Solid-State Terahertz High-Speed Wireless Communication Technology

Terahertz (THz) waves lie in the frequency range between 100GHz and 10THz. This section of the electromagnetic spectrum locates in the band sandwiched between the frequency regions that electronics and photonics conventionally focus on, thus leaving this area almost unexplored and unexploited. With the rapid evolution of wireless communication, the existing spectrum allocation has become increasingly crowded. It is widely acknowledged that exploiting new frequency resources is a straightforward approach to tackle this issue. Since there is huge availability of absolute bandwidth in THz band, wireless communication is a niche customer of these frequency resources.Among several technological enablers, solid-state electronics is regarded as one of the most promising owing to the possibility of adopting system-on-chip integration, which is a vital enabling feature for future commercial applications.This dissertation focuses on the realization of THz high-speed wireless communications by the use of solid-state technology. Two types of critical enabling components of THz wireless systems (subharmonic mixers and frequency doublers) were also discussed and developed. Based on this, THz communication systems were built and high-speed wireless data transmission was successfully achieved. In particular, the research of this dissertation is embodied in the following three regards.(1) THz subharmonic mixers. Subharmonic mixers fulfilling frequency conversion in the wireless systems play a key role in the system performance due to the lack of THz solid-state amplifiers. To better understanding the device, the fundamental physics of Schottky barrier diodes were looked into, and furthermore the equivalent circuit model,noise model and diode chip geometric model of mixer diodes were built. Effects of diode parameters and chip parasitics on the mixer performance were discussed. In the discussion,a method to extract the parasitic capacitance of an anti-parallel diode pair was proposed.In order to expedite the circuit design, an optimization procedure was proposed. The basic idea of the methodology is primarily to locate the optimum performance of the nonlinear device and obtain the corresponding embedding impedance conditions and the device impedances under this condition. The matching networks can then be synthesized in terms of S-parameter performance to meet the optimum embedding conditions. With the matching networks designed,the ultimate circuit's nonlinear performance can be optimized. This methodology was adopted to design a 220GHz subharmonic mixer. When carrying out the mixer design simulation, an external noise source was introduced to present the effects of hot electron noise in order to better characterize the mixer's noise performance. The mixer was constructed, assembled and measured. Y-factor measurements show that the mixer exhibited a double side band (DSB) noise temperature less than 1500K and DSB conversion loss lower than 10dB over 188-244GHz. The experimental results were in good agreement with the simulation predictions, which verifies the modeling of mixer diodes and the proposed circuit optimization methodology.(2) THz frequency doublers (×2 multiplier). Frequency multiplication is a common technique to generate THz frequencies, which makes a doubler one of the key components to form solid-state frequency sources in THz systems. In this part, the principles of varactors were analytically investigated. Based on these analytical models, effects of varactor parameters on the doubler performance were discussed and thus several design aspects of varactor parameters were presented. In response to the specific requirements of two doublers respectively at 190GHz and 180GHz, a varactor design model was proposed to quantitatively look into the effects of the device parameters on the doubler performance and to facilitate the varactor design for the two doublers. To validate the varactor design, the 190GHz and 180GHz doubler circuits were designed based on the two designed varactor chips. The circuit optimization methodology was adapted from that used for the mixer design. Both of the doublers were machined, assembled and measured.The measurement results show that the 190GHz doubler generated output over 190-198GHz and could handle up to 350mW input power. The maximum conversion efficiency of 8% was achieved at 193GHz when the input power was 200mW, resulting in the output power of 16mW. When the input power was increased to 350mW, it produced the output power of 24.12mW at 193GHz with the conversion efficiency of 6.89%. As for the 180GHz doubler, the measurement results indicated that it exhibited a conversion efficiency larger than 10% over 173-184GHz when the input power was 100mW with the maximum conversion efficiency of 15.5% at 183GHz. Its capability of power handling was further tested at 183GHz with the input power increased to 200mW,which yielded the output power of 24.17mW and conversion efficiency of 12.1%. The measurements of both the doublers agreed reasonably well with the design simulations,which demonstrates the effectiveness of the modeling of varactor diodes, the design of the two varactor chips and the circuit optimization methodology as well.(3) THz high-speed wireless communications. On the basis of key component realization, two wireless data links respectively at 120GHz and 220GHz were successfully accomplished. The proof-of-concept transmission experiment at 120GHz was conducted in the lab environment with the data rate up to 12.5Gbit/s. The successful transmission demonstrates the potential of the adequate bandwidth in THz band for wireless communication. The 220GHz experimental system was built based on the 220GHz low-noise subharmonic mixer and it was able to transmit up to 3.52Gbit/s wireless data at a distance of 200m in the outdoor environment. The bit error rate (BER)at 3.52Gbit/s was 1.92 × 10-6.The outcomes in this dissertation project the promising potential of THz waves for high-speed wireless communication, and also demonstrates the feasibility of the future practical application of THz wireless communication systems. This lays an important foundation in both theoretical and technological aspects to exploit THz frequency resources for future wireless communications.