In this work, we propose and investigate a novel technique for the generation of millimeter-wave (mm-wave), i.e. frequency sixuplexing technique. The proposed technique is comprised of two cascaded Mach-Zehnder modulators (MZMs). The first MZM, biased at maximum transmission, is only used for even-order optical harmonic generation, and then a second MZM, biased at minimum transmission, is used for both optical carrier suppression modulation and data signal modulation. As an example, we consider an RF at 10 GHz, which carries the data signal and drives the MZMs; and an mm-wave signal at 60 GHz, i.e. a frequency sixupler, is obtained. It is found that our proposed sixupler leads to an 8-dB higher RF power at 60 GHz and a 6-dB improvement in receiver sensitivity with comparison to the conventional technique, i.e. optical carrier suppression modulation. The generated mm-wave signal is robust to fiber chromatic dispersion. The proposed technique is verified by experiments.

Linearization technologies that can be used for linearizing RoF transmission are reviewed. Three main linearization methods, i.e. electrical analog linearization, optical linearization, and electrical digital linearization are presented and compared. Analog linearization can be achieved using analog predistortion circuits, and can be used for suppression of odd order nonlinear distortion components, such as third and fifth order. Optical linearization includes mixed-polarization, dual-wavelength, optical channelization and the others, implemented in optical domain, to suppress both even and odd order nonlinear distortion components, such as second and third order. Digital predistortion has been a widely used linearization method for RF power amplifiers. However, digital linearization that requires analog to digital converter is severely limited to hundreds of MHz bandwidth. Instead, analog and optical linearization provide broadband linearization with up to tens of GHz. Therefore, for broadband radio over fiber transmission that can be used for future broadband cloud radio access networks, analog and optical linearization are more appropriate than digital linearization. Generally speaking, both analog and optical linearization are able to improve spur-free dynamic range greater than 10 dB over tens of GHz. In order for current digital linearization to be used for broadband radio over fiber transmission, the reduced linearization complexity and increased linearization bandwidth are required. Moreover, some digital linearization methods in which the complexity can be reduced, such as Hammerstein type, may be more promising and require further investigation.

A substrate-integrated gap waveguide (SIGW) sequentially rotating phase (SRP)-fed 2 × 2 circularly polarized (CP) patch antenna array is presented for the wideband millimeter-wave application. The antenna element is composed of a truncated square patch, SIGW feedline with three layers of printed circuit board (PCB), and another layer of PCB with an air hole for supporting radiation patch. Each radiation patch is driven by an underneath coupling slot etched on SIGW. A wide axial-ratio (AR) bandwidth is achieved by deploying a 2 × 2 SIGW SRP feed network. Moreover, simple calculation expressions of the characteristic impedance and guide wavelength of SIGW are given for the design of SRP feed network. The fabricated prototype shows an impedance bandwidth of 25.6% covering 22.2-28.8 GHz, 3 dB AR bandwidth of 19% from 22.8 to 27.6 GHz, and peak gain of 11.53 dBic.

The first antenna based on substrate integrated gap waveguide (SIGW) is presented. Using two contacted metal strips to build the required conducting ridge, the SIGW addresses the defects inherent in the printed gap waveguide at millimeterwave frequencies. Moreover, a bandwidth enhancement method is proposed and demonstrated by tuning the cavity that excites the slot antenna. The achieved bandwidth can be more than 21 GHz centered at 60 GHz (35%). Different tuning configurations are carefully investigated, and a design guideline is provided, in which different substrates for the gap and via layers of the SIGW are considered. Two fabricated antennas using low-cost PCB verify the simulation results and design guidelines. The first one is a single-slot antenna, and the second is a four-slot linear array. The measured S ₁₁ , realized gain, and radiation patterns of the fabricated antennas have good agreement with the simulated counterparts.

In this paper, a new quasi-transverse electromagnetic (TEM) microstrip line, named as packaged microstrip line, is proposed for microwave and millimeter-wave applications. It is comprised of three dielectric substrates: a base layer to place the metal strip, a top layer to build the perfect magnetic conductor (PMC) shielding with periodic plated vias, and a middle layer to separate the PMC layer from the metal strip and the bottom ground plane. With the top PMC shielding, this novel microstrip line no longer suffers from issues of radiation losses, surface waves, or cavity resonances that exist in the classical microstrip line without or with a metallic shielding box. Also, the unwanted higher order mode created in the stripline by any vertical asymmetry between the ground planes will not be generated in the packaged microstrip line. In addition, the fundamental quasi-TEM mode will help this novel line avoid the mode conversion losses when integrated with other quasi-TEM lines, which happen to the substrate-integrated waveguide due to its fundamental TE ₁₀ mode. This paper focuses on the effects of the PMC-layer substrate on this line performance. We present how to adopt a proper substrate for each layer of the packaged microstrip line considering both stopband and transmission loss, and how to design the linewidth for a given characteristic impedance. Two different transition schemes of the packaged microstrip line to the standard microstrip line are taken into account. Two prototypes of the proposed packaged microstrip lines using different substrates for the PMC layer are fabricated. The measured and simulated results show good agreement.

High–speed, high–power photodiodes play a key role in wireless communication systems for the generation of millimeter wave (MMW) and terahertz (THz) waves based on photonics–based techniques. Uni–traveling–photodiode (UTC–PD) is an excellent candidate, not only meeting the above–mentioned requirements of broadband (3 GHz~1 THz) and high–frequency operation, but also exhibiting the high output power over mW–level at the 300 GHz band. This paper reviews the fundamentals of high–speed, high–power photodiodes, mirror–reflected photodiodes, microstructure photodiodes, photodiode–integrated devices, the related equivalent circuits, and design considerations. Those characteristics of photodiodes and the related photonic–based devices are analyzed and reviewed with comparisons in detail, which provides a new path for these devices with applications in short–range wireless communications in 6G and beyond.