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We proposed an analog eigenmode transmission technique, that realizes multi-stream parallel transmission in a short-range multipleinput multiple-output (SR-MIMO) communication using only 180-degree hybrid couplers. Provided the MIMO array is symmetric, this technique can orthogonalize MIMO channels independently of antenna spacing and Tx-Rx distance. Furthermore, due to wideband characteristics of a hybrid, the channel orthogonalization effect can hold over a broad band of frequencies. In this letter, its array parameter independency and wideband characteristics are quantitatively evaluated in term of channel capacity. It has been experimentally clarified that this technique achieves approximately 90-% performance of ideal eigenbeam space division multiplexing (E-SDM) by digitalbeamforming despite the deviations of the above factors.

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The usage of this PDF file must comply with the IEICE Provisions on Copyright. The author(s) can distribute this PDF file for research and educational (nonprofit) purposes only. Distribution by anyone other than the author(s) is prohibited.SUMMARY In-band full-duplex (FD) Multiple-Input and Multiple-Output (MIMO) communication performs uplink and downlink transmission at the same time using the same frequency. In this system, the spectral efficiency is theoretically double that of conventional duplex schemes, such as Time Division Duplex (TDD) and Frequency Division Duplex (FDD). However, this system suffers interference because the uplink and downlink streams coexist within the same channel. Especially at the terminal side, it is quite difficult for the terminal to eliminate the interference signals from other terminals since it has no knowledge about the contents of the interference signals. This paper presents an inter-terminal interference suppression method between the uplink and downlink signals assuming the multi-user environment. This method uses eigen-beamforming at the transmitting terminal to direct the null to the other terminal. Since this beamforming technique reduces the degrees of freedom available, the interference suppression performance and transmitting data-rate have a trade-off relation. This study investigates the system capacity characteristics in multi-user fullduplex MIMO communication using the propagation channel information measured in an actual outdoor experiment and shows that the proposed communication scheme offers higher system capacity than the conventional scheme. key words: MIMO, full duplex, interference reduction, inter-terminal interference

In order to realize a full-duplex wireless communication, the authors have proposed a spatial self-interference reduction technique with an end-fire array (EFA) which consumes only one degree of the multiple-input multiple-output (MIMO) freedom. However, this technique has a limited interference reduction performance due to an insufficient rank-degenerated interference channel because amplitude differences of the paths increase the rank of the channel in the EFA. In this letter, the authors propose a rotationally-symmetrical array (RSA) suitable for our conventionally proposing scenario to reduce the self-interference, where geometrical symmetry of the MIMO paths achieves a perfectly one-ranked channel and suppresses all of the self-interference in theory. Performance evaluation on the interference reduction level P BF clarifies that the RSA reduces more interference than the EFA: P BF = 47.3 dB (RSA), 23.0 dB (EFA) while array length of the transmitter (Tx), element spacing of the receiver (Rx), and distance between the Tx and Rx are 3.5 wavelengths, 0.5 wavelengths, and 5 wavelengths, respectively. Keywords: MIMO, full-duplex, interference reduction, rotationally-symmetrical array, eigen-beamforming Classification: Antennas and Propagation References[1] A. Sabharwal, P. Schniter, D. Guo, D. W. Bliss, S. Rangarajan, and R. Wichman, "In-band full-duplex wireless: challenges and opportunities," IEEE J. Sel. Areas