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Recently, research on the design of relay structure and optimal precoding matrix in massive multiple-input multiple-output (MIMO) relay systems has been proposed in many works assuming that partial or all of the channel state information (CSI) is known. However, in the practical communication systems, the CSI is unknown and needs to be estimated. In particular, channel estimation is more difficult when the relay adopts a hybrid precoding structure. In this paper, we propose an efficient channel estimation scheme based on Tucker-2 tensor model for a multi-way massive MIMO hybrid relay system. The proposed algorithm provides all the CSI involved in the communication links at each user end. Compared with traditional channel estimation algorithms, the proposed algorithm has better estimation performance and requires few iterations to achieve convergence. In addition, we derive the analytical expression of the Cramér-Rao lower bound (CRB) for reference. Simulation results verify the effectiveness of the proposed Tucker-2-based channel estimation approach. [ABSTRACT FROM AUTHOR]

Indirect tire pressure monitoring system monitors inflation pressures in pneumatic tires using wheel speed sensor (WSS) signals acquired from anti-lock braking system. In order to monitor the tire pressures indirectly, frequency analysis and wheel radius analysis are operating in parallel. This paper focuses on the frequency analysis. The resonance frequency of tire torsional vibration decreases as the tire pressure decreases. Therefore, the resonance frequency needs to be estimated to detect tire pressure decrease. This paper describes the resonance frequency estimation method with the WSS. Firstly, toothed wheel error of the WSS is calculated from the wheel speed signals and the wheel speed is corrected with the toothed wheel error. A notch filter is designed based on the engine rotational speed to eliminate the engine noise. The resonance frequency is estimated by model parameter estimation based on auto regressive model. Finally, compensation of the estimated frequency, detection of snow chain and monitoring of severe driving environments have been proposed to prevent false alarms at normal tire pressures. Vehicle tests have been conducted to evaluate the proposed estimation method of the resonance frequency. It has been shown that the proposed method provides satisfactory performance for warning indication of tire pressure loss from the test results. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses articles in the issue on topics including channel estimation; symbol error rate; and cognitive radio network (CRN).