# A Perspective on Terahertz Next-Generation Wireless Communications
## 3 Terahertz Devices
Several transceivers beyond 100 GHz have now been demonstrated using SiGe technologies [65; 67]. Some of these challenges can be overcome by using radically different front-end architectures. In radio-astronomy, where terahertz systems have been actively utilized for decades, passive mixers and harmonic frequency multipliers are used to detect extremely weak signals from distant objects [77]. As passive mixers and harmonic multipliers rely on device nonlinearity, their performance is not strictly bound to \(f_{\text{MAX}}\). Instead, the cutoff frequency (\(f_{T}\)), which is the first pole frequency of a nonlinear device, has a more significant correlation with the device performance. Even in CMOS processes, nonlinear passive devices with \(f_{T}\) close to 1 THz can be realized [78; 79]. Impressively, a 16 Gbps QPSK transceiver link at 240 GHz [63; 64], a 105 Gbps QAM transmitter front-end [60], and a 30-Gbps QPSK transmitter [61] have all been demonstrated using passive mixers and multipliers using a 65 nm CMOS process. However, their system-level energy efficiency and practical long-range operation with full system integration including baseband circuits has not yet been investigated. Finally, we mention a growing area of hybrid photonic-electronic devices [80; 81; 82; 83], in particular sources and detectors. This technology approach is quite relevant since its most commonly reported application is wireless communication [84; 85], although terahertz sensing utilizes the same basic devices [86]. Both terahertz transmitters [81; 87; 88; 81] and receivers [89; 90] have been implemented that leverage the high maturity of optical fiber technology, but transmitters in particular have received more attention [91]. In this approach, two slightly detuned, frequency-stabilized and phase-locked semiconductor lasers (\(\lambda=1.55\) um InP/InGaAs lasers are common) or an optical frequency comb generator [84] typically act as local oscillators, both illuminating an electronic photodiode. These generate a beat frequency in the terahertz regime, which through nonlinear effects in the photodiode produce a terahertz signal with a relatively narrow bandwidth of about 1-2 MHz, corresponding roughly to the linewidth of the lasers. The lasers are typically highly tunable, well beyond 1 THz [85], meaning the beat frequency and corresponding terahertz carrier can be tuned over a very wide range, from zero to over 1 THz. Modulation of the terahertz signal often occurs via modulation of one or both of the lasers with commercially-available electro-optic modulators, for example. The uni-travelling-carrier photodiode (UTC-PD) [87; 88] has been employed extensively as the photodiode of choice for terahertz generation; it supports fast modulation with high current density which is advantageous for generating terahertz radiation. With UTC-PDs, terahertz output powers of \(-2.7\) dBm at 0.35 THz are currently possible [87; 92], making wireless communication systems supporting \(\geq 100\) Gbps [93] over multiple tens of meters possible [84; 91]. The advantages and disadvantages of the hybrid photonic-electronic approach are clearly important. For advantages, they are based on mature fiber-optics technology. They are widely and easily tunable, being based on the mature tunability of the mixed lasers. They exhibit admirable output powers, particularly in the sub-0.5 THz regime, though not quite as good as all-electronic approaches. Data modulation is fast, again leveraging mature technology from optical fiber systems. Finally, most of the system parts are already commercially-available (e.g., lasers, UTC-PDs, modulators, etc.) in some form. As disadvantages for next-generation wireless, photomixer based devices also require laser stabilization and possibly synchronization between transmitter and receiver for real-time demodulation. Indeed, most demonstrations still use all-electronic terahertz receivers.