Due to the lack of spectral resource in traditional low frequency bands and the great demand for capacity of wireless communications, millimeter wave (mmWave) communications is considered as a key technology for the upcoming 5G networks. Large scale antenna arrays and hybrid precoding are employed to compensate the high path loss of mmWave propagation. In this paper, based on the decomposition of hybrid precoding problem, we propose a novel algorithm for mmWave massive multiple-input multiple-output (MIMO) system, which has low computational complexity. In addition, simulations show that the proposed algorithm can achieve near-optimal performance.

Low permittivity materials are widely used in many electronic systems. However, dielectric measurement on low permittivity material is challenging. To address this difficulty, a dual-band resonant sensor is developed. The proposed sensor is an improved design from the complementary square spiral resonator (CSSR). By adding a square patch on top of the resonant cell, the local field is significantly increased. Therefore, the interaction between field and material is strengthened, resulting in an enhancement in measurement sensitivity. Owing to the increased sensitivity, low permittivity samples down to 1.19 can be measured with an accuracy of better than 3%. Such results demonstrate that this sensor is promising for low permittivity characterization. Moreover, this sensor is also capable of measuring samples with permittivity in the range of 2–10, with an accuracy of 5%. Furthermore, two resonances are created, enabling dual-band operation. The unloaded resonances take place at 2.00 and 5.41 GHz. Particularly, the measurement can be conducted simultaneously in the two operation bands. Measured results demonstrate that the accuracy and sensitivity are satisfactorily high and comparatively superior to the designs in the literature for low permittivity measurement.

Millimeter wave (mmWave) communication is expected to be a key technology for 5G networks, because it can provide high data rates and large available bandwidth. Large antenna array is necessary to be applied to mmWave system to overcome the high path loss. Restricted by power consumption and system complexity, traditional digital precoding cannot be used for mmWave system. Therefore, analog radio frequency signal processing is introduced to simplify the architecture of precoder. In this paper, we propose dominant vector approaching (DVA) searching method for analog precoder design. Assuming that the channel state information is known at both transmitter and receiver sides, we decompose the analog precoding problem into a series of subproblems. For each subproblem, we just need to optimize one column of analog precoding matrix. Then we can apply DVA searching method to solve these subproblems under unit modulus constraint. For the fully-connected structure (FS) and the partially-connected structure (PS), we propose DVA-FS and DVA-PS as our precoding designs, respectively. In addition, we prove that DVA searching method finds the near-optimal solutions for the non-convex subproblems. Through simulation, we show that DVA algorithms achieve near-optimal performance with analog precoder in different scenarios. Besides, we investigate the effect of quantization and the energy efficiency of DVA algorithms, and find that they perform well with rough quantization.

An ultra-broadband angular-stable reflective linear to cross polarization converter based on metasurface is developed. The unit cell of the converter is formed by a slant end-loaded H-shaped resonator. The slant arrangement is to create polarization conversion. The end-loaded stubs are useful for miniaturization and the slots are responsible for enlarging bandwidth. The simulated results show that the polarization conversion ratio of the proposed design is better than 90% in the range of 9.83–29.37 GHz, corresponding to a relative bandwidth of 99.69%. It is also demonstrated that the mean polarization conversion ratio is larger than 80% even though the incident angle reaches 40° for both x-polarized and y-polarized incidences. To validate the design, a prototype of the proposed structure is fabricated and measured. Satisfactory agreement has been observed between measurement and simulation. Compared with the designs in the literature, the developed converter exhibits good performance of high efficiency, ultra-broadband and angular stability. Potential applications can be expected in polarization-controlled devices, stealth surfaces, antennas, etc.

Millimeter wave (mmWave) communications have been considered as a promising candidate for next generation network since it offers orders-of-magnitude wider underutilized spectrum than classical microwave bands. To combat the severe signal attenuation of the mmWave channel, hybrid precoding has recently received extensive attention. In this paper, we transform the hybrid precoding problem and develop a low complexity joint hybrid precoding algorithm. First, the analog precoder and combiner are obtained by iterative updating using the coordinate descent method. Then, the joint design of the digital precoder and combiner are computed based on the obtained analog precoder and combiner. Simulation results demonstrate that the proposed algorithm approaches optimal unconstrained percoding performance with low complexity. Finally, we investigate the performance of the proposed algorithm for the practical setting, in which only the quantized analog components are available, and find that it is still effective even with coarse quantization.