Today, the 6G standard is in the early phases. Candidate technologies are being researched, prototyped, and simulated to drive the direction of the 3GPP standards development activity. Applying digital twin workflows to simulations and pairing them with a prototype as it moves through the design phase opens up innovative ways to design and identify optimizations early. Modeling software tools that leverage as much real-world data as possible have the potential to solve significant challenges early by incorporating known insights from existing 5G deployments. Research testbeds will be critical to success if 6G aims to spur collaboration alongside innovation. The ability to prototype new AI, digital twin, and software-defined tools across the wireless ecosystem will provide significant results for applying those technologies wisely. This eBook looks at the design densification and test challenges from migrating 5G communications into the 6G sub-Terahertz frequency ranges. Keysight Technologies, the sponsor of this eBook, presents end-to-end capabilities for simulating, testing, and evaluating devices, subsystems, and systems for the evolution of the 5G systems as they evolve towards 6G.