A solar-blind multi-quantum well (MQW) structure wafer based on AlGaN materials is epitaxial growth by metal-organic chemical vapor deposition (MOCVD). The monolithically integrated photonic chips including light-emitting diodes (LEDs), waveguides, and photodetectors (PDs) are presented. The results of the finite-difference time-domain (FDTD) simulation confirm the strong light constraint of the waveguide designed with the triangular structure in the optical coupling region. Furthermore, in virtue of predominant ultraviolet transverse magnetic (TM) modes, the solar blind optical signal is more conducive to lateral transmission along the waveguide inside the integrated chip. The integrated PDs demonstrate sufficient photosensitivity to the optical signal from the integrated LEDs. When the LEDs are operated at 100 mA current, the photo-to-dark current ratio (PDCR) of the integrated PD is about seven orders of magnitude. The responsivity, specific detectivity, and external quantum efficiency of the integrated self-driven PD are 74.89 A/W, 4.22×10¹³ Jones, and 3.38×10⁴%, respectively. The stable on-chip optical information transmission capability of the monolithically integrated photonic chips confirms the great potential for application in large-scale on-chip optical communication in the future.

The fifth generation (5G) communication has been a hotspot of research in recent years, and both research institutions and industrial enterprises put a lot of interests in 5G communications at some new frequency bands. In this paper, we investigate the radio channels of 5G systems below 6 GHz according to the 5G communication requirements and scenarios. Channel measurements were conducted on the campus of Beijing Jiaotong University, China at two key optional frequency bands below 6 GHz. By using the measured data, we analyzed key channel parameters at 460 MHz and 3.5 GHz, such as power delay profile, path loss exponent, shadow fading, and delay spread. The results are helpful for the 5G communication system design.

In the new era of railways, infrastructure, trains and travelers will be interconnected. In order to realize a seamless high-data rate wireless connectivity, up to dozens of GHz bandwidth is required. This motivates the exploration of the underutilized millimeter wave (mmWave) as well as the largely unexplored THz band. In this paper, we first identify relevant communication scenarios for railway applications. Then the specific challenges and estimates of the bandwidth requirements for high-data rate railway connectivity in these communication scenarios are described. Finally, we outline the major challenges on propagation channel modeling and provide a technical route for further studies.