear that electronics operating in intense radiation environment should provide acceptable performance for a given total dose requirement. Another mechanism in radiation effects is single-event effects (SEE), which occur when high energy heavy ions or particles strike devices, causing abrupt voltage and/or current transients at device terminals [107]. An example of dynamic interference in a low-noise amplifier generated by pulsed-laser SEE testing is shown in Figure 4. This example also illustrates that devices can be designed differently to significantly reduce susceptibility to SEEs. Figure 3: Transfer characteristics of a SiGe HBT following various total radiation doses. Upper family of curves shows collector currents \(I_{C}\) and lower family shows base currents \(I_{B}\). Used with permission from [108]. While radiation-effects are mostly unique challenges to space-based systems, wide temperature variations are still possible even in terrestrial wireless links. Likewise, high peak voltages or currents can degrade device reliability, leading to leakage increases, performance degradation, or even device failure. These challenges are also not limited to the space environment but can affect all terahertz devices pushed to their maximum performance capabilities. The field of space-based terahertz communication has many open questions for characterization, design, and implementation. Breakthroughs in all aspects of performance, efficiency, channel allocation, beam directivity and small form factor in devices, circuits, and system level improvements have been in demand for the maturity of these technologies [104]. To support stable and robust operations in extreme environments, radiation effects and cryogenic/high temperature performance should be carefully investigated, and then, relevant protection/compensation techniques can be implemented [110]. While the radiation effects of conventional RF or optical components and systems have been studied in the literature [111; 112; 113], relatively little has been analyzed in the terahertz domain, which represents a future research opportunity. Importantly, many such studies would translate to better understood requirements in terrestrial wireless links.