A fast living-body localization algorithm using timedifferential channel suitable for multi-path environments is introduced. In this method, a time-differential channel is calculated from the difference among the observed channels that correspond to known biological activities such as respiration and heartbeat. The living-body locations are estimated by applying two-dimensional MUltiple SIgnal Classification (MUSIC) to the correlation matrix calculated from the time-differential channels. Experiments were carried out in an indoor environment, and the results show that the proposed localization algorithm could well estimate multiple device-free living-bodies simultaneously. The experimental results demonstrate that the locations of up to three persons could be estimated faster, within a few seconds, and more accurately than the conventional method. One, two or three targets yield 90% values of localization errors of 0.23 m, 0.41 m and 1.29 m, respectively.

The elderly are constantly in danger of falling and injuring themselves without anyone realizing it. A safety-monitoring system based on microwaves can ease these concerns. The authors have proposed safety-monitoring systems that use multiple-input multiple-output (MIMO) radar to localize persons by capturing their biological activities such as respiration. However, our studies to date have focused on localization, which is easier to achieve than an estimation of human postures. This paper proposes a human posture identification scheme based on height and a Doppler radar cross section (RCS) as estimated by a MIMO array. This scheme allows smart home applications to dispense with contact and wearable devices. Experiments demonstrate that this method can identify the supine position (i.e., after a fall) with 100% accuracy, and the average identification rate is 95.0%.

In this paper, we introduce an interference suppression technique that combines a multiple-input multiple-output antenna arrangement with eigen-beamforming suitable for space division duplex relay that uses separate transmitter (Tx) and receiver (Rx) in relay stations (RSs) to realize full-duplex communication. The proposed technique degenerates the rank of the interference channel matrix to basically one at the RSs by careful arrangement of the antenna, which allows the Tx and Rx arrays to use nearly zero space weights. This scheme offers significant interference suppression while consuming only one degree of spatial division multiplexing freedom. To implement this scheme, we adopt the end-fire arrangement, where all antenna elements are arranged in line. In line-of-sight environments, this arrangement degenerates the rank of the interference channel matrix. The results of a numerical analysis and an experiment show that the proposed method can reduce the interference by up to 75.2 and 36.5 dB, respectively. The simulation using measured data clarifies that the proposed system attains 1.81 times higher capacity (12.1 b/s/Hz) than the conventional system.

Multiple-input multiple-output (MIMO) antennas are commonly evaluated by using the correlation coefficient. However, the correlation has several different definitions and calculation methods, and this misleads the real performance of the MIMO antennas. This paper describes five definitions and their differences of the correlation, i.e., signal correlation, channel correlation, fading correlation, complex pattern correlation, and S-parameter-based correlation. The difference of these characteristics is theoretically and numerically discussed, and their relations are schematically presented. Furthermore, the relationship between the correlation coefficient and the antenna element spacing is numerically evaluated, and it is shown that correlation coefficient calculated only from the S-parameter yields the error greater than 10% when the efficiency of the antenna is lower than 97%.

This paper proposes a method of implementing analog eigenmode transmission for a short-range multiple-input-multiple-output (SR-MIMO) system that uses only 180° hybrid couplers. If the short-range MIMO system places transmitting and receiving arrays in line-of-sight (LOS) and symmetrically with respect to specific reference planes, the channel matrix in the system also becomes symmetrical. This allows the channel matrix to be diagonalized by connecting 180° hybrid couplers to both ports at the transmitter and receiver sides, i.e., eigenmode transmission can be realized. In this paper, first, the theory of the proposed technique is explained by taking a 2 $\times$ 2 MIMO as an example, based on the characteristics of 180° hybrid coupler and the theory of the cascade-connected network. Moreover, an extension to the method that offers 4 $\times$ 4 MIMO is also proposed. This paper clarifies that this proposed technique can diagonalize the channel matrix, regardless of array configuration, such as type of antennas and antenna spacing, as long as the symmetry of the system configuration is assured. Furthermore, numerical results verify that the distance between the transmitter and the receiver has no effect on channel orthogonality. As a result, this technique partially avoids the performance degradation caused by array displacement, which is a major disadvantage of the conventional analog-beamforming-based decoding technique for short-range MIMO systems. Thus, the superiority of the proposed technique is clear.

This paper presents an improvement of interference suppression between mobile terminals using the antenna selection method of a mobile terminal in a Full-duplex MIMO system. In the system, a method of simultaneously transmitting and receiving a signal on the same frequency band in uplink and downlink between a base station and a mobile terminal has been studied. As a method of terminal-to-terminal interference suppression, a method of null beamforming for the first eigenvalue of the terminal-to-terminal channel by eigen-beamforming has been studied. The authors propose a method to realize further interference suppression by applying the antenna selection method to the mobile terminal when performing eigenbeamforming. The examination results of simulation using the ray tracing method and measurement in outdoor environment are reported assuming 4×4 MIMO in the frequency band of 12.9GHz. From the evaluation results, the interference suppression effect by the antenna selection method is improved by about 3 to 5 dB compared with the conventional 4 element MIMO antenna.