Innovative Electric Components for Millimeter Wave Communication

The millimeter-wave (MMW) region of the electromagnetic spectrum is a region of frequency band between 30 gigahertz and 300 gigahertz, and it is called the Extremely High Frequency range. These high frequencies can carry huge amounts of data at extremely high speed but with little delay or loss. As we are becoming more digital-reliant, millimeter waves can be used for high speed wireless broadband communications, such as radio, cell phone, or satellite. Many telecommunication companies are now investing in millimeter wave spectrum for their 5G network.

However, one drawback of the MMW system is its large propagation loss in free space and cables, especially when deploying MMW technology in dense urban environments because of blockage from high buildings. Therefore, researches are investigating to extend the distance the signal can travel. Where longer paths are desired, the extremely short wavelengths of MMW signals can be concentrated into highly focused beams by very small antennas, with enough energy gain to overcome propagation losses.

Hence, the design of high gain antennas has been a central focus for researchers and technologists. In this blog post we will explore three such designs from research groups in the UK and Japan.

Empty Substrate Integrated Waveguide Slot Antenna

One popular design is the linear slot antenna array with four radiating slots, based on an empty substrate integrated waveguide (ESIW). The design was first introduced by Khan et al. [1] This antenna design not only facilitates easier integration with planar electronic components, as compared to conventional slotted waveguide antenna arrays, it also reduces the weight and fabrication cost.

ESIW structure is constructed by creating a void rectangular shape in the geometry of a planar substrate, as shown in Figure 1. The upper and lower surfaces as well as the lateral walls are metallized to create an empty waveguide-like geometry. Additionally, the structure has the microstrip-to-empty-waveguide transition at one end, and is terminated by short circuit at the opposite end to keep the standing waves inside. Longitudinal slots etched on the edge of the ESIW structure interrupt the transverse current flow and give rise to radiation.

Figure 1. Left: The schematics of ESIW slot antenna array. The end slot is placed at a distance of λ/4 from the short circuit termination, where λ is microstrip wavelength; Right: Dimension table of the structure. [1]

The design geometry is simple and the results from tests validate the consistent radiation pattern. The features of high bandwidth that cover intended 5G bands make this antenna capable of fulfilling the demands of future networks.

Polyethylene Terephthalate Based Flexible Antenna

Jilani [2] et al. presented an antenna geometry which contains a T-shaped patch integrated with symmetrically designed slot arrangements, on Polyethylene Terephthalate (PET) substrate. The defected ground structure (DGS) is utilized for bandwidth enhancement by creating resonant gaps throughout slots in the patch, to change the surface current distribution. A co-planar waveguide (CPW)-fed slotted monopole antenna is embedded within an aperture cut inside the ground plane. Moreover, the antenna prototyping uses silver nanoparticle ink for inkjet printing process, which directly controls the propagation loss and improves the radiation efficiency due to its conductivity.

Figure 2. Flexible inkjet-printed antenna on PET film

This antenna design possesses the characteristics of high gain profile, high efficiency, less complexity of feed systems, simplicity of the structure and accurate control over the radiation pattern, which make it a potential candidate for incorporating into MMW communication system.

An Electro-Optic Modulator for MMW

Otagaki [3] et al. proposed and developed an electro-optic modulator by using antenna-coupled-electrodes for the conversion of MMW wireless signals to optical signals in radio-over-fiber systems. Radio-over-fiber refers to a technology whereby light is modulated by a radio frequency signal and transmitted over an optical fiber link. This technique is attractive for the MMW wireless systems since MMW signals can be transferred as light wave signals through a silica optical fiber at extremely low loss and huge bandwidth.

The electro-optic modulator they designed uses a hybrid stacked substrate structure composed of a thin (50μm) LiNbO₃ film and a thick (250μm) SiO₂ base substrate. Optical waveguides and antenna-coupled electrodes are fabricated on the opposite side of the LiNbO₃ film. Each antenna-coupled electrode is composed of two planar microstrip patch antennas and a standing-wave resonant electrode, where a coupled microstrip line with electrically-shorted ends is embedded.

By arranging the antenna-coupled electrodes as an array along the optical waveguides, this modulator can be operated like a phased array antenna. The planar path antennas can detect and receive MMW signals and send them to the standing-wave resonant electrodes through the short microstrip lines. The phases of the two signals from the two antennas are mutually out of phase owing to the connecting points of feeding micro-strip lines. Then the electromagnetic field is coupled to the odd mode of the coupled microstrip line, which generates high efficiency light wave modulation. The light wave propagating in the optical wave guide is modulated under the resonant electrodes by MMW signals. This millimeter wave to light wave conversion system is a significant prototype for future MMW wireless communication systems.

Figure 3. Schematic view of the antenna-coupled electrode EOM [3]

Figure 4. Detailed Structure of an antenna-coupled electrode [3]

Conclusion

Newly designed components for millimeter wave communication should be easily integrated with planar circuits and should maximize the wave transmission performance. Millimeter wave frequencies are anticipated as highly promising for the realization of next generation mobile networks for better coverage with massive data delivery. It will continuously develop and offer broad extent of applications in the future.

References

[1] Z. U. Khan, Q. H. Abbasi, A. Belenguer, T. H. Loh and A. Alomainy, “Empty Substrate Integrated Waveguide Slot Antenna Array for 5G Applications,” 2018 IEEE MTT-S International Microwave Workshop Series on 5G Hardware and System Technologies (IMWS-5G), Dublin, 2018, pp. 1-3.

[2] S. F. Jilani, Q. H. Abbasi and A. Alomainy, “Inkjet-Printed Millimetre-Wave PET-Based Flexible Antenna for 5G Wireless Applications,” 2018 IEEE MTT-S International Microwave Workshop Series on 5G Hardware and System Technologies (IMWS-5G), Dublin, 2018, pp. 1-3.

[3] Y. Otagaki, Y. Matsukawa, H. M. A. Sanada and S. Kurokawa, “Design of antenna-coupled-electrode electro-optic modulators for 5G mobile systems,” 2017 IEEE Conference on Antenna Measurements & Applications (CAMA), Tsukuba, 2017, pp. 28-31.