RESUMO
A novel radio-over-fiber system with enhanced spectral efficiency is proposed by using a building-block-sparse-constellation-based orthogonal multiple access (BOMA) technique, and orthogonal frequency-division multiplexing with offset quadrature amplitude modulation (OFDM/OQAM) technique. With the help of BOMA, a user with good signal-to-noise ratio (SNR) can use the frequency band of a user with poor SNR to transmit extra information by sharing the same well-designed constellation. Moreover, the cycle prefix in OFDM is no longer required due to the properly designed prototype filters in OFDM/OQAM, contributing to maximum spectral efficiency (SE). To optimize transmission performance, four types of sparse constellations with different Euclidean distances between constellation points are designed. In our experiment, BOMA-OFDM/OQAM signals with a total net rate of 1.84 Gb/s and average SE of 4.6 bit/s/Hz are successfully transmitted over 25.2 km standard single-mode fiber for two users with wireless distances of 0.8 m and 0.3 m, respectively. Experimental results show that SE is increased by 37.7% compared with the traditional orthogonal frequency-division multiple access technique.
RESUMO
Piezoelectric aluminum nitride (AlN) thin film, as a commonly used material for high-frequency acoustic resonators, has been a research hotspot in the RF field. Doping Sc elements in AlN is one of most effective methods to improve the piezoelectricity of the material. In this work, the first principal calculation and Mori-Tanaka model are used to obtain the piezoelectric constants of AlN, ScAlN, and AlN/ScAlN composites. Then, five types of AlN/ScAlN thin films are prepared on 8 inch silicon substrates. The crystal quality, roughness, and stress distribution are measured to characterize the film quality. The results show that composite film can effectively solve the problem of abnormal grains and reduce the roughness. Finally, a lamb wave resonator with an AlN/Sc0.2Al0.8N composite working at 2.33 GHz is fabricated. The effective electromechanical coupling coefficient Keff2 is calculated to be 6.19%, which has the potential to design high-frequency broadband filters.
RESUMO
Film bulk acoustic resonators (FBARs) with a desired effective electromechanical coupling coefficient (Keff2) are essential for designing filter devices. Using AlN/AlScN composite film with the adjustable thickness ratio can be a feasible approach to obtain the required Keff2. In this work, we research the resonant characteristics of FBARs based on AlN/AlScN composite films with different thickness ratios by finite element method and fabricate FBAR devices in a micro-electromechanical systems process. Benefiting from the large piezoelectric constants, with a 1 µm-thick Al0.8Sc0.2N film, Keff2 can be twice compared with that of FBAR based on pure AlN films. For the composite films with different thickness ratios, Keff2 can be adjusted in a relatively wide range. In this case, a filter with the specific N77 sub-band is demonstrated using AlN/Al0.8Sc0.2N composite film, which verifies the enormous potential for AlN/AlScN composite film in design filters.