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Terahertz (THz) band (0.1-10 THz) communication is envisioned as a key wireless technology to support faster and denser wireless networks. However, absorption by water vapor molecules splits the THz band into multiple transmission windows, each tens to hundreds of Gigahertz (GHz) wide. Because of the broadening of the absorption lines, the available bandwidth within each transmission window changes drastically with distance. As a result, a large portion of the bandwidth close to the absorption lines is considered not practical for communications or, in the best case, practical only for short-range applications. In this paper, chirp spread spectrum communication is proposed as a way to enable ultra-broadband communication links spanning across absorption lines in the THz band. More specifically, first, the performance of Chirp-Spread Binary Phase Shift Keying (CS-BPSK) is analytically derived and compared to that of Binary Chirp Spread Spectrum (BCSS) as well as non-spread Binary Phase Shift Keying (BPSK). Extensive numerical results based on an analytical channel model of the THz band are provided to illustrate the performance of the proposed scheme. Finally, experimental results above conducted in the first absorption-defined window above 1 THz are provided, validating the original hypothesis and highlighting the opportunities and challenges in communication across absorption lines at THz frequencies.
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