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 wideband 3-dB coupler using substrate integrated gap waveguide (SIGW) is proposed based on the principle of the branch-line coupler structure. The SIGW coupler design is comprehensively studied. It is found that SIGW conducting ridge vias play a significant role and have a strong impact on the cavity resonance, passband and impedance matching. A prototype of the proposed SIGW coupler is fabricated and tested, and the measured agrees well with the simulated. The coupler, better than the reported, has a passband of ~5 GHz from 23.59 to 29 GHz with the isolation above 20 dB, covering the 24.25 GH - 27.5 GHz and 27.5 GHz - 28.5 GHz frequency bands of 5G.

At this work, we propose a magneto-electric (ME) dipole antenna with characteristics of low profile, easy integration, wide band and high gain. The ME dipole is composed of rectangular patches and metallic via holes, and is embedded in a square aperture etched on substrate integrated gap waveguide (SIGW). In particular, the ME dipole is fed by a coplanar strip line (CPS), which is designed in SIGW to realize a low profile of 0.19λg (λg is the guide-wavelength at center frequency). The measurements show that the 10-dB impedance bandwidth of the antenna is 29%, ranging from 22.4 to 30 GHz, with mean gain of 8.04 dBi. Furthermore, the proposed antenna emits a stable radiation pattern within the operating frequency band, validated by the far-field measurements.

In this article, an integrated substrate groove gap waveguide (Integrated-SGGW) is proposed, which is composed of gap and groove layer in printed circuit board technology, which is considered a highly integrated circuit design technology. Electromagnetic wave is transmitted via the dielectric groove in transverse electric mode. Firstly, the waveguide is analyzed, including dispersion characteristics, transmission characteristics, characteristic impedance and field distribution. Then, the theory of dispersion and characteristic impedance for the Integrated-SGGW are given, and the waveguide is comprehensively analyzed. An Integrated-SGGW is fabricated, verified, and compared with substrate integrated waveguide and other waveguides. To show Integrated-SGGW applications in the design of millimeter wave circuits, a third-order bandpass filter using the Integrated-SGGW is designed, fabricated, and verified.

For emerging 5G fronthaul, broadband radio over fiber (RoF) transmission has been found much more cost-effective than conventional digital fiber transmission. Current digital predistortion (DPD) techniques may not be effective in linearizing broadband RoF links. This work presents a simple envelope-assisted radio frequency (RF)/intermediate frequency (IF) DPD model for linearization of broadband/multiband RoF transmission. The model uses both RF/IF signals and baseband envelopes, resulting in the short-term and long-term memory effect mitigated efficiently even for high signal bandwidth. The model complexity is lower than the existing baseband DPDs and does not increase with the increase of the signal bands. The model is experimentally evaluated in three scenarios: two 20 MHz long-term evolution (LTE) signals spaced by 100 and 40 MHz, and three 20 MHz LTE signals spaced by 50 MHz. It is shown that the proposed DPD results in comparable performance as today's baseband models.