As a promising technology in the next generation mobile network, millimeter-wave (mmWave) communication can mitigate the spectrum crunch of improving the network capacity by exploiting the large underutilized spectrum bands of the mmWave frequencies. The hybrid (analog/digital) beamforming of multi-data streams are widely used to further the spectrum efficiency of mmWave relay system when faced with the complex environment or long distance communication. This paper investigates the hybrid beamforming scheme for the decode-and-forward (DF) mmWave massive multiple-input multiple-output (MIMO) relay system with mixed structure and full-connected structure. We optimize hybrid beamforming of relay system by maximizing the sum rate of the overall system as an objective function. To reduce the computational complexity, we reformulate the original problem as two single-hop mmWave MIMO sum-rate maximization subproblems. Then, the piecewise successive approximation method is proposed based on the criterion which jointly designs the analog and digital beamforming stages by trying to avoid the loss of information at each stage. The hybrid beamforming of the two subproblems can be solved by the proposed scheme united with the idea of successive interference cancelation (SIC), the baseband block diagonalization (BD) scheme, and waterfilling power allocation method. Finally, simulation results confirm that the proposed optimal method can achieve good performance in hybrid beamforming design of relay system with both mixed and full-connected structures.

We experimentally demonstrated a tunable terahertz bandpass filter based on microelectromechanical systems (MEMS) reconfigurable metamaterials. The unit cell of the filter consists of two split-ring resonators (SRRs) and a movable bar. Initially, the movable bar situates at the center of the unit cell, and the filter has two passbands whose central frequencies locate at 0.65 and 0.96 THz. The intensity of the two passbands can be actively modulated by the movable bar, and a maximum modulation depth of 96% is achieved at 0.96 THz. The mechanism of tunability is investigated using the finite-integration time-domain method. The result shows that the image currents induced on the movable bar are opposite the resonance currents induced on the SRRs and, thus, weaken the oscillating intensity of the resonance currents. This scheme paves the way to dynamically control and switch the terahertz wave at some constant frequencies utilizing induced image currents.

We experimentally demonstrate a broadband terahertz (THz) intensity modulator operating at low current and frequency range. The modulator consists of VO2 and meander–wire hybrid metamaterials. The measured 3-dB bandwidth for normal incidence is 0.5 THz, while the maximum modulation depth of 99% is achieved at 0.28 A. The physical mechanism of the device’s electrical tunability is attributed to an ohmic-heating-induced transition. An equivalent circuit model is proposed for the upper cut-off frequency. This scheme paves the way for a full electric control of the intensity of a THz wave at a broad band, low current, and low frequency range.

No abstract

For multiple-antenna systems, the technologies of joint symbol and channel parameter estimation have been developed in recent works. However, existing technologies have a number of problems, such as performance degradation and the large cost of prior information. In this paper, a tensor space-time coding scheme in multiple-antenna systems was considered. This scheme allowed spreading, multiplexing, and allocating information symbols associated with multiple transmitted data streams. We showed that the received signal was formulated as a third-order tensor satisfying a Tucker-2 model, and then a robust semi-blind receiver was developed based on the optimized Levenberg–Marquardt (LM) algorithm. Under the assumption that the instantaneous channel state information (CSI) is unknown at the receiving end, the proposed semi-blind receiver jointly estimates the information symbol and channel parameters efficiently. The proposed receiver had a better estimation performance compared with existing semi-blind receivers, and still performed well when the channel became strongly correlated. Moreover, the proposed semi-blind receiver could be extended to the multi-user massive multiple-input multiple-output (MIMO) system for joint symbol and channel estimation. Computer simulation results were shown to demonstrate the effectiveness of the proposed receiver.