title: Planar Liquid Crystal Beam-Steering and Beam-Switching Millimeter-Wave Networks with Slow-Wave Structures for Miniaturization and Fast Response
Creators: name: Wang, Dongwei
Publisher: name: UNSPECIFIED
Year: 2024

description: The upcoming generations of wireless communication hold great promise in providing ultra-fast data rate and very low latency by exploiting the large absolute bandwidths at millimeter-waves (mmW) frequencies. Hardware deployment requires highly directive antenna systems with beam steering capability, which can be provided by microwave liquid crystal (LC) technology at mmW in an analog manner. LC has relatively low dissipation factor above 5 GHz with a decreasing trend as frequency increases up to at
least several THz. As a functional material, LC performs continuous tunable permittivity to the propagating signal, depending on the relative orientation between LC molecules and signal polarization. LC-based transmission lines have tunable electrical length and are used as passive analog delay line phase shifters. In terms of planar LC phase shifter, the compromise between fast response and high RF performance arises as the major issue. This work focuses on innovative methods, namely defective ground structure (DGS) and nanowire-filled-membrane (NaM) technologies to realize planar microstrip LC phase shifters with miniaturized size, improved RF performance, and reduced response time. Both technologies show state-of-the-art performance in the realm of planar phase shifters. The one based on defective ground structures filled with 4.6 μm thick GT7-29001 type of LC is suitable for relatively low mmW frequencies, due to the low pass nature of defective ground structures. It achieves a response time of 51 ms and a figure-of-merit (FoM) of 79 °/dB at 30 GHz. The one based on NaM filled with 4.0 μm thick GT7-29001 is suitable for relatively high frequencies up to at least W-band and achieves 110 ms with 70 °/dB at 56 GHz. The
response time is related to LC layer thickness and surface anchoring condition. Furthermore, continuously beam steering planar phased antenna arrays are realized by combining the aforementioned LC phase shifters with antennas and feeding networks at around 28 GHz. The first phased array utilizes a 1×4 corporate feed network with four full 360° LC phase shifters, and the second utilizes a 4 × 4 Butler Matrix (BM) with four LC phase shifters of reduced length, and hence, losses, since they require only 135° maximum phase shift. For both phased arrays, each individual phase shifter is biased through a high-impedance DC conductive line to prevent RF leakage. DC blocks are integrated on both sides of each phase shifter to ensure DC isolation to other phase shifters and the non-tunable circuitry. While the former realizes a continuous beam scanning range of 110° by tuning phase shifters only, the latter can steer the beam continuously in a small range around the four predefined switchable directions, achieving a total beam scanning range of 120°. Both arrays have decent gain of 4.5 dBi and 5.5 dBi, respectively. The latter has higher gain due to the smaller phase shifters, resulting in lower loss. Both arrays are estimated to
be steered within 0.7 s or less for a full scan. Beside phased arrays, an interference-based single-pole double-throw (SPDT) is realized, using two LC phase shifters of 90°. The SPDT achieves not only on/off states, it can continuously adjust the power splitting ratio. When switched-on, the SPDT show 3.4 dB to 4.5 dB insertion loss within 26 GHz to 30 GHz and &gt; 10 dB isolation to the switched-off port. When switched-off, the SPDT requires 1.5s to recover the initial state.
Towards higher mmW band, NaM technology is promising as an interposer. Thus, a second Butler Matrix at W-band is realized on NaM as a proof-of-concept, and is to be integrated with LC NaM phase shifter. Taking the advantage of easy realization of through substrate via on NaM, the crossover as the key component of the Butler Matrix is proposed with compact size, low loss, high isolation and high balance, working from DC to 110 GHz. The final Butler Matrix with integrated patch antennas is measured at 100 GHz to have far field patterns at the desired discrete directions with good symmetry at ±12° and ±45°, with decent maximum gain of 5.4 dBi.